Direction Of Plate Convergence Research Articles

A subducting seamount beneath the Mediterranean Ridge

Two transverse morphological tracks across the front of the Mediterranean Ridge were swath mapped with a Hydrosweep instrument system. Along the tracks, ridges and troughs at the front of the accretionary prism are disrupted and bent in the direction of relative plate convergence. The longer of the two tracks extends to Bannock Basin, a brine-filled depression with a closed outer moat around a central mound. Seismic reflection data and a gravity anomaly show a buried circular seamout beneath Bannock Basin whose summit is the mound. This indicates an origin of the larger transverse track and Bannock Basin from a subducting seamount. Beneath Bannock Basin, the seamount has displaced about 340 km 3 of the Messinian section and yet the seafloor has not been uplifted. Therefore, considerable evaporitic material is inferred to have been displaced by the seamount and Bannock Basin is a likely vent for large quantities of dissolved materials from the disrupted Messinian evaporitic sequence.

Simplified analysis of horizontal stresses in a buttressed forearc sliver at an oblique subduction zone

Forearc slivers are commonly present at obliquely convergent margins. Such slivers are often buttressed at their leading edges, but no quantitative model of the buttress effect exists. I consider the horizontal stresses in a buttressed sliver using a simple plan‐view two‐dimensional elastic model. The driving force created by the subducting plate is approximated by a body force in the direction of plate convergence. An Airy stress function solution relates the stress field to the boundary and body forces that allows the relative importance of different forces driving and resisting the sliver to be evaluated. Oblique subduction can easily cause arc‐parallel compression in the buttressed sliver, especially near the leading edge, because the arc‐parallel component of the driving force by the subducting plate is integrated over a much larger distance than its arc‐normal component. For the Southwest Kuril arc, the stress field predicted by the simple model agrees with that inferred from geological structures. It is possible that the model can also explain the stress field of the Cascadia forearc as constrained by earthquake focal mechanisms and in situ stress measurements.

Spatial distribution of intermediate-depth earthquakes with horizontal or vertical nodal planes beneath northeastern Japan

The 1993 Kushiro-oki (off-Kushiro) earthquake ( M JMA = 7.8) is one of the largest intermediate-depth earthquakes that have occurred in northeastern Japan. This earthquake is a notable event that has a nearly horizontal fault plane extended from the lower seismic plane of the double-planed seismic zone toward the upper seismic plane. We have examined the spatial distribution of focal mechanism solutions of deep and intermediate-depth earthquakes beneath northeastern Japan concerning this peculiar event. Analyses of 127 new solutions along with existing solutions confirm the previous results on essential features of their spatial distribution. The predominant types of solutions are down-dip compression in the upper plane except beneath eastern Hokkaido, and down-dip extension in the lower plane. P-Axes in the upper seismic plane and T-axes in the lower plane are in the direction of plate convergence. Beneath Hokkaido, however, T-axes show significant deviation from the direction of plate convergence. The stress axes beneath the junction between the northeastern Japan arc and the Kuril arc exhibit slight deviation from those in the adjacent areas, suggesting slab distortion beneath the junction. We next investigate the spatial distribution of events with horizontal nodal planes as candidates for events with similar focal mechanisms to the Kushiro-oki earthquake. The activity of earthquakes with horizontal nodal planes is definitely high in the lower plane, especially beneath Hokkaido. Many of them have similar focal mechanisms to the Kushiro-oki earthquake, indicating that the Kushiro-oki earthquake is not exceptional but is a typical event in the region. However, the fault planes of these events are variable in space; some events are known to have vertical fault planes. There are shallow and deep depth bounds and a southwestern boundary for the events with horizontal fault planes. These facts suggest that the horizontal faulting is probably caused by some regional stress concentration owing to the unbending and the distortion of the segmented slab at the junction.

