The Neo-tectonic structure of the southwestern tip of the Okinawa Trough

The Ilan Plain is located at the southwestern tip of the Okinawa Trough backarc basin, which propagates westward into the Taiwan orogen. From three moderate earthquakes which occurred near the coastline of the Ilan Plain, one of magnitude 6.2 on May 15, 2002 and the other two of magnitudes 5.51 and 5.49 on March 5, 2005, we attempted to understand the relationship between the earthquake mechanisms and the geological context. Seven seismic reflection profiles collected roughly parallel to the coastline of the Ilan Plain were used in this work. A structural fault, with a significant normal faulting component trending approximately ENE-WSW, has been identified to the east of the doublet earthquakes of March 5, 2005. Because this fault follows the ENE-WSW trend of the aftershock seismicity, we named it the Ilan Shelf Fault, which might extend to the east beyond the Ilan continental shelf spur. However, the centroid seismic moment tensor solutions for the doublet earthquakes of March 5, 2005 are of a left-lateral strike-slip faulting type which are consistent with onland GPS observations, such that the Ilan Shelf Fault may have a left-lateral strike-slip component. Based on GPS data, we suggest that the northern Central Range block, bounded in the west by the Lishan Fault and in the north by the Ilan Shelf Fault, rotates clockwise. This rotation is ascribed to the northwestward collision of the Luzon Arc against Taiwan. Thus, there is a left-lateral strike-slip and an extensional motion along the Ilan Shelf Fault. The northern Central Range block is tilting northward which may also induce the normal faulting component of the Ilan Shelf Fault. In short, the Ilan Shelf Fault could be a major tectonic and structural feature bounding the southern end of the Okinawa Trough backarc basin.


INTRODUCTION
The Ryukyu trench-arc-backarc system results from the N309° convergence and subduction of the Philippine Sea plate with respect to the Eurasian plate, while the Taiwan orogen results from the collision between the Luzon Arc and the Eurasian continental margin. The Okinawa Trough backarc basin extends from the Kyushu Island (Japan) to the Ilan Plain (Taiwan) and by extension has been created within the Eurasian continental lithosphere (e.g. Lee et al., 1980;Sibuet et al., 1987;Sibuet et al., 1998). The last two phases of the opening of the southwestern Okinawa Trough occurred during the Pleistocene (2-0.1 Ma) and Pleistocene-Holocene (0.1-0 Ma) (Sibuet et al., 1998). The recent extension phase (0.1-0 Ma) is demonstrated by numerous E-W trending normal faults dipping toward the Okinawa Trough axis with a few meters to a few tens meters vertical offsets in the western Okinawa Trough (Sibuet et al., 1998), which turns to ENE-WSW trending normal faults in the westernmost part of the trough (Figure 1). The morphology and distribution pattern of the normal faults reflect that the westward propagation of the southwestern Okinawa Trough has extended into the Ilan Plain (Sibuet et al., 1998;Wang et al., 2000). Recent alluvial deposits carried by the Lanyanghsi River and accumulated in the Ilan Plain hide the surficial expression of the normal faults. Two seismic reflection and refraction profiles collected across the Ilan Plain by the Chinese Petroleum Corporation (CPC) indicate that ENE-WSW faults are present beneath the sedimentary cover of the Ilan Plain (Chiang, 1976) (Figure 2). The presence of that fault pattern in the Ilan Plain is also indicated by magnetic anomaly analyses (Hsu et al., 1996). In the Ilan Plain, several basement faults have been identified but none have been identified within the sedimentary cover (Chiang, 1976). These ENE-WSW faults are also in the prolongation of thrust faults outcropped around the Ilan Plain (Deffontaines et al., 2001).
Offshore of the Ilan Plain, Kueishantao Island mainly consists of andesitic rocks (Chen et al., 1995) with a subducted Philippine Sea plate slab component, and it is regarded as the westernmost island of the Ryukyu volcanic front (Sibuet et al., 1998). Seismic tomography indicates that a magma body or partial melting exists beneath the Ilan Plain and has moved upward to the subsurface of Kueishantao Island (Lin et al., 2004). Radiometric dating indicates that the Kueishantao Island was formed in the last 200,000 years, and the last eruption was 8,000 years ago (Chung et al., 2000).
According to recent seismicity cataloged by the Central Weather Bureau Seismic Network In order to better define the structural characteristics in this area and to link geological features to the recorded seismicity, we have collected seven seismic reflection profiles between the Kueishantao Island and the Ilan Plain, roughly parallel to the Ilan Plain shoreline (Figure 2). The aim of this study is to identify the presence of active faults in the nearshore area of the Ilan Plain and to understand the structural geodynamics in this region.

SEISMIC DATA ACQUISITION AND PROCESSING
Seven high-resolution multi-channel seismic reflection profiles trending in a NNW-SSE direction were collected by the R/V Ocean Researcher 2 (OR2) during the OR2-1292 cruise in June 2005 ( Figure   2). The seismic reflection profiles were set up to be perpendicular to the trend of the southwestern Okinawa Trough normal faults (Sibuet et al., 1998) and the Ilan continental shelf spur, which is a bathymetric ridge protruding eastward to the Ilan continental shelf ( Figure 2). One GI (generator and injector) gun of 45/45 cubic inches was used as the seismic source, which was fired at 10-second intervals with the ship steaming along at a mean speed of 5 knots. Seismic signals were received by a 150-meter long 12-channel streamer with 3 s recorded length of data and a 0.5 ms sampling rate.
All seismic reflection data were filtered with a 10-64-128-256 Hz band-pass filter. The normal moveout correction and stack were performed using a constant seawater velocity of 1480 m/s. The seven stacked seismic profiles displayed with AGC and their interpretations are shown in Figures 3 to 9.

