Showing papers on "Fault (geology) published in 2004"

Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults

Abstract: [1] We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface-cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 Mw = 6.7 Coalinga earthquake brought the subsequent 1983 Mw = 6.0 Nunez and 1985 Mw = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust-faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 Mw = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 Mw = 9.5 and 1995 Mw = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2–20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike-slip systems modulates the stress acting on nearby thrust and strike-slip faults. We calculate that the 1857 Mw = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault ∼1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw = 7.3 Kern County shock but inhibited slip on the left-lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales.

A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran

Abstract: SUMMARY The Zagros mountains of SW Iran are one of the most seismically active intra-continental fold-and-thrust belts on Earth, and an important element in the active tectonics of the Middle East. Surface faulting associated with earthquakes is extremely rare, and so most information about the active faulting comes from earthquakes. We use long-period teleseismic P and SH body waves to determine the orientation and depth of faulting in 16 new earthquakes, and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the Zagros. We use this information to investigate the style and distribution of active faulting in the Zagros, and how it contributes to the N–S shortening of the Arabia–Eurasia collision. When the data are ranked in quality and carefully evaluated, simple patterns are seen that are not apparent when routine catalogue data are taken at face value. An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation (‘partitioning’) of the orthogonal strike-slip and shortening components on separate parallel fault systems. By contrast, in the SE, overall convergence is orthogonal to the regional strike and achieved purely by thrusting. In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt. The overall configuration is similar to that seen in other curved shortening belts, such as the Himalaya and the Java–Sumatra trench. All the Zagros earthquakes we have been able to check have centroids shallower than ∼20 km and are confined to the upper crust. Many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover, which is active even beneath areas of known shallow structural decollement such as the Dezful embayment. The dominant style of shortening is high-angle reverse faulting with dips >30° though some lower-angle thrusting occurs in places. Active thrust and reverse faulting is relatively confined to the lower topography on the SW edge of the belt today, and only strike-slip faulting affects the higher topography. Profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the Simple Folded Belt, but has progressively migrated SW over the last 5 Ma. There is no evidence for a seismically active structural decollement, such as a low-angle thrust, beneath the Zagros, nor is there any seismic evidence for active subduction, either beneath the Zagros or beneath central Iran. Instead the Arabian margin seems to have shortened by distributed thickening of the basement. Only in the syntaxis of the Oman Line, at the SE end of the Zagros, is there any evidence for a low-angle thrust of regional extent. Here, earthquakes continue 50 km north of the Zagros Thrust Line (the geological suture between the Arabian margin and central Iran) reaching depths of ∼30 km, and may represent thrusting of Arabian basement beneath central Iran to this extent.

New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps

Abstract: Direct observation and extensive sampling in ancient margins exposed in the Alps, combined with drill-hole and geophysical data from the present-day Iberia margin, result in new concepts for the strain evolution and near-surface response to lithospheric rupturing at magma-poor rifted margins. This paper reviews data and tectonic concepts derived from these two margins and proposes that extension, leading to thinning and final rupturing of the continental lithosphere, is accommodated by three fault systems, each of them characterized by a specific temporal and spatial evolution during rifting of the margin, by its fault geometry, and its surface response. The data presented in this paper suggest that margin architecture and distribution of rift structures within the future margin are controlled first by inherited heterogeneities within the lithosphere leading to a contrasting behaviour of the future distal and proximal margins during an initial stage of rifting. The place of final break-up appears to be determined early in the evolution of the margin and occurs where the crust has been thinned during a first stage to less than 10 kilometres. During final break-up, the rheology of the extending lithosphere is controlled by the thermal structure related to the rise of the asthenosphere and by serpentinization and magmatic processes.

