Showing papers on "Fault (geology) published in 1997"

Response of regional seismicity to the static stress change produced by the loma prieta earthquake

Abstract: The 1989 Loma Prieta, California, earthquake perturbed the static stress field over a large area of central California. The pattern of stress changes on major faults in the region predicted by models of the earthquake9s dislocation agrees closely with changes in the regional seismicity rate after the earthquake. The agreement is best for models with low values of the coefficient of friction (0.1 ≤ µ ≤ 0.3) on Bay Area faults. Both the stress models and measurements suggest that stresses were increased on the San Andreas fault north of the Loma Prieta rupture, but decreased slightly on the Hayward fault. This relaxation does not warrant lower probability estimates for large earthquakes on the Hayward fault in the next 30 years, however.

Quantitative Fault Seal Prediction

Abstract: Fault seal can arise from reservoir/nonreservoir juxtaposition or by development of fault rock having high entry pressure. The methodology for evaluating these possibilities uses detailed seismic mapping and well analysis. A first-order seal analysis involves identifying reservoir juxtaposition areas over the fault surface by using the mapped horizons and a refined reservoir stratigraphy defined by isochores at the fault surface. The second-order phase of the analysis assesses whether the sand/sand contacts are likely to support a pressure difference. We define two types of lithology-dependent attributes: gouge ratio and smear factor. Gouge ratio is an estimate of the proportion of fine-grained material entrained into the fault gouge from the wall rocks. Smear factor methods (including clay smear potential and shale smear factor) estimate the profile thickness of a shale drawn along the fault zone during faulting. All of these parameters vary over the fault surface, implying that faults cannot simply be designated sealing or nonsealing. An important step in using these parameters is to calibrate them in areas where across-fault pressure differences are explicitly known from wells on both sides of a fault. Our calibration for a number of data sets shows remarkably consistent results, despite their diverse settings (e.g., Brent province, Niger Delta, Columbus basin). For example, a shale gouge ratio of about 20% (volume of shale in the slipped interval) is a typical threshold between minimal across-fault pressure difference and significant seal.

Mapping the frequency-magnitude distribution in asperities: An improved technique to calculate recurrence times?

Abstract: We hypothesize that highly stressed asperities may be defined by mapping anomalously low b values. Along the San Andreas fault near Parkfield the asperity under Middle Mountain, with its b=0.46, can be distinguished from all other parts of the fault surface. Likewise, along the Calaveras fault the northern asperity of the Morgan Hill 1984 (M6.2) rupture can be identified by its low b of 0.5 as a high stress patch along the fault. We add further evidence to the observations that the b value of the frequency-magnitude relationship of earthquakes is inversely proportional to stress by showing that it decreases with depth in the Parkfield segment of the San Andreas and along the Calaveras fault. In both of these areas, b values above and below 5 km depth are ∼1.2 and 0.8, respectively. We propose that probabilistic recurrence times Tr, based on the seismicity parameters a and b, should be calculated from their values within asperities only, instead of from the values of the entire rupture area of the maximum expected earthquake. The strong patches on faults control the time of rupture because they are capable of accumulating larger stresses than the rest of the fault zone, which slips along passively when an asperity breaks. Therefore no information on Tr is contained in the passive fault segments, only in the asperities. At Parkfield the probabilistic estimates of Tr derived from the data in the whole rupture and in the asperity only are 72 (−18/+24) and 23 (−12/+18) years, respectively, compared to the historically observed repeat time of 22 years. At Morgan Hill the Tr estimates are 122 (−46/+76) and 78 (−47/+110) years, respectively, compared to the observed repeat time of 72 years.

