Showing papers on "Fault (geology) published in 1993"

Updated interpretation of magnetic anomalies and seafloor spreading stages in the south China Sea: Implications for the Tertiary tectonics of Southeast Asia

Abstract: We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ≈50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ≈16 Ma) and the average spreading rate was slower, ≈35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons, we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale “domino” models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion along the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia.

Internal structure and weakening mechanisms of the San Andreas Fault

Abstract: New observations of the internal structure of the San Gabriel fault (SGF) are combined with previous characterizations of the Punchbowl fault (PF) to evaluate possible explanations for the low frictional strength and seismic characteristics of the San Andreas fault (SAF). The SGF and PF are ancient, large-displacement faults of the SAF system exhumed to depths of 2 to 5 km. These fault zones are internally zoned; the majority of slip was confined to the cores of principal faults, which typically consist of a narrow layer (less than tens of centimeters) of ultracataclasite within a zone of foliated cataclasite several meters thick. Each fault core is bounded by a zone of damaged host rock of the order of 100 m thick. Orientations of subsidiary faults and other fabric elements imply that (1) the maximum principal stress was oriented at large angles to principal fault planes, (2) strain was partitioned between simple shear in the fault cores and nearly fault-normal contraction in the damaged zones and surrounding host rock, and (3) the principal faults were weak. Microstructures and particle size distributions in the damaged zone of the SGF imply deformation was almost entirely cataclastic and can be modeled as constrained comminution. In contrast, cataclastic and fluid-assisted processes were significant in the cores of the faults as shown by pervasive syntectonic alteration of the host rock minerals to zeolites and clays and by folded, sheared, and attenuated cross-cutting veins of laumontite, albite, quartz, and calcite. Total volume of veins and neocrystallized material reaches 50% in the fault core, and vein structure implies episodic fracture and sealing with time-varying and anisotropic permeability in the fault zone. The structure of the ultracataclasite layer reflects extreme slip localization and probably repeated reworking by particulate flow at low effective stresses. The extreme slip localization reflects a mature internal fault structure resulting from a positive feedback between comminution and transformation weakening. The structural, mechanical, and hydrologic characteristics of the Punchbowl and San Gabriel faults support the model for a weak San Andreas based on inhomogeneous stress and elevated pore fluid pressures contained within the core of a seismogenic fault. Elevated fluid pressures could be repeatedly generated in the core of the fault by a combination of processes including coseismic dilatancy and creation of fracture permeability, fault-valve behavior to recharge the fault with fluid, post-seismic self-sealing of fracture networks to reduce permeability and trap fluids, and time-dependent compaction of the core to generate high pore pressure. The localized slip and fluid-saturated conditions are wholly compatible with additional dynamic weakening by thermal pressurization of fluids during large seismic slip events, which can help explain both the low average strength of the San Andreas and seismogenic characteristics such as large stress relief. In addition, such a dynamic weakening mechanism is expected only in mature fault zones and thus could help explain the apparent difference in strength of large-displacement faults from smaller-displacement, subsidiary seismogenic faults.

Growth of normal faults: Displacement-length scaling

Abstract: The form of the scaling relation between the displacement and length of faults has been a subject of considerable controversy because of insufficient scale range and scattered data. Here we report on displacement and length data collected from well-exposed normal faults located on the Volcanic Tableland in northern Owens Valley, California. These data, which exhibit little scatter, are from a fault population that spans three orders of magnitude in fault length and were gathered in a relatively uniform lithologic and tectonic setting. With the upper cooling surface of the middle Quaternary Bishop Tuff used as a marker, the displacement distribution along individual faults can be mapped in detail. The displacement distribution profiles are consistent with a linear relation between displacement and fault length.

Dynamics of fault interaction: parallel strike‐slip faults

Abstract: We use a two-dimensional finite difference computer program to study the effect of fault steps on dynamic ruptures. Our results indicate that a strike-slip earthquake is unlikely to jump a fault step wider than 5 km, in correlation with field observations of moderate to great-sized earthquakes. We also find that dynamically propagating ruptures can jump both compressional and dilational fault steps, although wider dilational fault steps can be jumped. Dilational steps tend to delay the rupture for a longer time than compressional steps do. This delay leads to a slower apparent rupture velocity in the vicinity of dilational steps. These “dry” cases assumed hydrostatic or greater pore-pressures but did not include the effects of changing pore pressures. In an additional study, we simulated the dynamic effects of a fault rupture on ‘undrained’ pore fluids to test Sibson's (1985, 1986) suggestion that “wet” dilational steps are a barrier to rupture propagation. Our numerical results validate Sibson's hypothesis by demonstrating that the effect of the rupture on the ‘undrained’ pore fluids is to inhibit the rupture from jumping dilational stepovers. The basis of our result differs from Sibson's hypothesis in that our model is purely elastic and does not necessitate the opening of extension fractures between the fault segments.

