Honn Kao

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones

Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity. Citation: Brown, J.R., G. C. Beroza, S. Ide, K. Ohta, D. R. Shelly, S. Y. Schwartz, W. Rabbel, M. Thorwart, and H. Kao (2009), Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones, Geophys. Res. Lett., 36, L19306, doi:10.1029/2009GL040027.

Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography

We use 110 000 small earthquakes to locate and map active faults in three dimensions within the Taiwan arc-continent collision. The structure is dominated by a nearly horizontal band of small earthquakes at ∼10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt, with other illuminated fault zones abutting the detachment zone. The zone steepens below eastern Taiwan to 30°–90° and reaches depths of 30–60 km. The three-dimensional shape of the detachment zone in relation to topography allows a new test of critical-taper wedge mechanics and suggests that the reversal of topographic slope across Taiwan is controlled by the shape of the detachment.

Transition from oblique subduction to collision: Earthquakes in the southernmost Ryukyu arc‐Taiwan region

Tectonic characteristics of the region between Taiwan and the southernmost Ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5.5≤mb≤6.6. Five major seismogenic structures can be delineated: the Collision Seismic Zone (CSZ), the Interface Seismic Zone (ISZ), the Wadati-Benioff Seismic Zone (WBSZ), the Lateral Compression Seismic Zone (LCSZ), and the Okinawa Seismic Zone (OSZ). In the CSZ, located along the east coast of Taiwan and offshore, earthquake focal mechanisms show horizontal P axes distributed in two directions, 287°±10° and 333°±16°, possibly reflecting a strain partition associated with the relative plate convergence between the Eurasia plate and the Philippine Sea plate. The corresponding seismic strain tensor indicates a maximum compressive strain rate of 1.2×10−7 yr−1 along 293° and a comparable extension in vertical direction, presumably resulted from plate collision in the region. The geometry of the ISZ, which is distorted significantly at its westernmost end, can be approximated by a north dipping plane that is gradually pushed northward with increasing dip. The seismogenic portion of the interface spans a short depth range from ∼10 km to ∼35 km. A clear pattern of earthquake slip partition is observed; the average slip vector residual is as large as 35°. Seismic strain patterns within the subducted Philippine Sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at ∼270 km, different from the pattern of strain segmentation observed for the rest of the Ryukyu arc where the northern and southern portions are dominated by downdip extension and compression, respectively. Owing to the large convergence obliquity, the slab is descending at a rate significantly slower near Taiwan than in the southern Ryukyu. Thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slabs negative buoyancy. A number of thrust or oblique strike-slip earthquakes between ISZ and WBSZ show a consistent pattern of lateral compression with P axes oriented roughly parallel to the local strike of the trench-arc system. They are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere. Shallow normal-faulting earthquakes show successive rotation of T axes from approximately N-S in the Okinawa trough to approximately E-W in northeast Taiwan, possibly as a result of interaction between the extension from the opening of the Okinawa trough and the compression from collision. One normal event (January 18, 1991, mb=5.9) occurred in the Central Range with T axis roughly parallel to the structural trend of Taiwan, implying that the nature of the orogeny in Taiwan has changed from “thin-skinned” deformation to lithospheric collision involving the whole crust and uppermost mantle.

Moment-tensor inversion for offshore earthquakes east of Taiwan and their implications to regional collision

Reliable determination of source parameters for offshore earthquakes east of Taiwan with mb<5.5 was a difficult task because of the poor azimuthal coverage by local network and the lack of signals at teleseismic distances. We take advantage of the recently established “Broadband Array in Taiwan for Seismology” (BATS) to invert seismic moment tensors for 7 such events occurred in 1996. To cope with different patterns of background noise and unknown structural details, we utilize variable frequency bands in the inversion and adapt a two-step procedure to select best velocity models for individual epicenter-station paths. Our results are consistent with the overall patterns of regional collision and indicate that the resulting compressive stress has caused significant intraplate deformation within the Philippine Sea plate. Simulation of the regions geological evolution and orogenic processes should take this factor into account and allow the Philippine Sea plate to deform internally.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

