Interplate Thrust Zone Research Articles

Seismic Characteristics of the Nootka Fault Zone: Results from the Seafloor Earthquake Array Japan–Canada Cascadia Experiment (SeaJade)

Abstract The Nootka fault zone (NFZ) divides the incoming Explorer and Juan de Fuca plates of the Cascadia subduction zone. Three months of seafloor monitoring using 33 ocean‐bottom seismometers off the west coast of Vancouver Island has allowed us to better understand the tectonic configuration and seismogenic characteristics of the NFZ. We have learned that the NFZ is comprised of northern and southern primary bounding faults, and several conjugate faults developed subperpendicular to the primary faults. Earthquakes typically occur over the depth ranges of 15–20 and 6–15 km along the primary bounding and conjugate faults, respectively. Focal mechanisms reveal that the most common modes of failure in this region are left‐lateral strike slip, with normal faulting occurring along the southwestern extent of the NFZ and thrust faulting to the northeast before the subduction front. Seismic tomography suggests that the oceanic Moho is at a depth of 12–14 km below sea level (10–12 km below seafloor) just seaward of the Cascadia deformation front, and that it deepens to 19 km (17 km below seafloor) approximately 20 km landward of the deformation front. Converted phase analysis illuminates four velocity‐contrasting interfaces with average depths below sea level deepening landward of the subduction front at ∼4–6, ∼6–9, ∼11–14, and ∼14–18 km. We interpret them as the sedimentary basement, upper–lower crust boundary, oceanic Moho, and the base of the highly fractured and seawater or mineral enriched veins within mantle. The precipitation of minerals such as quartz or the formation of talc, which is made possible by the intense degree of fracturing within the NFZ facilitating the infiltration of seawater, may reduce mantle velocities, as well as VP/VS ratios. The lack of seismicity observed along the interplate thrust zone in northern Cascadia may suggest that the megathrust fault is completely locked, consistent with prior studies.

Arc segmentation and seismicity in the Solomon Islands arc, SW Pacific

This paper evaluates neotectonic segmentation in the Solomon Islands forearc, and considers how it relates to regional tectonic evolution and the extent of ruptures of large megathrust earthquakes. We first consider regional geomorphology and Quaternary vertical displacements, especially uplifted coral reef terraces. Then we consider geographic seismicity patterns, aftershock areas and vertical displacements for large earthquakes, focal mechanisms, and along-arc variations in seismic moment release to evaluate the relationship between neotectonically defined segments and seismicity. Notably, one major limitation of using seismicity to evaluate arc segmentation is the matter of accurately defining earthquake rupture zones. For example, shoreline uplifts associated with the 1 April 2007 M w 8.1 Western Solomons earthquake indicate that the along-arc extent of rupture was about 50 km smaller than the aftershock area. Thus if we had relied on aftershocks alone to identify the 2007 rupture zone, as we do for most historical earthquakes, we would have missed the rupture's relationship to a major morphologic feature. In many cases, the imprecision of defining rupture zones without surface deformation data may be largely responsible for the poor mismatches to neotectonic boundaries. However, when a precise paleoseismic vertical deformation history is absent, aftershocks are often the best available tool for inferring rupture geometries. Altogether we identify 16 segments in the Solomon Islands. These comprise three major tectonic regimes or supersegments that correspond respectively to the forearc areas of Guadalcanal–Makira, the New Georgia island group, and Bougainville Islands. Subduction of the young and relatively shallow and buoyant Woodlark Basin and spreading system distinguishes the central New Georgia supersegment from the two neighboring supersegments. The physiographic expression of the San Cristobal trench is largely absent, but bathymetric mapping of the surface trace of the interplate thrust zone defines it adequately. The New Georgia supersegment has smaller arc segments, and more islands due to general late Quaternary forearc uplift very close to the trench where vertical displacement rates tend to be faster; prior to the 2007 earthquake it had much lower rates of seismic activity than the neighboring supersegments. Generally the mean along-arc lateral extent of Solomon arc segments is about 75 km, somewhat smaller than the segments reported in some other island arcs such as Japan (~ 100–260 km), but larger than those of the Tonga (30–80 km) and Central New Hebrides arcs (30–110 km). These differences may be real but it may occur simply because the coral-friendly tropical environment of the South Pacific arcs, numerous emerged forearc islands, and high seismicity rates provide an unusually favorable situation for observing variations in vertical tectonic activity and thus for identifying segment boundaries. Over the past century seismic slip in the Solomons, as indicated by seismic moment release, has corresponded to about half the plate convergence rate; however, there are notable variations along the arc. Even with the 2007 earthquake, the long-term moment release rate in the New Georgia supersegment is relatively low, and this may indicate that large earthquakes are imminent.

