Tzay-Chyn Shin

Relationship between Peak Ground Acceleration, Peak Ground Velocity, and Intensity in Taiwan

Based on the strong-motion data set from the 1999 Chi-Chi, Taiwan, earthquake and a shaking damage statistics database, we investigated the correlations between strong ground motions and earthquake damage (fatalities and building collapses) through a regression analysis. As a result, the current earthquake intensity scale I t is placed on a more reliable instrumental basis. This is necessary for the real-time seismic monitoring operation in Taiwan where programs for earthquake rapid reporting (RRS) and earthquake early warning (EWS) are actively pursued. It is found that the earthquake damage statistics give a much closer correlation with the peak ground velocity (PGV) than with the peak ground acceleration (PGA). The empirical relationship between PGV and the intensity I t determined in this study can be expressed as \[I_{\mathrm{t}}=2.14{\times}\mathrm{log}_{10}(\mathrm{PGV})+1.89.\] This PGV-based intensity is particularly useful in real-time applications for damage prediction and assessment, as the damage impact of high PGV is much more important for mid-rise and high-rise buildings that are characteristic of a modern society. For smaller earthquakes ( M M

Near Real-Time Mapping of Peak Ground Acceleration and Peak Ground Velocity Following a Strong Earthquake

During a disastrous earthquake, the early assessment and timely reporting of the peak ground acceleration (PGA) and peak ground velocity (PGV) maps will be crucial in an effective emergency response operation. In this study, we first derive an empirical relationship between M L and M W. The PGA and PGV attenuation relationships are deduced with data from the Taiwan Strong Motion Instrumentation Program (TSMIP) and the Taiwan Rapid Earthquake Information Release System (TREIRS). Site corrections of the attenuation relationships for shallow and large earthquakes in Taiwan region are also obtained. Peak values of earthquake strong ground motion can be well determined in Taiwan as soon as the earthquake location is determined, and magnitudes are calculated by the TREIRS. This peak ground motion value information can be immediately turned into the calculated PGA and PGV maps that can be issued within two minutes of the earthquake origin time. During any disastrous earthquake, these maps are found to be very useful for immediate seismic damage assessment and dispatching of emergency response missions. Manuscript received 1 December 2000.

CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake

The Chi-Chi earthquake occurred at 17:47 on 20 September 1999 and is the largest earthquake ( M W 7.6) to have occurred on land in Taiwan in the twentieth century. This earthquake caused considerable damage and was named the “921 Chi-Chi Great Earthquake” by the Taiwan government, as the local date was 21 September. Because an extensive strong-motion instrumentation program in Taiwan was completed by the Central Weather Bureau (CWB) in 1996, over 30,000 digital strong-motion records have been obtained from the Chi-Chi earthquake and thousands of its aftershocks. These records form the largest set of strong-motion data recorded from a major earthquake since strong-motion seismology studies began in the 1930s. This data set is important to seismology and earthquake engineering because it includes over 60 recording sites within 20 km of the fault ruptures, which provides a five-fold increase of such near-field records available for the entire world. A prepublication CD was made available in mid-December 1999, and the data on it have been used by many authors in dozens of articles published so far. Since then, we examined about 10,000 strong-motion records and conducted a first-order quality assurance procedure for all the records obtained on 20 September 1999, including the mainshock and hundreds of the early aftershocks. We performed extensive data processing for quality assurance and selected a total of 663 strong-motion data files from 441 accelerographs to construct strong-motion records up to 4-min long for the mainshock whenever possible. In this article, we present a brief description of the processed acceleration data from the Chi-Chi earthquake. The data set (about 500 megabytes) and an extensive 562-page report (documenting the data processing and results of the processed data) are archived on the attached CD in this Special Issue so that users can quickly access this valuable data set for their research. Manuscript received 7 April 2001.

Development of earthquake early warning system in Taiwan

[1] With the implementation of a real-time strong-motion network by the Central Weather Bureau (CWB), an earthquake early warning (EEW) system has been developed in Taiwan. In order to shorten the earthquake response time, a virtual sub-network method based on the regional early warning approach was utilized at first stage. Since 2001, this EEW system has responded to a total of 225 events with magnitude greater than 4.5 occurred inland or off the coast of Taiwan. The system is capable of issuing an earthquake report within 20 sec of its occurrence with good magnitude estimations for events up to magnitude 6.5. Currently, a P-wave method is adopted by the CWB system. Base on the results from 596 M > 4.0 earthquakes recorded by the real-time strong-motion network, we found that peak displacement amplitudes from initial P waves (P d ) can be used for the identification of M > 6.0 events. Characteristic periods τ c and τ max p of the initial P waves can be used for magnitude determination with an uncertainty less than 0.4. We expect to achieve a 10-second response time by the EEW system in Taiwan in the near future.

Review: Progress in Rotational Ground-Motion Observations from Explosions and Local Earthquakes in Taiwan

Rotational motions generated by large earthquakes in the far field have been successfully measured, and observations agree well with the classical elasticity theory. However, recent rotational measurements in the near field of earthquakes in Japan and in Taiwan indicate that rotational ground motions are 10 to 100 times larger than expected from the classical elasticity theory. The near-field strong-motion records of the 1999 Mw 7:6 Chi-Chi, Taiwan, earthquake suggest that the ground motions along the 100 km rupture are complex. Some rather arbitrary baseline corrections are necessary in order to obtain reasonable displacement values from double integra- tion of the acceleration data. Because rotational motions can contaminate acceleration observations due to the induced perturbation of the Earths gravitational field, we started a modest program to observe rotational ground motions in Taiwan. Three papers have reported the rotational observations in Taiwan: (1) at the HGSD station (Liu et al., 2009), (2) at the N3 site from two TAiwan Integrated GEodynamics Research (TAIGER) explosions (Lin et al., 2009), and (3) at the Taiwan campus of the National Chung-Cheng University (NCCU )( Wuet al., 2009). In addition, Langston et al. (2009) reported the results of analyzing the TAIGER explosion data. As noted by several authors before, we found a linear relationship between peak rotational rate (PRR in mrad=sec) and peak ground acceleration (PGA in m=sec 2 ) from local earthquakes in Taiwan, PRR 0:002 1:301 PGA, with a correlation coefficient of 0.988.