Kinematics around a large‐scale oblique ramp, southern Pyrenees, Spain

The Gavarnie thrust sheet of the southern Pyrenees is bound to the east and west by ∼40‐km‐long transverse zones producing a rectilinear thrust sheet. Within the Boltana region the northwest corner of the Gavarnie thrust sheet is well exposed. The locale is used to characterize the strain and kinematic patterns and detailed geometry of an oblique ramp over a variety of scales. The hanging wall of the Gavarnie thrust fault in the area was transported toward ∼200° and contains two regional‐scale, north‐south trending anticlines. In the vicinity of the oblique ramp the strike of the Gavarnie thrust fault and the trend of hanging wall fold axes are ∼0°–180°. The magnitude of shortening accommodated by regional‐scale fold and fault development is constant at ∼25% along the length of the ramp. The orientation of instantaneous and finite shortening axes on meso and grain scales was inferred from the attitudes of cleavage, mesoscale folds and faults, carbonate mechanical twins, and the geometry of finite strain. Finite and instantaneous shortening axes are subhorizontal and remain perpendicular to the Gavarnie thrust ramp within a ∼10‐km‐wide zone in the footwall and a ∼20‐km‐wide zone in the hanging wall. Outside of the halo of footwall and hanging wall ramp deformation, the principal shortening directions consistently trend ∼200°, parallel to thrust transport directions throughout the south Pyrenean thrust belt. The parallelism of instantaneous and finite principal strain axes suggests that grain‐scale deformation was rionrotational. Total shortening measured within the footwall of the Gavarnie thrust fault decreases from ∼60% adjacent to the thrust ramp to <10% approximately 10 km away from the ramp. While principal shortening directions determined over scales ranging from millimeters to kilometers show a consistent pattern in the Boltana region, they differ by as much as 90° from tectonic convergence directions between Iberia and Eurasia during the Alpine orogeny (∼80–18 Ma) determined from magnetic anomaly patterns in the central Atlantic and Bay of Biscay. Divergence between plate convergence directions and Alpine south Pyrenean shortening directions suggests that ∼200 km of NW–SE convergence was apparently partitioned into the north and south Pyrenean thrust zones which were shortened ∼167 km, the Iberian Range which accomodated ∼30 km of shortening, <10 km of shortening accomodated in the Catalan Coastal Range, and ∼100 km of right‐lateral strike slip accomodated within the internides.

Distribution of volcanoes in active margins

We report a comprehensive study of the distribution of volcanoes in 16 active plate margins, corresponding to a total of 479 volcanic systems. The active volcanic arcs are found to have a ribbon geometry with an average length / width ratio of around 10. The shape of the volcanic arc is compared to the shape of the associated trench by projecting one onto the other. The projection direction agrees with the direction of plate convergence, which shows that plate motion is the main factor constraining magma generation on the lithospheric scale. There is no characteristic spacing between volcanoes. In each arc, the distribution of volcano spacing is best represented by a Gamma distribution which corresponds to randomly generated points in the same geometrical conditions. In order to explain how such distributions may be generated, we have investigated in the laboratory the gravitational instability of a layer of buoyant liquid which is fed at a constant rate at the bottom of denser fluid. Different dynamic regimes are reached for different values of the viscosity ratio between the two fluids. For low viscosity contrasts, there is only one initial instability event, with buoyant plumes organized in a periodic pattern. These plumes are fed continuously by the source, even at large times. For high viscosity contrasts, which is relevant to the geological problem, plumes are produced intermittently by individual instability events. They take the form of “cavity” plumes fed by narrow tails. Once the plumes have reached the surface, they continue to be fed from below by material rising through their shrinking tail. In successive instability events, plume generation is not repeated at the same locations which leads to a complex plan‐form. With time, and hence with increasing number of instability events, the cumulative distribution of plume spacing changes from periodic to random. Material from a single plume is able to sustain volcanic eruptions for a period longer than the time between two instability events, and hence the distribution of active volcanic centers reflects the cumulative distribution of several instability events. This is consistent with the observed distributions.