Hydrological signatures of earthquake strain

Abstract: The character of the hydrological changes that follow major earthquakes has been investigated and found to be dependent on the style of faulting. The most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake, and typically, excess flow is sustained for a period of 6–12 months. In contrast, hydrological changes accompanying pure reverse fault earthquakes are either undetected or indicate lowering of well levels and spring flows. Strike-slip and oblique-slip fault movements are associated with a mixture of responses but appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakes in the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is sufficient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The discharge has been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distances greater than 50 km. The total volume of water released in these earthquakes was around 0.3 km3 (Borah Peak) and 0.5 km3 (Hebgen Lake) Qualitative information on other major normal fault earthquakes, in both the western United States and Italy, indicates that the size, duration, and range of their hydrological signatures have been similar. The magnitude and distribution of the water discharge for these events are compared with deformation models calibrated using seismic and geodetic information. The quantity of water released over a time period of 6–12 months suggests that crustal volume strain to a depth of at least 5 km is involved. The rise and decay times of the discharge are shown to be critically dependent on crack widths, and it is concluded that the dominant cracks have a high aspect ratio and cannot be much wider than 0.03 mm. Using the estimated depth to which water is mobilized, the modeled crack size, and the measured volumes of water expelled, it is concluded that even at distances of 50 km from the earthquake epicenters, cracks must be separated by no more than 10 or 20 m. In regions of highest discharge nearer the earthquake epicenters, separations of 1 or 2 m are required. These results suggest that water-filled cracks are ubiquitous throughout the brittle continental crust and that these cracks open and close throughout the earthquake cycle. The existence of tectonically induced fluid flows on the scale that we demonstrate has major implications for our understanding of the mechanical and chemical behavior of crustal rocks.

Fault growth and fault scaling laws: Preliminary results

Abstract: We report progress made in the last few years on the general problem of the mechanism of fault growth and the scaling laws that result Results are now conclusive that fault growth is a self-similar process in which fault displacement d scales linearly with fault length L Both this result and the overall nature of along-strike fault displacement profiles are consistent with the Dugdale-Barenblatt elastic-plastic fracture mechanics model In this model there is a region of inelastic deformation near the crack tip in which there is a breakdown from the yield strength of the unfractured rock to the residual frictional strength of the fault over a breakdown length S and displacement d0 Limited data also indicate that S and d0 also scale linearly with L, which implies that fracture energy G increases linearly with L The scaling parameters in these relationships depend on rock properties and are therefore not universal In our prime field locality, the Volcanic Tableland of eastern California, we have collected data over 2 orders of magnitude in scale range that show that faults obey a power law size distribution in which the exponent C in the cumulative distribution is ∼13 If the fault is growing within the brittle field, the zone of inelastic deformation consists of a brittle process zone which leaves a wake of fractured rock adjacent to the fault Preliminary results of modeling the process zone are consistent with observations now in hand both in predicting the preferred orientation of cracks in the process zone wake and the rate of falloff of crack density as a function of distance from the fault The preferred orientation of these cracks may be used to infer the mode and direction of propagation of the fault tip past the point in question According to the model, the width of the process zone wake may be used to infer the length of the fault at the time its tip passed the measurement point, but data have not yet been collected to verify this prediction If the fault displacement has been accumulated by repeated seismic slips, each of these will sweep the fault with a crack tip stress field of a smaller spatial extent than that of the fault tip stress field, producing an inner, more intensely fractured, process zone wake This may be the mechanism that creates the cataclasite zone, rather than simple frictional wear, as has been previously supposed

Model for episodic flow of high-pressure water in fault zones before earthquakes

Abstract: In this model for the evolution of large crustal faults, water that originally came from the country rock saturates the initially highly porous and permeable fault zone. During shearing, the fault zone compacts and water flows back into the country rock, but the flow is arrested by silicate deposition that forms very low permeability seals between the fault zone and the country rock. Because of variations in temperature and mineralogical composition and the complex structure of the fault zone, a three-dimensional network of seals is formed in the fault zone itself; thus, the high-pressure fluid is not evenly distributed. As in deep oil reservoirs, the fluid will be confined to seal-bounded fluid compartments of various sizes and porosity that are not hydraulically connected with each other or with the hydrostatic regime in the country rock. When the seal between two of these compartments is ruptured, an electrical streaming potential will be generated by the sudden movement of fluid from the high-pressure compartment to the low-pressure compartment. When the pore pressure in the two compartments reaches its final equilibrium state, the average effective normal stress across them may be lower than it was initially, and, if the two compartments are large enough, this condition may trigger an earthquake. During an earthquake, many of the remaining seals will be ruptured, and the width of the fault zone will increase by failure of the geometric irregularities on the fault. This newly created, highly porous and permeable, but now wider fault zone will fill with water, and the process described above will be repeated. Thus, the process is an episodic one, with the water moving in and out of the fault zone, and each large earthquake should be preceded by an electrical and/or magnetic signal.

Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

Abstract: THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1–5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7–10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, D c has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (∼10−5m) and values obtained from modelling earthquakes (∼10−2m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102–103 m thick12–17, whereas laboratory gouge layers are 1–10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

Transfer zones in extensional basins: their structural style and influence on drainage development and stratigraphy

Abstract: Transfer zones form important structural elements in extensional basins, accommodating displacement changes between individual fault and basin segments. Transfer zone geometry is related to the extension direction and the displacement, dip polarity, overlap and overstep of fault/basin segments adjacent to the zone. Topographic changes associated with transfer zones have a direct influence on drainage basin evolution, sediment transport and stratigraphy. Two main categories of transfer zone can be identified: (i) interbasin transfer zones, linking individual half graben, and (ii) intrabasin transfer zones, linking individual fault segments within a half graben. Interbasin transfer zones range from interbasin ridges to broad faulted highs and major relay ramps. They have a marked influence on basin stratigraphy and drainage evolution, often separating half graben with distinct stratigraphies and acting as conduits through which major axial depositional systems enter the rift zone. Intrabasin transfer zones range from relay ramps separating adjacent en echelon normal faults to discrete fault jogs. Intrabasin transfer zones commonly act as a conduit for local sediment transport, but have minimal effect on basin-scale stratigraphy. Transfer zones also affect early post-rift sedimentation and are important elements in controlling fluid migration in the subsurface.

Determination of earthquake energy release and ML using TERRAscope

Abstract: We estimated the energy radiated by earthquakes in southern California using on-scale very broadband recordings from TERRAscope. The method we used involves time integration of the squared ground-motion velocity and empirical determination of the distance attenuation function and the station corrections. The time integral is typically taken over a duration of 2 min after the P-wave arrival. The attenuation curve for the energy integral we obtained is given by q(r) = cr^(−n)exp(−kr)(r^2 = Δ^2 + h_(ref)^2) with c = 0.49710, n = 1.0322, k = 0.0035 km^(−1), and h_(ref) = 8 km, where Δ is the epicentral distance. A similar method was used to determine M_L using TERRAscope data. The station corrections for M_L are determined such that the M_L values determined from TERRAscope agree with those from the traditional optical Wood-Anderson seismographs. For 1.5 6.5, M_L saturates. The ratio E_S/M_0 (M_0: seismic moment), a measure of the average stress drop, for six earthquakes, the 1989 Montebello earthquake (M_L = 4.6), the 1989 Pasadena earthquake (M_L = 4.9), the 1990 Upland earthquake (M_L = 5.2), the 1991 Sierra Madre earthquake (M_L = 5.8), the 1992 Joshua Tree earthquake (M_L = 6.1), and the 1992 Landers earthquake (M_w = 7.3), are about 10 times larger than those of the others that include the aftershocks of the 1987 Whittier Narrows earthquake, the Sierra Madre earthquake, the Joshua Tree earthquake, and the two earthquakes on the San Jacinto fault. The difference in the stress drop between the mainshock and their large aftershocks may be similar to that between earthquakes on a fault with long and short repeat times. The aftershocks, which occurred on the fault plane where the mainshock slippage occurred, had a very short time to heal, hence a low stress drop. The repeat time of the major earthquakes on the frontal fault systems in the Transverse Ranges in southern California is believed to be very long, a few thousand years. Hence, the events in the Transverse Ranges may have higher stress drops than those of the events occurring on faults with shorter repeat times, such as the San Andreas fault and the San Jacinto fault. The observation that very high stress-drop events occur in the Transverse Ranges and the Los Angeles Basin has important implications for the regional seismic potential. The occurrence of these high stress-drop events near the bottom of the seismogenic zone strongly suggests that these fault systems are capable of supporting high stress that will eventually be released in major seismic events. Characterization of earthquakes in terms of the E_S/M_0 ratio using broadband data will help delineate the spatial distribution of seismogenic stresses in the Los Angeles basin and the Transverse Ranges.