Mapping the mantle wedge and interplate thrust zone of the northeast Japan arc

We detected sP depth phases from three-component, short-period seismograms from the forearc earthquakes under the Pacific Ocean recorded by the dense seismic network on the Japan Islands. Then we use the sP depth-phase data together with P and S wave arrival times to locate the suboceanic earthquakes accurately. A large number of P and S wave arrival times from the relocated suboceanic events and earthquakes under the land area are used to determine the 3-D P and S wave velocity and Poisson's ratio structures under the entire NE Japan arc from the Japan Trench to the Japan Sea. Our results exhibit strong lateral heterogeneity under the forearc region and a clear correlation between the structural heterogeneity and distribution of large earthquakes (M 7.0–8.2) occurred in the interplate thrust zone. Prominent low-velocity and high Poisson's ratio zones are visible in the mantle wedge under the volcanic front and back-arc region, which show nicely the images of arc magma caused by the slab dehydration and corner flow in the mantle wedge.

Rapid forearc uplift and subsidence caused by impinging bathymetric features: Examples from the New Hebrides and Solomon arcs

Isotopically dated corals from the central New Hebrides and New Georgia Island Group, Solomon Islands, indicate that both forearcs underwent rapid late Quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to ∼8 mm/yr, while total plate convergence was only a few kilometers. Two mechanisms that might account for these rapid reversals in vertical motion include (1) a “displacement” mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate (as the object moves deeper and vacates the base of the forearc, the forearc subsides to near its original position) and (2) a “crustal shortening” mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone. Rapid subsidence follows when the impinging object is broken or otherwise decoupled, shallow interplate coupling becomes weak, and the uplifted forearc extends and subsides. The displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers, but it fails to explain the geographic pattern, short time frame, and abruptness of the change from subsidence to uplift that we observe. The crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone.

Seismogenic structures along continental convergent zones: from oblique subduction to mature collision

We summarize seismogenic structures in four regions of active convergence, each at a different stage of the collision process, with particular emphases on unusual, deep-seated seismogenic zones that were recently discovered. Along the eastern Hellenic arc near Crete, an additional seismogenic zone seems to occur below the seismogenic portion of the interplate thrust zone—a configuration found in several other oblique subduction zones that terminate laterally against collision belts. The unusual earthquakes show lateral compression, probably reflecting convergence between the subducting lithosphere's flank and the collision zone nearby. Along oblique zones of recent collision, the equivalence between space and time reveals the transition from subduction to full collision. In particular, intense seismicity beneath western Taiwan indicates that along the incipient zone of arc–continent collision, major earthquakes occur along high-angle reverse faults that reach deep into the crust or even the uppermost mantle. The seismogenic structures are likely to be reactivated normal faults on the passive continental margin of southeastern China. Since high-angle faults are ineffective in accommodating horizontal motion, it is not surprising that in the developed portion of the central Taiwan orogen (<5 Ma), seismogenic faulting occurs mainly along moderate-dipping (20–30°) thrusts. This is probably the only well-documented case of concurrent earthquake faulting on two major thrust faults, with the second seismogenic zone reaching down to depths of 30 km. Furthermore, the dual thrusts are out-of-sequence, being active in the hinterland of the deformation front. Along the mature Himalayan collision zone, where collision initiated about 50 Ma ago, current data are insufficient to distinguish whether most earthquakes occurred along multiple, out-of-sequence thrusts or along a major ramp thrust. Intriguingly, a very active seismic zone, including a large ( M w=6.7) earthquake in 1988, occurs at depths near 50 km beneath the foreland. Such a configuration may indicate the onset of a crustal nappe, involving the entire cratonic crust. In all cases of collision discussed here, the basal decollement, a key feature in the critical taper model of mountain building, appears to be aseismic. It seems that right at the onset of collision, earthquakes reflect reactivation of high-angle faults. For mature collision belts, earthquake faulting on moderate-dipping thrust accommodates a significant portion of convergence—a process involving the bulk of crust and possibly the uppermost mantle.