Recurrence Intervals for Great Earthquakes of the Past 3,500 Years at Northeastern Willapa Bay, Washington

Abstract: Plate-boundary earthquakes have occurred repeatedly in the past several thousand years at the Cascadia subduction zone, where they are widely recorded by buried marsh and forest soils beneath estuarine wetlands. This report adds to previous accounts of such soils along the Pacific coast of southern Washington State. Our new evidence comes from outcrop surveys, diatom analyses, and radiocarbon dating of soils exposed about 10 km apart in banks of the Niawiakum and Willapa Rivers, tidal arms of Willapa Bay. This new evidence clarifies the timing of great (magnitude 8 or larger) earthquakes during the past 3,500 years at this part of the Cascadia subduction zone. All the surveyed outcrops display buried soils that probably record tectonic subsidence during earthquakes. Although in many cased we cannot rule out every alternative to such coseismic subsidence, we found no buried soil that is better explained by stream migration, storm, river flood, sea-level rise, barrier breaching, or sediment compaction. Each of the soils probably represents a marsh or forest that suddenly became a tidal flat and consequently was buried by tidal mud. In nearly every case the soils have too much lateral continuity and too little relief to record cutting and filling by tidal streams. Vascular plant fossils preserved within and above many of the soils show that storms or floods, if unaccompanied by lasting submergence, cannot account for burial of the soils. Where their remains are preserved, plants that had lived on the soils belong to species indicative of high parts of tidal marshes, or of tidal swamps or uplands. By contrast, the main vascular -plant species preserved in mud above the soils is Triglochin maritimum, a colonist of saltwater mudflats in southern coastal Washington. Assemblages of diatoms verify vascular-plant evidence for lasting submergence and show that such submergence is recorded also by buried soils with which vascular-plant fossils are not preserved. In an outcrop along the Niawiakum River, for example, diatoms show that each of six successive soils represents a high marsh or an upland, and that mud above each of these soils represents an intertidal or subtidal mudflat. Fossils further show that the change from high marsh or upland to mudflat probably happened too fast to have resulted from a gradual rise in sea level. For every soil studied for fossil diatoms, diatom assemblages imply that this change happened suddenly, without transition through low marsh. Gradual sea-level rise is further precluded for some soils by remains of vascular plants that had lived on buried soils. These remains include stems and leaves that were surrounded by mudflat deposits before they had time to decompose. Other alternatives to coseismic subsidence can be discounted as well. Submergence from breaching of a baymouth barrier is ruled out by soils of brackish marshes that require tidal connection with the sea. Localized settlement from earthquake-induced compaction of unconsolidated Holocene deposits does not explain the presence of buried soils directly above well-consolidated Pleistocene deposits. Evidence against alternative explanation at one outcrop can be extrapolated to other outcrops by radiocarbon and stratigraphic correlation of buried soils. Plate-boundary earthquakes probably account for all the subsidence events. The plate boundary is the only recognized fault common to all areas having evidence of coseismic subsidence in southern coastal Washington. Although some of these areas coincide with mapped late Cenozoic synclines, where coseismic subsidence might accompany earthquakes on faults in the North American plate, others are outside such synclines. The coseismically subsided areas include part of welt of Eocene basement rock 40 km long and 15 km wide, herein termed the South Bend antiform. This structural high, the largest in southern coastal Washington, has probably grown in late Cenozoic time. The South Bend Antiform should subside during plate-boundary earthquakes that flex the North American plate throughout southern coastal Washington. By contrast, the antiform might grow upward during an upper-plate earthquake that produces subsidence only in late Cenozoic synclines that flank the antiform. With these expectations in mind, we compared outcrops off the South Bend antiform (Niawiakum River) with outcrops on the antiform (Willapa River). We found no difference in sense of timing of earthquake-induced changes in land level. Like the surveyed outcrops along the Niawiakum River, the surveyed outcrops along the Willapa River contain buried soils indicative of earthquake-induced subsidence. the longest stratigraphic sequence exposed along both streams contain six or seven soils less than 3,500 years old, and these sequences correlate with one another on the basis of soil horizons, spruce roots, and radiocarbon ages. Though every buried soil identified in the surveyed outcrops probably records a plate-boundary earthquake is widely recorded by a buried soil. One buried soil was obliterated at many sites through centuries of decomposition, probably because shallow burial left the soil high in the weathering profile of a succeeding soil. Such decomposition also destroyed most of the organic matter associated with two other soils. In addition, a widespread soil might not be available for burial if an earthquake occurs too soon after its predecessor for much rebuilding of tidal marshes. Despite these limitations, the composite record from buried soils in large outcrops of northeastern Willapa Bay probably includes every earthquake in the area during the past 3,500 years that caused at least .5 meters of widespread coseismic subsidence and followed the preceding great earthquake by more than a century. Willapa Bay's earthquake history in the past 3,500 years probably includes seven events, each comprising a single rupture or multiple contiguous ruptures on the Washington part of the Cascadia plate boundary. Each event probably included at least one great earthquake, as judged from likely rupture widths inferred from modern geophysical evidence, likely rupture lengths inferred from coastwise correlation of buried soils, extensive sea-floor displacement inferred from a tsunami in Japan, and seismic-moment release deduced from plate motions and average recurrence intervals. The history begins with three events between 3,300-3,500 years ago and 2,400-2,800 years ago (ranges include estimated 95-percent confidence interval). The next recorded events occurred 1,500-1,700 and 1.130-1,350 years ago. They were followed by a poorly dated event that probably occurred before 900 years ago and may have been associated with rupture in inland faults in the North American plate 1,000-1,100 years ago. The most recent of the events happened close to 300 years ago, probably in January 1700. The six intervals between events in this inferred history average 500-540 years but range from about one to three centuries to about a millennium. The earliest two intervals sum to 540-1,100 years. The first of them may have been the longer, as judged from spruce roots that may indicate prolonged interseismic emergence. Next came an interval of 700-1,300 years, when spruce forests spread onto emerging tidal marshes and decomposition largely destroyed an underlying buried soil. This exceptionally long interval was followed by two short ones that together spanned no more than 800 years. The most recent complete interval, marked by another spreading of spruce forests and decomposition of earlier buried soils, lasted 600-1,000 years. This pattern of long and short recurrence intervals at Willapa Bay may match the pattern of intervals between turbidity currents in Cascadia Channel , on the abyssal sea floor 200 km off the central Oregon coast. Previous work showed that these currents largely originated at submarine canyon heads about 50 km west of Willapa Bay, and that great earthquakes may have generated 13 currents in the past 7,500 years. Although pelagic layers between the turbidites have been interpreted as evidence for recurrence intervals of fairly uniform duration, borrows in the turbidites suggest variability in recurrence intervals inferred from buried soils at Willapa Bay.

