| dc.description.abstract | Receiver function (RF) waveform observation, analysis and simulation approaches are used to investigate the crustal thickness and main crustal structure discontinuity beneath each broadband station in Taiwan. The main goal of this study is to resolve the spatial depth and dip angle variations of Moho discontinuity. The proposed Taiwan Reference Moho Discontinuity Model (TRMDM) is based on the analysis of teleseismic data collected by stations deployed and maintained by the Institute of Earth Sciences (BATS) and Central Weather Bureau (BBCWB) with additional five temporary broadband arrays deployed along east-west and north-south transect lines across island organized under TAIGER (TAiwan Integrated GEodynamic Research) project. More than 500 teleseismic events from Incorporated Research Institutions for Seismology (IRIS) report were recorded by 104 BB stations during 2006-2008. The data selection criteria are that the teleseismic events with mb ≧ 5.5 and epicentral distance is from 30 to 95 degree.
From synthetic modeling studies of RF profile for uniform dip, planar interface with sufficient velocity contrast, the radial component shows symmetric waveform variation with back azimuth angle. That is, the amplitude variation with back azimuth angle of P- phase has inverse relationship with Ps phase. For the same polarity, the maximum amplitude of P- phase will corresponds to minimum amplitude for Ps- phase. For transverse component, the anti-symmetric property, coincidence of amplitude variation and inverse polarity change between P- and Ps- phase varying with back azimuth angle can help to determine the dipping direction. In addition, for both components, the amplitude variation with incidence angle for direct P phase is more apparent than the changes in Ps phase.
For teleseismic data analysis, interpretation and identification of Moho conversion phase from stacked RF through different stacking criteria and travel-time picks were performed. Time-to-depth conversion on the manually picked Ps phase arrival time, determination of dipping direction and angle variation of TRMDM beneath each station are studied. The single stacked RF trace from all incoming plane waves and stacked four traces from four major directions impinging upon each station are examined and compared their waveform, amplitude, polarity and arrival time variations in order to determined their dipping angle and direction. Ps arrival time information is used to constrain Moho depth.
Relative thin crust (21-24-28 km) in the northernmost corner (TIPB-TWBB-WFSB) of the island may correspond to slab budge and/or back-arc opening of Okinawa trough. In northern Taiwan, the Moho depth derived from ten stations indicates that: Moho depth varying from 27 km (TGN05) to 33 km (TGN12) in region covers latitude 24.6o to 25o and longitude 121.1o to 121.6o. In the region close to the northern end of central mountain range, between latitude 24o to 24.5o and longitude 121.3o to 121.8o, significant deeper Moho depth varying from 42 km (NACB) to 53 km (TGN09). In the northern end of the backbone range close to Ilan and Hualien county border, the northeastward subduction and flexure bending of the Phillipine Sea plate causing crustal thickening while thin crust behind the plate bending corresponding to in the northernmost Taiwan. At southwestern end of Shuieh Shan and close to central Taiwan, Moho reaches depth of 50 km (TGC06) to its maximum of 56 km (Shunglong station, SSLB) and become shallow towards east coast. The mechanism involving significant Moho depth variations may attributes to lithospheric thickening due to arc-continent collision in central Taiwan. In southern Taiwan (south of latitude 23.5o), average Moho depth is 35 km which is consistence with average global Moho depth. In northern Taiwan, analyzing polarity variations of six BB stations show clear southward dipping of Moho surface which is consistent with result derived from Ps arrival time.
Simulation of common-receiver RF stacking profile at each station provide more detailed information on their spatial azimuthal variation of Moho discontinuity beneath Taiwan island. For Kimen (KMNB) and Matsu (MATB) stations, the clear Ps and PpPmS phases, at 4.1 and 13 sec respectively, show that the average Moho depth is around 34 km. At Kimen station a northwestward dipping shallow interface at depth of 3 km (0.5 sec) is identified from transverse component. At Penhu (PHUB) station, although an obvious 0.5 second shift occurred for P- arrival, the clear arrivals at 3.0 and 4.5 seconds (Ps1 and Ps2) may indicate partial melting of felsic basaltic magma intrusion produce high elastic impedance contrasts in the upper and lower intrusive boundary. Further detail analyses on the conversion phase for other stations are required in the future. The proposed TRMDM can be further constrained base on the broadband data available from TEC data center.
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