Applicability of Mwp for broadband seismographs located close to the epicenter for Mar 11, 2011 Tohoku earthquake
Abstract
To issue rapid and accurate Tsunami Warnings it is necessary to estimate the location and size of the causative earthquake as soon as possible after the event begins. This is necessary in order to rapidly warn coastal residents in the near field of the rupture and sea floor deformation. Mwp has been used to issue Tsunami Warnings within 5 to 10 minutes of origin time, by both US Tsunami Warning Centers since about 2000. Because Mwp [Tsuboi et. al., 1995] is based on the far-field approximation to the P-wave displacement due to a double couple point source, we should very carefully apply Mwp to data obtained at distances of only about a fault length from the source. However, in the case of a huge earthquake, which occurs just offshore of populated area, like the 2011 Tohoku earthquake, it was reported that seismographs located at close epicentral distances were clipped by large surface waves arrivals. This causes problems when determining earthquake magnitude based on the amplitude of surface waves. Mwp could be useful even for these station located close to the epicenter, because we do not expect the P-waves will clip. We examine broadband STS-1 seismograms around the Japanese Islands, to determine if Mwp can usefully estimate magnitude of great earthquakes using the unclipped portion of broadband seismograms obtained close to the epicenter. Specifically in case of Tohoku, since the duration is less than 200 sec, if the time window to estimate Mwp is around 120-150 sec, there should be a good chance to get reliable Mwp value for epicentral distance 9 to 10 degree stations. Our observations indicate that Mwp gives reliable results as close as 8.3 degrees from the source. At 8.3-9.5 degrees, Mwp gives reliable results of Mw 8.9-9.1 for the Mw9.1 Off East Coat of Tohuku event of March 11, 2011, 4-5 minutes after origin time. We also have examined for the Mw8.8 Chile earthquake of February 27, 2010, and obtained Mwp8.8 at 8.9 degrees from the source, at 4 minutes after origin time. These results demonstrate that Mwp should be applicable to estimate earthquake magnitude promptly using seismographs located close to the epicenter even for magnitude 9 class events. We use broadband seismograms recorded by F-net broadband network operated by NIED in Tsukuba, Japan.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S51F..07H
- Keywords:
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- 7215 SEISMOLOGY / Earthquake source observations