SEISMIC INTERPRETATION
Due to usual fishing activity in the surveyed area and the large width of the Okinawa Trough, it is difficult to realize the full geographical extent of the Okinawa Trough backarc basin. However, three profiles (1292-3, 1292-5 and 1292-7) reasonably covered the northern flank of the backarc basin. We Thus, the Ilan Shelf Fault contains both normal and left-lateral strike-slip components and extends from onshore to the offshore area. The onshore prolongation of the Ilan Shelf Fault was not seen on the two CPC reflection and refraction profiles in the Ilan Plain (Chiang, 1976), this is probably due to the poor quality of the seismic profiles obtained at that time.

Figures 10 and 11a show the horizontal velocity vectors in the Ilan Plain and in Taiwan deduced
from GPS measurements (Hou et al., in press;Yu et al., 1997) with reference to the Paisha station located on the Penghu Island west of Taiwan. The velocity vectors describe the motions of a region relative to the stable Chinese continental platform. The GPS data were acquired from 2002 to 2006 at 34 annually surveyed geodetic sites located in the Ilan Plain region (Hou et al., in press) ( Figure 10) and in 131 permanent stations over Taiwan recorded from 1990 to 1995 (Yu et al., 1997) (Figure 11a).
The GPS velocity field pattern in Figure 10 differs from that shown in Figure 11a because of the different recording periods. Between these two GPS observations, the Chi-Chi earthquake (ML=7.3) occurred in 1999 (Ma et al., 1999) which caused an important crustal deformation in middle Taiwan (Lo and Hsu, 2005). As several large earthquakes of magnitude larger than 5 occurred during the GPS recording periods, particularly in the Ilan Plain region, the measured horizontal velocity vectors reflect not only interseismic but also co-seismic displacements.
From the GPS observations in the Ilan Plain (Figure 10), the onshore extension of the Ilan Shelf Fault roughly separates a northern region with very small eastward velocity vectors from a southern region with velocity vectors about ten times larger pointing eastward to southeastward (Figure 10). This suggests that a large amount of left-lateral shear motion occurs along the onshore extension of the Ilan Shelf Fault. In the middle portion of the Central Range near 23.8° N latitude, a NW-SE trending feature separates a northern region with velocity vectors rotating clockwise relative to a southern region with the velocity vectors generally pointing towards the northwest (Figure 11a). This NW-SE trending feature was also suggested by . A pseudo-rigid block located between the Ilan Shelf Fault and the NW-SE trending feature described above is named as the "Northern Central Range" block ( Figure 12). The western boundary of the Northern Central Range block follows the Lishan Fault. The eastern boundary, however, lies offshore of eastern Taiwan (Figures 11 and 12) but is not identified in this study.
The velocity vectors located just south of the Ilan Shelf Fault ( Figure 10) changes their directions and amplitudes laterally. This means that the "Northern Central Range" block is not fully rigid, or at least portions of the block located close to its boundaries are deformed due to the irregular shape of the block. The distribution of velocity vectors inside the block with respect to Penghu Island projected along the strike of the Ilan Shelf Fault (N076.5° direction) (Figure 11b) shows that the velocity vectors point eastward in the northern part of the block and westward in the southern part of the block. It is reasonable to suggest that the block rotates clockwise with respect to the rest of the Taiwan orogen. If this is true, then there should be an extensional regime in the northern portion of the Lishan Fault, and a compressional regime in the southern portion of the Fault (Figure 12). This interpretation is consistent with the analyses of earthquake focal mechanisms conducted by Yeh et al. (1991). In addition, Lee et al. (1997) also suggested that back-thrusting occurs further south along the Lishan Fault as a combination of Quaternary uplifting of the Hsueshan Range and horizontal compression. Thus, the southern Hsueshan Range is highly deformed by contraction in the northwestern direction (Figure 11a) due to a "lock" or a "collision" in the southern part of the block. We suggest that the clockwise rotation of the northern Central Range block is probably due to the N309° motion of the Luzon Arc against the Eurasia plate. In that case, the northeastern end of the northern Central Range block should have pulled apart from the Okinawa Trough and created an extensional normal faulting component. That is why we can observe normal faulting along the Ilan Shelf Fault. Another explanation is that the northern Central Range block could have been tilted northward that could also create a normal faulting component along the Ilan Shelf Fault.
To summarize, the impinging Luzon Arc colliding with Eurasia continental margin along the N309° direction has caused the clockwise rotation of the northern Central Range block, giving rise to a lateral extrusion of the northern portion of the block along the Ilan Shelf Fault. In this scheme, the Ilan Shelf Fault is considered the southern boundary of the Okinawa Trough extensional system. The left-lateral motion along the Ilan Shelf Fault results from the clockwise rotation of the northern Central Range block. As the Ilan Shelf Fault is located south of the Ilan Plain, this left-lateral motion may favor 7 a propagation of the western tip of the Okinawa Trough into Taiwan. According to Sibuet et al. (2004), the western tip of the Okinawa Trough has remained approximately above the edge of the Ryukyu slab through time. As the Ryukyu Islands moved westward at a mean velocity of 4.5 cm/yr in the last 9 Ma , the westward propagation of the Okinawa Trough might also proceed at the same velocity. The clockwise rotation of the northern Central Range block and the rifting mechanism of the Okinawa Trough provide a simple interpretation for the triangular shape of the Ilan Plain. In other words, the southwestern Okinawa Trough consists of N-S normal faulting extension and left-lateral motion along the Ilan Shelf Fault.