Three-dimensional velocity structure, seismicity, and fault structure in the Parkfield Region, central California

Abstract: This study examines the three-dimensional velocity structure in a 60- by 80-km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three-dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across-fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4-km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low-velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20-km-long high-velocity slice is imaged northeast of the SAF near Gold Hill. Its along-fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right-lateral strike-slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.

The interaction between normal faulting and drainage in active extensional basins, with examples from the western United States and central Greece

Abstract: Faulting exerts an important control upon drainage development in active extensional basins and thus helps determine the architecture of the sedimentary infill to a synrift basin. Examples of the interaction between faulting and drainage from the western United States and central Greece may be grouped into a relatively small number of classes based upon the structural position of a drainage catchment: footwall, hangingwall, fault offset and axial. Our examples illustrate the diversity of erosional effects that might arise because of variations in the spacing, orientation and segmentation of faults and their interactions. Where basement lithology is similar, footwall catchments are generally smaller, shorter and steeper than those of the hangingwall. Footwall-sourced alluvial fans and fan deltas are generally smaller in area than those sourced from similar lithologies in the hangingwall. Wide fault offsets often give rise to large drainage catchments in the footwall. The development of axial drainage depends upon the breaching of transverse bedrock ridges by headward stream erosion or by lake overflow. Once breaching has occurred the direction of axial streamflow is controlled by the potential developed between basins of contrasting widths. Fault migration and propagation leads to the uplift, erosion and resedimentation of the sedimentary infill to formerly active basins, leading to the cutting of footwall unconformities. The outward sediment flux from structurally controlled catchments is modulated in an important way by lithology and runoff. The greatest contrasts in basement lithology arise when fault migration and propagation have occurred, such that the sedimentary fill to previously active basins is uplifted, incised and eroded by the establishment of large new drainage systems in the footwalls of younger faults. Drainage patterns in areas.where faults interact can shed light on the relative timing of activity and therefore the occurrence of fault migration and propagation. Facies and palaeocurrent trends in ancient grabens may only be correctly interpreted when observations are made on a length scale of 10-20 km, comparable to that of the largest fault segments.

Fission track analysis of the Late Cenozoic vertical kinematics of continental pacific crust, South Island, New Zealand

Abstract: During the late Cenozoic the Pacific plate has been converging obliquely with the Australia plate in South Island, New Zealand A result of this convergence has been the growth of a major mountain range (the Southern Alps) at the leading edge of the Pacific plate The results of fission track analysis of 140 samples from 13 transects across the Alps reported here establish the late Cenozoic vertical kinematics (amount, age, and rate of rock uplift) of the Pacific crust underlying the Alps The late Cenozoic rock uplift of the Pacific crust is asymmetrical with respect to the Alpine fault, being a maximum (19 km) immediately east of the central part of the fault, with lesser values at the eastern (3 km), northern (10 km), and southern (8 km) extremities of the Alps The age of the start of rock uplift varies spatially across the Southern Alps, the earliest indications from fission track analysis being at 8 Ma at the southern end of the Alps, decreasing to 5 Ma at the northern end and 3 Ma along the southeastern margin This age variation reflects the longer time over which the southern parts of the Alps have been in collision The rate of propagation of rock uplift southeastward into the Pacific plate has been 30 mm/yr, nearly 4 times the late Cenozoic average rate of convergence normal to the plate boundary Late Cenozoic mean rock uplift rates range from a maximum of ∼28 mm/yr at the Alpine fault to a minimum of ∼10 mm/yr in the east and have been sustained for periods of 3–8 my Accompanying denudation has exhumed amphibolite grade rocks immediately east of the Alpine fault The rock uplift has been controlled by oblique-slip displacement on the Alpine fault A continental crustal section at least 19 km thick has been uplifted on the Alpine fault Comparison of the late Cenozoic mean rock uplift rates with uplift rates derived from reset zircon data (2–10 mm/yr) near the Alpine fault shows that uplift has accelerated over time, but only significantly since 13 ± 03 Ma The amount of Mesozoic uplift ranged from minimal amounts north of Arthur's Pass, to ∼3 km near Mount Cook, to 10 km in the south at Lake Wanaka

Structural, petrological and thermal evolution of a tertiary ductile strike-slip shear zone, Diancang Shan, Yunnan

