The Philippine Fault system is a result of the oblique convergence between the Philippine Sea plate and the recently-discovered Sundaland Block/Eurasia plate. It is a left-lateral strike slip fault that trends in N 30°- 40°W and transects the Philippine archipelago from the northwest corner of Luzon to the southeast end of Mindanao for about 1200 km. The Philippine Fault branches into several splays: the San Jose Fault, the San Manuel Fault, the Gabaldon Fault and the Digdig Fault all in Central Luzon. For studying the strain partitioning along this segment of the Philippine Fault system, a densely-deployed GPS array composed of 56 survey-mode sites and 22 continuous GPS (CGPS) stations were set up in the area in 2008-2011, respectively. This project will maintain and operate these CGPS stations and repeated GPS surveys on the densely-deployed array will be carried out annually. The continuous and survey-mode GPS data in Luzon will be integrated with that from Taiwan Continuous GPS Array and processed together using RunGamit automated GPS processing system. The derived velocity field and crustal strain rates will be utilized to study the mechanism and characteristics of present-day interseismic crustal deformation along the Philippine Fault system. Then the spatial and temporal variations of crustal strain and its relationship with seismic activity may be realized. Based on the GPS observed velocity field, a dislocation model for the interseismic deformation by Matsu'ura et al. (1986) will be employed to estimate the fault geometries, slip rates and locking depths on the branch faults of the Philippine Fault system. The results of modeling studies can provide the important information for seismic hazard analysis.（Fig）
The "Taiwan Continuous GPS Array" is now composed of more than 360 stations operated by the Seismological Center, Central Weather Bureau, Institute of Earth Sciences, Academia Sinica, Ministry of the Interior, Central Geological Survey and other institutions. The enormous continuous GPS data collected daily by the array need to be processed carefully. Only after the systematic and random errors or noises are removed, we may be able to detect the weak signals of crustal strain such as earthquake precursors or transient aseismic slip. GPS position time series from these continuous stations will be modeled to estimate the annual and semi-annual periodic motion, offsets due to coseismic displacement or instrument cahnge, postseismic deformation, and site velocity with more realistic uncertainties. Finally, the strain rates will be estimated directly from the precise velocity field or position time series. Then we can study the spatial and temporal variations of crustal strain and their correlation with seismic and faulting activities in and around Taiwan. The results may provide the important information for the probability evaluation of earthquake occurrence and detection of any possible earthquake precursors.（Fig1）（Fig2）
Taiwan is an exceptional active orogeny, due to its extremely rapid rates of deformation and high erosion rates. Plenty data from a dense continuous GPS array, seismic network and geological investigations, provide an unprecedented opportunity for integrated modeling studies. This project aims to build a seismic cycle model that addresses both the misfits of existing models to their respective data sets. By testing different boundary conditions and rock rheology, we attempt to understand the influences of plate motion and various rheology models on the fault zone evolution, the deformation field of lithosphere, as well as the long-term evolution of mountain range. This work could shed new light on the mountain building process in Taiwan. In addition, we analyze the postseismic deformation associated with the 1999 Chi-Chi earthquake and the 2003 Chengkung earthquake to give a better estimate of frictional properties on the fault.
Evaluating the scenario of a large earthquake to occur along the Manila Trench is important because of the threat of tsunamis to the large population cities bordering the South China Sea. The Manila Trench is the convergent boundary where the Sunda Plate subducts eastward underneath the Philippine Sea Plate. The maximum convergence rate of about 81 mm/yr in the NW direction is observed in the northern Luzon area of Philippines. The subduction is replaced by the collision tectonic in southern Taiwan to the north and near the Palawan Island to the south. The oblique convergence has resulted in the Philippine Fault, a primary left-lateral strike-slip fault with a length of about 1200 km in Philippines. In spite of its recognition as a major geological structure and sources of destructive earthquakes (Ms=7.5 1973 Ragay Gulf earthquake, Ms=7.9 1990 Luzon earthquake, Ms=6.2 2002 Masbate earthquake), many characteristics of the fault such as precise fault location, segmentation, fault slip rates, seismicity, and earthquake recurrence intervals, are poorly understood. Earthquake focal mechanisms and geological field surveys suggest that the strike-slip faulting prevails in the Luzon area. The Philippine fault is likely to consume most of accumulated strike-slip motion along the plate boundary. On the other hand, the dip-slip motion is possibly taken up by seismic or aseismic fault slip on the plate interface along the Manila trench. The historic earthquake records in Philippines have not observed any large earthquake with magnitude greater than 8 in the past 400 years. However, the accumulated strain in this area has to be released by aseismic slip or seismic ruptures. Due to the paucity of GPS and seismic stations in the Manila subduction zone, the seismogenic behavior remains poorly understood. The preliminary analysis suggests that the plate interface between the Manila Trench and the western Luzon is possibly aseismic. The maximum coupling ratio obtained from the GPS inversion is about 0.3 in the region from the West Luzon Tough to the east of the Scarborough Seamount.（Fig1）（Fig2）
The earth sciences is founded on accurate observations, however human activities on the earth's surface have produced noises on tectonic signals. To detect subsurface signals of earthquakes and fault motions, the downhole instruments would significantly increase the signal-to-noise ratio. The borehole strainmeter is embedded in the rock at a depth range of 200-1000 m, it is therefore an ideal tool to detect subtle signals from tremors or fault slip transients. Strainmeters can resolve strain changes of less than one part per billion (1 mm in 1000 km) at short periods with a high short-term stability (10-2 ~ 103 sec); while at longer periods, GPS techniques are more stable. An integrated network of borehole strainmeter and GPS would provide critical insights into a whole spectrum of tectonic motions such as slow earthquake and slow slip event, the earthquake nucleation process, precursory strain changes before, during, and after earthquakes, as well as seismic and strain budgets during the seismic cycle.
