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SS1-005
14:20
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14:40
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DEVELOPMENTS OF THE SEISMIC MONITORING IN TAIWAN AFTER THE 1999 CHI-CHI EARTHQUAKE
After the 1999 Chi-Chi earthquake, for enhancing the capabilities of seismic monitoring in Taiwan, 20 years’ continuous efforts have been made at the Central Weather Bureau (CWB). There have been great progresses and major accomplishments include the modernization of seismic instruments and the improvement of monitoring operations. Nowadays, there are three major seismic networks in operation. These networks are the Central Weather Bureau Seismographic Network (CWBSN), the Taiwan Strong Motion Instrumentation Program network (TSMIP), and the Taiwan Geophysical Network for Seismology (TGNS). Based on the stable running of high-density networks, very large quantities of seismic data have been collected and analyzed. Moreover, from the seismological researches and seismic hazard mitigation points of view, essential services have been developed and operated. Through the improvement of resolution, density, and coverage of seismic network, the more complete earthquake catalog is compiled. Currently, 40,000 events could be averagely detected per year and the cumulative amount has achieved 600,000. The valuable data are not only utilized to understand the seismic activities, the seismotectonics around the Taiwan region were researched in detail as well. Earthquake is one of the most threaten natural disaster in Taiwan. In order to provide effective information of strong earthquakes, three main reporting systems are developed and operated now. While a notable earthquake occurred in or very near Taiwan, the earthquake early warning (EEW) system can acquire the information and issue the warning message in about 15 seconds averagely. Then, the general public could be notified through the message from Internet applications, Cell Broadcast (CB) on mobile device, TV, and so forth. About 5 minutes later, an official earthquake report is generated by earthquake report releasing system. Quick confirmed earthquake report is crucial for the emergent responses of society, and meanwhile, reduce the social panic effectively. Besides, more detailed and observed seismic intensities around strong shaking areas could be quickly provided by the township intensity reporting system. Consequently, the reporting capability of felt earthquake information for seismic hazard mitigation is effectively strengthened. After the Chi-Chi earthquake, the earthquake precursory studies have been emphasized as well. In cooperation with scholarship, various methods were developed and tested. The data observed and analyzed include seismicity, crustal deformation, ionospheric electron density, geomagnetism, groundwater, and geoelectricity.
Nai-Chi Hsiao
earthquake catalog, felt earthquake information, precursory study, seismic data
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SS1-003
14:40
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15:00
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CGS’S INVESTIGATIONS ON ACTIVE FAULT IN TAIWAN SINCE 1997
Locating in an ongoing orogenic belt, there are frequent seismic activities and many active faults in Taiwan. Large earthquakes are usually accompanied with the recurrent movement of active faults, which may cause surface fractures and severe hazards. The 1995 Kobe Earthquake in Japan gave the lesson that the study of active fault was also a critical issue in Taiwan. Central Geological Survey (CGS), as the responsible government unit, proposed the first program in 1997 and started the systematic investigations on active fault. In the beginning, the progress was slow due to the limited resource. In 1998, CGS published the 1st edition of Active Fault Map of Taiwan (scale 1: 500,000), compiling the existing references. The 1999 Chi-Chi Earthquake marked the turning point in earthquake geology studies in Taiwan. With strong government support and community awareness, numerous researchers devoted themselves to relative fields. The 2nd edition of Active Fault Map of Taiwan (scale 1: 500,000) was soon published in 2000, only one year after the Chi-Chi earthquake. CGS also proposed new projects continuously and introduced several leading-edge methods to get the locations, displaced landforms, structural characteristics, slip rates and paleoearthquake events of the active faults. The strip maps of each individual active fault (scale 1: 25,000) were then completed region-by-region during 2007 to 2009. In CGS’s most updated 2012 edition of Active Fault Map of Taiwan (scale 1: 500,000), 33 active faults were identified. All CGS’s investigations were opened to the public and provided for governments’ reference in policy making. Based on the Geology Act issued and effective in 2011, the land development in four types of geologically sensitive areas would have to undergo a specific site investigation in advance. From 2014 to 2018, 17 active fault geologically sensitive areas have been announced by MOEA.CGS’s further investigations will focus on the fault activity and earthquake probability assessment for each individual active fault and will make more efforts on those probable and possible active faults. We hope that the risk of fault activities and disastrous earthquakes can be reliably assessed and managed to reduce the threats and losses in the near future.