Active tectonics of the Eastern Mediterranean region: deduced from GPS, neotectonic and seismicity data

Abstract: This paper reviews the main tectonic features of the Eastern Mediterranean region combining the recent information obtained from GPS measurements, seismicity and neotectonic studies. GPS measurements reveal that the Arabian plate moves northward with respect to Eurasia at a rate of 23 ± 1 mm/yr, 10 mm/yr of this rate is taken up by shortening in the Caucasus. The internal deformation in Eastern Anatolia by conjugate strike-slip faulting and E-W trending thrusts, including the Bitlis frontal thrust, accommodates approximately a 15 mm/yr slip rate. The Northeast Anatolian fault, which extends from the Erzincan basin to Caucasus accommodates about 8 ± 5 mm/yr of left-lateral motion. The neotectonic fault pattern in Eastern Anatolia suggests that the NE Anatolian block moves in an E-ENE direction towards the South Caspian Sea. According to the same data, the Anatolian-Aegean block is undergoing a counter-clockwise rotation. However, from the residuals it appears that this solution can only be taken as a preliminary approximation. The Eulerian rotation pole indicates that slip rate along the North Anatolian fault is about 26 ± 3 mm/yr. This value is 10 mm/yr higher than slip rates obtained from geological data and historical earthquake records and it includes westward drift of the Pontides of a few millimetres/year or more. GPS measurements reveal that the East Anatolian fault accommodates an 11 ± 1 mm/yr relative motion. GPS data suggest that Central Anatolia behaves as a rigid block, but from neotectonic studies, it clearly appears that it is sliced by a number of conjugate strike-slip faults. The Isparta Angle area might be considered a major obstacle for the westward motion of the Anatolian block (Central and Eastern Anatolia). The western flank of this geological structure, the Fethiye-Burdur fault zone appears to be a major boundary with a slip rate of 15-20 mm/yr. The Western Anatolian grabens take up a total of 15 mm/yr NE-SW extension. The fact that motions in Central Anatolia relative to Eurasia, are 15-20 mm/yr while in Western Anatolia and Aegean Sea they are 30-40 mm/yr could suggest that Western Anatolia decouples from Central Anatolia and the Isparta Angle by the Fethiye-Burdur fault zone and Eski?ehir fault. It is also hypothesized that the differentiation of tectonic styles and velocities in the Anatolian-Aegean block are related to differences between the slabs lying under the Cyprus and Hellenic arcs.