Evolving force balance during incipient subduction

Abstract: Nearly half of all active subduction zones initiated during the Cenozoic. All subduction zones associated with active back arc extension have initiated since the Eocene, hinting that back arc extension may be intimately associated with an interval (several tens of Myr) following subduction initiation. That such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates. Subduction initiation is known to have occurred in a variety of tectonic settings: old fracture zones, transform faults, and extinct spreading centers and through polarity reversal behind active subduction zones. Although occurring within different tectonic settings, four known subduction initiation events (Izu-Bonin-Mariana (IBM) along a fracture zone, Tonga-Kermadec along an extinct subduction boundary, New Hebrides within a back arc, and Puysegur-Fiordland along a spreading center) were typified by rapid uplift within the forearc followed by sudden subsidence. Other constraints corroborate the compressive nature of IBM and Tonga-Kermadec during initiation. Using an explicit finite element method within a two-dimensional domain, we explore the evolving force balance during initiation in which elastic flexure, viscous flow, plastic failure, and heat transport are all considered. In order to tie theory with observation, known tectonic settings of subduction initiation are used as initial and boundary conditions. We systematically explore incipient compression of a homogeneous plate, a former spreading center, and a fracture zone. The force balance is typified by a rapid growth in resisting force as the plate begins bending, reaching a maximum value dependent on plate thickness, but typically ranging from 2 to 3 × 1012 N/m for cases that become self-sustaining. This is followed by a drop in stress once a shear zone extends through the plate. The formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall. Cumulative convergence, not the rate of convergence, is the dominant control on the force balance. Viscous tractions influence the force balance only if the viscosity of the asthenosphere is >1020 Pa s, and then only after plate failure. Following plate failure, buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction. The total work done is also influenced by the rate of lithospheric failure. A self-sustaining subduction zone does not form from a homogeneous plate. A ridge placed under compression localizes subduction initiation, but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate. The large initial bending resistance can be entirely eliminated in ridge models, explaining the propensity for new subduction zones to form through polarity reversals. A fracture zone (FZ) placed in compression leads to subduction initiation with rapid extension of the overriding plate. A FZ must be underthrust by the older plate for ~100–150 km before a transition from forced to self-sustaining states is reached. In FZ models the change in force during transition is reflected by a shift from forearc uplift to subsidence. Subduction initiation is followed by trench retreat and back arc extension. Moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of Pacific subduction zones. The models provide an explanation for the compressive state of western Pacific margins before and during subduction initiation, including IBM and Tonga-Kermadec in the Eocene, and the association of active back arcs with young subduction zones. On the basis of our dynamic models and the relative poles of rotation between Pacific and Australia during the Eocene, we predict that the northern segment of the Tonga-Kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self-sustaining states.

Pyrenean orogeny and plate kinematics

Abstract: [1] The evolution of the Pyrenees, a mountain range between Iberia and Eurasia, has remained the subject of many debates between geologists and geophysicists for a long time. By combining the identification of seafloor spreading anomalies A33o to M0 in the Bay of Biscay with those in the North Atlantic, we have derived a position of a mean pole of rotation for the entire opening of the Bay of Biscay. Four hundred kilometers of shortening took place between the Iberian and Eurasian plates in the Pyrenean domain during the opening of the Bay of Biscay, from chrons M0 to A33o time (118 to 80 Ma). The deep seismic Etude Continentale et Oceanique par Reflexion et refraction Sismique (ECORS) profile shot across the Pyrenees and teleseismic data show the presence of two distinct slabs, which dip to the north. The southern slab is linked to the subduction of the neo-Tethys Ocean, which was created from late Jurassic to early Aptian. Simultaneously, elongated back arc basins formed along the future Pyrenean domain. This slab was active from at least 118 Ma (early Aptian) to 100 Ma (late Albian). The northern slab, active since 85 Ma, is linked to the subduction of the lower continental crust located south of the Pyrenean domain. In the upper crust, normal faults as well as the north Pyrenean fault became reverse faults, and former back arc basins were inverted, giving rise to the uplift of the Pyrenees as a double asymmetrical wedge.

