As a part of the U .S.-R.O.C. collaborative deep seismic imaging experi ment, six ocean-bottom seismographs (OBSs) were deployed along the 12°12'N parallel west of Hengchun Peninsula and wide-offset seismic sig nals from a large air-gun array shot over the line were recorded. A pre liminary analysis of these data together with those from another OBS to the east of the peninsula reveals that a 11-km thick crustal layer, inter preted to be an extended continental crust of the southern margin of the Chinese mainland, lies under most of the line and dips toward the east at low angle. A complex margin wedge is observed at depth immediate west and underneath the peninsula, which may have resulted from the thick sedimentary layer, quite likely including the basement complex, pushed against the strong backstop provided by the Philippine Sea plate converg ing from the east.

The authors conducted interviews about the Chi-Chi earthquake dur ing a geological field investigation in order to build records about its pos sible precursors and after effects for future pres�rvation. The precursors reported include aberrant animal behavior and non-animal related activ ity. Phenomena observed during and after the earthquake may be related to seismic energy released and displacement of the crustal structure. We, therefore, suggest establishing a warning system based on a watch network under proper government jurisdiction .. Such a warning system would be beneficial in the prevention of future prevent casualties.

Three different types of seismic data have been examined for seismic events occurring within the zone called the accreted wedge or forearc marginal wedge that underlies the inner trench wall of some arcs. These types of data are (1) teleseismically recorded earthquakes that have been reported in the literature as occurring in major arc‐trench regions; these events fail to demonstrate that earthquakes occur within the accreted wedge because the uncertainty of focal depth usually exceeds the depth dimension of the accreted wedge; these data include many tsunamigenic earthquakes, (2) local earthquakes located by combined ocean bottom seismograph and land networks in the arc‐trench region in the New Hebrides and the central and eastern Aleutian Trench; none of the more reliable of these hypocenters lies within the accreted wedge; (3) S‐P intervals measured at stations on islands located on the outer ridge or at ocean bottom seismograph stations on the forearc marginal wedge; these data do not show the existence of events occurring within the accreted wedge; e.g., from 18 ocean bottom seismograph stations with a cumulative operation time of about 1 year, the smallest S‐P time is about 2.5 s for events in the New Hebrides and about 4 s for events in the Adak and Kodiak regions. We found no S‐P time smaller than 2 s from 6 years of seismograms recorded at Middleton Island, Alaska, and no S‐P time smaller than 4 s from 25 years of seismograms recorded on Barbados. All of the events could have occurred outside the forearc marginal wedge. Although we have located no events with hypocenters which are situated unambiguously within the accreted wedge, we have located many events which occurred in the vicinity of the accreted wedge. Some occur in the upper portion of the Benioff zone shallower than about 40 km and others occur in the leading edge of the overriding plate. These locations reported in this paper which are within or close to the seismic networks comprise some of the most reliably located shallow events ever reported in convergent margins of island arcs. The absence of seismic activity within the accreted wedge in any island arc that we studied suggests that the deformation evident in this region occurs aseismically.

The nature of the plate boundar)'• between the Eurasian a11d Philippine Sea Plates changes from subduction to collision along the northern exten sion of the 1\ifanila T1•ench, north of 21°N, offshore southwest Tai\van. To det .ermine \\'hether the collision-formed deformation front passes through the Tainan Basin, a region ()f petroleum inte1 .. est, seismic profiles that cross a proposed ''deformation front'' are interpreted. However, only a ma. jor normal fault, that is a growth fault that forms the north,ivestern side of a tilted horst structure can be positivel)• identified. r-fhe nature of this nor mal faulti11g is also supported by the modeling of gravity and magnetic data.The observation of 011ly tensional faults in the Tai11an Basin suggests that the deformation t"ront is located to the S(lutheast, near Kaohsiung ()r even further to the south.The economic basement in the Tainan Basin region, defined as pre Tertiary rocks that u11derlie the thick Tertia1 .. y sequences alc>ng the seismic.lines \ive examined, maintains a depth of 3 to 5 km in a tilted horst-like zone, kno\vn as the Central Uplit" t, and deepens in either direction away f" rom it.The thick Tertiary deposits provide excellent potential fo1 .. source rc>ck and seal. Petroleum exploration in this area should be directed toward gener ating prospects related to normal faulting if struct11ral cl(>Sures can be iden tified.

Six ocean bottom seismographs (OBSs) were deployed along an east west seismic line at 22°12' N latitude in the eastern offshore region of south ern Taiwan to record air-gun shots over the instruments. The line extends from 122°00'E in the Philippine Sea Basin to 120°55'E near the Hengchun Peninsula and crossed the North Luzon Arc, the North Luzon Trough, the Huatung Ridge and the Southern Longitudinal Trough, from east to west. A seismic velocity model along the profile derived from the acquired data includes two layers of sediments with a P-wave velocity of less than 3.6 km/ s and upper and lower crusts with velocities of up to 5.3 km/s and 7 .0 kmls, respectively. The sediment layer is very thin on the North Luzon Arc and Trough, but is as thick as 5 km under the Huatung Ridge and the Southern Longitudinal Trough. The crust, which is about 6 km thick under the Phil ippine Sea Basin, thickens westward and is more than 15 km under the North Luzon Arc and again towards the Hengchun Peninsula, with corre sponding variations in Moho depth. The seismic structure in the area is thus consistent with a hypothesis that once a trench located along the South ern Longitudinal Trough might retreat farther west at a later stage.