Linked sequence stratigraphic and structural evolution of propagating normal faults

Abstract: Two distinct phases in the structural evolution of normal faults can be identified in the Miocene Gulf of Suez rift: (1) an initial growth fold stage when the fault is a buried structure and (2) a subsequent surface faulting stage. During the growth fold stage, strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines. In contrast, once the fault breaks surface, strata form a divergent wedge, which is rotated and thickens into the fault. The two tectono-stratigraphic styles also occur contemporaneously along the length of a single fault segment. Growth folding characterizes deformation around the ends of fault segments where the fault is blind, whereas the center of fault segments are characterized by surface faulting. These observations suggest that marked along-strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting.

Numerical modeling of trishear fault propagation folding

Abstract: In contrast to kink band migration modeling methods, trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges. Modeled fold hinges tighten and converge downward, within a triangular zone of distributed deformation which is focused on the fault tip. Such features have been reported from field studies and are also seen in analogue models of compressional deformation. However, apart from its initial application to Laramide folds, little quantitative work has been undertaken on trishear fault propagation folding in other settings. In addition, no study has been undertaken into the growth strata which might be associated with such structures. This paper uses an equivalent velocity description of the geometric model of trishear, together with models of erosion and sedimentation, to investigate trishear fault propagation folding of both pregrowth and growth strata. The trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement. The models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata. When total slip on a structure is high, the model predicts overturned pregrowth and growth strata. During the initial stages of deformation, beds in the forelimb thicken but later thin when they become steep or overturned. The effect of variations in fault propagation to slip ratios on two-dimensional finite strain in the models is assessed by the use of initially circular strain markers. High fault propagation to slip (p/s) ratios lead to narrow zones of high finite strain, while lower p/s ratios lead to more ductile deformation and broader zones of high strain. In all cases, hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault. Modeled structures are compared with natural examples.

Mantle fluids in the San Andreas fault system, California

Abstract: Fluids associated with the San Andreas and companion faults in central and south-central California have high3He/4He ratios. The lack of correlation between helium isotopes and fluid chemistry or local geology requires that fluids enter the fault system from the mantle. Mantle fluids passing through the ductile lower crust must enter the brittle fault zone at or near lithostatic pressures; estimates of fluid flux based on helium isotopes suggest that they may thus contribute directly to fault-weakening high-fluid pressures at seismogenic depths.