Seismicity in the outer rise–forearc region and configuration of the subducting lithosphere with special reference to the Japan Trench

Along nine subduction zones where the largest (Mw or Ms ≥ 7.6) normal‐faulting earthquakes have occurred beneath the outer rise, there is an apparent correlation between a large curvature of the subducting slab and regions of low background seismicity in the forearc. In six cases, a gentle inner trench is also observed. Taking advantage of the excellent coverage of teleseismic stations and extensive previous studies using local data, we investigate the nature of this spatial correlation between seismicity and bathymetry along the Honshu forearc in some detail. An aseismic region in the forearc lies directly west of the rupture zone of the 1933 Sanriku earthquake (Mw = 8.4), a great normal‐faulting earthquake that most likely has ruptured the entire brittle portion of the lithosphere in the outer rise. Along this particular cross section of the Japan trench, large to moderate‐sized interplate earthquakes span a short interval of depths between approximately 14 and 31 km, about half the interval observed along an adjacent profile to the south (12–45 km). The aseismic portion of the plate interface (between depths of approximately 30 and 45 km) has persisted for over 100 years. The occurrence of lithospheric normal‐faulting earthquakes in the outer rise region indicates that bending moment within the subducting slab, limited by yield strength of the lithosphere, has nearly reached an asymptotic value (moment saturation). At this state, the curvature of the plate can increase with little increase in bending moment. As such, any stresses transmitted across the plate interface will be relaxed by a permanent increase in the plate's curvature and little elastic strain can accumulate across the interplate thrust zone to produce large earthquakes. Thus the saturation of bending moment in the subducting lithosphere offers a straightforward interpretation for the observed correlation between seismicity in the outer rise–forearc region and configuration of the subducting lithosphere.

Large accelerations and tectonic setting of the May 1993 Shumagin Islands earthquake sequence

Abstract Variability of earthquake rupture can lead to large disparities between expected and observed ground motion. To better understand the range of ground motions generated by subduction zone earthquakes, we examine teleseismic and nearsource strong-motion records from an interplate thrust earthquake sequence that occurred near the Shumagin Islands, Alaska Peninsula, in May 1993. The two largest events show source depths of 36 km (May 13) and 42 km (May 25) from inversion of teleseismic waveforms, which place them in the deepest part of the interplate thrust zone. This inference is verified by single-station epicenter estimates, derived from P-wave polarization and S-P times, and by the focal mechanisms. The 13 May 1993 mainshock (MS 6.9) shows peak accelerations of 0.7 to 1.4 m · sec−2 within 75 km of the epicenter, consistent with scaling relations observed elsewhere. The largest aftershock, 25 May (MW 6.1), shows peak acceleration near 8 m · sec−2 at a station 25 km away. The 8 m · sec−2 acceleration is far larger than expected for an earthquake of this size and depth and is perhaps the largest recorded in Alaska. Simple explanations such as site effects and rupture directivity cannot fully account for the anomalous accelerations. The short rupture time of the 25 May event (∼2.8 sec) contributes to high accelerations but cannot explain the factor of 12 variability observed in peak acceleration near the epicenter. Unusual focusing effects and rupture patterns are suspected but cannot be confirmed. Whatever the cause, the unusually high accelerations demonstrate that deep and moderate-sized earthquakes can occasionally produce significant ground motions in subduction zones.

Source scaling of earthquakes in the shumagin region, Alaska: time-domain inversions of regional waveforms

The scaling of pulse duration to seismic moment is estimated for earthquakes along an interplate thrust zone, from digital waveforms recorded by short-period and broad-band instruments of the East Aleutian (Shumagin) Seismic Network. We measure pulse duration using an empirical Green's function technique based on damped time-domain deconvolution. From several thousand events, 22 earthquakes with magnitudes 3.0–7.0 and depths 23-56 km are found to give reliable estimates of pulse duration. Durations are also determined directly from one-parameter nonlinear inversions, for a variety of simple functional forms of source time functions. Symmetric source pulses (boxcar or triangle shapes) fit waveforms better than an asymmetric model [t exp (-2t/D)] for most (62 per cent) of the waveform pairs, while the asymmetric model fits best for only 8 per cent of the data. Pulse duration increases with the size of events, from 0.1 to 10s over the seismic moment (M0) range of 1014 to 3 × 1019 Nm. When normalized by the cube root of seismic moment, pulse durations show ~8 × variation; comparable static stress drop estimates range from 0.2 to 135 MPa. Contrary to predictions of some laboratory and theoretical studies, earthquakes at the deepest part of the thrust zone do not show significantly higher stress drops than do shallower events. Rupture properties, however, show a strong dependence on earthquake size. The three largest events (M0>5 × 1018Nm) have the three longest normalized durations, on average 3.8 times longer than those for smaller events. The durations require smaller events to have 10–100 × larger static stress drops, or ~4 × faster rupture velocities, or some combination of the two. Possibly, the largest events rupture both strong and weak patches while smaller events just rupture strong patches on the fault surface. The characteristic dimension that separates large from small events, 3-15 km, is comparable to characteristic wavelengths of Pacific basin bathymetry and may reflect the influence of the subducted sea-floor upon fault-zone heterogeneity.

