Seismogenic Zones in the Convergent Margin , Eastern Taiwan and Its Implications in the Luzon Forearc Deformation

Three-dimensional relocated seismicity in eastern Taiwan reveals two north-to-northeast trending seismogenic zones, one located close to the Longitudinal Valley and the other near to the crest of the Luzon arc. Earth­ quake focal mechanisms obtained from P-wave first motion polarity data are presented in this study for 70 events in these two seismic zones. The focal mechanism solutions for both seismic zones show mainly thrust and strike-slip faulting in the area south of 23.2°N. The horizontal projection of the events' P-axis indicates a pattern consistent with the regional NW-SE compression. However, the orientation of the events' T-axes indicates that there are different patterns for the two seismic zones. By combining evi­ dence from seismicity, Pand T-axes, as well as detailed bathymetry, we infer that the two seismic zones (fault systems) might mark the east and west boundaries of the Luzon forearc. According to the transpressional strain model proposed in this study, the Luzon forearc represents a de­ forming zone, between two relatively rigid blocks, that undergoes shearing from the transcurrent component of oblique convergence in eastern Taiwan. (


INTRODUCTION
In eastern Taiwan, the Longitudinal Valley fault is considered to represent the oblique collisional boundary between the Philippine Sea plate, which carries the Luzon volcanic arc, and the Eurasian plate (Fig. 1) (e.g., Barr ier and Angelier 1986;Ho 1986;Tsai 1986).Global Positioning System (GPS) measurements, collected from 1990 to 1995 (Yu et al. 1997), sug gested that about 60% of the plate convergence is accommodated by deformation between the eastern flank of the Central Range and the Luzon arc (Fig. 2).It has been thought that the shortening in this region is accomplished by transfer of slip (e.g., Lundberg et al. 1997) and forearc deformation (e.g., Chemenda et al. 1997;Cheng et al. 1999).Oblique convergence also gives way to high seismicity, including the left-lateral slip observed in the northern Lon gitudinal Valley (Hsu 1962) and thrust events (Wu 1989;Kao et al. 1998;Kao et al. 2000).The deformation of the Luzon forearc basin is also evident in seismic reflection data (Fuh et al. 1997;Lundberg et al. 1997).A recent study on earthquake relocation and tomography of the eastern Taiwan area (Cheng et al. 1999) reported two sub-parallel seismic zones-one located close to the Longitudinal Valley and the other located west of the Luzon arc's crest (Fig. 1).However, what is the main type of faulting for these two seismic zones?Do their mechanisms vary as the collision between the Luzon volcanic arc and Taiwan increases from south to north?And, what is the relationship between the genesis of these two seismic zones and the deformation of the Luzon forearc?
Here we address these question:s by studying the focal mechanisms of earthquakes of magnitude M L :::: : 3 and mostly shallower than 30 km, in the eastern Taiwan area.We use hypocenters relocated with three-dimensional velocity model (Cheng et al. 1999).Horizontal and depth standard errors for these events are mostly less than 2 km and 3 km, respectively.This is also the level of the largest changes caused by small changes in the model, such as whether or not station corrections are included.The events we have studied are relatively small and do not represent the total convergence in the region due to relative motion of the Philippine Sea and Eurasian plates.This will occur in large earthquakes on the major faults (e. g., 1951 Longitudinal earthquakes, M L =7.3, 7.1, and 7.3) that are presently quiescent, a fact that is reflected in the present diffuse seismicity throughout the region.However, we believe that this type of study can give a useful insight into the present state of convergence in this area.