Faulting Induced by Forced Fluid Injection and Fluid Flow Forced by Faulting: An Interpretation of Hydraulic-Fracture Microseismicity, Carthage Cotton Valley Gas Field, Texas

Abstract: We analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the Carthage gas field of east Texas. The treat- ments were conducted in two adjacent boreholes within interbedded sands and shales of the Upper Cotton Valley formation. The microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers. Events throughout all the treatments show strike-slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction. These events are consistent with the reservoir's prevalent natural fractures, known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orien- tation. When this uniform fracture system was activated exclusively, the detected shear deformation, measured as the moment release per unit volume of fluid injected, was constant, independent of various fluid viscosities and flow rates used. Within the base of the Upper Cotton Valley formation, anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones. The mechanisms are also strike-slip, but the fault planes are more favorably oriented for failure. The dense clusters show location patterns diverging in time, suggesting the expulsion of fluids from compressive fault jogs. Fluid flow forced by the adjacent slip-induced loading appears to initially extend the treatments, but the loading also tends to lock up and concentrate stress at the jogs, as evident by fewer but larger events populating the structures as treatments progress. As a result, effec- tive drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs. These high-moment asperities are similar to dense patches of seismicity observed along creeping sections of the San Andreas fault, where they have been attributed to localized zones of strength or stress con- centration, surrounded by larger regions undergoing stable, aseismic slip. This sim- ilarity, plus large moment deficits in terms of volume injected, suggests a large component of aseismic slip is induced by the Cotton Valley treatments.

Pacific–Antarctic–Australia motion and the formation of the Macquarie Plate

Abstract: SUMMARY Magnetic anomaly and fracture zone data on the Southeast Indian Ridge (SEIR) are analysed in order to constrain the kinematic history of the Macquarie Plate, the region of the Australian Plate roughly east of 145 ◦ E and south of 52 ◦ S. Finite rotations for Australia‐Antarctic motion are determined for nine chrons (2Ay, 3Ay, 5o, 6o, 8o, 10o, 12o, 13o and 17o) using data limited to the region between 88 ◦ E and 139 ◦ E. These rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the SEIR. An analysis of the synthetic flowlines shows that the Macquarie Plate region has behaved as an independent rigid plate for roughly the last 6 Myr. Finite rotations for Macquarie‐ Antarctic motion are determined for chrons 2Ay and 3Ay. These rotations are summed with Australia‐Antarctic rotations to determine Macquarie‐Australia rotations. We find that the best-fit Macquarie‐Australia rotation poles lie within the zone of diffuse intraplate seismicity in the South Tasman Sea separating the Macquarie Plate from the main part of the Australian Plate. Motion of the Macquarie Plate relative to the Pacific Plate for chrons 2Ay and 3Ay is determined by summing Macquarie‐Antarctic and Antarctic‐Pacific rotations. The Pacific‐ Macquarie rotations predict a smaller rate of convergence perpendicular to the Hjort Trench than the Pacific‐Australia rotations. The onset of the deformation of the South Tasman Sea and the development of the Macquarie Plate appears to have been triggered by the subduction of young, buoyant oceanic crust near the Hjort Trench and coincided with a clockwise change in Pacific‐Australia motion around 6 Ma. The revised Pacific‐Australia rotations also have implications for the tectonics of the Alpine Fault Zone of New Zealand. We find that changes in relative displacement along the Alpine Fault have been small over the last 20 Myr. The average rate of convergence over the last 6 Myr is about 40 per cent smaller than in previous models.

Nonlinear Dynamics, Granular Media and Dynamic Earthquake Triggering

Abstract: The 1992 magnitude 7.3 Landers earthquake triggered an exceptional number of additional earthquakes within California and as far north as Yellowstone and Montana. Since this observation, other large earthquakes have been shown to induce dynamic triggering at remote distances--for example, after the 1999 magnitude 7.1 Hector Mine and the 2002 magnitude 7.9 Denali earthquakes--and in the near-field as aftershocks. The physical origin of dynamic triggering, however, remains one of the least understood aspects of earthquake nucleation. The dynamic strain amplitudes from a large earthquake are exceedingly small once the waves have propagated more than several fault radii. For example, a strain wave amplitude of 10(-6) and wavelength 1 m corresponds to a displacement amplitude of about 10(-7) m. Here we show that the dynamic, elastic-nonlinear behaviour of fault gouge perturbed by a seismic wave may trigger earthquakes, even with such small strains. We base our hypothesis on recent laboratory dynamic experiments conducted in granular media, a fault gouge surrogate. From these we infer that, if the fault is weak, seismic waves cause the fault core modulus to decrease abruptly and weaken further. If the fault is already near failure, this process could therefore induce fault slip.

