Detecting the b asement and the fault s under Ilan plain , northeastern Taiwan, using seismic reflection method
Speaker: Jun-We Zhang
Abstract
XXIlan plain is located at the northeastern Taiwan and is considered as a delta fan formed under the influence of Okinawa Trough. There exists a conspicuous linear distribution of earthquakes in the southern part of Ilan plain, a significant indication of buried faults. Although many faults were identified in the surrounding mountain areas, their features are not examined under the plain due to thick sediment deposit s . In t his study , we use the seismic reflection method to depict the subsurface structure under the Ilan plain and to infer its possible structur al evolution.
XXA ‘M inivibe ' source and a 192-channel system are used to do the survey. For comparing with Chiang's (1976) seismic results, the surveys are divided into three parts to detect different structural distributions: basement shape, sediment layers, and faults . The seismic profiles in 1976 are 3 folds and the receiver interval 20 m . However, this study used 24 folds and 4 m interval. It is expected to reveal much detailed structure features.
XXThe results can be described in two parts : basement depth s and fault distribution s . In the part of the basement depth, the profiles near the mountain show that the basement depths are about 250 to 350 m. The profiles near the coast show that the basement depth near the Lanyang River reach es 760 m and gets shallower southward. Quaternary strata above the basement have two part s: the lower one is full of gravel deposits and the upper one is sand and silt . T he strong reflect or ‘A' in Chiang's profile s is identified as a separator of these two parts. The reflection in the gravel deposit is complex which even leads a broken distribution of the basement. This represents a high energy deposit environment in the earl y stage of delta fan deposit . Several faults appear in seismic profile s , among them, two faults are quite distinct. After comparing with the 1976 seismic profile, we recognize them as the Zhuoshui fault , which is probably related to the Lishan fault , and the Sansing fault. T he Sansing fault is a strike-slip fault and is found to cut though the Quaternary strata to very shallow depth. This implies that it could be an active fault. This fault may be the main fault of the earthquake doublet (M=6.2) happened at 2005. The seismic profile s across the Zhuosui fault show shat the basement depths change suddenly . To the nor th of the Zhuosui fault the basement depth is 76 0 m, but decrease s abruptly to the sou th of the fault a s 64 0 m. T he sediments above the basement are quite flat and stable . T he fault should be the boundary to distinguish the different rock contact and could be the north ern boundary of the Central Range geologic province.
Reference
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