G32A-01 10:30h
Interplate Coupling Derived From The GPS Traverse Across the Hinge-Line in Kii Pennisula and its Tectonic Implication
The hinge-line in the south Kii peninsula, southwest Japan, which is the boundary between uplift and subsidence due to the 1946 Nankai earthquake, is considered to be related to the lower margin of the coupling zone. We established 10 GPS observation sites to fill the gaps in the GEONET operated by GSI along two lines nearly parallel to the relative motion between the Philippine Sea and Amurian plates. Resultant average spacing is 5~10km. We have repeated the campaign survey of this traverse using dual-frequency receivers since March 2001. So far, we have collected data from 4 campaigns done every March and can discuss displacement/velocity field during the recent 3 years. We calculate coordinates of our campaign sites with GSI_fs permanent sites referring to the surrounding IGS sites such as Tsukuba, Usuda, Suwon, etc in ITRF97. We use Bernese 4.2 software with IGS precise ephemeredes in the analysis. Zenith delays are estimated every 2 hours. Velocity of each site is calculated referring to the first campaign and converted to the relative velocity to the Amurian plate using the Euler vector by Heki et al. (1999). Unfortunately for campaign survey, antennas of GSI_fs sites were replaced in 2003. Therefore discontinuity arose in time series before and after 2003. We try to remedy this discontinuity to obtain continuous time series. Velocities derived from 2001 and 2002 campaigns are about 20mm/yr in the middle part of Kii peninsula and 35mm/yr at its southern tip, respectively, relative to the Amurian plate. Their directions are WNW. The gradient is almost linear and there is no significant change in gradient across the hinge-line. Velocities along the eastern line are a little smaller than those on the western line. We apply a model with multiple fault segments to this observed velocity field in order to estimate interplate coupling beneath the Kii peninsula. We adopt the fault model of Sagiya and Thatcher (1999) and estimate slip deficit rates on each segment around the Kii peninsula. They are as large as 70 mm/yr that is slightly larger than the relative plate motion. If we add deeper extension to the modeled fault, slip deficit rates of about ~60 mm/yr are obtained. This implies the coupling zone between the Philippine Sea and Amurian plates might be extended much deeper than the depth previously suggested from the thermal model and others, although the estimated slip deficit rates are still large. A westward block motion of the forearc sliver of southwest Japan is found. If we subtract a 12mm/yr nearly westward block motion from the observed velocities, slip deficit rates are significantly reduced.
G32A-02 10:45h
Postseismic deformations associated with the 2003 Tokachi-oki earthquake derived from dense GPS observation
A MJMA=8.0 earthquake hit Hokkaido, northern Japan, on September 26, 2003 (JST). This is the first great interplate earthquake since the deployment of the nation-wide continuous GPS observation network in Japan. Up to 1m coseismic displacements were detected in the southeastern Hokkaido by this network. Since there may be a seismic gap or unbroken asperity on the plate interface east of the source region of this earthquake, how released stress is redistributed is crucial. It is necessary to monitor the postseismic deformations as well as seismicity. Postseismic deformations can provide us with frictional characteristics of plate interface, which is critically important to the physics of earthquake. Postseismic movements have been observed by the nation-wide network as well. However this network is a bit sparser in Hokkaido than in other areas of Japan. We can not expect enough resolving power of slip distribution on plate interface, especially its deeper extension. Therefore, we newly established GPS sites just after the mainshock in the eastern part of Hokkaido to fill gaps of the network as a collaborative work of researchers from Japanese universities. Observation at some sites started on September 28. Thirty GPS sites with dual frequency receivers were established by the end of October in the region of about 250 km x 150 km in southeastern Hokkaido. Some sites are telemetrically monitored. Data are stored in receiver or PC and collected periodically at the rest of them. All the data are archived in the server of Hokkaido University. Rapid data analysis for one month after the mainshock clearly indicated postseismic displacements only in the horizontal components. The direction is almost SW. Observed maximum horizontal displacement was 6.6 cm from 28 September to 24 October, 2003. Absence of the vertical suggests that afterslip occurred in and around the coseismic fault rather than at downdip extension. Time series of coordinates are characterized by logarithmic decay functions with 4-11 days relaxation times. This suggests that postseismic deformation was due to afterslip on the fault following the large earthquake. Several fluctuations are recognized in the time series. Especially, deceleration of postseismic movement is significant from the end of October to November. However movements reactivated and still continue in February, 2004, reaching up to 7cm in SE. It is considered that deformations with two different characteristic times are superposed on the observed postseismic deformations.