TECTONIC SETTING
Taiwan is the result of active, oblique collision between the Eurasian and Philippine Sea plates.Oblique collision has formed several mountain ranges and can be divided into two geological provinces by the Longitudinal Valley fault (Fig. 1).To the east of the fa ult is the Coastal Range which is mainly composed of Miocene to Pliocene andesitic volcanic units and associated flyschoid and turbidite sediments.To the west of the fault is the main body of Taiwan, distinguishable into the Central Range and the Hsi.iehshanRange, which is composed of a pre-Tertiary metamorphic basement overlain by Paleogene low-grade metamorphosed sediments, Neogene folded and thrusted sedimentary rock layers and Quaternary alluvial de posits (Ho 1986).The Hsilehshan Range is separated from the Central Range by the Lishan fault (LF), which branches off from the Chuchih fault (CF).The Chuchih fault is a major upthrust fault which follows the contact between the slat belt and the fold-and-thrust belt of the western foothill region (Ho 1982).The Foothill zone is a province of typical cover tecton ics in which only late Tertiary and Quaternary rocks are involved (Fig. 1).
The southeastern Taiwan area is a transitional zone from a typical subduction system to the south into a collision zone to the north (Fig. 1).Based on a morphological study, Chen and (a)  Juang (1986) divided the southeastern offshore area of Taiwan into five physiographic zones: the Hengchun Ridge, the Southern Longitudinal Trough, the Huatung Ridge, the Taitung Trough and the Lanhsu Ridge, from west to east (Fig. 1).Using magnetic and gravity data, Liu et al. (1992) indicated that the material at the basement of the Taitung Trough belongs to a volcanic arc, whereas the basements of the Southern Longitudinal Trough and Huatung Ridge do not.The Taitung Trough, the northern part of the North Luzon Trough, narrows with observable shoals toward the north and ends at the southern Coastal Range on eastern Taiwan (Page and Suppe 1981).Lutao and Lanhsu are the two northernmost islands of the Lutao-Babuyan ridge of the Luzon volcanic arc (Bowin et al. 1978).Radiometric dating shows that as you move southward along this ridge, the volcanoes are younger in this ridge belt (Richard et al. 1986).
Based on magnetic basement boundary analysis, Shyu et al. (1996) also concluded that the Luzon Arc has started subsiding northward.In this part of the Luzon arc, the main volcanic activity ceased in the middle Miocene (H o 1982, 1986).The Luzon arc presumably continues northward to include the Chimei Igneous Complex in the �iddle of the Coastal Range of Taiwan (H o 1986).

DATA AND ANALYSIS
We study the focal mechanisms of earthquakes of magnitude M L greater than 3.0, and mostly shallower than 30 km, in eastern Taiwan.Mechanisms have been determined for indi vidual events using first motion polarity data.Arrival times and polarities were picked from digital seismic data recorded by the Central Weather Bureau Seismographic Network ( CWBSN) during 1990 to 1998.The hypocenters were relocated with the local earthquake tomographic scheme (Thurber and Eberhart-Phillips 2000) using the three-dimensional velocity model de termined for eastern Taiwan (Cheng et al. 1999).We have obtained good values of root mean square (RMS) residuals for most earthquakes analyzed.
The focal mechanism solutions were calculated with the FPFIT program (Resenberg and Oppenheimer 1985).The estimated uncertainties on strike, dip and rake of the fault-plane solutions are less than 20° and involve at least ten polarities.In order to test further the struc tural significance of the solutions, we selected ten of the events and analyzed them in detail.We decided to test the focal mechanisms of fixed hypocentral depths, shifting the hypocenter to 15 km and then to I km.In general, more stable solutions were obtained, for earthquakes where we have both good azimuthal coverage and many polarity data.Large azimuthal gaps, however, led to larger variations.