Case for very low coupling stress on the Cascadia Ssubduction Fault

A fundamental problem in plate tectonics is the shear strength of major plate boundary faults. This translates to the question whether the generally observed small earthquake stress drops of 3–10 MPa on major faults release most of the accumulated stress or only a small fraction of it. There is strong evidence that the San Andreas fault, a major transform plate boundary, is weak (<20 MPa shear resistance). It is not yet clear whether subduction thrust faults are also weak. We present two types of evidence from the northern Cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault, comparable to that estimated for the San Andreas fault. First, the well‐defined surface heat flow and heat generation allow negligible frictional heating on the plate interface. The average shear stress on the fault must thus be very low over a time scale of a few million years. Second, focal mechanism solutions for small crustal earthquakes in the southern Vancouver Island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress. This inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low. One explanation for the weakness of the fault is the presence of near‐lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence. The conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked, accumulating strain energy toward a future great earthquake. The surface deformation responds to the small (<20 MPa) temporal changes of the stress field associated with the subduction earthquake cycle. This transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin. The weakness of the Cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards. For example, a subduction earthquake may induce large strike‐slip earthquakes in the forearc that affect a large area.

Shallow subduction tectonics and fault interaction: The Weber, New Zealand, earthquake sequence of 1990–1992

A series of four large earthquakes with epicenters very close to one another occurred in 1990–1992 in the Weber region of the southeast coast of the North Island, New Zealand. The region is one of oblique plate convergence and subduction, the plate interface being at about 20 km depth. The first event, February 19, 1990, ML 6.1, Mw 6.2, occurred within the upper part of the subducting Pacific plate on a steeply northwest dipping normal fault, as defined by relocated aftershocks. The mechanism of this event reflects downdip tension within the subducting plate due to “slab pull” forces, as is common in New Zealand. The second event, May 13, 1990, ML 6.2, Mw 6.4, occurred on an imbricate fault in the overlying Australian plate, dipping shallowly northwest, with components of both thrusting and dextral strike‐slip motion. The slip vector is similar to the direction of plate convergence, showing no arc‐normal rotation as observed for subduction thrust events in other areas of oblique convergence. It is unlikely that the spatial and temporal closeness of these events is coincidence (a similar pair occurred to the south in 1942), but the coupling mechanism is not clear, perhaps being related to aseismic slip downdip on the subduction interface. Two subsequent events, both of ML 5.5, occurred on August 15, 1990, and March 2, 1992, and extended the aftershock zone of the deeper first main shock to the northeast, matching the extent of the shallower second main shock. Mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to the relevant mainshock. The time history of the aftershocks of the first (deeper) event shows a period of quiescence beginning 35 days before the second (shallow) event; the mean magnitude of those events that did occur was high. This observation cannot be explained by station biases, weather, or changes in analysis methods and may be due to stress changes induced by precursory aseismic slip which would discourage aftershock occurrence.

Strain accumulation in the Shumagin Islands: Results of initial GPS measurements

Deformation in the Shumagin seismic gap has been monitored with repeated trilateration (EDM) in the 1980–1987 interval and with the Global Positioning System (GPS) in the 1987–1991 interval. The geodetic network extends for 100‐km across the Shumagin Islands to the Alaska Peninsula. Results from the GPS surveys are consistent with those previously reported for the EDM surveys: we failed to detect significant strain accumulation in the N30°W direction of plate convergence. Using the method of simultaneous reduction for position and strain rates, we found the average rate of extension in the direction of plate convergence to be −25±25 nanostrain/yr (nstrain/yr) during the 1987–1991 interval of GPS surveys compared with −20±15 nstrain/yr during the 1981–1987 interval of complete EDM surveys. We found a marginally significant −26±12 nstrain/yr extension rate in the 1981–1991 interval covered by the combined EDM and GPS surveys. Strain rates are higher, but not significantly so, in the part of the network closest to the trench. Spatial variation in the deformation is observed in the 1980–1991 average station velocities, where three of the four stations closest to the trench have an arcward velocity of a few mm/yr. The observed strain rates are an order of magnitude lower than the −200 nstrain/yr rate predicted by dislocation models.

