Ya-Chuan Lai

Multiple Diving Waves and Steep Velocity Gradients in the Western Taiwan Coastal Plain: An Investigation Based on the TAIGER Experiment

Seismic data collected during explosion experiments performed as part of the TAiwan Integrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high‐resolution images of the structure of the crust and upper mantle beneath Taiwan. The most significant feature observed at near‐source stations located on the western coastal plain in Taiwan is high‐energy later arrivals. These high‐amplitude multiples almost completely mask the lower‐amplitude signals (seismic refraction and wide‐angle reflection) from the deep crust. The later arrivals are identified as free‐surface‐reflected multiples. The nature and generation of these high‐energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer‐velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P ‐wave velocity of ![Graphic][1] , and an average velocity gradient of about 0.72/s from the surface to 3.0‐km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multiple diving waves with the aim of enhancing the deep‐surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [1]: /embed/inline-graphic-1.gif

A west-dipping seismogenic boundary beneath the Chia-Yi area of western Taiwan

Taiwan is one of the most typical arc-continental collision systems in the world. The Taiwan orogeny is often explained by the thin-skin model, where a shallow decollement, with depths of less than 10 km, is considered as the crustal deformation boundary for generating significant mountains on the island of Taiwan. In this study, the seismic data generated by crustal earthquakes, recorded by seismic stations in the Chia-Yi area of western Taiwan was examined to find the crustal structures beneath the Taiwan orogeny. A significant velocity boundary below the decollement was found from unambiguous multiple secondary waves and phases of primary waves converted to secondary waves. Careful comparison between the observations and calculated results of the travel-time and ray-path indicates that this boundary is dipping to the west at ~10 degrees, which, surprisingly, is dipping to the opposite direction of the proposed decollement. It is also interesting to note that this boundary is not only a velocity boundary but also consistent with a seismicity boundary. This westdipping boundary is the major seismogenic boundary in the upper crust, while the east-dipping decollement proposed earlier is a geological boundary. As a result, the westward dipping boundary observed in this study is more suitable, than the eastward dipping decollement, to be the seismogenic boundary in the Chia-Yi area. Article history: Received 25 December 2017