RESULTS
Spatial distribution of 8334 relocated hypocenters shows that there are two sub-parallel seismic groupings in eastern Taiwan-the Longitudinal Valley seismic zone (hereinafter re ferred to as the L VSZ) and the arc-parallel seismic zone (hereinafter referred to as the APSZ) (Fig. 1).
Figure 3 shows fault plane solutions for 70 events in eastern Taiwan.Details of the focal parameters, locations and references to these earthquakes are given in Table 1.It is clear from  Fig. 3 that the focal mechanisms of events determined in this study show a complex pattern for the APSZ and L VSZ.However, if we roughly divide these events into two groups, they pre dominantly show thrust (events 1, 2, 4, 9, 13, 22, 24, 29, 31, 40, 41, 44, 48, 49, and 59) and strike-slip solutions (events 21, 60, 66, 68, 69, and 70) for the APSZ and LVSZ in the area south of23.2°N.On the other hand, there are several normal solutions observed (events 5, 8, 14, 27, 55, and 62) for both seismic zones in the area north of 23.2°N.Strike-slip solutions (events 15, 17, 19, 28, 47, 54, 63, and 65) are observed in the area between the A�SZ and LVSZ.The P-and T-axes for all the mechanisms mentioned above in eastern Taiwan are shown in Fig. 4 and Table 1.Compilation of Fig. 4 has involved specifying which of the nodal planes in each solution was the fault plane and needs some justification.In case of almost pure dip slip faulting (such as events 9, 13, and 34) the direction is insensitive to the choice of fault plane.Some choices were made because the earthquake occurred close to known active or historical surface faulting (events 30, 31, and 37) or were associated with topography that make the choice more likely (events 7, 23, 29, 49, 69, and 70).Apart from three events (events 4, 20, and 28), the pattern for the region shows mainly NW-SE trending (Fig. 3).Similarly, there is broad clustering towards near-horizontal NE-SW extension.However, the APSZ shows T-axis directions that are quite distinct from the horizontal ones found in the L VSZ.A depth section in the E-W direction of the P-and T-axes of the events located south of 23.2°N is shown in Fig. 5.

DISCUSSION
As shown in the earthquake focal mechanisms (Fig. 3), frequent seismicity (Fig. 1), and GPS observations (Fig. 2), the eastern Taiwan collision zone is characterized by strong com pression oriented approximately perpendicular to the major structural trend of the Luzon arc.Oblique plate-convergence would also produce arc-parallel gradients in the horizontal shear stress on plate-boundary faults, that in tum results in arc-parallel stretching or compression of the forearc (McCaffrey 1992).For example, GPS measurements (Fig. 2b) indicate that the velocities of Lutao and Lanhsu are larger than those of other stations to the east of Longitudi nal Valley and are in the order of about 20 mm/yr.In other words, part of the plate conver gence is accommodated by deformation between the eastern flank of the Central Range and the Luzon arc, i.e., the Luzon forearc area.In fact, previous studies (e.g., Dahlen et al. 1984;Byrne and Fisher 1990;Shyu et al. 1996;Kao et al. 2000), have noted that the forearc basin and the accretionary prism located to the west of the Luzon arc are of low strength and can deform easily in response to plate convergence.However, the deformation pattern, and its boundaries, for the closing Luzon forearc are relatively unknown.

Significance of the Longitudinal Valley and Arc-Parallel Seismic Zones
The oblique collision between the Luzon arc and continental shelf started near the Hualien area about 4 million years ago, and moved progressively southward to reach Taitung about 1 million years ago (Lee et al. 1991).The spacing between the Luzon arc and Taiwan, measured from Lutao to the Longitudinal Valley, is about 50 km and decreases northwards (Fig. 1).In a modern arc-trench system, the minimum distance cif the spacing between arc and trench is about 100 km.If the Longitudinal Valley represents an ancient trench, the closing between the Luzon arc and Longitudinal Valley implies that most of the forearc block has been shortened or plunged into the mantle (e.g., Cheng et al. 1999;Malavieille et al. 1999).
The stress regime of the L VSZ and APSZ, as inferred from the earthquake strain axes, appears to be dominated by subhorizontal compression (Fig. 4a and Table 1).In general, the P axes of events in the APSZ and L VSZ show an orientation of 300° ± 30°.This is indistin-  and Table 1.