Abstract: The Diancang Shan, a horst massif within the Red River fault zone in Yunnan, People's Republic of China, preserves a structural, petrological, and thermal record of two distinct phases of tectonic activity: a left-lateral ductile shear that terminated between 20 and 17 Ma and a ductile-to-brittle phase of normal faulting which began at 4.7 Ma and remains active. Mylonitic rocks in the core of the range display an early, steep, high-temperature (HT), schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt. Kinematic indicators indicate that shear was left-lateral. The complex shape of the HT schistosity at the southern termination of the massif likely results from a large-scale, oblique, left-lateral C' shear plane that dismembered the shear zone and separated the Ailao Shan and the Diancang Shan as left-lateral deformation terminated. Thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event. Along the eastern edge of the Diancang Shan, the HT fabrics were overprinted by low-temperature structures during activation of east dipping normal faults. Cooling associated with this normal/right-lateral faulting along the Diancang Shan (and perhaps activation of the right-lateral/normal movement on the Range Front fault farther south along the Ailao Shan) began at 4.7±0.1 Ma. These results tend to support the view that extrusion of Indochina occurred along the left-lateral Red River shear zone between 35 and 19–17 Ma. Initiation of right-lateral/normal slip during the late Miocene may relate to eastward extensional collapse of the thickened Tibetan crust or, more probably, to initiation of the second phase of extrusion.

Earthquake Failure Sequences Along a Cellular Fault Zone in a Three-Dimensional Elastic Solid Containing Asperity and Nonasperity Regions

Abstract: Numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three-dimensional (3-D) model representing approximately the central San Andreas fault. The model consists of an upper crust overlying a lower crust and mantle region, together defining an elastic half-space with a vertical half-plane fault. The fault contains a region where slip is calculated on a uniform grid of cells governed by a static/kinetic friction law and regions where slip is prescribed so as to represent tectonic loading, aseismic fault creep, and adjacent great earthquakes. The computational region models a 70-km-long and 17.5-km-deep section of the San Andreas fault to the NW of the great 1857 rupture zone. Different distributions of stress drops on failing computational cells are used to model asperity (“Parkfield asperity”) and nonasperity fault regions. The model is “inherently discrete” and corresponds to a situation in which a characteristic size of geometric disorder within the fault (i.e., cell size, here a few hundreds of meters) is much larger than the “nucleation size” (of the order of tens of centimeters to tens of meters) based on slip weakening or state evolution slip distances. The computational grid is loaded by a constant plate motion imposed at the lower crust, upper mantle, and creeping fault regions and by a “staircase” slip history imposed at the 1857 and 1906 rupture zones. Stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3-D elastic dislocation theory. The resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops. The frequency-magnitude statistics of the simulated failure episodes are approximately self-similar for small events, with b ≈ 1.2 (the b value of statistics based on rupture area bA is about 1) but are strongly enhanced with respect to self-similarity for events larger than a critical size. This is interpreted as a direct manifestation of our 3-D elastic stress transfer calculations; beyond certain rupture area and potency (seismic moment divided by rigidity) release values, the event is usually unstoppable, and it continues to grow to a size limited by a characteristic model dimension. This effect is not accounted for by cellular automata and block-spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size. The simulations suggest that local maxima in observed frequency-magnitude statistics correspond to dimensions of coherent brittle zones, such as the width of the seismogenic layer or the length of a fault segment bounded by barriers. The analysis indicates that a single cell size, representing approximately a single scale of geometric disorder, cannot induce self-similarity in a 3-D elastic model over a broad range of magnitudes. A representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self-similar Gutenberg-Richter statistics. Our simulations show a great diversity in the mode of failure of the Parkfield asperity; the earthquakes themselves define an irregular sequence of events. The modeling, like many other discrete fault models, suggests that expectations for periodic Parkfield earthquakes and/or simple precursory patterns repeating from one event to the other are unrealistic.

