A striking high magnetic anomaly belt (HMAB) trending almost SW-NE is situated along the continental shelf edge off the southeastern Chinese continent. It extends northeastward to central Taiwan and gradually vanishes there. In this paper, magnetic simulation is performed and compared with other geological and geophysical data to establish a better understanding of the nature of the magnetic source of HMAB and how it makes HMAB vanish. The magnetic simulation was performed with a geometrical constraint from the seismic velocity structure published in literature. Our simulation shows that there is a layer of material with high magnetic susceptibility (which should be the source of HMAB), situated near the center of the thick crust, along the trend of HMAB. This source layer should be the magmatic relic intruded along the continental shelf edge off the Chinese continent during the initial stage of the opening of the South China Sea. The change in physical conditions or large-scale dislocation of the source material due to intensive and complicated crustal deformation are concluded to be the possible causes of the gradual vanishing of HMAB near its northeastern end.

When using ground-penetrating radar to identify underground pipelines of similar dielectric constants (i.e. PE and PVC), misidentification is quite common. In this study, we apply reflected travel time of radar waves into the PE- and PVC- dielectric constants and the differentiation becomes possible. We have conducted the experiments using the non-metal PE- and PVC- pipelines as well as the heavy metal (iron) pipelines in a water environment. Based on a travel- time calculation, the dielectric constants of non-metal pipelines with similar composition (PE = 2.3 and PVC = 3.0) were quite close. The error between the experimental and theoretical values is acceptable in general engineering projects. It is also compliant with the standard error by the U.S. ASTMD4748-98 (the error range is about +/- 0.2 inches or +/- 0.508 cm). Therefore, the result of this study not only can be applied to detect the metal pipes, but also may be used to distinguish non-metallic pipes in a water environment.

In 1995, a combined ocean-bottom-seismometric (OBS) and multichannel seismic (MCS) survey with strong air-gun shots was carried out in the southernmost Ryukyu subduction zone. A crustal velocity structure constructed from the layer-stripping Monte Carlo inversion of three OBS/MCS profiles and the associated density models inverted from gravity data in the SW end of the Ryukyu arc-trench system are presented. Parallel to the arc in the southernmost Ryukyu subduction system, the OBS/MCS profiles show sedimentary layers of the Hoping, Nanao and East-Nanao forearc basins from west to east, warping of the arc basement and buckling of the subducted slab beneath the Hoping basement rise. The arc-parallel variation of the crustal structure may result from increasing lateral compression westward due to oblique subduction of the Philippine Sea plate and collision with the Luzon arc near the northwestern edge of the forearc region. Northward subduction and arc-parallel compression of the slab also have generated thrust faulting along the subduction interface and strike-slip faulting within the subducted slab, respectively. On 2002 March 31, an earthquake with a moment magnitude of 6.84 was induced by buckling of the subducted slab and strongly affected cities within an epicentral distance of 100 kin. The velocity-interface models, the density models and the focal mechanisms presented in this paper therefore suggest that earthquakes induced by slab buckling or arc-parallel compression have been stronger but less frequent than those generated by northward subduction in the Ryukyu seismogenic zone off Taiwan.

Three-dimensional relocated seismicity in eastern Taiwan reveals two north-to-northeast trending seismogenic zones, one located close to the Longitudinal Valley and the ether near to the crest of the Luzon are. Earthquake 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 degreesN. 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 evidence from seismicity, P- and 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 deforming zone, between two relatively rigid blocks, that undergoes shearing from the transcurrent component of oblique convergence in eastern Taiwan.

Using travel time data from local earthquakes and air-gun shots recorded by the Central Weather Bureau Seismographic Network, the transition from a typical subduction to a collision suture in the southeastern Taiwan area is imaged in terms of a three-dimensional Vp structure. The southern prolongation of the Longitudinal Valley Fault (PLVF), which is characterized by a sharp contrast in velocity on either side, is the primary feature in the velocity structure. West of the PLVF, a high velocity volume exists from the surface to about 9-km in depth, which can be interpreted as being related to the Central Range. The Central Range structure seems to end near 22.2 degrees N beneath the Hengchun Peninsula. East of the PLVF, a major high velocity anomaly in the middle- to lower-crust beneath the Southern Longitudinal Trough and Huatung Ridge is observed. According to the velocity structure and the estimated composition, the high velocity body could be the forearc oceanic crust, which might have been torn off and separated from the Philippine Sea plate after the Luzon are was formed, and has been shortened during the collision of the Eurasian and Philippine Sea plates. The other conspicuous feature of the Vp model is a clearly lateral velocity variation across the Taitung Canyon from the surface to about 25-km in depth, which might be associated with the segmentation of the Luzon are. Using the three-dimensional Vp model, earthquake events that occurred from 1990 to 1997 were relocated. Most of the relocated hypocenters in the study area tend to lie on the locations where there is a greater gradient in the Vp model.

