Ta-Liang Teng

A Virtual Subnetwork Approach to Earthquake Early Warning

Progress has been made toward the goal of earthquake early warning in Taiwan. By applying the concept of a virtual subnetwork (VSN) to the Taiwan Central Weather Bureau seismic network, the earthquake rapid-reporting time has been reduced to about 30 sec or less by a new system called VSN. This represents a significant step toward realistic earthquake early-warning capability. The VSN system described here was put into operation from December 2000 to June 2001. A total of 54 earthquakes (100% correct detection) were detected and processed successfully during this period. Comprehensive earthquake reports are issued mostly in less than 30 sec, with an average of about 22 sec after the origin time. The 22-sec reporting time will offer more than 20 sec of early-warning time to cities at distances greater than 145 km from the source, for which the shear-wave strong-shaking arrival time is about 44 sec. This 20 sec is an adequate amount of time to carry out numerous preprogrammed emergency-response measures prior to the arrival of strong shaking. Manuscript received 27 July 2001.

A Comprehensive Relocation of Earthquakes in Taiwan from 1991 to 2005

We have carried out a comprehensive relocation of a total of 267,210 earthquakes in Taiwan that occurred during the past 15 yr. We based our relocation process on the earthquake catalog of the Taiwan Central Weather Bureau Seismic Network (CWBSN) and made improvements in three aspects. First, we incorporated a large dataset of the S-P times from 680 Taiwan Strong-Motion Instrumentation Pro- gram (TSMIP) stations distributed throughout the island of Taiwan to improve the coverage of earthquakes on the island. Secondly, we added 18 Japan Meteorological Agency (JMA) stations in the southern Ryukyu Island chain to enhance the station coverage for eastern offshore events, especially around the subduction zone northeast of Taiwan. Thirdly, we adopted 3D VP and VP=VS models in predicting the travel times of P and S waves. The effectiveness of these improvements in earthquake re- location can be seen in three aspects: (1) the reduction in the residuals of P-wave arrival times and S-P times, (2) a better understanding of the attenuation relationship between the peak-ground acceleration and epicentral distance, and (3) the geologically meaningful patterns of station corrections to P-wave arrival times and S-P times. Online Material: Catalog of relocated earthquakes in Taiwan from January 1991 to

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

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.

3D Velocity Structure around the Source Area of the 1999 Chi-Chi, Taiwan, Earthquake: Before and After the Mainshock

3D V P and V P / V S models for the source area before and after the Chi-Chi earthquake are derived from a tomographic inversion of seismic travel-time data from the Taiwan Central Weather Bureau Seismic Network (CWBSN). Resulting 3D velocity models reveal a strong lateral variation in the crust, particularly at a depth range of 5 to 25 km along the trend of the dipping Chelungpu thrust. In general, low V P and high V P / V S anomalies are found in and around the source volume of the Chi-Chi earthquake, a volume centered around the entire 100 km × 40 km Chelungpu rupture surface. We pay special attention to the crustal properties and their changes due to the Chi-Chi event and the association of the crustal anomalies with seismicity. Combining the results with those of other geological and geophysical surveys, the crustal anomalies are interpreted as images of a highly fractured and fluid-filled Chi-Chi source region. Fluid overpressure is known to reduce the strength of the fault zone and in this case may have initiated the nucleation of the Chi-Chi earthquake. The V P models obtained before and after the Chi-Chi event show some interesting heterogeneities. Regions of low V P anomaly appear to have further expanded with the occurrence of the Chi-Chi earthquake. The V P / V S model shows high anomalies near the source rupture, especially near where the rupture is believed to have nucleated. After the Chi-Chi event, the V P / V S model shows a substantial change that may reflect an outward shift of the fluid-filled fractured source region. This shift is shown by a general expansion in V P / V S anomalies to a broader region (e.g., where two intensive aftershock clusters occurred) and a significant V P / V S reduction to more normal values near the Chelungpu rupture. We attribute these changes in V P and V P / V S anomalies, as well as the expansion and shift of the anomalous regions, to be a response to the localized stress change caused by the slip of the Chi-Chi rupture. The location of the seismic events is correlated with that of the crustal V P and V P / V S anomalies. Large events appear to occur in the transition zones of both V P and V P / V S anomalies. This finding may be of use in the identification of weakened crustal regions in a seismic zone, thus suggesting where impending large earthquakes are likely to occur. All input data and initial and final models of this article are given as electronic files in the CD attached to the end of this issue.