Seismic Data, Geometry, Evolution, and Shortening in the Active Sulaiman Fold-and-Thrust Belt of Pakistan, Southwest of the Himalayas

Surface and subsurface data from the Sulaiman fold-and-thrust belt are integrated to analyze the deep structure, tectonic shortening, and kinematics of the Sulaiman fold-and-thrust belt at the western margin of the Indian subcontinent. Seismic reflection data show that nearly all the 10-km-thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland. A balanced structural cross-section suggests that structural uplift in the Sulaiman fold-and-thrust belt is a result of a thin-skinned, passive-roof duplex style of deformation. The duplex sequence consists of Jurassic and older strata. It is separated from the roof equence by a passive-roof thrust in thick Cretaceous shales that has a backthrust sense of displacement over foreland-propagating duplexes. The roof sequence with incipient out-of- sequence thrusting is intact for about 150 km northwards, where it emerges along a passive backthrust. Sequential restoration of the balanced section reveals a series of structural and geometrical features including: (1) development of low-amplitude, broad concentric (detachment) folds (Sui and Loti) at the tip of the decollement; (2) increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures; and (3) out-of-sequence thrusting to create required critical taper for an outward translation of the foreland fold-and-thrust belt. A balanced structural cross-section 349 km long from the Sulaiman fold-and-thrust belt restores to an original length of 727 km, suggesting a maximum of 378 km of shortening since 21 Ma in the cover strata of the Indian subcontinent. Calculation of displacement rates over the Sulaiman fold-and-thrust belt (18 mm/yr) added to the resolved rate of the Chaman fault vector for the component parallel to the plate convergence direction (15 mm/yr) are close to the current India-Asia plate convergence rate (37 mm/yr).

Neogene folding and faulting in southern Monterey Bay, Central California, USA

The goal of this study was to determine the Neogene structural history of southern Monterey Bay by mapping and correlating the shallow tectonic structures with previously identified deeper occurring structures. Side scan sonographs and Uniboom seismic reflection profiles collected in the region suggest that deformation associated with both compressional and transcurrent movement is occurring. Strike-slip movement between the North American and Pacific plates started as subduction ceased 21 Ma, creating the San Andreas fault system. Clockwise rotation of the Pacific plate occurred between 3.4 and 3.9 Ma causing orthogonal convergence between the two plates. This plate rotation is responsible for compressional Neogene structures along the central California coast. Structures exhibit transpressional tectonic characteristics such as thrust faulting, reverse faulting and asymmetrical folding. Folding and faulting are confined to middle Miocene and younger strata. Shallow Mesozoic granitic basement rocks either crop out or lie near the surface in most of the region and form a possible décollement along which the Miocene Monterey Formation has decoupled and been folded. Over 50% of the shallow faults strike normal (NE-SW) to the previously identified faults. Wrench fault tectonics complicated by compression, gradual uplift of the basement rocks, and a change in plate convergence direction are responsible for the observed structures in southern Monterey Bay.

Fine structure of seismotectonics in western Shillong massif, north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.

Interplate coupling and relative plate motion in the Tokai district, central Japan, deduced from geodetic data inversion using ABIC

SUMMARY The spatial distribution of the strength of interplate coupling between the subducting Philippine Sea and overlying continental plates in the Tokai district, central Japan, was investigated in detail through the inversion analysis of geodetic data using Akaike's Bayesian Information Criterion (ABIC). The geodetic data used for the analysis are annual rates of level changes (1972-1984) and horizontal length changes (1977-1988), which presumably represent average crustal movements during the interseismic period. The result of the inversion analysis shows the existence of a strongly coupled region extending from 10 to 30 km in depth. The total seismic moment accumulated in this area since the last event (the 1854 Ansei earthquake) is roughly estimated to be 5.5 × 1027 dyne cm, which corresponds to Ms= 7.8. The interplate coupling becomes weaker toward the shallower and deeper portions. This is consistent in general tendency with a rheological model inferred from petrological viewpoints. The strength of coupling also tends to decrease toward northeast over the west coast of Suruga Bay. The direction of plate convergence inferred from the inversion analysis is oriented N30°W. This is significantly different from the direction of relative plate motion between the Philippine Sea and Eurasian plates but concordant with that between the Philippine Sea and North American plates.