G32A-03 11:00h
Interseismic and Postseismic Slips Along a Decollement in Western Taiwan
Continuous and survey-mode GPS data collected before and after the devastating 1999 Chi-Chi, Taiwan, earthquake (Mw 7.6) were utilized to infer the geometry of seismogenic faults and realize the coseismic slip as well as afterslip distributions. Based on the GPS-derived velocity field of Taiwan for the time period between 1993 and 1999, a two-dimensional elastic dislocation model was employed to infer interseismic slip rates on subsurface faults. Elastic dislocation modeling studies on the interseismic, coseismic, and postseismic deformations associated with the Chi-Chi earthquake indicate the existence of a sub-horizontal decollement at a depth of 8-10 km. This is consistent with the geological balanced cross sections, aftershock distribution and microearthquake seismicity, as well as seismic reflection results. Interseismic slip along the decollement may cause elastic strain accumulation in the Western Foothills that will be released in the future earthquake. The coseismic slip on the decollement contributes 21% of the total modeled moment release, while the afterslip on the decollement in the first 3 months and over 15 months after main shock contribute 68% and 80% of the total modeled moment release, respectively. The afterslip moment inferred from 15 months of GPS data is approximately 2.3 times the seismic moment released by aftershocks. This implies that a major part of postseismic deformation is aseismic.
G32A-04 11:15h
A Decade of GPS Measurements in SE Asia: Tectonic Motions on Sundaland and near the Triple Plate Junction
South-East Asia is a complex natural "laboratory", with high tectonic activity, caused by the relative motion of the Eurasian, Phillipine, Indian and Australian plates. As part of the joint EC-ASEAN GEODYSSEA project, which studied the geodynamics in this region, a GPS network including 45 systematically well spread locations was observed 3 times between 1994 and 1998. To expand the position time series, many of these sites in Indonesia, Malaysia and Thailand have been re-observed between 1998 and 2003. Also 3 years (1999-2001) of permanent GPS data from Thai, Malaysian and Indonesian stations were added to the GPS database. The island of Sulawesi, Indonesia, is situated in a complex region near the triple junction of the Eu/Ph/Au tectonic plates. The existing GEODYSSEA network here was gradually densified and yearly observed since 1997. The Sulawesi GPS network also includes transects of 2 faults, of which one poses an acute seismic hazard for the nearby city of Palu. To monitor the behaviour of the Palu-Koro fault continuously, 4 permanent GPS stations have been set-up since 2000. All together, an unique and high-quality GPS data set, spanning almost a decade, was (re-)analyzed using the latest state-of-the-art GPS techniques, and combined into a kinematic model. This model contains over 100 station motions in ITRF-2000, with horizontal accuracies between 1-3 mm/yr. The tectonic block and fault motions were compared with previous results and were geophysically interpreted. Highlights are the relative motion of the Sundaland block (and it's rigid/deforming parts) with respect to the Eurasian plate, the confirmation of at least 2 mirco-blocks in Sulawesi and a better insight into the Palu-Koro fault system.