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

Abstract: We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

Segmented normal faults: Correspondence between three-dimensional mechanical models and field data

Abstract: Traces of many normal faults form an array of closely spaced overstepping segments. In three dimensions, fault segments may either be unconnected or link vertically or laterally into a single continuous fault surface. The slip distribution along segmented faults is complex and asymmetric, and the point of maximum slip generally is not located at the center of a segment. In relay zones between segments, slip gradients may be gentler or steeper, depending on the spatial fault arrangement. Branch points are characterized by steep slip gradients. One explanation for these observations is mechanical interaction between neighboring faults which occurs through local perturbation of the stress field. Three-dimensional (3-D) boundary element models show that the degree of fault interaction and hence the degree of asymmetry in the slip distribution increases with increasing fault overlap and downdip fault height and with decreasing fault spacing and Poisson's ratio. Interaction is strongest for faults with uniform shear strength and decreases if there exists a zone of greater shear strength near the tip line. This analysis provides a mechanical rationale for more frequent occurrence of overlapping segments relative to underlapping segments and for the limited range of the ratio between segment overlap and spacing along natural faults. Echelon segment configurations promote interaction, maximize the capacity to accommodate slip, and do not necessarily require a strike-slip movement component. Model idealizations of some outcropping fault arrays and of branching/merging faults capture a wide variety of common field observations. Consistent, mechanically based 3-D normal fault models can be obtained by combining different types of field data such as fault slip-to-length ratios, location of maximum slip, segment overlap-to-spacing ratios, and footwall uplift/hanging wall subsidence. By capitalizing on these data one can understand the mechanics of faulting, constrain the boundary conditions that govern the formation and growth of faults, and provide a rationale for interpreting normal faults in seismic surveys.

Patterns of basement structure and reactivation along the NE Atlantic margin

Abstract: A lineament pattern on the NE Atlantic margin is discussed, illustrated by gravity and magnetic images in the Norwegian Sea, and reviewed in the context of onshore field evidence. While most possible fault trends exist, three major sets predominate. A NE-SW left-stepping lineament set defines the gross geometry of the margin, while interposing northerly trends impose a rhomboidal geometry at a variety of scales. The margin is segmented by NW-SE transfer zones, sometimes involving significant offsets. The principal trends are primarily a function of Mesozoic-Cenozoic plate-wide extensional stress fields. Certain Proterozoic and Caledonian lineaments were, however, opportunistically reactivated according to the extension direction. Caledonian NE-SW orogen-oblique shears, typified by the More-Trondelag Fault Zone, were reactivated via (?Jurassic) strike-slip or oblique-slip, and were further exploited during Cretaceous-early Cenozoic extensional episodes leading to continental break-up. Jurassic E-W extension may also have reactivated N-S faults existing in the basement or generated in duplex systems between the NE-SW shears. Precambrian and Caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones.

Tertiary diachronic extrusion and deformation of western Indochina: Structural and 40Ar/39Ar evidence from NW Thailand

Abstract: The Wang Chao and Three Pagodas fault zones cut the western part of the Indochina block and run parallel to the Red River Fault. Evidence of intense ductile left-lateral shear is found in the Lansang gneisses, which form a 5 km wide elongated core along the Wang Chao fault zone. Dating by 40 Ar/ 39 Ar shows that such deformation probably terminated around 30.5 Ma. The Wang Chao and Three Pagodas faults offset the north striking lower Mesozoic metamorphic and magmatic belt of northern Thailand. 40 Ar/ 39 Ar results suggest that this belt suffered rapid cooling in the Tertiary, probably around 23 Ma. These results imply that the extrusion of the southwestern part of Indochina occurred in the upper Eocene-lower Oligocene. It probably induced rifting in some basins of the Gulf of Thailand and in the Malay and Mekong basins. In the Oligo-Miocene, the continuing penetration of India into Asia culminated with the extrusion of all of Indochina along the Ailao Shan - Red River fault. This occurred concurrently with the onset of E-W extension more to the south. Plotting in a geographical reference frame the diachronic time spans of movement on left-lateral faults east and southeast of Tibet implies that the northward movement of the Indian indenter successively initiated new strike-slip faults located farther and farther north along its path.