The 1992 Landers Earthquake Sequence: Seismological observations

Abstract: The (M_W 6.1, 7.3, 6.2) 1992 Landers earthquakes began on April 23 with the M_W6.1 1992 Joshua Tree preshock and form the most substantial earthquake sequence to occur in California in the last 40 years. This sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long. The faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the San Andreas fault. The M_W6.1 Joshua Tree preshock at 33°N58′ and 116°W19′ on 0451 UT April 23 was preceded by a tightly clustered foreshock sequence (M≤4.6) beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks. The aftershocks extended along a northerly trend from about 10 km north of the San Andreas fault, northwest of Indio, to the east-striking Pinto Mountain fault. The M_w7.3 Landers mainshock occurred at 34°N13′ and 116°W26′ at 1158 UT, June 28, 1992, and was preceded for 12 hours by 25 small M≤3 earthquakes at the mainshock epicenter. The distribution of more than 20,000 aftershocks, analyzed in this study, and short-period focal mechanisms illuminate a complex sequence of faulting. The aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults, and 40 km to the south, crossing the Pinto Mountain fault through the Joshua Tree aftershock zone towards the San Andreas fault near Indio. The rupture initiated in the depth range of 3–6 km, similar to previous M∼5 earthquakes in the region, although the maximum depth of aftershocks is about 15 km. The mainshock focal mechanism showed right-lateral strike-slip faulting with a strike of N10°W on an almost vertical fault. The rupture formed an arclike zone well defined by both surficial faulting and aftershocks, with more westerly faulting to the north. This change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west. A 20-km-long linear cluster of aftershocks occurred 10–20 km north of Barstow, or 30–40 km north of the end of the mainshock rupture. The most prominent off-fault aftershock cluster occurred 30 km to the west of the Landers mainshock. The largest aftershock was within this cluster, the M_w6.2 Big Bear aftershock occurring at 34°N10′ and 116°W49′ at 1505 UT June 28. It exhibited left-lateral strike-slip faulting on a northeast striking and steeply dipping plane. The Big Bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north. The Landers mainshock occurred near the southernmost extent of the Eastern California Shear Zone, an 80-km-wide, more than 400-km-long zone of deformation. This zone extends into the Death Valley region and accommodates about 10 to 20% of the plate motion between the Pacific and North American plates. The Joshua Tree preshock, its aftershocks, and Landers aftershocks form a previously missing link that connects the Eastern California Shear Zone to the southern San Andreas fault.

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

Abstract: The Irpinia fault was the source of the Ms 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre-1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

Earthquakes on the Kazerun Line in the Zagros Mountains of Iran: strike‐slip faulting within a fold‐and‐thrust belt

Abstract: Summary The Kazerun Line is a transverse valley of about 200 km long that obliquely crosses the regular anticlines of the Zagros fold belt in SW Iran. At its northern end it is a clear fault which can be mapped on the surface. Anticline axes die out or bend towards this valley but do not cross it. Six moderate-sized earthquakes that occurred close the the Kazerun Line, and within a 25 km area involved right-lateral strike-slip motion parallel to the strike of the valley. They indicate that the Kazerun Line is the surface expression of a buried strike-slip fault. Slip vectors in these strike-slip earthquakes are different from those of neighbouring reverse-fault earthquakes, suggesting that the Kazerun Line accommodates some of the shortening between Arabia and central Iran by an elongation of the Zagros mountains parallel to strike. The centroid depths and the source dimensions of these earthquakes, combined with the lack of seismogenic surface faulting in the Zagros, suggest to us that all these earthquakes involve faulting in the metamorphic basement beneath the sedimentary cover. The sedimentary cover is almost certainly decoupled from the basement by several thick evaporite horizons. The seismicity of the Kazerun Line thus demonstrates how lateral interruptions to the regularity of a fold belt can arise from faulting in the basement, and not just from lateral ramps between the thrust sheets that deform the sedimentary cover.

Causes and consequences of variations in faulting style at the Mid‐Atlantic Ridge

Abstract: Both volcanism and faulting contribute to the rugged topography that is created at the Mid-Atlantic Ridge (MAR) and preserved off-axis in Atlantic abyssal hill terrain. Distinguishing volcanic from fault-generated topography is essential to understanding the variations in these processes and how these variations are affected by the three-dimensional pattern of mantle upwelling, ridge segmentation, and offsets. Here we describe a new quantitative method for identifying fault-generated topography in swath bathymetry data by measuring topographic curvature. The curvature method can distinguish large normal faults from volcanic features, whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes. The combination of curvature and slope information allows inward and outward facing fault faces to be mapped. We apply the method to Sea Beam data collected along the MAR between 28° and 29°30′N. The fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework: long, linear, small-throw faults occur toward segment centers, while shorter, larger-throw, curved faults occur toward ends; these variations reflect those of active faults within the axial valley. We investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the MAR. In the first model only one fault is active at a time on each side of the rift valley. Each fault grows while migrating away from the volcanic center due to dike injection; extension across the fault causes a flexural rotation of nearby inactive faults. The amount of stress necessary to displace the fault increases as the fault grows. When reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley. This model yields a realistic terracelike morphology from the rift valley floor into the rift mountains; the relief is caused by the net rotation accumulated in the lithosphere from the active faults (e.g., 10° reached 20 km from the active fault). Fault spacing is controlled by lithospheric thickness, fault angle, and the ratio of amagmatic to magmatic extension. We hypothesize that this mechanism may be dominant toward ridge segment offsets. An alternative model considers multiple active faults; each fault relieves stresses as it grows and inhibits the growth of nearby faults, causing a characteristic fault spacing. Such fault interactions would occur in a region of necking instability involving deformation over an extended area. This mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults. We hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints, causing the homogeneous abyssal hill fabric in these regions.

