Vertical Crustal Motion of Taiwan Determined from Tide Gauge and Altimeter Data

Taiwan is located at the western bordure of the convergent boundary between the Philippine Sea plate and Eurasian plate. Processings of plate collision and mountain building is obvious and occur uncountable earthquakes here. In this study, we applied the method of comparing the tide gauge (TG) and altimeter (ALT) data to resolve land vertical motion at 20 TG sites along Taiwan coast. Sea surface height measured by altimetry is related to an absolute reference frame, while TG is related to the coast where they are attached on land. The difference of time series Alt(t)-TG(t) contains information about vertical crustal motion. This method has been used to many applications, such as the global image of vertical ground motion [Cazenave et al., 1999; Nerem and Mitchum, 2002], the tectonic activity around Mediterranean [Garcia et al., 2007], and glacial isostatic rebound occurred in Scandinavian Peninsula, Great Lakes region, and Alaska [Kuo, 2004], etc. In this study, the tide gauge records are adopted from the Marine database of Central Weather Bureau, and altimetry data are the Geophysical Data Record (GDR) which conducted from the tracks of TOPEX/Poseidon(1992 - 2002) and Jason-1 (2002 - Present), charging from JPL's Physical Oceanography Distributed Active Archive Center (PO.DAAC). We calculate the linear trend of the ALT-TG record for each of the TG locations. Seasonal signals, primarily periodic tides are removed from both TG and ALT data as the beforehand treatment for linear regression. We can therefore figure out a whole image of vertical motion along the Taiwan coast. The majority of these movements tend to descend in the latest decade. However, some TG stations reveal an extreme large uplift rate which may be affected by local effects. Sites in the eastern Taiwan generally have a smaller descending rate in comparison to the western sites. Furthermore, our estimations in the southwestern Taiwan show a clear land subsidence from -6.2mm/yr to -34.3mm/yr. Another focus of our study is to determine the coseismic motion in the TG sequence. Step-function with least- square method is used to measure the magnitude of transient motion probably caused by earthquakes neighboring to TG station. In terms of geometrical spreading of seismic energy, the determined coseismic transient motion should be function of hypocentral distance and earthquake size. A forward rupture modeling is also taken to predict coseismic vertical offset with the preset parameters of focal mechanism solution. Further application of the determined vertical offset from TG sequence can help to discuss the coseismic movement of historical earthquakes, which occurred in the period prior to the modern rapid response of coseismic measurement.