Results - A Case Study for M W 7.6 Chi-Chi Earthquake

5 A Case Study for M W 7.6 Chi-Chi Earthquake

5.3 Results

The raw records of the Chi-Chi earthquake were replayed in the proposed EEW system (Hsiao at al. 2011). The P arrival times of each station were used for locating the earthquake. The parameters Pd andτc of each station were used to estimate magnitude by

the empirical formulas of MPd (Hsiao et al. 2011) and Mτc (Wu et al. 2007). Figure 5-4 shows six progressive EEW reports. The first event report is available 11.7 seconds after the earthquake origin time. The reporting time is significantly reduced compared to the present average EEW reporting time of 20 seconds. Therefore, the radius of the warning blind zone is shortened from 70 km to about 40 km. The estimated earthquake location is quite satisfactory even in the first report. In each report, MPd are all smaller than 7.0, implying that Pd may saturate for large earthquakes. On the other hand, the estimated Mτc

between 7.2 and 7.5 is rather close to the reported Mw of 7.6.

Figure 5-3. System configuration for a case study of Chi-Chi earthquake. (A) Hardware and (B) software configurations of the CWB P-wave earthquake early warning system.

Figure 5-4. Simulation results for six stages after the earthquake occurrence. Open circles and stars indicate the epicenter of the Chi-Chi earthquake from the CWB catalog and simulations, respectively. Large triangles indicate the stations (types A and B) used in simulations. Tr is the reporting time after the earthquake occurrence, and D is the focal depth from simulations. M Pd

and Mτc represent the Pd magnitude andτc magnitude, respectively.

5.4 Summary

After learning the lesson of the 1999 Chi-Chi earthquake, Taiwan has improved the hardware of its seismic networks. The station density and the recording devices have been

gradually improved. Each station now is equipped with an uninterruptible power supply to provide steady electrical power in case of a power failure due to an electrical power tower collapse or a disconnected communication line. The real-time EEW system is easy to implement based on the Earthworm environment. Thanks to its open-source software, users can construct a user designed real-time seismic network and also can easily modify the Earthworm modules for their own data processing tasks. P-wave methods are an effective tool for EEW because only a few seconds of the initial portion of P-wave are needed. In the case of the Chi-Chi earthquake, the first report was generated in only about 12 seconds by the proposed EEW system. The use of the initial P-wave turns out to be a robust system even in those cases in which large ground-shaking may provoke data interruption. Wu and Kanamori (2005b) found the empirical relationship between the peak ground velocity (PGV) and Pd. By taking advantage of the PGV versus Pd surface trace of the rupturing fault for the Chi-Chi earthquake. Pd values are larger in the northern part of the fault plane, which is consistent with the rupture directivity of the earthquake. The results in Figure 5-4 suggest that Pd is not as sensitive asτc for the

large-magnitude earthquakes because of the saturation problem. The suggested upper limit of the Pd methods is about 6.5 MW (Wu et al. 2006; Wu and Zhao 2006). Estimated by the relationship of Pd and PGV (Wu and Kanamori 2005b, 2008a), the PGV of the Chi-Chi earthquake are underestimated again, suggesting the Pd saturation. The study of Lancieri and Zollo (2008) shows that extending the P-wave window to four seconds or more drastically reduces the saturation effect. We also tested the P-wave window at four seconds. We obtained an MPd of 6.9, suggesting that the saturation problem really is reduced. Nevertheless, MPd can build more magnitude redundancy into the EEW system for earthquakes with magnitudes less than 6.5 or 7.0 (it depends on the P-wave window).

In real-time operation, when MPd is determined to be larger than 6.5, Mτc will be used for early warning purposes.

79 Figure 5-5. Pd values of the Chi-Chi earthquake.

80 uncertainty of earthquake location is large. For example, the offshore earthquakes in Taiwan usually have large location error in the initial stage of the EEW updated procedure. Here we discuss the station coverage of the CWBSN.

Figure 6-1(a) shows the distance variations with six stations. The areas with red color means that within 25 km there at least six stations. These areas also represent the areas that the P-wave arrivals can reach at least six stations about 4 seconds. Figure 6-1(b) shows the GAP variations with six stations. Because the offshore areas have poor station coverage, the EEW system may take longer time to locate offshore events. This figure illustrates the weakness of our EEW system. Comparing Figure 6-1 to the Figure 6-2, it is clear that the areas with high potential of damage earthquake should be deployed more stations.

Figure 6-1. Station coverage and density. (a) Distance variations with six stations. (b) GAP variations with six stations.

Figure 6-2. Damage earthquakes in Taiwan (Hsiao et al., 2011).

6.2 Magnitude Saturation

Earthquake Early Warning (EEW) systems provide warnings to people or pre-programmed systems before the intense ground shakings may cause damage to target areas. With a timely issuance of earthquake information (location and magnitude) provided by EEW systems after large earthquakes, we can take immediate precautions against seismic hazards. Currently, the earthquake locations can be well determined by the P-wave arrivals obtained by dense stations around the source area (Rydelek and Pujol 2004; Satriano, 2008). However, the most challenging work in EEW system is to improve the reliability and accuracy of the empirical method for estimating earthquake magnitude since only the initial portion of seismic waves are used. Based on the precise magnitude and hypocenter estimates, the ground motion can be predicted reliably. On the other hand, overestimation or underestimation of earthquake magnitude may lead to releases of false or missed alarms, respectively, that would result in additional economic loss and societal impacts. For EEW purposes, it is necessary to detect earthquake magnitude in the beginning stage of the earthquake occurrence. However, the 2011, Mw 9.0 Tohoku earthquake demonstrated that for a large earthquake the magnitude cannot be determined by the signals of only the initial several seconds (Hoshiba and Iwakiri, 2011; Colombelli et al., 2012). The on-scale magnitude determination approaches such as W-phase fast source inversion (Kanamori and Rivera 2008; Duputel et al., 2012) and quick Mw

determination using total effective shakings (Wu and Teng 2004; Lin and Wu 2012) could be considered in the future system. It is recommended that the eastern Taiwan area need more stations for faster gathering more P-wave arrivals.