Shih-Ting Lu, Chii-Wen Lin, Po-Tsun Chen, Yen-Chiu Liu
1999 Chi-Chi Earthquake, active fault, geologically sensitive area, Geology Act
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SS1-006
15:00
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15:20
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SEISMOGENIC STRUCTURES IN WESTERN TAIWAN
After the 1999 Chi-Chi earthquake, several damaging earthquakes occurred in Taiwan, such as 2010 Chiashian earthquake, 2013 Nantou earthquake, 2016 Meinong earthquake, and 2018 Hualien earthquake. Except for the 2018 Hualien event, all the other earthquakes resulted from the activations of blind faults which had not been recognized before the earthquake occurred, indicating that recognition of concealed active structures is an important issue for the earthquake hazard assessment in Taiwan. The 1999 Chi-Chi earthquake is an important landmark for the active fault or earthquake studies in Taiwan, not only increases of research funding invested from the government after the Chi-Chi earthquake, but the destructive earthquakes occur continuously that provided valuable information about crustal deformation characteristics. Here we would like to reveal those active structures in western Taiwan and discuss what we have learned after the Chi-Chi earthquake and what we need to pay more attention in the future. The Chi-Chi earthquake sequences and the following 2013 Nantou earthquake provided a unique opportunity to identify the detachment geometry which can extend to ca. 15-18km in depth. In addition, many structures were triggered by the 1999 earthquake such as the Meishan fault, Tuntzuchiao fault, and the other unknown blind faults that could help us to recognize the possible seismogenic structures. In southwestern Taiwan instead of the NE trending active thrust, the preexisting E-W to NW trending structures play an important role for the crustal deformation. How the preexisting structures interact with regional stress also will be discussed? In southern Taiwan, we recognize an important tectonic boundary, Tainan-Taitung Transfer zone (3T zone), which divides the tectonic process from collision to subduction and can be distinguished by different seismogenic structure and geological structures across this boundary and the 2016 Meinong earthquake could be related with this 3T zone.
Ruey-Juin Rau, Yuan-Hsi Lee, Yi-Ying Wen, Strong Wen
SEISMOGENIC STRUCTURES
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B3-011
15:20
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15:35
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CO- AND POST-SEISMIC RESPONSES IN AMBIENT SEISMIC VELOCITY TO THE 1999 M W 7.6 CHI-CHI EARTHQUAKE IN CENTRAL TAIWAN
The 1999 M w 7.6 Chi-Chi earthquake ruptured about 100 km of ground surface in Central Taiwan, and produced hydrologic changes throughout Taiwan. In this study, we collect near two decades of continuous seismic records from the Broadband Seismic Array for Taiwan (BATS) network. We calculate cross-correlations of ambient seismic noise at the stations and analyze the temporal changes in the coda of the resulting empirical Green's functions. From the 17 seismic stations that are available in the Incorporated Research Institutions for Seismology (IRIS), we find that 6 stations have significant velocity drop immediately following the Chi-Chi earthquake. The closest station, SSLB (< 5 km from the epicenter) recorded the largest change in seismic velocity up to 0.45%. In general, there is much greater velocity drop north of the Chi-Chi surface rupture, even for station TATO (0.2%) that is ~100 km north of the north end of the surface rupture. In contrast, station TPUB that is ~10 km south of the south-most surface rupture shows only 0.08% velocity drop. The ambient seismic velocity drop is highly correlated with peak ground velocity (PGV) which, in turn, was significantly affected by the northward seismic directivity. We also observe gradual recovery of ambient velocity after the eartqhauke for most of the stations. This may suggest healing of the upper crust and is consistent with observations of hydrologic recovery after the earthquake. However, the ambient velocity of station SSLB, where the maximum velocity drop occurred, remains ~0.2% lower than pre-Chi-Chi level. This may suggest permanent damage or crustal movement in the near-field. We also find seasonal modulation in ambient seismic velocity that correlates with the mean annual precipitation in Taiwan. Our observations show how upper crustal material properties are affected by earthquakes and rainfall.
Mong-Han Huang, Avinash Nayak, Noah Randolph-Flagg
1999 Chi-Chi Earthquake, hydrology, seismic interferometry, Seismology
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B3-016
15:35
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15:50
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THE STRUCTURE CHANGES OF THE 1999 CHI-CHI EARTHQUAKE FROM 4D TOMOGRAPHY
Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. While there is growing number of observations on co-seismic velocity reductions, detailed observations of rock healing are generally lacking. Here we apply joint tomography method for Pg-wave travel time data at different time periods before and after the Chi-Chi earthquake to find their co-seismic velocity changes and post-seismic velocity healing. The preliminary results will be shown in the meeting.
Shunping Pei, Xiaotian Xue
1999 Chi-Chi Earthquake, 4D Tomography, Velocity Changes
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