http://www.deos.tudelft.nl/sunda
G32A-05 11:30h
The SEAMERGES Project: Applying Geodetic Space Techniques to Study Environmental Risks in SE Asia
The "South-East Asia: Mastering Environmental Research with GEodetic Space Techniques" (www.deos.tudelft.nl/seamerges) project is a new EC-ASEAN project which started in January 2004. The aim of this project is the transfer of knowledge, expertise and technology to SE Asia to locally enable higher-level geodetic research and initiate implementation of applications for water management and risk assessment. It also aims at stimulating scientific cooperation among the different Asian countries. The project involves 5 major universities from France, Indonesia, Malaysia, the Netherlands and Thailand. The first part of the project focuses on human resource development (HRD) in 3 areas of high-precision space geodetic technology. These are the Global Positioning System (GPS), Synthetic Aperture Radar Interferometry (InSAR) and Satellite Radar Altimetry (SALT). This will be achieved by providing a number of seminar/workshops at the Asian universities, including the necessary computer platforms and scientific softwares. The second part of the project includes 3 multi-disciplinary common applied research (CAR) projects, where the EC and ASEAN partners jointly cooperate on topics of common interest and research priority. These projects are: 1) Monitoring land subsidence for 3 Asian cities using InSAR/GPS/SALT. 2) Study of regional crustal deformations and tectonic block motions, including associated environmental risk to densely populated areas using GPS/InSAR. 3) Tidal prediction improvement in shallow waters employing SALT, tide gauges and models. These will be long-term projects, and the SEAMERGES team welcomes any of the other ASEAN nations to participate in the future. The SEAMERGES project will be presented in general, while the 3 research projects will be discussed in more detail. This includes both the required information/data and the envisaged results.
http://www.deos.tudelft.nl/seamerges
G32A-06 11:45h
Study of the Philippine Fault in the Visayas through Spatial Geodesy
The objective of this study is to analyze the behavior of the Philippine Fault in the Visayas, using GPS data gathered for a total period of 11 years, from January 1991 to February 2002. The Philippine Fault is a 1200 km long, left lateral strike-slip fault that transects the whole of the Philippine archipelago. In the Visayas region, this fault forms an arc that spans 500 km, between the Bondoc Peninsula in the north and Surigao in the south. Three GPS networks were installed in the central part of this region: (1) the Masbate network, installed in the transfer zone between the Philippine Fault and the Sibuyan Sea Fault, (2) the Leyte network, and (3) the Tongonan network, which covers a transfer zone where the Philippine fault separates into three individual sub-parallel faults. These networks were repeatedly measured between 1991 and 2002: 5 times for Masbate, 6 times for Leyte and 7 times for Tongonan. The results showed that the Philippine Fault in Masbate has an average slip rate of 2.3 + 0.2 cm/yr in 11 years. The fault on this region has an intermediate mechanical behavior, with a part of its movement absorbed through creeping (\# 60%) and a part absorbed during 6-magnitude earthquakes (\#40%), whose recurrence is from 15-20 years. This intermediate behavior characterizes a transition zone between the creeping zone in Leyte in the south and the Bondoc Peninsula and Ragay Gulf in the north, whose movement is released only during important seismic events. In Masbate, GPS data showed the existence of a rigid or semi-rigid block SW of the island. It appears that the Sibuyan Sea Fault is also active and has a slip rate of 2.3 + 0.2 cm/yr. A 2.5+ 0.6 cm/yr has also been calculated in the Legazpi Lineament. In the north of Leyte, results showed that the bulk of the 3.6 + 0.2 cm/yr slip rate 10 years) is absorbed just along the fault. This movement is essentially through creeping since no important seismic activity was recorded during the GPS campaigns. Seventy-five percent of this creeping movement is absorbed along the fault and the remaining twenty-five percent is absorbed around the immediate vicinity of the fault. The results in Tongonan network showed that the movement in this transfer zone is partitioned along the 3 branches sub-parallel to the Philippine Fault. In 10 years of measure, 20% is absorbed by the SW branch (0.7 + 0.2 cm/yr) and 30% along the NE branch (1.2 + 0.2 cm/yr). The remaining 50% is absorbed by the external eastern branch (2.0 + 0.5 cm/yr) measured in 5 years. One of the most important results in this study showed that what appears to be a creeping movement in Leyte, if viewed in a more detailed scale, is in reality, a succession of periods in which the Philippine Fault in this region is either blocked or slipping with varying displacement rates. Through this study, we have established the behavior of the Philippine Fault in the Visayas, evaluated the mechanisms of its deformation as well as quantified its relative movements.