Study of plate deformation and stress in subduction processes using two‐dimensional numerical models

Abstract: Two-dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere. The subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence. The lithosphere is assumed to be underlain by an in viscid asthenosphere. Different rheological laws have been considered for the lithosphere, including elasticity and elastoplasticity. The modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters: the density contrast between lithosphere and asthenosphere (Δρ = ρL - ρA) and the coefficient of friction along the subduction plane. Varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc-back arc system. Extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction, while negative Δρ values and/or high friction leads to a compressional regime. The coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0.1. The results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions.

Juxtaposition and Seal Diagrams to Help Analyze Fault Seals in Hydrocarbon Reservoirs

Abstract: A new set of diagrams aids in analyzing fault juxtaposition and sealing. The diagrams are based on the interaction of rock lithology and the fault displacement (throw) magnitude to control juxtapositions and fault seal types. The advantages of the diagrams are that they allow an evaluation of a fault seal without the need for detailed three-dimensional mapping of stratigraphic horizons and fault planes, and can be used to contour permeability, sealing capacity, and transmissibility of fault zones. These diagrams may be used to rapidly identify the critical fault throw and juxtapositions that require mapping to identify compartments in hydrocarbon reservoirs.

Propagation of rifting along the Arabia‐Somalia Plate Boundary: The Gulfs of Aden and Tadjoura

Abstract: The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large-scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia-Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates >10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia-Somalia plate. The longest pause was at the Shukra-El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia-Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra-El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en echelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large-scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small-scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia-Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea-Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self-sustaining, plate-scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

Cenozoic geodynamics of the Ross Sea region, Antarctica: Crustal extension, intraplate strike-slip faulting, and tectonic inheritance

Abstract: An integrated study of onshore and offshore geology of the Ross Sea region (namely, Victoria Land, north of Ross Island, and the Ross Sea, Antarctica) has revealed a complex, post-Eocene tectonic framework. Regional NW-SE right-lateral, strike-slip faults are the outstanding feature of this framework and overprint an older Mesozoic extensional event, responsible for formation of N-S basins in the Ross Sea. The Cenozoic framework includes kinematic deformation and reactivation along the NW-SE faults, including formation of pull-apart basins, both positive and negative flower structures, and push-up ridges. N-S extensional faults are well developed between NW-SE faults and indicate E-W extension during the Cenozoic, produced by the NW-SE right-lateral strike-slip motion together with regional crustal extension. NNW-SSE compression, induced by the right-lateral, strike-slip kinematics, is indicated by locally inverted NE-SW faults and basins. The evolution, geometry, and location of the Rennick Graben and the Lanterman Range fit well into this model. Variations in the deformational style across the region can be linked to corresponding variations in the bulk crustal rheology, from brittle behavior in the west, to ductile deformation (at subseismic-scale resolution) near the Eastern Basin. A semibrittle region that favors N-S clustering of Cenozoic magmatic activity lies in between. In this region, Cenozoic volcanoes develop at the intersections of the NW-SE and the major N-S faults. The NW-SE faults cut almost continually from the Ross Sea to East Antarctica through lithospheric sectors with different rheology and thickness. At least two of the NW-SE faults correspond to older Paleozoic terrane boundaries in northern Victoria Land. The NW-SE faults link in the Southern Ocean with major transform faults related to the plate motions of Australia, New Zealand, and Antarctica.

The state of stress on some faults of the San Andreas system as inferred from near-field strong motion data