Paleostress adjacent to the Alpine Fault of New Zealand: Fault, vein, and styolite data from the Doctors Dome Area

Doctors Dome, 75 km north of Christchurch, New Zealand, is an early Pleistocene to Recent structure being deformed along the southeast edge of the Pacific‐Australian plate boundary. Paleostress in the area has been determined in basement rocks of the Mesozoic meta‐graywacke Torlesse Supergroup which lies unconformably beneath Cretaceous and younger cover rocks. Pre‐Late Cretaceous multiple folding and bedding plane faulting produced a complex, low grade, metasedimentary basement with predominantly steeply plunging fold hinges and largely isotropic overall behavior with respect to later deformation. Probable Late Cretaceous WNW‐ESE shortening and NNE‐WSW extension of the basement produced widespread laumontite, calcite, and silica veins. Widespread but sparse slickensided minor fault surfaces are probably related to the same stress system. During the late Cenozoic dome stage, basement was uplifted, rotated and severely thrust along some of the range fronts to produce local high concentrations of small slickensided surfaces in the frontal areas. Inversion of basement fault data for the area indicates a general northwest compression with two peaks, one WNW‐ESE parallel to the shortening suggested by the older vein system and the other parallel to southeast‐northwest stylolite columns in the cover rocks. This direction is approximately parallel to regional indicators of contemporary deformation in and adjacent to the Alpine Fault Zone and suggests that the stress field affecting these rocks has not changed significantly since the late Pliocene‐early Pleistocene. Like the San Andreas system, this compression is at a high angle to the strike of the zone as a whole, but is compatible with the direction of plate convergence and motion of the major faults. Between the overlapping ends of the Alpine Fault and the Hikurangi Subduction Zone the Alpine Fault may become subhorizontal at middle‐lower crustal levels, partially decoupling the crust from underlying structures, and thus allowing oblique motion to be transferred directly onto the fault from the subduction complex, while aiding the change from subduction to continental collision.

Structure of the Nankai accretionary prism as revealed from IZANAGI sidescan imagery and multichannel seismic reflection profiling

Abstract The Nankai accretionary prism, off southwest Japan represents one of the best developed clastic prisms in the world. A combination of swath mapping including Sea Beam and ‘IZANAGI’ sidescan sonar and closely spaced seismic reflection data was used to investigate the relationship between the progressive landward change in surface morphology and the internal structural evolution of the prism. The prism surface is divided into three zones sub‐parallel to the trough axis on the basis of the IZANAGI backscattering image. The frontal part of the prism is characterized by several continuous lineaments that are approximately perpendicular to the plate convergence direction. These lineaments correspond to anticlinal ridges caused by active imbricate thrusting. Landward, these anticlinal ridges become progressively masked by fine‐grained hemipelagic slope sediments that are constantly supplied to the entire prism slope. However, these overlying sediments show little deformation. This implies a change in deformation style from frontal thrusting with fault‐bend folds to internal refolding of thrust sheets. In the middle to upper prism slope, the IZANAGI image shows numerous landslide features and large fault scarps, suggesting that exposed sediments are lithified enough to fail in brittle mode compared with the wet sediment deformation at the prism toe. Prism evolution is strongly affected by the decollement depth which may be indirectly controlled by oceanic basement relief; a topographic embayment coincides with a regional minimum of sediment offscraping where a basement high has been subducted. The small tapered prism observed in the embayment may be due to lateral supply of overpressured pore fluids from the adjacent prism. Strain caused by the differential rate of prism growth across the basement relief forms faults trending at high angles to the trough axis.

