Improved centroid moment tensor analyses in the NIED AQUA (Accurate and QUick Analysis system for source parameters)
Abstract
The rapid determination of hypocentral parameters and their transmission to the public are valuable components of disaster mitigation. We have operated an automatic system for this purpose—termed the Accurate and QUick Analysis system for source parameters (AQUA)—since 2005 (Matsumura et al., 2006). In this system, the initial hypocenter, the moment tensor (MT), and the centroid moment tensor (CMT) solutions are automatically determined and posted on the NIED Hi-net Web site (www.hinet.bosai.go.jp). This paper describes improvements made to the AQUA to overcome limitations that became apparent after the 2011 Tohoku Earthquake (05:46:17, March 11, 2011 in UTC). The improvements included the processing of NIED F-net velocity-type strong motion records, because NIED F-net broadband seismographs are saturated for great earthquakes such as the 2011 Tohoku Earthquake. These velocity-type strong motion seismographs provide unsaturated records not only for the 2011 Tohoku Earthquake, but also for recording stations located close to the epicenters of M>7 earthquakes. We used 0.005-0.020 Hz records for M>7.5 earthquakes, in contrast to the 0.01-0.05 Hz records employed in the original system. The initial hypocenters determined based on arrival times picked by using seismograms recorded by NIED Hi-net stations can have large errors in terms of magnitude and hypocenter location, especially for great earthquakes or earthquakes located far from the onland Hi-net network. The size of the 2011 Tohoku Earthquake was initially underestimated in the AQUA to be around M5 at the initial stage of rupture. Numerous aftershocks occurred at the outer rise east of the Japan trench, where a great earthquake is anticipated to occur. Hence, we modified the system to repeat the MT analyses assuming a larger size, for all earthquakes for which the magnitude was initially underestimated. We also broadened the search range of centroid depth for earthquakes located far from the onland Hi-net network. After implementing the above improvements, the CMT solution for the 2011 Tohoku Earthquake was successfully determined with a moment magnitude (Mw) of 8.6 (9.04 × 10^21 Nm). The focal mechanisms and centroid depths of the 2011 Tohoku Earthquake and M>7 aftershocks, as obtained using the improved system, are in agreement with those from the GlobalCMT. The sizes of these earthquakes are also consistent with those of GlobalCMT, with differences of less than Mw 0.1 except for the mainshock (Mw9.1, 5.31 × 10^22 Nm, GlobalCMT). This discrepancy may indicate that the bandwidth used in the analysis is insufficient for an earthquake of this size. To address this shortcoming, we used 0.0025-0.0100 Hz records and obtained a magnitude of Mw8.9 (3.35 × 10^22 Nm). This result is consistent with the GlobalCMT and other results (e.g., Mw 9.0, 3.43 × 10^22 Nm reported by Ozawa et al., 2011; Mw9.0, 4.42 × 10^22 Nm reported by Suzuki et al., 2011). Using the improved system, the CMT analysis for the 2011 Tohoku Earthquake is estimated to be completed within 12 minutes of the origin time.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.S54C..08K
- Keywords:
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- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology;
- 7294 SEISMOLOGY / Seismic instruments and networks