We have compiled new free-air gravity anomaly (FAA) and magnetic anomaly maps, shedding light on the tectonics in the Taiwan-Luzon region. To have a suitable datum level for both the available gravity and magnetic anomaly data, the set of data from an ACT cruise, conducted during May 27 to June 21, 1996, was chosen as a reference. Based on the cross-over error analysis, all the other data were adjusted accordingly. Some satellite-derived, airborne or land data were also added to the compilation to obtain better coverage.Several major new insights into the Taiwan-Luzon region are revealed by the new maps. (1) A prominent NE-SW trending belt of gravity and magnetic anomalies is present in the onshore and offshore areas of southwestern Taiwan. The Peikang High is located on this belt. (2) Located in the offshore region west of Taiwan and to the north of the belt described above, the Taishi Basin, in contrast, occupies a relatively low FAA area. It could be regarded as a flexural basin on account of the loading of a thrust-and-fold belt in western Taiwan. (3) A probable NW-SE trending old transform fault is well imaged off southwestern Taiwan, which separates the lithosphere (plate) of the South China Sea from a trapped piece of the Philippine Sea plate. (4) Located east of the Luzon Are, the Huatung Basin contains several E-W trending magnetic reversals and two N-S trending old fracture zones. The Huatung Basin is separated from the West Philippine Basin by the "123E Fracture Zone". Accordingly, the Gagua Ridge corresponds to a transverse ridge bounding the 123E Fracture Zone. (5) The Luzon Are is abnormally concave toward the Manila Trench and becomes wider toward the south. The internal deformation of the Luzon Are in terms of several NE-SW discontinuities could be related to its collision with eastern Taiwan, (6) Three previously proposed NW-SE trending strike-slip faults in the southern Okinawa Trough, west of 123.5 degrees E, are well imaged on both the FAA and magnetic anomaly maps. Post-collisional volcanism off northeastern Taiwan occurs along these NW-SE trending faults.

Abstract To the northeast of Taiwan, northwestward subduction of the Philippine Sea plate is occurring beneath the Eurasian plate along the Ryukyu Trench. The Ryukyu Trench, which is well defined along the northeastern part of the Ryukyu arc, cannot be easily defined west of 123° east. This is an area where the Gagua Ridge (whose origin is controversial) enters the trench from the south. On the basis of the marine geophysical survey data the following results have been obtained. The structural elements associated with the Ryukyu subduction system deform and partially disappear west of 123° east. Among other things the Ryukyu Trench terminates close to the western slope of the Gagua Ridge. The Gagua Ridge is the result of tectonic heaping and is likely to be an uplifted sliver of oceanic crust. The interaction between the Ryukyu subduction system and the Taiwan collision zone encompasses a wide region from Taiwan to the longitude 124.5° east. The Gagua Ridge is a boundary between the active deformation zone related to the collision in Taiwan and the West Philippine Basin. It is proposed that there is a tectonic zone that can be traced from the Okinawa Trough on the north to the southern termination of the Gagua Ridge on the south.

A numerical hybrid method was developed to model elastic wave propagation.This algorithm was implemented with both the pseudo-spectrumand the finite-element methods.The pseudo-spectrum is currently a popularnumerical method in earthquake seismology studies due to its high efficiencyand accuracy.On the other hand,its most significant drawback isthe difficulty of implementing a free surface or absorption boundary owingto the nature of its periodic boundary.In addition,since the grid spacemust be defined globally within a model to prevent grid dispersion dependingon the region of strong velocity contrasts,computations may becomevery expensive.However,these drawbacks can be overcome with a hybridof the pseudo-spectrum and the finite-element techniques.With the implementationbased on the finite-element formulation,grid spacing can be determinedaccording to local velocity within a velocity model.In so doing,the coding of the boundary conditions becomes much easier as well.Theadvantages of this proposed hybrid method consist of both reducing theamount of computational time and memory needed and obtaining both accurateand stable results during calculation.Some examples are shown todemonstrate the advantages of the hybrid method.This method can also beeasily expanded to 3-D situations with minor modifications.

Seismic refraction data from onshore and offshore experiments in the eastern-northeastern Taiwan region \\'ere used to study the velocity struc ture by the two-dimensional ray-t .racing method.In the \1elocity model, a structural fault boundary located beneath the Longitudinal Valley "ras used to separate the northern Coastal Range (CR) on the eastern side from the eastern flank of the Central Range (EFCR) on the western side.The P 'vave 'relocities from the surface to the dept .h of 12-15 km varied from 3.9 to 5.8 km/s beneath the CR and from 4.8 to 6.1 km/s beneath the EFCR.Com paring the velocity structures along various latitudes, it \\'as found that the CR extends northward to 24.2 <> N. The velocity structures of the CR, the Hsinchen Ridge (HR) and the Yaeyama Ridge (YR) indicate that the HR and the YR both belong to t .he same type of tectonic unit as the CR.To the north of 24.2 <• N, the velocit)' structure of the Ilan Ridge (IR), located be t,veen the EFCR and the southwest .ern end of the Ryukyu arc, is similar to that . of the EFCR; hence, probably indicating it is the northeastern exten sion of the EFCR.This suggests that the EFCR bends eastward and be longs to the same tectonic unit as the southwestern Ryukyu arc.From a comparison of the velocity structures of the CR, EFCR and of other typical continental arcs, orogens and oceanic arcs in the literature, it can be con cluded that the northern CR b�longs to an oceanic arc and that the EFCR is a continental arc.Further more, from the analysis of the velocity struc tures beneath the CR and EFCR, it is believed that the upper crust of the CR is weaker in strength than the EFCR, which means that the arc-conti nent collision is not an appropriate model for the formation of Tai\\ran is land.