The regional intraplate stress field in South America

A compilation of lithospheric stress directions for continental South America and the inferred major patterns of the regional intraplate stress field are presented. Stress orientations are based primarily on earthquake focal mechanisms and Quaternary fault slip inversion published in the literature. Four new focal mechanisms based on short‐period P wave modeling, and selected centroid‐moment‐tensor solutions (published by U.S. Geological Survey) consistent with P wave first motions at South American and other World‐Wide Standard Seismograph Network stations are also included in the data base. The observed patterns of intraplate stresses may be useful in constraining numerical models of the plate driving forces in the South American plate. In the Andean plateau (altitudes greater than 3000 m), N‐S extensional stresses predominate. E‐W compressional stresses are observed in the sub‐Andean and platform regions extending up to about 1000 km east of the Andes. The maximum horizontal stress (SHmax) is uniformly oriented in the E‐W direction throughout western South America. Averages of the SHmax direction at grid points spaced 2.5° were taken as representing the “regional” field. The E‐W direction of this regional field is not affected by the change of strike of the Andean chain nor by the contact of a flat subducted slab underneath. The regional SHmax direction is oriented 15° more clockwise than the direction of the Nazca plate convergence. The difference between the regional SHmax orientations and the absolute plate motion may be only about 6°. This may indicate that contact forces with the Nazca plate may not be the only major contributor to the intraplate stresses in western South America. The eastern limit of this Andean stress province seems to coincide with aseismic regions in the Upper Amazon basin and in the Parana basin. In the central Amazonian region, seismicity and stress data suggest a seismic province with N‐S compressional stresses. The origin of these stresses is not yet clearly understood but could possibly be related to lower crustal loading along the middle Amazon basin. In northeastern Brazil, seismicity is characterized by upper crustal strike‐slip earthquakes bordering the Potiguar marginal basin. A model is proposed for this region in which the stress field is the result of a superposition of regional E‐W compressional stresses and local extensional stresses (oriented perpendicular to the continental margin) possibly related to density contrasts and sediment loading in the continental shelf.

Active faults and state of stress in the central part of the Trans‐Mexican Volcanic Belt, Mexico 1. The Venta de Bravo Fault

The central part of the Trans‐Mexican volcanic belt is being deformed by seismically active, west‐east striking normal faults that cause pronounced scarps. Structural and seismic along‐strike discontinuities allow the following segmentation: (1) The western segment, the area between Morelia and the Los Azufres volcanic center, is characterized by ≤ 25‐km‐long fault scarps that define the Cuitzeo graben, and the Santa Ana Maya horst which has a relief ≤ 850 m with respect to the Cuitzeo graben. The faults cut several Quaternary scoria cones as well as alluvium. (2) The central neotectonic segment extends between Maravatio in the west and Altamirano volcano in the east. The most prominent structure of this segment is the 45‐km‐long Venta de Bravo fault with a relief ≤ 300 m. The February 22, 1979, mb = 5.3 Maravatio earthquake occurred on this fault. The late Quaternary vertical slip rate along the easternmost part of the Venta de Bravo fault is estimated at 2 mm/yr. The lateral extension of the segment is defined by the limits of the Venta de Bravo fault and the eastern termination of the Cuitzeo graben. (3) The eastern segment is made up of the Acambay graben. The limit between the central and eastern segments is defined by an extensional jog between the Venta de Bravo and Pastores faults, which indicates a left‐lateral strike‐slip component. The most recent rupture of this segment occurred in the November 19, 1912, Ms = 6.9 Acambay earthquake that caused vertical displacements ≤ 50 cm along the faults flanking the graben. Stress determinations based on Quaternary scoria cone alignments, the focal mechanism of the 1979 earthquake, and the inversion of striation measurements indicate a stress field with Sv>SENE>SNNW. The fault striations as well as the focal mechanism indicate a minor but consistent left‐lateral strike‐slip component. The observed stress and deformation can be explained by the superposition of two sources of stress. Isostatically compensated surface loads related to the high elevation of the Trans‐Mexican volcanic belt probably cause horizontal deviatoric tension and the observed crestal normal faults which strike parallel to the axis of the volcanic belt. The observed left‐lateral strike‐slip component, on the other hand, can be explained by compressional far‐field stresses caused by loads at the boundary between the Cocos and North American plates; the orientation of the faults with respect to the direction of plate convergence is a prefered orientation for left‐lateral slip along the observed faults. Alternatively, the observed left‐lateral strike‐slip component may accommodate trench‐parallel motion of crustal blocks in southwestern Mexico.