Fault strain and seismic coupling on mid‐ocean ridges

Abstract: The contribution of extensional faulting to seafloor spreading along the East Pacific Rise (EPR) axis near 3°S and between 13°N and 15°N is calculated using data on the displacement and length distributions of faults obtained from side scan sonar and bathymetric data. It is found that faulting may account for of the order of 5–10% of the total spreading rate, which is comparable to a previous estimate from the EPR near 19°S. Given the paucity of normal faulting earthquakes on the EPR axis, a maximum estimate of the seismic moment release shows that seismicity can account for only 1% of the strain due to faulting. This result leads us to conclude that most of the slip on active faults must be occurring by stable sliding. Laboratory observations of the stability of frictional sliding show that increasing normal stress promotes unstable sliding, while increasing temperature promotes stable sliding. By applying a simple frictional model to mid-ocean ridge faults it is shown that at fast spreading ridges (≥90 mm/yr) the seismic portion of a fault (Ws) is a small proportion of the total downdip width of the fault (Wƒ). The ratio Ws/ Wƒ interpreted as the seismic coupling coefficient X, and in this case X≈ 0. In contrast, at slow spreading rates (≤40 mm/yr), Ws≈Wƒ, and therefore X≈ 1, which is consistent with the occurrence of large-magnitude earthquakes (mb= 5.0 to 6.0) occurring, for example, along the Mid-Atlantic Ridge axis.

The interpretation of inverted metamorphic isograds using simple physical calculations

Abstract: Several zones of major thrust faulting exhibit a juxtaposition of rocks of higher metamorphic grade over rocks of lower grade. This configuration may indicate, but does not require, that temperature gradients were temporarily inverted near the fault. We examine the physical conditions under which such a temperature inversion could occur. The overthrusting of hotter rock on colder rock can cause a temporarily inverted gradient both above and below the fault only if the time taken to underthrust rock from the land surface to a given depth, zƒ, on the fault is less than π times the characteristic time, , for diffusion of heat through the block above the fault, where κ is thermal diffusivity. An inverted gradient cannot form without heat generation in the fault zone unless V × zƒ × sin δ ≳ 100, where V is the rate of underthrusting (in millimeters per year), zƒ is in kilometers, and δ is the dip of the fault. This simple criterion is sufficient to demonstrate that several examples of inverted metamorphic gradients cannot be explained simply by the thrusting of hot on cold rock without heat sources in the fault zone. Dissipative heating accompanying deformation can cause an inverted temperature gradient within and beneath the thrust zone, but whereas the overthrusting of hot upon cold rocks cools the fault zone, dissipation heats it. Thus the overthrusting of hot on cold rock and dissipative heating affect the temperature gradient and the maximum temperatures differently. We show that the magnitudes of the inverted temperature gradient and of the maximum temperature above the inverted gradient yield independent estimates of the rate of dissipative heating. Discrepancy between these estimates implies that some additional process must have occurred, such as the post-thrusting disruption of the isograds. If there is such a discrepancy, the maximum temperature probably provides the more reliable estimate of the rate of heating at the fault. We illustrate this analysis by applying it to reported inverted metamorphic zonation in the Pelona Schist, the St. Anthony Complex, the Mt. Everest region of the Main Central Thrust, and the Olympos Thrust. Petrological inferences of maximum temperatures, depths of metamorphism, and magnitudes of apparent inverted gradients, imply that shear stresses of about 100 MPa accompanied thrust faulting in some of these regions, and that some zones of inverted metamorphism have been tectonically thinned after the metamorphism occured.

Does magmatism influence low-angle normal faulting?

Abstract: Synextensional magmatism has long been recognized as a ubiquitous characteristic of highly extended terranes in the western Cordillera of the United States. Intrusive magmatism can have severe effects on the local stress field of the rocks intruded. Because a lower angle fault undergoes increased normal stress from the weight of the upper plate, it becomes more difficult for such a fault to slide. However, if the principal stress orientations are rotated away from vertical and horizontal, then a low-angle fault plane becomes more favored. We suggest that igneous midcrustal inflation occurring at rates faster than regional extension causes increased horizontal stresses in the crust that alter and rotate the principal stresses. Isostatic forces and continued magmatism can work together to create the antiformal or domed detachment surface commonly observed in the metamorphic core complexes of the western Cordillera. Thermal softening caused by magmatism may allow a more mobile mid-crustal isostatic response to normal faulting.