279
guishable from the average P-axis azimuth of 295° and 297° for the region north and south of 23°, respectively, determined by Kao et al. (2000).It is also consistent with the azimuth of the axes of maximum relative shortening in eastern Taiwan determined geodetically (Yu et al. 1997).In addition, low-angle thrusting mechanisms near the plate interface, indicative of strike slip motion between the plates, do not extend deeper (Fig. 5).On the other hand, the orienta-    Table 1), P-and T-axes (Figs. 4 and 5) and detailed bathymetry, we infer that the development of the APSZ and LVSZ has important tectonic implication.Our study suggests that both might mark obvious east and west boundaries of the Luzon forearc.It has been proposed that for high angles of oblique plate convergence, the thrust wedge will move laterally relative to the underthrust slab and sepa rates from the upper plate by a strike-slip fault, defining a forearc sliver (e.g., Platt 1993).Although the plate-convergence angle between the Eurasian and Philippine Sea plates is larger than 60° if the Longitudinal Valley plays as a plate interface, we can regard the APSZ as one component accommodating the transfer of oblique slip.This is physically reasonable because the more the Luzon forearc sliver is indenting the Eurasian continental margin, the higher resistance against the forearc sliver expected.This also probably explains why the seismicity of the APSZ is more active than that of the LVSZ (Fig. 1).

Luzon Forearc Deformation
Figure 3 indicates that several moderate strike-slip events (events 17, 28, 60, 66, and 68) occurred in the APSZ in the area south of 23.2° (Fig. 3).In thinegion, the oblique conver gence between Philippine Sea and Eurasian plates might have been partitioned into shortening in the Luzon forearc and right-lateral strike-slip in eastern Taiwan.However, the partitioning is not perfect, as the complex pattern of focal mechanisms north of about 23.2°N shows.Be cause the right-lateral strike-slip motion in eastern Taiwan might be accommodated by seis mic slip on the major Longitudinal Valley faults, the Luzon foreatc must be deforining mostly aseismically, either by creeps on faults or by folding as revealed in seismic reflection data (Fuh et al. 1997;Lundberg et al. 1997).Therefore, it might be expected that the shear strength of the Luzon forearc is relatively small, such that the deformation and large strike-slip fault in the forearc would take place due to the lateral component of the relative plate convergence.Many low-angle thrust events lie within the dipping seismic zone (Fig. 5) and thus represent the deformation of the forearc area.
As mentioned above, the Luzon forearc is located between the Luzon arc and Central Range in eastern Taiwan.The seismological data presented in this paper confirm that the Luzon forearc is undergoing a marked tectonic evolution and provides further evidence of active compression along its external front, as shown in Fig. 3: We propose that the obliquity of the plate convergence vector resulted in (1) a stress field with NE-SW extension, (2) gen eration of NW-SE fractures, and (3) sinistral shear along the LVSZ and APSZ. Figure 6 is not intended as a realistic sketch of the tectonic evolution of the region, but merely as an illustra tion of the sort of processes that might occur.This model is based on focal mechanisms, seismicity, as well as geodetic, geological, and topographic data.Given the Luzon arc trend of 5°, a convergence rate of 7 cm/yr, and total partitioning of the strike-slip component of the plate slip vector (310°) into the Luzon arc, this tectonic setting likely resulted in sinistral slip along the Luzon forearc deformation zone and generation of a stress field with NE-SW extension.
During sinistral deformation, a1 and a3 lay in the horizontal plane, oriented approximately NW-SE and NE-SW respectively, and a2 is vertical.In Fig. 6, the Central Range and Luzon arc units represent relatively rigid blocks for the upper crust, which has also been proposed by seismic tomographic study in southeastern Taiwan (Cheng et al. 1998).
We know that the existence of a stress field in the lithosphere is the result of the interac tion of tectonic forces whose magnitude and direction vary extremely slowly.Moreover, earth quakes are mostly related to already existing faults, or fault systems in the lithosphere, and these serve as stress concentrators.The forearc unit represents a deforming zone between the two rigid blocks that undergoes simple shearing from the transcurrent component of oblique collision (Fig. 6).The typical effects should occur, such as the formation of en-echelon fold and thrust belts and extensional features that parallel the directions of minimum and maximum compression (e.g., Teyssier et al. 1995).The existence of tensile fracture in a fault system can play an important role because the strike slip displacement can take place not only along the primary faults, but also along the planes of tensile fractures (e.g., Spicik, and Lokajicek, 1986).
In addition, strike-slip faults, might also develop or coincide with thrust faults (Sanderson and Marchini 1984 ).The oblique fold and fault systems of the Coastal Range (left in Fig. 6) and the APSZ and L VSZ in eastern Taiwan could be explained with this model (right in Fig. 6).