6.3 Multi-Events

The near real-time Peak Ground Acceleration (PGA) map can be obtained by the PSN within one minute from the occurrence of a large earthquake (Wu et al., 2002, 2013b;

Hsieh et al., 2014; Wu, 2014). By incorporating the PSN into the CWBSN, the ISN can generate a PGA map with more details. The PGA map can be used as an indicator for the most damaged areas, the rupture direction of the fault, and the potential aftershock distribution (Hsieh et al., 2014; Wu, 2014). Moreover, a dense seismic network provides another solution for earthquake magnitude determination. Using the distribution area of the PGA or the Pd is a quick and robust method for estimating earthquake magnitude (Lin and Wu, 2010; Lin et al., 2011). The EEW system can implement this approach without locating earthquakes. It means the source location error will not be included in the magnitude estimation procedure. In addition, this method is also quite useful for detecting consequent earthquakes and provide warnings, especially for two consecutive earthquakes occurred in a very short time. In this case it is difficult to detect clear P-wave onset time of each event because one event’s P-wave phase may be involved in the surface wave of other events. The CWBSN-EEW or ISN-EEW may failed to detect each earthquake due to phase picking problems. However, the distribution area of the PGA or the Pd can reveal the location and the size of the damage. With a real-time dense seismic network, these observable information will become readily available for the purpose of emergency response after the occurrence of a large earthquake.

6.4 Application to Earthquake Rapid Reporting System

Figure 6-3 shows the timeline of the 2015 Hualien earthquake. The first information was issued at 13.4 s after the earthquake occurrence. This is an early warning message that provide warnings to target areas at 50 km away from the epicenter. The following information were created by the Earthquake Rapid Report (ERR) system. The ERR system applied P- and S-wave auto-picking and used those arrivals for locating the earthquake. Meanwhile, the entire waveform records were used for estimating magnitude.

This Auto-Report was given in 51 seconds. People on duty in CWB manually checked the quality of waveforms and make sure the intensity of records are not affected by noise or spike. After this procedure, an official earthquake report was released in 3 minute and 17 seconds. Finally, the contour of intensity was given in about 8 minute.

Figure 6-3. Timeline of the 2015 Hualien earthquake.

The EEW system is the initial point in the procedure of earthquake information issuance, shown in Figure 6-4. In the data processing center the data acquisition modules receives real-time data streams from seismic stations maintained by CWB or external institutes. Although those data streams coming from different kind of sensors, taking advantage of the Earthworm software can integrate all of them in the same platform.

Meanwhile, the Earthworm software provides two types for serving data streams, WaveServer and WaveRing. The WaveServer stores data for a period of time. Thus, it is usually used for archiving event file. The WaveRing only stores latest data. Thus, it is usually used for real-time data processing.

The EEW system process real-time data from the WaveRing. If an earthquake was detected by the system, there are two procedures will be triggered. One is the EEW procedure. The EEW message will be sent by email, APP and user display. The other is the ERRS procedure. When the EEW message was sent to the ERRS, the ERRS will archive an event and process the file to obtain an auto-report like Figure 6-3. Then, the CWB staff will manually check the report and modify some unreasonable records. Finally, an official report will be released by Website, Fax, and TV.

6.5 Conclusions

In this study, the new Earthworm modules, pick_eew, tcpd and dcsn were created for EEW purposes. The pick_eew is able to detect P-wave arrival and estimated Pd value in the 3-s time window after P arrival. A set of parameters are used for automatically detecting the onset of P wave. It is necessary to have a series of offline test to determine those parameters for each station because the background noise and the instrument

sensitivity are different involved in each station. The tcpd module is able to determine location and magnitude of earthquakes using the P-wave arrivals and Pd values. The dcsn module receives earthquake information from the shared memory in the Earthworm system and creates XML formatted file for EEW issuance. Although the whole system is very simple, it indeed work very well for providing timely earthquake in formation after events occurrences. The online results from EEW system are display in web site, shown in Appendix D.

The Palert sensor is a low-cost accelerometer which can be installed and maintained easily. In this study an Earthworm module, named eew_svr, was created for receiving real-time data streams from all Palerts and transferring all of them into the Earthworm’s shared memory. In this way, it is possible to incorporate Palert Seismic Network into the Central Weather Bureau Seismic Network. Based on the integrated seismic network, EEW system can be implemented faster and more robust.

There are two reasons that the eBEAR system is able to be distributed to any seismic network all over the world. One is that the Earthworm is good at integrating different kinds of seismic sensors. The other is that the eBEAR system is based on Earthworm software. Currently, the eBEAR system has been distributed and tested in India, Korea and Pacific Tsunami Warning Center.

Figure 6-4. System architecture of the ERR system and EEW system.

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在文檔中 Earthworm平台應用於臺灣地震速報預警研究 (頁 75-0)