The high resolution records from borehole strainmeters make the detection of subtle strain variations possible and give an opportunity to investigate strain changes prior to earthquakes. Previous studies have shown a close relationship with the fluid pressure change in the fault zone and earthquake occurrences. Observations of borehole strain, together with ground water level changes, earthquake numbers, and earthquake occurrence time would capture the details of rupture behaviors and evaluate the possibility of earthquake prediction. Additionally, the seismic budget from earthquakes is usually an order of magnitude smaller than the strain budget derived from geodetic measurements. This discrepancy may result from aseismic slip on the fault zone such as postseismic slip after large earthquakes or fault creep near the surface. Alternatively, the inconsistency may correspond to the periodic slow slip on the fault zones. Observations of episodic tremor and slip have been found at both the Cascadia (Canada) and Nankai (Japan) subduction zones. These slow events mainly occur on the down-dip portion of the subduction zone and possibly caused by increased pore pressure during mineral dehydration reactions. The energy radiated from the deep event has a very low signal-to-noise ratio such that they are difficult to detect on the surface. The high sensitivity of borehole strainmeter is able to detect smaller slip events than GPS at periods of hours to weeks and is ideally suited for detections of slow earthquakes. While both tremor and slow slip have been well-documented in a variety of environments worldwide, the underlying physical mechanisms remain poorly understood. Important issues such as do tremor and slow slip occur simultaneously, are they two manifestations of the same physical phenomenon, or do they represent two phenomena that are linked by a physical process, why tremor and slow slip rarely occur on continental faults, what are the role of slow slip in the strain/seismic budget during the seismic cycle, are there precursory strain changes before large earthquakes. These questions will be addressed by exploiting the new data from borehole strainmeter, GPS, and seismometers in Taiwan.
The institute of Earth Sciences, Academia Sinica (IESAS), in cooperation with the Department of Terrestral Magnetism, Carnegie institution of Washington, have deployed 11 borehole strainmeters at a depth range of 200–270 m near Taroko, Juishi-Chimei, and Chengkung- Chihshang in eastern Taiwan (Figure 1). The borehole strainmeters have recorded tens of tremors between 2003 and 2007 and eleven of them are related to typhoons. These tremors are associated with slow slip with a time interval of few hours to tens of hours. According to the barometric pressure record and seismic data, slow earthquakes are likely to be triggered by typhoons (Liu et al., 2009). Their findings shed a new light on the capability of discover tremor and slow slip in east Taiwan. Future studies aim to focus on the relationship among slow slip, aseismic slip, tremor, water-level change, and rainfall on the Longitudinal Fault in eastern Taiwan. The Longitudinal Valley is situated at plate suture zone between the Eurasian and Philippine plates. About half-amount of accumulated crustal strain in Taiwan is released on the Longitudinal fault, it is important to estimate the strain and seismic budget released by various tectonic motions, detect strain fluctuations in the seismic cycle, and evaluate the earthquake recurrence interval based on the data collected from borehole strainmeters.
Preliminary results show strain changes are strongly affected by the precipitation and resulted from at least two effects (Figure 2). The recorded permanent strain changes are more frequently in the wet season. Further numerical analysis is needed in order to understand better the influence of the precipitation on the strain change.（Fig1）（Fig2）
At the southern part of the Longitudinal Valley, the Longitudinal Valley Fault (LVF) lies mostly along the Peinan River and partitions into the Luyeh thrust Fault to the west and the sinistral fault, called the Coastal Range Fault, to the east. The active Luyeh fault generated deformation recorded by geomorphic features along the western edge of Peinanshan tablelands. The main purpose of this project is to study the present-day crustal deformation in the southernmost segment of the LVF by utilizing the GPS, precise leveling and Differential Interferometric Synthetic Aperature Radar (DInSAR) as the tools to detect the near-fault deformation. Furthermore, these geodetic data will be used to identify the deformation patterns, and understand the strain accumulation in the interseismic period. Hopefully, the results can be utilized on the seismic hazard evaluation of the area.（Fig1）（Fig2）