CONCLUDING REMARKS
Knowledge of the state of stress in the convergent plate boundary is an important con straint on our understanding on seismic and aseismic faulting, the force of resisting plate motion, and the rheology of the crust.To better understand the nature of the seismogenic zone in eastern Taiwan and crustal deformation, focal mechanisms of 70 relocated events generally shallower than 30 km have been determined using P-wave first motion polarity data.The geographical distribution of such events has allowed us to map the seismic strain field of the seismogenic zones in eastern Taiwan.In general, focal mechanisms of events located in the arc-parallel seismic zone (APSZ) and the Longitudinal Valley seismic zone (L VSZ) show mainly thrust and strike-slip solutions in the area south of 23.2°N.
The P-axes for events in both the APSZ and L VSZ are generally oriented in the direction of plate convergence, suggesting some degree of coupling of the plate interface at this depth.However, the orientation of the T-axes indicates that there are different patterns for the APSZ and L VSZ in eastern Tai wan.We infer that these two seismic zones might mark east and west boundaries of the Luzon forearc.
We have developed a transpressional model to explain important tectonic features of the Luzon forearc.Our tectonic model illustrates the pervasive effects of oblique collision processes, combined with subsequent transpressive forces generated by the motion of the forearc sliver parallel to the continental margin.This model suggests that the Luzon forearc deformation plays an important role in accommodating the oblique convergence in eastern Taiwan.
from discussions with Honn Kao, Cheng-Sein Liaw and Chao-Shing Lee.The anonymous reviewers also provided helpful comments.The research was supported by the National Sci ence Council, under grant NSC 89-2 1 16-M-052-001.
Fig. 3.Epicenters and focal mecha nisms of earthquakes deter mined for eastern Taiwan.All equal-area projections of the lower focal hemispheres with compressional quadrants are shaded.The numbers show the event's number in Table 1.Events are from this work and other sources mentioned in the text.

Fig. 4 .
Fig. 4. Map of horizontal projections and locations of the P-(left) and T-(right) axes for earthquakes shown in Fig. 3. Details are the same as for Fig. 3 Fig. 4. Map of horizontal projections and locations of the P-(left) and T-(right) axes for earthquakes shown in Fig. 3. Details are the same as for Fig. 3

Fig. 5 .
Fig. 5. Depth section of the seismiogenic zones in eastern Taiwan, together with projections of the P-and T-axes for the events located south of 23.2°N determined in this study.HR: Hengchun Ridge; SLT: Southern Longitu dinal Trough.'Solid dots represent hypocenters of earthquakes relocated with 3-D velocity model.

Fig. 6 .
Fig. 6.Schematic diagram showing the Luzon forearc deformation.Left shows several strike-slip solutions inside the Luzon forearc obtained in this study.Heavy dashed lines represent roughly the east and west boundaries of the forearc.Thin dashed lines indicate active strike-slip faults observed in reflection data (e.g., Fuh et al. 1997; Lallemand et al. 1999; Malavieille et al. 1999).Right figure shows transpressional model for the Luzon forearc block.Note the strike-slip faults inside the forearc block also develop or coincide with thrust faults occurring on both sides.

Table 1 .
Summary of Source Parameters.
I t:t:i' T s• T � 16