Quaternary uplift of the peruvian coast related to the subduction of the Nazca Ridge: 13.5 to 15.6 degrees south latitude

Marine terraces between 13.5 and 15.6°S. latitude on the Peruvian coast preserve a history of Quaternary sea-level fluctuations and forearc movement above the subducting Nazca Ridge. Field mapping and amino acid racemization/epimerization, electron spin resonance, radiocarbon, and uranium-series disequilibrium ages from mollusk shells from terrace deposits are used to define a marine terrace chronology. Terraces on the Peruvian coast formed during global high stands of Quaternary sea level, in a similar way to the marine terraces on the west coast of the United States, and Baja California, Mexico. A last interglacial marine terrace (125 ka) is preserved on nearly every terrace flight. Along the coast from northeast to southwest, each terrace varies in elevation because of variations in the rate of coastal uplift. Variations in uplift rate from northeast to southwest in the study area are explained by a simple geometric model in which the variables are the rate of plate convergence, morphology of the Nazca Ridge, and orientation of the ridge relative to the Peruvian coast. The oblique northeastward orientation of the ridge relative to the east-west direction of plate convergence results in a ca. 70 mm/yr southeastward migration of the zone of Nazca Ridge subduction beneath the Peruvian coast. The coastal area above the northern flank and crest of the Nazca Ridge is stable or has undergone net subsidence, while the coast above the ridge crest is rising at about 0.3 m/kyr. Above the southern flank of the Nazca Ridge, the coast is rising at a rate of ca. 0.5 m/kyr, the fastest rate determined from studies of marine terraces on the coasts of Peru and Chile. South of the influence of the Nazca Ridge, uplift rates are ⩽ 0.2 m/kyr. The pattern of faster uplift above the southern flank and slower uplift above the northern flank of the ridge is a predictable consequence of oblique ridge subduction.

Transverse structural trends along the Oregon convergent margin: Implications for Cascadia earthquake potential and crustal rotations

A remarkable set of west-northwest-trending left-lateral strike-slip faults intersects the Cascadia subduction zone. Three of these faults have been mapped off northern and central Oregon by using seismic reflection, SeaMARC-1A sidescan sonar, and SeaBeam bathymetry. These faults are highly oblique to the north-south structural grain of the active accretionary wedge. One of them has 6 km of horizontal slip; the average slip rate is 7-10 mm/yr. The faults cut the subducting Juan de Fuca plate, and can be traced into the North American plate. Folds that deform late Pleistocene and Holocene sediments on the upper continental slope and shelf strike north-northwest to west-northwest. Some of the west-northwest-trending folds are associated with the throughgoing strike-slip faults, whereas other northwest-trending folds are approximately normal to the plate convergence direction. Many of these folds are mapped across the shelf, and several active shelf synclines project toward Oregon's coastal bays, where marsh subsidence events are inferred to be the result of great subduction-zone earthquakes. These subsidence events may actually record the growth of local synclines, possibly as secondary effects of slip on the megathrusts. The authors postulate that shortening of the forearc region by clockwise tectonic rotation, associated with movement of the left-lateral faultsmore » and folding of the upper plate, may accommodate a significant amount of plate convergence.« less

Earthquakes along the Ryukyu‐Kyushu Arc: Strain segmentation, lateral compression, and the thermomechanical state of the plate interface

