16:15 〜 16:30
[SCG40-10] Seismic observation and automatic hypocenter location system in Nepal
キーワード：ネパール SATREPS、地震観測システム、自動震源決定システム
1. Introduction
SATREPS program of Integrated Research on Great Earthquake and Disaster Mitigation in Nepal was started in 2015. One of the objectives of this project is to enhance the eismic observation system in Nepal. Our group installed eight seismic stations in the area called central Himalayan seismic gaps. Seismic waveform data are collected in near real-time. We developed an automatic location system, which determines hypocenter locations, magnitude, and seismic intensity distribution within a few minutes after the felt earthquake occurrence. The computed earthquake parameters are disclosed on a website for the damage reduction by a larger earthquake. We present our group activity and scientific results from this subject.
2. Seismic observation system
We installed 8 seismic stations with three-component velocity and acceleration sensors. Broadband velocity seismometers were installed at four stations and 1 Hz seismometers were installed at remaining four stations. A modem with a SIM card was installed in the seismic observation device and continuous waveform data is transmitted every minute to a server at the Department of Mines & Geology (DMG) in Kathmandu. The observation device operates with a solar panel and a built-in battery. There is a function to update the software remotely from the cental server.
3. Automatic processing system
Since the seismic stations were installed in the area of central Himalayan earthquake gap, seismicityinside and around the seismic network is relatively low. For security reasons, we need to install seismometers inside the public facilities, such as schools and parks. Therefore, the noise level during the daytime is relatively high for most of the stations. We found that the events created by noise are several ten times larger than seismic events without a noise filter. The low seismicity and high noise level require a good algorithm of noise signal discrimination.
Since the seismic stations are located on the premises of schools, etc., the main source of noises is human activity. The noise durations are mostly 1-2 seconds or less and high-frequency components are dominant. We set the following criteria for the noise-signal discrimination and removed noise events after hypocenter location.
The event is classified as noise if:
(1) Standard error of arrival time residual > 2.0 seconds.
(2) No P-wave readings at the closest 3 stations if the number of P-wave readings (Np) < 5.
(3) No P wave reading at the closest station if Np < 4.
(4) Nerr>Np, where Nerr is the number of the inappropriate stations with (a) (f-P) time <(S-P) time and (b) predominant period(Tp) and epicentral distance(D) are following combinations; Tp>1.2 and D>100km, Tp>0.7 and D<100km, Tp>0.3 and D<50km. These parameters are set empirically by checking noise events and seismic events to eliminate noise events.
4. Result
Hypocenters are located automatically when more than three P-wave arrival times are observed. Many noise events are included in events, which are located by three P wave arrivals.However, if the number of P wave readings is 4 or more, the event is most likely an earthquake and accurate location can be determined. The processing results can be monitored on the website. When an earthquake with a magnitude > 4.5 occurs, our system displays the distribution of the MMI seismic intensity calculated from the epicenter location and magnitude.
In addition to the SATREPS seismic stations, DMG collects continuous waveform data from seismic stations installed by France, China, and RIMES in a mini-seed format. Currently, we are developing an automatic hypocenter location system using all the data.
SATREPS program of Integrated Research on Great Earthquake and Disaster Mitigation in Nepal was started in 2015. One of the objectives of this project is to enhance the eismic observation system in Nepal. Our group installed eight seismic stations in the area called central Himalayan seismic gaps. Seismic waveform data are collected in near real-time. We developed an automatic location system, which determines hypocenter locations, magnitude, and seismic intensity distribution within a few minutes after the felt earthquake occurrence. The computed earthquake parameters are disclosed on a website for the damage reduction by a larger earthquake. We present our group activity and scientific results from this subject.
2. Seismic observation system
We installed 8 seismic stations with three-component velocity and acceleration sensors. Broadband velocity seismometers were installed at four stations and 1 Hz seismometers were installed at remaining four stations. A modem with a SIM card was installed in the seismic observation device and continuous waveform data is transmitted every minute to a server at the Department of Mines & Geology (DMG) in Kathmandu. The observation device operates with a solar panel and a built-in battery. There is a function to update the software remotely from the cental server.
3. Automatic processing system
Since the seismic stations were installed in the area of central Himalayan earthquake gap, seismicityinside and around the seismic network is relatively low. For security reasons, we need to install seismometers inside the public facilities, such as schools and parks. Therefore, the noise level during the daytime is relatively high for most of the stations. We found that the events created by noise are several ten times larger than seismic events without a noise filter. The low seismicity and high noise level require a good algorithm of noise signal discrimination.
Since the seismic stations are located on the premises of schools, etc., the main source of noises is human activity. The noise durations are mostly 1-2 seconds or less and high-frequency components are dominant. We set the following criteria for the noise-signal discrimination and removed noise events after hypocenter location.
The event is classified as noise if:
(1) Standard error of arrival time residual > 2.0 seconds.
(2) No P-wave readings at the closest 3 stations if the number of P-wave readings (Np) < 5.
(3) No P wave reading at the closest station if Np < 4.
(4) Nerr>Np, where Nerr is the number of the inappropriate stations with (a) (f-P) time <(S-P) time and (b) predominant period(Tp) and epicentral distance(D) are following combinations; Tp>1.2 and D>100km, Tp>0.7 and D<100km, Tp>0.3 and D<50km. These parameters are set empirically by checking noise events and seismic events to eliminate noise events.
4. Result
Hypocenters are located automatically when more than three P-wave arrival times are observed. Many noise events are included in events, which are located by three P wave arrivals.However, if the number of P wave readings is 4 or more, the event is most likely an earthquake and accurate location can be determined. The processing results can be monitored on the website. When an earthquake with a magnitude > 4.5 occurs, our system displays the distribution of the MMI seismic intensity calculated from the epicenter location and magnitude.
In addition to the SATREPS seismic stations, DMG collects continuous waveform data from seismic stations installed by France, China, and RIMES in a mini-seed format. Currently, we are developing an automatic hypocenter location system using all the data.