An Overview of the 1999 Chi-Chi, Taiwan, Earthquake

Abstract: The Chi-Chi earthquake was the largest onland earthquake to occur in Taiwan in the twentieth century. It inflicted severe damage in central western Taiwan: the excited strong shaking projected impact at cities as far as 150 km away and destroyed several high-rise buildings in the Taipei basin. Having a very complex source, the Chi-Chi earthquake ruptured the 100-km-long Chelungpu fault in a series of jumping dislocations that did not follow a process commonly assumed for an orderly propagating rupture. Furthermore, the rupture developed over a surface that was by no means planar. Principally a N-S-trending thrust of shallow (30°) dip to the east, the northern end of the ruptured Chelungpu fault turned into a more easterly trending rupture surface with an oblique-slip motion. The recently completed Taiwan Strong-Motion Instrumentation Program (TSMIP) with more than 600 modern digital instruments scored the historically largest and most significant strong-motion data recovery. In the meantime, the upgraded Taiwan Central Weather Bureau Seismic Network (CWBSN), all with digital telemetered stations each having six components in both high- and low-gain operations, electronically issued earthquake information (hypocenter, magnitude, and isoseismal map) within minutes of the mainshock to all pertinent emergency management agencies. This rapid reporting significantly improved timely emergency response and effective dispatching of rescue missions. Based on these local data, as well as on GPS, leveling, and geological ground truth observations of the surface rupture, preliminary results show that the rupture started at the southern part of the Chelungpu fault. Dislocations (or the rupture of asperities) jumped around behind the S -wave front in a rather random spatial distribution by which no rupture propagating velocity can be properly defined. Large ground acceleration, some over 1 g , occurred in the southern part of the Chelungpu fault where the rupture initiated. Toward the northern section of the Chelungpu fault, a decrease in ground acceleration was accompanied with an increase in ground velocity (to as much as 300 cm/sec) and ground displacement (to as much as 8 m). Most of the large motions were confined to the hanging wall (i.e., the eastern block of the thrust), and relatively small ground motions occurred in the footwall. Thus the hanging wall contributed the most to the rupture process. During the later part of the rupture, the N-S-trending Chelungpu fault made an easterly bend and the thrust motion turned into a more oblique-slip motion at the northern end. Meanwhile, the strong dynamic rupturing process triggered two M 6 events, each one having occurred in the vicinity of a known fault: one off the southern end and the other off the northern end of the Chelungpu fault. Manuscript received 12 December 2000.

Regional tectonic stress near the San Andreas fault in central and southern California

Abstract: [1] Throughout central and southern California, a uniform NNE-SSW direction of maximum horizontal compressive stress is observed that is remarkably consistent with the superposition of stresses arising from lateral variations in lithospheric buoyancy in the western United States, and far-field Pacific-North America plate interaction. In central California, the axis of maximum horizontal compressive stress lies at a high angle to the San Andreas fault (SAF). Despite relatively few observations near (±10 km) the fault, observations in the greater San Francisco Bay area indicate an angle of as much as 85°, implying extremely low fault strength. In southern California, observations of stress orientations near the SAF are rotated slightly counter-clockwise with respect to the regional field. Nevertheless, we observe an approximately constant angle between the SAF and the maximum horizontal stress direction of 68 ± 7° along ∼400 km of the fault, indicating that the SAF has moderately low frictional strength in southern California.