Focal mechanisms of large earthquakes in the South Island of New Zealand: implications for the accommodation of Pacific‐Australia plate motion

Abstract: b The plate motion model NUVEL-1 predicts oblique convergence between the Pacific and Australian plates in the South Island of New Zealand. We used P and SH body waveform analysis to constrain the focal mechanisms of the 15 largest earthquakes (Ms > 5.8) that have occurred in this region since 1964, in order to see how the plate motion is accommodated. At the southern end of the Alpine Fault, convergence is achieved by oblique slip movement along a concentrated zone of deformation. In the southern offshore region one event may be related to thrusting of the Australian plate beneath the Pacific plate, and another strike-slip event probably demonstrates movement on an active strike-slip fault system parallel to, but offset from, the southern limit of the Alpine Fault. This geometry provides a possible mechanism for the rapid uplift of the Fiordland region. Deformation in the northern South Island is more distributed. In the south-west Marlborough region partitioning occurs between strike-slip faulting in the SE and reverse faulting farther NW in the Buller region. We suggest that the partitioning developed as a consequence of an increasing component of shortening that was accommodated by slip on reactivated pre-existing normal faults in the Buller region. Shortening in the Buller region may have deflected the NE end of the Alpine Fault towards the NW, forming the prominent bend. The Marlborough Fault System, with its youngest and most active faults to the SE, probably developed in an attempt to maintain a through-going strike-slip structure as each of the strike-slip faults was transported towards the north-west. Partitioning of the opposite polarity (with reverse faulting SE of the strike-slip faulting) occurs in north-east Marlborough. The boundary between the two different styles of partitioning in NE and SW Marlborough appears to coincide with a change in the nature of the downgoing slab and a change in strike of faults of the Marlborough Fault System. A normal faulting earthquake on the northern edge of the Chatham rise probably results from a complex interaction of the buoyant continental crust in that region with the subduction zone and the overlying Marlborough Fault System.

The effect of basement faulting on diapirism

Abstract: Experimental and natural examples illustrate the inf1uence of sub-salt horizon basement faults on diapirism. In a series of experimental models, viscous diapirs were observed to form above or close to basement faults. In all the models, basement faults initiated a half-graben, where thicker overburden units enhanced differential loading on an underlying buoyant layer. The buoyant material flowed updip to the low-pressure zones in the uplifted block, and updip along the tilted upper boundary of the hanging-wall. Basement faulting extended the overburden, and provided the space through which the buoyant layer could rise. Subsidence and faulting of overburden layers allowed diapirism along the faulted zones. In all cases, the deformation in the overburden was accommodated within a wider zone of faulting than the discrete basement fault which initiated the deformation. Differential compaction enhances differential loading and accumulation of thicker overburden on the downthrown sides of basement faults.

Tectonic role of active faulting in central Oregon

Abstract: Geologic and geodetic studies in California indicate that about 1 cm yr−1 of right-lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is ∼N60° ± 20°W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction ∼N90° ± 30°W at a rate of ∼0.5 mm yr−1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 ± 1 mm yr−1 in a direction N55° ± 10°W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr−1 oriented ∼N60° to 70°W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific-North American transform motion and much of the lateral component of relative motion between the Juan de Fuca and North American plates.

The metamorphic signature of contemporaneous extension and shortening in the central Himalayan orogen: data from the Nyalam transect, southern Tibet

Abstract: Geological relationships and geochronological data suggest that in Miocene time the metamorphic core of the central Himalayan orogen was a wedge-shaped body bounded below by the N-dipping Main Central thrust system and above the N-dipping South Tibetan detachment system. We infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the Indian foreland, thereby moderating the extreme topographic gradient at the southern margin of the Tibetan Plateau. Reaction textures, thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the Nyalam transect in southern Tibet (28°N, 86°E) imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core. Amphibolite facies metamorphism and anatexis at temperatures of 950 K and depths of at least 30 km accompanied the early stages of displacement on the Main Central thrust system. Our findings suggest that the late metamorphic history of these rocks was characterized by high-T decompression associated with roughly 15 km of unroofing by movement on the South Tibetan detachment system. In the middle of the metamorphic core, roughly 7–8 km below the basal detachment of the South Tibetan system, the decompression was essentially isothermal. Near the base of the metamorphic core, roughly 4–6 km above the Main Central thrust, the decompression was accompanied by about 150 K of cooling. We attribute the disparity between the P–T paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued (and probably accelerated) underthrusting of cooler rocks in the footwall of the Main Central thrust at the same time as movement on the South Tibetan detachment system.