Transition from interplate slip to double seismic zone along the Kuril‐Kamchatka arc

During subduction, the downgoing slab interacts with a strong overriding plate at shallow depths and with the surrounding asthenosphere at intermediate depths. Thus the transition from earthquakes representing interplate slip to those reflecting intraplate deformation within subducting lithosphere is important for understanding how the mechanics of subduction varies with depth. To characterize this transition, we studied 44 large to moderate sized earthquakes that occurred between 1964 and 1990 in two regions along the central portion of the Kuril‐Kamchatka arc. Precise earthquake source parameters are determined by analyzing body waveforms recorded at teleseismic distances. At depths of 30–50 km, our results delineate a transitional thrust zone, characterized by earthquakes showing thrust faulting on moderately dipping nodal planes. The northwesterly dipping nodal plane dips approximately 40°, some 10°–15° steeper than the dip of low‐angle earthquake faulting along the interplate thrust zone at depths shallower than 30 km. For earthquakes along the transitional thrust zone, secondary features in observed waveforms suggest that either a thick layer of sediments is present in the forearc region or the sources are embedded in materials with crustal seismic wave speeds. In the former case, the transitional thrust zone is interpreted as the deep‐seated portion of the plate interface whose dip increases with depth. The alternative implies that the transitional thrust zone occurrs below the plate interface within subducted crust of the Pacific plate and that the occurrence of earthquakes along the transitional zone may indicate a locked plate interface between depths of 30 and 50 km. The inference of a locked plate interface is consistent with the occurrence of compressional earthquakes beneath the outer rise region. However, a locked interface does not seem to affect the state of strain in the double seismic zone. The upper, compressional layer of the double seismic zone abuts the transitional thrust zone at a depth of approximately 50 km. At this depth, the two layers of seismicity in the double seismic zone are separated by as much as 30–40 km, too large a distance to be accommodated by models of seismogenesis invoking either the gabbro‐eclogite transition or dehydration of subducted oceanic crust.

Geodetic measurements of convergence at the New Hebrides island arc indicate arc fragmentation caused by an impinging aseismic ridge

Global positioning system (GPS) measurements in 1990 and 1992 from two sites on the southern New Hebrides island arc give convergence rates with the Australian plate of 103 ± 5 mm/yr and 118 ± 10 mm/yr. In contrast, GPS measurements in the central New Hebrides indicate anomalously low convergence rates of ≈42 mm/yr. On geologic time scales, the mean central New Hebrides convergence rate has been 85–132 mm/yr. Elastic fault models with a locked interplate thrust zone indicate that maximum possible rates of horizontal elastic strain are insufficient to account for the anomalously slow convergence. Therefore, we propose that the central New Hebrides segment is moving eastward relative to adjacent arc segments at a rate of ≈36–83 mm/yr. This displacement is accommodated by crustal shortening at the eastern margin of the arc and strike-slip faults crosscutting the arc. Resistance to subduction of the aseismic D9Entrecasteaux Ridge system is the likely cause for horizontal forces sufficient to shove a large segment eastward and fragment the arc. This process demonstrates that subducting bathymetric features can impose fundamental structural modifications on an arc that may represent the initial stages of arc polarity reversal.