The nature of the plate boundary between the Eurasian and PhilippineSea Plates changes from subduction to collision along the northern extension of the Manila Trench,north of 21°N,offshore southwest Taiwan.Todetermine whether the collision-formed deformation front passes throughthe Tainan Basin,a region of petroleum interest,seismic profiles that crossa proposed”deformation front”are interpreted.However,only a majornormal fault,that is a growth fault that forms the northwestern side of atilted horst structure can be positively identified.The nature of this normal faulting is also supported by the modeling of gravity and magnetic data.The observation of only tensional faults in the Tainan Basin suggeststhat the deformation front is located to the southeast,near Kaohsiung oreven further to the south.The economic basement in the Tainan Basin region,defined as pre-Tertiary rocks that underlie the thick Tertiary sequences along the seismiclines we examined,maintains a depth of 3 to 5 km in a tilted horst-like zone,known as the Central Uplift,and deepens in either direction away from it.The thick Tertiary deposits provide excellent potential for source rock andseal.Petroleum exploration in this area should be directed toward generating prospects related to normal faulting if structural closures can be identified.

Dynamic interpretation of seismic waves is traditionally based on the model of geological medium as a combination of layers and blocks bonded rigidly to each other.However, a discrepancy has been found between the ac cumulated theoretical data and experimental data in seismic wave dynamics.Geophysical and rheological study of the Earth's crustal structure has indi cated an important role of layering and fracturing.So, some boundaries in the Earth's crust can be treated as a non-rigid contact between media, i.e., as a mechanically weakened contact or with thin intermediary layer filled with loose or viscous materials.The theory of non-rigid contact between media has been developed in the literature, but the study of the dynamic char acteristics of waves generated at a boundary with non-rigid contact is still required.Here we present the results of numerical and physical ultrasonic experiments on the reflected and transmitted seismic body wave dynamics for the model of non-rigid contact at the boundary between isotropic elastic media.These calculations are based on the non-rigid contact theory which is defined by the boundary conditions with discontinuities of the tangent and normal components of time derivative of the displacement vector across the interface.Our results demonstrate that the amplitude and phase behavior of the generated waves at the non-rigid contact is considerably different from those at the rigid contact between media.The physical ultrasonic experiment was performed for a model of a single fracture on a sheet of duraluminium with a thin filled-plasticine layer.The comparison of physical experimental data with theoretical computations shows a high level of agreement.

A 24-hour continuous sound recording was made after the 1986 May 20 earth quake.During the 24 hour period, 25 aftershocks with a magnitude greater than 2 were recorded by a temporary seismographic network.Comparing the seismograms with the recorded sounds, we found that 5 of these aftershocks were accompanied by earthquake sounds, and all of these sounds preceded the P-wave arr ivals by 0.2 to 1.1 seconds.These 5 aftershocks possessed common characteristics of greater magnitude, shallower focus, and being closer to the recording site than other after shocks.The predominant frequencies of the earthquake sounds ranged from 25 to 100 Hz.These predominant frequencies were relatively lower when the focus was deeper.Based on these discoveries, we infer that, before the main faulting which re sulted in the earthquakes, there were associated cracks that sent out acoustic waves with frequencies higher than those detected by the seismographs.

The aftershocks that followed three moderate size earthquakes occurring on June 12, 1985, off the east coast of Taiwan were monitored by land and ocean bottom seismographs. Hypocenters of aftershocks located by this sea‐land joint array outline a rupture plane striking northeast and dipping steeply to the southeast. The result agrees well with fault plane solutions of the three main shocks. These results combine to indicate a right‐lateral strike‐slip mechanism for the rupture. The earthquake sequence studied occurred behind an active subduction zone and is explained in terms of a forearc "sliver plate" movement resulting from the strong oblique convergence between the Phillipine Sea plate and the Eurasian plate along the trench just east of Taiwan.

A group of earthquakes occurred to the southeast of Taiwan in an area where an abyssal plain deeper than 4500 m is recognizable. According to some focal mechanism solutions in literature and data from the telemetered seismographic network in Taiwan, it is believed that these earthquakes are caused mainly by left-lateral faults striking NW. These faults are interpretable in terms of the horizontal bending of the Philippine Sea plate near Taiwan due to the oblique blockage of its northwestward movement by the Taiwan island