Coseismic Surface GPS Displacement and Ground Shaking Associated with the 2006 Pingtung Earthquake Doublet, Offshore Southern Taiwan

Abstract

Two successive large earthquakes (ML = 6.96 and 6.99) occurred in southern Taiwan offshore of the Hengchun town, Pingtung county, with the two main shocks separated by an interval of only 8 minutes. Based on a dense network of continuously recording GPS stations (CORS) in Taiwan and adopting two different post-processing methods, we estimate the coseismic displacements and characterize their ground motions. Daily solution algorithm is used to determine the total coseismic displacements of the dual main shocks from 30-second sampling rate data; however the coseismic displacement for the individual main shock cannot be resolved.We thus adopt the kinematic positioning technique using 1-second sampling rate (1 Hz) data to determine the individual coseismic displacements for each main shock as well as the evolution of the ground shaking. The results show only three stations near the epicenters with significant total coseismic displacements of 3 - 5 centimeters. We find that the stations farther north of the epicenters area indeed reveal significant coseismic displacements but moving in the opposite direction between the two main shocks. The coseismic displacement of the first main shock is consistent with a NNE-trending normal faulting in the lower crust offshore of southern Taiwan, while that of the second main shock likely agrees with an ENE-trending right-lateral strike-slip faulting, although the possibility of a NNW-trending left-lateral strike-slip faulting cannot be ruled out. The 1 Hz GPS data can record the coseismic ground shaking in great detail, including the first motion direction and the amplitude and arrival time, which are comparable to the seismometer data. By applying an exponential attenuation behavior with hypocenter distance we observe that four stations in the coastal plain exhibit relatively larger amplitudes of the ground shaking, implying a significant influence of thick unconsolidated deposits in that area.

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