Probing the mechanical properties of seismically active crust with space geodesy: Study of the coseismic deformation due to the 1992 Mw7.3 Landers (southern California) earthquake

Abstract: [1] The coseismic deformation due to the 1992 M(w)7.3 Landers earthquake, southern California, is investigated using synthetic aperture radar (SAR) and Global Positioning System (GPS) measurements. The ERS-1 satellite data from the ascending and descending orbits are used to generate contiguous maps of three orthogonal components ( east, north, up) of the coseismic surface displacement field. The coseismic displacement field exhibits symmetries with respect to the rupture plane that are suggestive of a linear relationship between stress and strain in the crust. Interferometric synthetic aperture radar (InSAR) data show small-scale deformation on nearby faults of the Eastern California Shear Zone. Some of these faults ( in particular, the Calico, Rodman, and Pinto Mountain faults) were also subsequently strained by the 1999 M(w)7.1 Hector Mine earthquake. I test the hypothesis that the anomalous fault strain represents essentially an elastic response of kilometer-scale compliant fault zones to stressing by nearby earthquakes [Fialko et al., 2002]. The coseismic stress perturbations due to the Landers earthquake are computed using a slip model derived from inversions of the InSAR and GPS data. Calculations are performed for both homogeneous and transversely isotropic half-space models. The compliant zone model that best explains the deformation on the Calico and Pinto Mountain faults due to the Hector Mine earthquake successfully predicts the coseismic displacements on these faults induced by the Landers earthquake. Deformation on the Calico and Pinto Mountain faults implies about a factor of 2 reduction in the effective shear modulus within the similar to 2 km wide fault zones. The depth extent of the low-rigidity zones is poorly constrained but is likely in excess of a few kilometers. The same type of structure is able to explain high gradients in the radar line of sight displacements observed on other faults adjacent to the Landers rupture. In particular, the Lenwood fault north of the Soggy Lake has likely experienced a few centimeters of left-lateral motion across < 1-km-wide compliant fault zone having the rigidity reduction of more than a factor of 2. The inferred compliant fault zones are interpreted to be a result of extensive damage due to past earthquakes.

Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China

Abstract: The late Cenozoic Kongur Shan extensional system lies along the northeastern margin of the Pamir at the western end of the Himalayan-Tibetan orogen, accommodating east-west extension in the Pamir. At the northern end of the extensional system, the Kongur Shan normal fault juxtaposes medium- to high-grade metamorphic rocks in both its hanging wall and footwall, which record several Mesozoic to Cenozoic tectonic events. Schists within the hanging wall preserve a Buchan metamorphic sequence, dated as Late Triassic to Early Jurassic (230–200 Ma) from monazite inclusions in garnet. Metamorphic ages overlap with U-Pb zircon ages from local granite bodies and are interpreted to be the result of regional arc magmatism created by subduction of the Paleo-Tethys ocean. The northern portion of the footwall of the extensional system exposes an upper-amphibolite-facies unit (~650 °C, 8 kbar), which structurally overlies a lowgrade metagraywacke unit. The high-grade unit records late Early Cretaceous crustal thickening at ca. 125–110 Ma, followed by emplacement over the low-grade metagraywacke along a north-northeast–directed thrust prior to ca. 100 Ma. Together these results indicate signifi cant middle Cretaceous crustal thickening and shortening in the northern Pamir prior to the Indo-Asian collision. A third Late Miocene (ca. 9 Ma) amphibolite-facies metamorphic event (~650–700 °C, 8 kbar) is recorded in footwall gneisses of the Kongur Shan massif. North of the Kongur Shan massif, rapid cooling in the footwall beginning at 7–8 Ma is interpreted to date the initiation of exhumation along the Kongur Shan normal fault. A minimum of 34 km of east-west extension is inferred along the Kongur Shan massif based on the magnitude of exhumation since the Late Miocene (~29 km) and the present dip of the Kongur Shan normal fault (~40°). Field observations and interpretation of satellite images along the southernmost segment of the Kongur Shan extensional system indicate that the magnitude of late Cenozoic east-west extension decreases signifi cantly toward the south. This observation is inconsistent with models in which east-west extension in the Pamir is driven by northward propagation of the right-slip Karakoram fault, suggesting instead that extension is driven by vertical extrusion due to topographic collapse, radial thrusting along the Main Pamir Thrust, or oroclinal bending of the entire Pamir region.

