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.
Highlights
Two successive major earthquakes with magnitudes ML = 6.96 and 6.99 occurred only eight minutes apart in time approximate 40 and 33 km, respectively, offshore of the Hengchun Peninsula, Pingtung county, in the southern tip of Taiwan (Fig. 1) on 26 December 2006 (CWB 2006)
The data processing of the continuously recording GPS stations (CORS) for kinematic positioning of daily solutions is in the post processing mode, under the standard algorithm of Bernese V5.0 (Dach et al 2007)
The ground motion recorded by the two different instruments showed a very similar pattern, with a good agreement for all three components for the first main shock (Fig. 7a) and the second main shock (Fig. 7b)
Summary
Two successive major earthquakes with magnitudes ML = 6.96 and 6.99 occurred only eight minutes apart in time approximate 40 and 33 km, respectively, offshore of the Hengchun Peninsula, Pingtung county, in the southern tip of Taiwan (Fig. 1) on 26 December 2006 (CWB 2006). This earthquake sequence was the largest magnitude of dual main shocks during Taiwan’s century-long history of instrumental record of earthquakes (TEC 2007). Because of the dual events, the coseismic displacements for the individual main shock are difficult to determine with solely the 30-second data in the conventional way. We compare the ground shaking records of GPS to seismometers and discuss their differences, and the likely exponential attenuation of the coseismic GPS ground movements with respect to the distance to the hypocenters
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