We systematically determined the focal depths and mechanisms of 49 large to moderate‐sized earthquakes (mb≥5.4) that occurred along the Ryukyu‐Kyushu arc since 1963 by inverting the waveforms and amplitudes of P and SH wave trains at teleseismic distances. The results are sufficiently precise to delineate seismogenic structures near the plate interface in detail, revealing features not predicted by plate kinematics. In contrast to previous studies of this arc and those along other subduction zones, shallow seismicity along the plate interface is systematic, showing two distinct layers of activities. The second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km, some 10–20 km directly beneath the seismogenic portion of the interplate thrust zone. These earthquakes indicate lateral compressional strain within the subducted slab as their P axes are subparallel to the local strike of the arc, not perpendicular to the arc as one would expect from the direction of plate convergence. The occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab. To our knowledge, similar earthquakes occurred only beneath the northern Indoburman ranges and along the Banda arc where subducted slabs, as part of the Indian plate, are being dragged northward with their northern edges bumping into east‐west trending collision zones nearby. By drawing an analogy between the tectonic settings of these three regions, we interpret events beneath the plate interface along the Ryukyu‐Kyushu arc as a consequence of ongoing collision between the Philippine Sea plate and Eurasia near Taiwan. The interplate thrust zone is largely aseismic down to a depth of approximately 30 km. A large number of earthquakes showing low‐angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20–25 km. These observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25±5 km. Based upon available heat flow measurements between the trench axis and the volcanic arc, we estimated the temperature field and magnitude of shear traction along the interplate thrust zone, the deepest interplate earthquakes (∼40–50 km) correspond to temperatures of approximately 730–980°C, comparable to the limiting temperature of intraplate mantle earthquakes (∼800°C) and that of intermediate‐ and deep‐focus earthquakes at Wadati‐Benioff zones (potential temperature ∼900°K). Therefore, the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions. The magnitude of shear traction along the seismogenic portion (∼40–50 km depth) of the interplate thrust zone seems to be of the order of 100 MPa which, in turn, implies a low average coefficient of friction of only 0.10±0.05, considerably less than those observed for laboratory specimens. The inception of intermediate‐focus earthquakes within the subducted slab is at a depth of about 100 km. Whereas shallow earthquakes show no apparent variations along the entire arc, intermediate‐depth earthquakes indicate downdip extension along the northern end of the arc near Kyushu but abruptly change to downdip compression along the rest of the arc. All available evidence indicates that this is a spatial pattern unrelated to earthquake cycles. At the moment, tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic. Without the presence of clear discontinuities in the age of the subducted slab, the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc. In any case, this pattern of strain segmentation requires the presence of a major accommodation structure, such as a tear fault, in the subducted slab near the Tokara channel.Appendix is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Ave., N.W., Washington, DC 20009. Document B91‐004; $2.50. Payment must accompany order.

Stress tensor analysis in the Taiwan area from focal mechanisms of earthquakes

We produce a map of the stress pattern in and around Taiwan based on 200 earthquake focal mechanism solutions. These solutions were determined by using data from Taiwan Telemetered Seismographic Network, microearthquake surveys and WWSSN. The stresses are derived through a minimization of angles between the slip vector and the shear stress on each nodal plane considered as a fault, employing appropriate weighting factors. The whole set of focal mechanisms is divided into several groups, mainly according to apparent clustering of the event locations. The results show that the direction of maximum principal stress in Taiwan area is nearly horizontal and SE-NW on average. This is in good agreement with the direction of relative motion between the Philippine Sea plate and the Eurasian plate. In western Taiwan, the fan-shaped distribution of the maximum principal stress is consistent with the direction of Philippine Sea-Eurasian plate convergence through a simple model of viscous material indented by a rigid wedge. In the northeastern part of Taiwan, a nearly horizontal minimum principal stress oriented N–S is found for shallow depths; it occurs in a region of low seismic velocities, probably related to the back-arc activity of the Okinawa Trough. Down-dip compressional and down-dip extensional stresses have been identified in different depth ranges within the subducting slab of the Philippine Sea plate in the northern Taiwan; this may reflect the slab characteristics in this area. A complex stress pattern prevails in the Hualien area, at the junction between the Ryukyu subduction system and the Taiwan collision zone.