Preseismic deformation and coseismic displacements associated With the 1999 Chi-Chi, Taiwan, Earthquake

Abstract: The destructive 1999 Chi-Chi, Taiwan, earthquake ( M w 7.5) produced an approximately 100-km-long surface rupture, mostly along the previously recognized north-south-trending Chelungpu fault. Preseismic deformation in central Taiwan is realized from annually repeated Global Positioning System (GPS) data acquired during the 1992-1999 period. The total WNW-ESE shortening rate in the vicinity of the epicentral region, that is from the west coast to the western boundary of the Central Range, is up to 25 mm/yr. The crustal deformation before the Chi-Chi earthquake was essentially a uniaxial compressional strain of 0.36 μstrain/yr in the direction of 114°. The GPS measurements taken 0.2-2.7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements. Horizontal movements of 1.1-9.1 m in the NW-NNW directions are observed on the hanging wall (eastern side) of the fault. There is a northward-increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50° toward the east along the fault strike. In contrast, much smaller SE-SEE movements of 0.1-1.5 m are found on the footwall (western side) of the fault. The GPS data show 2.4-10.1 m of total horizontal offsets across the Chelungpu fault. Vertical offsets of 1.2-4.4 m with the eastern side up are also observed along the surface rupture. The uplift on the hanging wall decreases rapidly toward the east. It becomes subsidence at Sun Moon Lake and in the Puli-Wushe area. The stations on the footwall show subsidence of 0.02-0.26 m. The width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north. Manuscript received 13 October 2000.

Remotely Triggered Seismicity on the United States West Coast following the Mw 7.9 Denali Fault Earthquake

Abstract: The Mw 7.9 Denali fault earthquake in central Alaska of 3 November 2002 triggered earthquakes across western North America at epicentral distances of up to at least 3660 km. We describe the spatial and temporal development of triggered activity in California and the Pacific Northwest, focusing on Mount Rainier, the Geysers geothermal field, the Long Valley caldera, and the Coso geothermal field. The onset of triggered seismicity at each of these areas began during the Love and Raleigh waves of the Mw 7.9 wave train, which had dominant periods of 15 to 40 sec, indicating that earthquakes were triggered locally by dynamic stress changes due to low-frequency surface wave arrivals. Swarms during the wave train continued for 4 min (Mount Rainier) to 40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small (M 2.5) earthquakes. Dy- namic stresses within the surface wave train at the time of the first triggered earth- quakes ranged from 0.01 MPa (Coso) to 0.09 MPa (Mount Rainier). In addition to the swarms that began during the surface wave arrivals, Long Valley caldera and Mount Rainier experienced unusually large seismic swarms hours to days after the Denali fault earthquake. These swarms seem to represent a delayed response to the Denali fault earthquake. The occurrence of spatially and temporally distinct swarms of triggered seismicity at the same site suggests that earthquakes may be triggered by more than one physical process.