Abstract: We present a simple method to calculate the stress produced on an earthquake fault during rupture. This method allows the complete evaluation of the stress spatio-temporal history over the fault. We apply this approach to study the changes in shear stress produced during four of the largest earthquakes which occurred along the San Andreas fault system over the last 20 years: the Imperial Valley earthquake of 1979, the Morgan Hill earthquake of 1984, the Loma Prieta earthquake of 1989, and the Northridge earthquake of 1994. We use for this study the tomographic models of the fault rupture obtained from the inversion of the near-field seismic data recorded during these earthquakes. The results obtained show that the static and the dynamic stress drops vary greatly over the fault. The peak values obtained for the four earthquakes studied range from about 20 to 100 MPa. These high values imply that the initial shear stress level on the fault at the onset of the earthquake was high on at least a significant portion of the fault. The regions of the fault which break with a high stress drop are also the regions where slip is large. This suggests that most of the slip produced in a large earthquake takes place over the “strong” areas of the fault. Low slip regions tend to break with low stress drops. After the earthquake, the shear stress is increased over a significant portion of the fault, which corresponds to low slip regions. Aftershock activity tends to be concentrated in these areas of stress increase. The apparent strength of the fault before the earthquake (that is the local shear stress increase which is required for rupture) is also extremely heterogeneous. The rupture velocity seems to be inversely correlated with this apparent fault strength, the rupture accelerating over the “weak” areas of the fault and slowing down over the high strength areas.

Continental subduction and three-dimensional crustal structure: The northern South Island, New Zealand

Abstract: The three-dimensional Vp and Vp/Vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km and 100 km depth, the subducted plate is imaged as a relatively low-velocity feature in the uppermost mantle, reflecting the continental nature of the subducted crust in this region. An increase in amplitude of this low-velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction. Velocity variations within the subducted and overlying plates show some spatial correlation. This suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath Marlborough is currently not accommodating much active subduction. In the overlying plate, the Awatere fault is a major structural feature, associated with a low-velocity zone extending to 23 km depth. There is a marked change in structure near this fault, with seismic velocities being lower to the southeast. A relatively high level of seismicity occurs in this region of lower seismic velocities, suggesting a relationship between the two. A possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust. The low-velocity region in the overlying plate coincides with the region of most intense active deformation, suggesting it is relatively weak.

Three‐dimensional dislocation model for great earthquakes of the Cascadia Subduction Zone

Abstract: There have been no historical Cascadia great subduction thrust earthquakes, but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event. Geodetic measurements in the coastal region from northern California to southern British Columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault. The segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two-dimensional (2-D) elastic dislocation modeling of interseismic deformation geodetic data. In this study, a general 3-D dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture. The model is based on die surface deformation due to shear faulting in an elastic half space. The 3-D model of the Cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin. The bend in the margin trend and subducting slab end effects are included. There is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement. An initial 3-D model based upon 2-D dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data. The best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km. The two zones lie mainly offshore beneath the continental shelf and slope. The locked and transition zone widths vary smoothly along the margin, being greater off northern Washington where the thrust dip is shallow and narrower off central Oregon. Assuming that the locked plus transition zones approximate the maximum coseismic rupture area, these widths permit aMw=9 earthquake.

Exhumation in a convergent orogen: the western Tauern window

Abstract: Unroofing of the western Tauern window involved both low-angle detachment faulting (Brenner Fault) and enhanced footwall erosion, contemporaneous with upright antiformal folding. This combination reflects orogen-parallel (˜E–W) extension during continued ˜N–S Alpine convergence. New fission-track ages establish the relative chronology of folding and faulting and demonstrate that displacement was not always accommodated on the same surface. During exhumation, some units migrated from the footwall to the hanging wall of the main detachment fault, due to the interplay between folding and faulting. The region can effectively be divided into 3 distinct domains. (1) The Penninic units of the western Tauern Window were always in the footwall to the fault, with maximum exhumation in the core of the dome, due to folding and erosion. (2) The Lower Austroalpine unit north of the Tauern Window was first part of the hanging wall to the Brenner Fault. At a later stage this unit was exhumed by a further 4–5 km as part of the footwall to a more discrete, through-going fault (the Silltal Fault). (3) The Middle and Upper Austroalpine units west of the Tauern Window were always within the hanging wall. Exhumation of the footwall from an initial depth of ∼ 25 km led to a transition in mechanical behaviour. The curviplanar (folded) ductile shear zone marking the boundary to the Tauern window was eventually transected by a more planar discrete brittle fault (Silltal Fault, with unit 2 now in the footwall), along which the pre-existing mylonites were passively exhumed to the surface.