Geologic implications of great interplate earthquakes along the Aleutian Arc

The Aleutian arc is one of the most seismically active regions of the world. It consists of a number of segments or blocks that are rotating clockwise and translating westward along the North American‐Pacific plate boundary. Two of the largest recorded earthquakes have occurred along the interplate boundary south of the Andreanof Islands in 1957 and the Rat Islands in 1965. The large magnitudes of these earthquakes implied that the probability of the recurrence of a large‐magnitude event at these same locations was low. However, the occurrence of a great earthquake south of the Andreanof Islands in 1986 less than 30 years after the 1957 event has prompted a reevaluation of the seismic potential of the arc. We present new marine geophysical observations and synthesize previous geologic interpretations of the Aleutian arc to show that the epicenters of these great thrust‐type earthquakes coincide with upper plate segments of the arc characterized by a coherent forearc structural fabric. These segments, which are associated with large plutonic masses, are mechanically stronger and are related to the location of asperities positioned at depth along the main thrust zone; they appear to be more strongly attached to the North American plate than adjacent blocks. The tectonically controlled locations of these asperities thus appear to influence the seismicity along the arc. The Andreanof and Rat segments of the arc are separated by the Delarof segment, which is considered to have a high probability of a large‐magnitude earthquake in the near future based on probabilistic estimates of seismic potential (Nishenko, 1991). The Delarof segment of the arc, however, is highly disrupted by arc‐transverse faults. This segment is structurally weak and does not appear to be capable of accumulating the interplate strain necessary to generate a great, subduction zone earthquake. We propose that variations in upper plate structural strength and mobility affect the mechanical properties of the interplate thrust zone and need to be considered in localizing interplate asperities. Forearc tectonic segmentation associated with the partitioning of strike‐slip and thrust motions may exert long‐term controls on the rates of seismic moment release.

Relationship between shallow‐ and intermediate‐depth seismicity in the Eastern Aleutian Subduction Zone

The transition from shallow interplate thrusting to intermediate‐depth seismicity is often poorly observed, but critical for understanding the fate of the downgoing slab. In order to better examine the transition, 1448 earthquakes are relocated from data recorded by a regional seismic network in the eastern Aleutian arc, using an improved three‐dimensional velocity model and accurate ray tracing. Single‐event first‐motion solutions are determined from these rays for 31 slab events. The interplate thrust zone is a planar fault zone, dipping 10–15° at 25–35 km depth, and is no more than 5–10 km wide. Most intermediate‐depth earthquakes are localized to a plane no wider than 5 km near the top of the descending plate. Fault‐plane solution orientations for these events vary by several tens of degrees in orientation, although 73% show T axes aligned within 45° of the slab dip. A parallel seismic zone, 20–25 km deeper into the slab, also shows down‐dip plunges of T axes for 3 of 5 solutions. The fault‐plane solutions are poorly explained by plate bending or unbending about a neutral fiber. Hypocenters show that intermediate‐depth events are confined near the subducted oceanic crust, supporting compositional rather than pure thermal control of intermediate‐depth seismicity. One explanation is that the upper‐plane events are an indirect consequence of phase changes in subducted crust. Perhaps similar processes are important in producing earthquakes in the lower, parallel zone.

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.

Seismotectonics of the Himalayan Collision Zone: Geometry of the underthrusting Indian Plate beneath the Himalaya

Fault plane solutions and well‐determined focal depths of medium‐sized earthquakes, topography, and Landsat imagery in conjunction with seismicity maps, cross sections, and available geological information are used to investigate the present tectonics of the Himalayan continental collision zone. Most of the accurately located epicenters of events along the Himalayan arc (78°E–95°E) that occurred between 1961 and 1981 are concentrated in a narrow zone, about 50 km wide, lying between the northerly dipping Main Boundary (MBT) and Main Central (MCT) thrusts. Most of these events are located just south of the MCT. Though the epicenters of the events are, in general, well located, their depths as determined by teleseismic travel time data are very unreliable. Events with accurately determined depths obtained from identification of surface‐reflected phases define a simple, planar zone from about 10‐km and 20‐km depth, with an apparent dip of about 15°. This result is all the more remarkable considering that the events used were located along about an 1800‐km length of the Himalyan arc. Except for one, all available focal mechanisms of events within this zone indicate shallow ( ≲30°), north dipping thrusts. This shallow, north dipping zone apparently defines a part of the detachment that separates the underthrusting Indian plate from the Lesser Himalayan crustal block. The spatial extent and the geometry of this interplate thrust zone strongly indicate that the MBT and nearby subsidiary surface and blind thrusts, rather than the MCT, are currently the most active structures of the Himalayan arc. We suggest that the great Himalayan earthquakes (M&gt;8) occur along the same detachment surface as defined by the thrust‐type, medium‐sized events. Events located to the south of the MBT and beneath the Ganges foredeep show normal faulting withTaxes perpendicular to the Himalayan trend. The above results suggest that the Indian continental plate is underthrusting the Himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones. The November 19, 1980, earthquake that occurred near the MCT (near 88.5°E) shows a predominantly strike‐slip focal mechanism. One of the nodal planes of this mechanism is transverse to the Himalayan structural grain, and moreover, this plane has a trend similar to that of the recently mapped Yadong‐Gulu rift in the Tethyan Himalaya and in southern Tibet just northeast of the earthquake. We interpret this predominantly left‐lateral, strike‐slip mechanism to indicate a possible genetic relationship between transverse structural features in the Underthrusting Indian plate (the Kishangang basement fault) and the upper Himalayan blocks and Tibet.