New constraints on the active tectonic deformation of the Aegean

Abstract: [1] Site velocities from six separate Global Positioning System (GPS) networks comprising 374 stations have been referred to a single common Eurasia-fixed reference frame to map the velocity distribution over the entire Aegean. We use the GPS velocity field to identify deforming regions, rigid elements, and potential microplate boundaries, and build upon previous work by others to initially specify rigid elements in central Greece, the South Aegean, Anatolia, and the Sea of Marmara. We apply an iterative approach, tentatively defining microplate boundaries, determining best fit rigid rotations, examining misfit patterns, and revising the boundaries to achieve a better match between model and data. Short-term seismic cycle effects are minor contaminants of the data that we remove when necessary to isolate the long-term kinematics. We find that present day Aegean deformation is due to the relative motions of four microplates and straining in several isolated zones internal to them. The RMS misfit of model to data is about 2-sigma, very good when compared to the typical match between coseismic fault models and GPS data. The simplicity of the microplate description of the deformation and its good fit to the GPS data are surprising and were not anticipated by previous work, which had suggested either many rigid elements or broad deforming zones that comprise much of the Aegean region. The isolated deforming zones are also unexpected and cannot be explained by the kinematics of the microplate motions. Strain rates within internally deforming zones are extensional and range from 30 to 50 nanostrain/year (nstrain/year, 10−9/year), 1 to 2 orders of magnitude lower than rates observed across the major microplate boundaries. Lower strain rates may exist elsewhere within the microplates but are only resolved in Anatolia, where extension of 13 ± 4 nstrain/year is required by the data. Our results suggest that despite the detailed complexity of active continental deformation revealed by seismicity, active faulting, fault geomorphology, and earthquake fault plane solutions, continental tectonics, at least in the Aegean, is to first order very similar to global plate tectonics and obeys the same simple kinematic rules. Although the widespread distribution of Aegean seismicity and active faulting might suggest a rather spatially homogeneous seismic hazard, the focusing of deformation near microplate boundaries implies the highest hazard is comparably localized.

Shear zones and magma ascent: A model based on a review of the Tertiary magmatism in the Alps

Abstract: [1] The Alpine Oligocene plutons are spatially and temporally associated with the activity of the Periadriatic Fault System (PFS), an orogen-parallel, crustal-scale transpressive mylonitic belt. Excellent three-dimensional exposure, combined with a wealth of structural, seismic, petrological, geochronological, geochemical, and paleomagnetic data collected over the last decades help to constrain the relationships between deformation, ascent, and emplacement of the plutons. Magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the Periadriatic Fault System, which was used as ascent pathway to cover vertical lengths of 20 to 40 km. Therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth. Ascent of the melts is controlled by the mylonitic foliation of the PFS, which forms the only steep anisotropy, continuously traversing the entire Alpine crust. In contrast, the flow direction is not influenced by the specific kinematics of the faults. Final emplacement of the plutons occurred by extrusion from the Periadriatic Fault System into the adjacent country rocks. The transition from ascent to final emplacement is favored by partitioning of transpressive deformation.

Global relations between seismic fault parameters and moment magnitude of earthquakes

Abstract: The most reliable of the globally available relative data have been used to derive empirical formulas which relate the subsurface fault length, L, the fault area, S, and fault width, w, with the moment magnitude, M. Separate such formulas have been derived for earthquakes generated by strike-slip faulting, by dip-slip faulting in continental regions and by dip-slip faulting in lithospheric subduction regions. The formula which relates the fault area with the magnitude is combined with the definition formulas of seismic moment and moment magnitude to derive also relations between the fault slip, u, and the moment magnitude for each of the three seismotectonic regimes. For a certain magnitude, the fault length is larger for strike-slip faults than for dip-slip faults, while the fault width is small for strike-slip faults, larger for dip-slip faults in continental regions and much larger for dip-slip faults in regions of lithospheric subduction. For a certain magnitude, fault slip is about the same for strike-slip faults and dip-slip faults in continental regions and smaller for dip-slip faults in regions of lithospheric subduction.

Seismic reflection imaging of a major strike-slip fault zone in a rift system: Paleogene structure and evolution of the Tan-Lu fault system, Liaodong Bay, Bohai, offshore China