Variations of radon content in groundwaters and possible correlation with seismic activities in northern Taiwan

Radon (222Rn) concentration in geothermal waters and CO2-rich cold springwaters collected weekly in duplicate samples from four stations in northern Taiwan were measured from July 1980 to December 1983. Seven spike-like radon anomalies (increases of 2 to 3 times the standard deviation above the mean) were observed at three stations. Following every anomaly except one, an earthquake ofML above 4.6 occurred within 4 to 51 days, at an epicentral distance 14 to 45 km, and at a focal depth of less than 10 km. The distribution of the earthquakes preceded by radon anomalies is skewed in certain directions from the radon stations; the radon stations seem to be insensitive to earthquakes occurring in the other directions. At the fourth station, near a volcanic area, much gas (mainly CO2) is discharged from the well, together with hot water. A very high concentration of radon was detected in the discharged gas; therefore trapping of gas in the water can result in anomalously high radon contents. According to limited measurements, the radon concentration in water appears to be undersaturated with respect to that in gas. This suggests that hot water is very susceptible to radon loss, and monitoring of radon in gas is more desirable.

Preface to the 1999 Chi-Chi, Taiwan, Earthquake Dedicated Issue

The 1999 Chi-Chi, Taiwan, earthquake ( M w 7.6) struck central western Taiwan at 1:47 a.m. local time (on 21 September or at UTC 17:47 20 September). The causal fault was immediately identified to be the mapped active Chelungpu thrust fault as it produced a remarkable 100-km-long surface break, with fault scarps as high as 8 m in places. The nucleation point of this long rupture was defined by an epicenter at 120.82 °E and 23.85 °N, with a focal depth of 8 km. A first-motion solution was reported to be striking N20 °E, dipping 30 °SE with an average rake of 85° (Chang et al., 2000). The event was officially named after the nearest town Chi-Chi by the Central Weather Bureau (CWB) (Shin, 2000). This was the most devastating earthquake to hit Taiwan in modern times. The Chi-Chi earthquake and its energetic aftershock sequence inflicted a casualty toll of 2435 and an estimated US

Ten-second Love-wave propagation and strong ground motions in Taiwan

4 billion property loss (Tsai et al., this issue). This catastrophe was nonetheless fortunate in several aspects. The mainshock struck when schools were not in session; the loss of student lives would have been unthinkable in view of the total collapse of hundreds of school buildings. In emergency response, it was fortunate that the CWB completed in 1995 the Taiwan Rapid Earthquake Information Release System that was capable of electronically releasing pertinent information within a couple minutes of an earthquake occurrence. Rapid earthquake information greatly facilitated the disaster relief missions, and it mitigated further secondary losses. Scientifically, because of the completion of the Taiwan Strong-Motion Instrumentation Program (TSMIP), the occurrence of the Chi-Chi earthquake sequence has resulted in the richest seismological data recovery in the world to date, in both quality and quantity. These data were open to the world with a data CD of the …

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

Applying the surface-wave Gaussian beam method to a three-dimensional (3-D) structure of Taiwan, we have studied the generation and propagation of short-period (10-s) surface waves in a region of pronounced crustal heterogeneity, especially in a region defined to be at an epicentral distance larger than a few focal depths away, where high-frequency near-field motions are largely attenuated and surface waves are adequately developed. By perturbing the source model as well as the crustal model in an iterative inversion process, we have achieved an excellent fit to the recent strong-motion observations recorded during the Tung-Ao earthquake (June 5, 1994, ML = 6.0) in Taiwan. This allows us to (1) obtain a refined 3-D crustal model of Taiwan from an initial model that was derived from recent tomographic results and, more important, (2) explain the distribution patterns of the strong shaking reported in terms of intensity maps for historical large earthquakes, and predict the long-period strong-motion distribution patterns for future large earthquakes in Taiwan. With an improved 3-D structure, we have gained better insight into short-period surface-wave propagations as they are modified by crustal lateral heterogeneity. Focusing and defocusing are clearly demonstrated. Of particular interest is the Central Mountain Range, a massive Tertiary metamorphic body forming the backbone of Taiwan, that seems to serve as a “divergent lens” for the propagating surface waves, whereas the large sedimentary basin in southwestern Taiwan seems to focus the wavefronts instead. The computed distortions of the propagating surface-wave field due to lateral heterogeneity are correctly reflected by the observations.