[1] We study in detail the two consecutive episodic tremor-and-slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source-Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30-km and 50-km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50-55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high-frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Transition from oblique subduction to collision in the northern Luzon arc-Taiwan region : Constraints from bathymetry and seismic observations

We systematically study the detailed bathymetry, seismicity, and source parameters of large- and moderate-sized earthquakes that occurred in the northern Luzon arc-Taiwan region between 1964 and 1996. The purpose is to characterize the transition from oblique subduction to regional collision in terms of the distribution of morphological features, the characteristics of seismogenic structures, and the corresponding state of strain. To the south of 21.5°N, the existence of a subduction zone is clearly shown by both bathymetry and seismicity. The subducted Eurasia slab reaches a maximum depth of ∼200 km, and the depth decreases sharply as it approaches Taiwan. Focal mechanisms indicate that the subducted slab is in downdip extension for depths <150 km but switches to downdip compression at greater depths. To the north of 23°N, collision is clearly the predominant process with broad deformation on both sides of the suture. In between, the transition is accommodated by a distributed thrust deformation zone in the frontal portion of the accretionary prism, closure of the forearc basin, and the right-lateral NE-SW striking Taitung canyon fault zone (TCFZ). These structures resolve the differential movements between the subduction and collision zones and will repeatedly change their positions as the collision propagates southwestward. The present-day plate boundary follows the axis of the Manila trench in the south, broadens significantly in the zone of transition, and finally becomes a zone of several tens of kilometers in width across eastern Taiwan. The deformation front, on the other hand, is located between the Western Foothills and Coastal plain in Taiwan, then connects to the Manila trench to the south. Seismic strain tensors indicate that the region to the north of TCFZ has a compression rate ∼15 times larger than that to the south. The northern Luzon arc shows characteristics of oblique subduction, but the effects of collision are not as significant within the subducted Eurasian lithosphere, as comparing to the subducted Philippine Sea plate beneath the southernmost Ryukyu arc.

Strain and stress field in Taiwan oblique convergent system: constraints from GPS observation and tectonic data

Abstract This paper focuses on the stress and strain rate field of the Taiwan area. The strain rate field in the Taiwan region is studied qualitatively and quantitatively, based on the GPS observation in 1990–1995. It reflects the accommodation of the ongoing lithospheric deformation within the seismogenic portion of lithosphere and exhibits zones with contrasting deformation modes and amounts. We then compare the obtained strain rate field with the tectonic information provided by studies of borehole breakouts and earthquake focal mechanisms for the Present, and by fault slip data analyses for the Quaternary period. In the first approximation, the stress and strain rate fields show spatial similarity. The orientation of principal shortening is generally consistent with the compressive stress orientation that reflects the oblique indentation of the Luzon Arc into the Eurasian continental margin. In more detail, significant anomalies in the deformation pattern deserve consideration in that they may reveal ongoing stress accumulation. Despite the short-term variations related to the earthquake cycle, some major features of the strain rate field, including the distribution of extension and compression, highlight the long-term tectonic behavior of the mountain belt at the lithospheric scale. The time and space variations of strain should be a function of local heterogeneity and be transferred between interseismic and coseismic periods.

Seismic image of the Tarim basin and its collision with Tibet

A broadband seismic deployment in 1998–1999 in southwestern Tarim provided data for imaging the crust and upper mantle across the contact between the Tarim block and the Tibetan Plateau. A profile composed of migrated teleseismic receiver functions clearly shows lateral structural changes. The crust under the Tarim basin is relatively simple. The Moho discontinuity is mapped at a depth of 42 km near the northern end of the array and dips gently toward the south to ∼50 km under the Kunlun foreland. The Tarim basin appears to be rigid, with little shortening. Farther to the south, the imaging reveals a complex of reflectors in the lower crust and the upper mantle. There are both north- and south-dipping upper mantle structures under the Kunlun foreland and Kunlun Shan region. We found the observations to be more consistent with a model of lithospheric collision in which the crust and the upper mantle on both sides interpenetrate and deform.

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;