Abstract: The Tan-Lu fault system in the Liaodong Bay, Bohai, offshore China, affords an exceptional opportunity to document the structural features of a major strike-slip fault using two- and three-dimensional seismic reflection data, as well as evolution of a strike-slip fault developed coeval with a rift system. The fault zone displays a relatively straight, throughgoing trace longitudinally bisecting the rift valley. It consists of positive and negative flower structures and en echelon folds in the south bay, and three parallel, flower-structure systems northward. The middle fault bifurcates northward into two semiparallel vertical fault strands. To the north, the west strand bends clockwise and merges with the east strand. The stepping pattern and orientation of en echelon structures indicate right-lateral sense and about N10–35E azimuth of slip. The fault apparently accrued about 30–40 km (20–25 mi) of post-early Eocene slip based on the current distribution of deformation zones and depocenters. Tan-Lu fault segments with clockwise and counterclockwise orientation relative to the regional slip direction are characterized by divergent and convergent structures (i.e., restraining and releasing bends), respectively. Waning of rifting eliminated the cause of a major restraining bend, putting an end to development of associated convergent structures in the south bay area. Near the central-north bay, deformation occurred along major normal faults related to the basin rifting. Although a prominent feature reflecting regional strain partitioning, the Tan-Lu fault apparently was not a major factor in the Paleogene opening of the Liaodong Bay basin and the larger North China rift basin.

Long-Range and Long-Term Fault Interactions in Southern California

Abstract: Paleoseismological data suggest the occurrence of four bursts of seismic moment release in the Los Angeles region during the past 12,000 yr. The historic period appears to be part of an ongoing lull that has persisted for about the past 1000 yr. These periods of rapid seismic displacement in the Los Angeles region have occurred during the lulls between similar bursts of activity observed on the eastern California shear zone in the Mojave Desert, which is now seismically active. A kinematic model in which the faults of the greater San Andreas system suppress activity on faults in the eastern California shear zone, and vice versa, can explain the apparent switching of activity between the two fault networks. Combined with the observation that short-term geodetic and longer-term geologic rates co-vary on major southern California fault systems, this suggests that either (1) a temporal cluster of seismic displacements on upper-crustal faults increases ductile deformation on their downward extensions, or (2) rapid ductile slip in the lower crust beneath faults loads the upper crust, driving a seismic cluster. We suggest that alternating periods of rapid seismic displacement may be the expected mode of seismicity when two fault systems accommodate the same plate-boundary motion, and slip on one system suppresses slip on the other.

Tectonostratigraphic subdivisions of the Himalaya: A view from the west

Abstract: [1] Indian plate rocks in the central Himalaya have traditionally been divided into orogen-parallel, fault-bound tectonostratigraphic zones. A straightforward westward extrapolation of these zones has proved problematic in part because of a lack of consensus on the existence or significance of major faults within the metamorphic zone of the Indian plate in Pakistan where more than 10 locations for the Main Central thrust (MCT) have been proposed. We address this ambiguity by systematically tracing established central Himalayan tectonostratigraphy around the western Himalayan syntaxis and across Pakistan. This exercise reveals the following stratigraphic and structural relationships: (1) There is a westward decrease in Neogene shortening across the Himalayan fold and thrust belt such that there is no age equivalent thrust in Pakistan with displacement and metamorphic juxtaposition equivalent to the central Himalayan MCT. (2) Shortening across the fold and thrust belt in western Pakistan is concentrated in the unmetamorphosed foreland as opposed to the metamorphic zone in the central Himalaya. (3) Lesser Himalayan, Higher Himalayan, and Tethyan rocks are in stratigraphic order within the metamorphic zone of Pakistan which appears to be the metamorphic equivalent of Kashmir Tethyan stratigraphy. (4) The combination of early Paleozoic and late Paleozoic tectonism in Pakistan has locally eliminated Upper Proterozoic Higher Himalayan rock and lower to middle Paleozoic Tethyan rock from the metamorphic zone of Pakistan. (5) Late Cretaceous and/or early Paleocene proto-Himalayan deformation in the Pakistan foreland telescoped and eroded stratigraphy prior to the main phase of Himalayan orogeny. (6) Tectonostratigraphic zones are offset in eastern Pakistan by the transverse Jhelum-Balakot fault. (7) There is no evidence within the Indian plate of Pakistan for a large-scale normal fault system comparable to the South Tibetan detachment system. (8) Stratigraphy, as well as the age and tectonic setting of deformation and metamorphism, must be taken into account when drawing tectonostratigraphic zones.