Detection of landslides and submarine slumps using broadband seismic networks

[1] We detected 52 seismic events associated with landslides and submarine slumps on 8 August 2009 when Typhoon Morakot hit Taiwan. These events were neither felt nor reported because their seismic energy was primarily in the long period band (20-50 s). Most of these events were located in mountain areas where the accumulated rainfall was extremely high, though some were found offshore southern Taiwan. Among all events, the fatal landslide earthquake was located at Hsiaolin Village, where 474 people were buried. Inversion modelling of the seismic waveforms generated by the Hsiaolin landslide shows that the seismic source was represented by single force, consistent with downhill sliding. Some offshore seismic events indicate that submarine slumps probably occurred along submarine canyons and steep slopes. Our study shows that broadband seismic monitoring can be used to issue early warnings for mitigating disasters generated by inland and offshore landslides.

Split S waveforms observed in northern Taiwan: Implications for crustal anisotropy

Short-period stations ENT and NSK in northern Taiwan have frequently recorded split S waveforms from earthquakes in the underlying subduction zone. To determine splitting parameters together with objective, straightforward error estimations, we employ a waveform cross-correlation method. The error for each measurement is estimated by translating the lower limit of the 95% confidence interval of the cross-correlation coefficient into azimuth and time. Only measurements with 95% confidence regions excluding zero delay time are accepted as splitting data. We also use an energy minimization method to constrain the solutions with minor splitting. We interpret the splitting as caused by stress-aligned cracks in the upper crust. The average delay times are 0.03–0.04 s for ENT and 0.07 s for NSK. The fast directions at the two stations are predominantly N-S, which roughly match the stress trajectories predicted by a model simulating the arc-continent collision in the Taiwan area. However, at ENT there is a subset of data with fast directions oriented in roughly E-W. The nearly 90° rotation of the inferred crack orientation occurs in a horizontal distance of 10–15 km in the upper crust. Both the predicted heterogeneous stress regime and the local structural setting in northeastern Taiwan favor a lateral variation with a similar length scale.

Marine Cable Hosted Observatory (MACHO) Project in Taiwan

Taiwan is located in a junction corner between the Philippine sea plate and Eurasian plate. Because of active convergence, numerous earthquakes have taken place in and around Taiwan. On average, there are about two earthquakes greater than magnitude 6 each year and over 70% of earthquakes occurred in the offshore area. Because of the subduction of Philippine Sea Plate beneath the western end of the Ryukyu Arc and northern Taiwan, both the tectonics and seismic activity are intensive. The 2004 Sumatra earthquake has induced giant tsunami attacking coastal countries of South Asia. In a similar geodynamic context, the Sumatra event has aroused the attention of Taiwan government. Specialists from Taiwan earth scientists and ocean engineers have quickly teamed up to discuss the potential and mitigation of natural hazards from the western end of the Ryukyu subduction zone. To construct a submarine cable observatory off eastern Taiwan (MACHO project) was proposed. MACHO means a sea goddess who protects people at sea. The purpose of MACHO project has several folds. Firstly, the extension of seismic stations on land to offshore area can increase the resolution of earthquake relocating. Secondly, the extension of seismic stations may obtain tens of second before the destructing seismic waves arrive on land or tens of minute before the arrival of giant tsunami, which is helpful for earthquake or tsunami warning. Thirdly, the seafloor scientific station can monitor the active volcanoes in the Okinawa Trough, which is directly adjacent to the Ilan plain in northeastern Taiwan. Fourthly, the seafloor observatory can be used to continuously study the Kurosho current, off eastern Taiwan. The MACHO project has been granted for the fiscal year of 2007. The MACHO project is expected to be fulfilled in 2009.

Data Files from “CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake”

The Central Weather Bureau (CWB) of Taiwan completed a deployment of 1200 modern digital strong-motion instruments in 1996 at free-field sites and in buildings and bridges. Consequently, a very extensive set of strong-motion records were obtained for the M W = 7.6 Chi-Chi earthquake in 1999, including over 60 near-field records within 20 km of the fault ruptures. For documentation purposes, we included all relevant data files on the attached CD-ROM from our article, “CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake” in this issue. We presented the data in four different ways to make the data more user friendly: (1) the original recorded data by accelerograph type; (2) the processed data in ASCII text format; (3) the processed data in SUDS format; and (4) the processed …

Ground displacements around the fault of the September 20th, 1999, Chi-Chi Taiwan Earthquake

Ground displacements in the meizoseismal area of the September 20, 1999, Chi-chi earthquake are derived from accelerograms at 39 stations. On the hanging wall side a maximum horizontal, NW-directed motion of 8 meters and a vertical rise of 3.7 meters are found near the northern end of the N-S trending surface trace of the fault. On the footwall side and next to the fault a maximum horizontal motion of 1.3 meters and a maximum downward vertical motion of 1 meter are observed. The displacement field describes the source as a thrust fault with left-lateral motion. The displacements increase from south to north along the fault, and so does the left-lateral component of the fault motion, from about 10% to 60% of the total horizontal displacement. The recorded motions, especially ground velocities and accelerations, are noticeably stronger on the hanging wall side than on the footwall side.