Leveling arrays as multicomponent tiltmeters: Slow deformation in the New Hebrides Island Arc

Two benchmark arrays located on the islands of Santo and Efate in the New Hebrides Island Arc have been releveled 2 to 4 times per year during the past five years (1975–1980). Each array is thereby used as a multicomponent tiltmeter. The arrays are located within 35–50 km of the interplate thrust zone of this convergent plate boundary. Each array has an aperture of about 1 km and in addition contains small subarrays with apertures of about 70 m. This configuration leads to a resolving power (for tilt) of about one microradian. The most important result of this study is that, at the Devils Point array on Efate Island, there has been a consistent tilting down of about five microradians in a direction subparallel to the strike of the arc. This tilting is clearly resolved by both the large array and by the small aperture sub‐arrays. The observations are accounted for by a combination of planar tilting and random error consistent with the observed misclosures. Although tilting is clearly established at the Devils Point array, it is not possible at this stage to establish a unique cause for the observed tilt. Hypotheses consistent with the tectonic setting include interseismic strain accumulation associated with the earthquake cycle of loading and interplate boundary rupture, deformations near a transition between a creeping and a locked segment of the interplate boundary, and near surface deformations associated with magmatic activity and/or block movements also reflected in the pattern of faulting of the uplifted Holocene coral terraces. At the second array at Ratard (Santo Island) no tilt trend clearly emerges from the noise level. However, two marginally significant tile excursions precede in each case large earthquakes located several hundred km north of the array. At the Ratard array the ground noise (small wavelength elevation disturbances and benchmark instability) contributes to residuals almost as much as the measurement error, thereby degrading the threshold level for detection of tilt signals. The difference between the two arrays is not clearly established but seems to reflect an inherently larger level of short wavelength ground noise associated with geological properties of the Ratard site.

Spatial distribution and mechanisms of earthquakes in the southern New Hebrides arc from a temporary land and ocean bottom seismic network and from worldwide observations

In 1977 a wide‐aperture seismic network of land and ocean bottom stations (OBS) was operated for 6 weeks in the southern New Hebrides island arc. Data on the spatial distribution and mechanisms of small events recorded by this local network were integrated with worldwide observations of moderate to large size New Hebrides earthquakes of the past 17 years to study the structure and deformational processes in the New Hebrides subduction zone. Surprisingly, small differences (less than about 10 km) were found between the locations of shallow‐ and intermediate‐depth earthquakes as located by the temporary local network and by the International Seismological Centre using worldwide stations. Analysis of the P wave travel time data from the OBS station closest to the trench shows limited evidence for a high‐velocity zone (about 5% higher than the surrounding mantle) associated with the descending plate. The thrust zone between the descending and upper plates is defined mainly by the spatial distribution and focal mechanisms of moderate to large size events and their aftershocks, but it is not very well defined by the spatial distribution of small size events located in this study. Shallow seismic activity located beneath the trench and beneath the interplate thrust zone indicates a lower limit of about 40 km for the thickness of the seismically active part of the descending plate beneath the thrust zone. Focal mechanisms of moderate to large size earthquakes located within the upper plate and geological observations on the islands suggest that the upper plate in the region of Erromango and Tanna is divided into a series of blocks that are differentially uplifted along mainly NW‐SE striking faults. During the OBS experiment several shallow events were well located beneath the Coriolis trough, a riftlike feature located to the east of the volcanic arc. Well‐determined depths of these events are between 11 and 22 km. The well‐located intermediate‐depth events define a 20‐km‐thick Benioff zone that has a dip of about 70°. The spatial distribution of events within the descending slab is very strongly clustered in a persistent pattern that is seen in both the short‐term sample of small events determined by the local network and the long‐term sample of locations based on worldwide data. Several focal mechanisms of moderate to large size intermediate‐depth events show components of lateral extension along the strike of the arc and some show components of lateral compression, both of which could be interpreted as the result of a lateral bending of the descending plate in the part of the southern New Hebrides arc where the trench begins to curve around to the east.