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2011年3月11日 東北地方太平洋沖地震
The 2011 off the Pacific coast of Tohoku Earthquake
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1.地震について (Earthquake Information)
This earthquake caused significant damage in over a large area of eastern Honshu. An earthquake early warning was issued to the public in Tohoku region about 8 seconds after the first P-arrival, which is 31 seconds after the origin time. The warning was received by cell phone, TV, and radio. There is no blind zone where no warning was received before S-wave arrival since the earthquake was fairly far offshore. The EEW system did not work for aftershocks during the first 3 hours after the mainshock, since many earthquakes were occurring simultaneously, causing problems for the system.----------------------------------------------------
Origin time: 2011/03/11,14:46:18.1
Latitude: 38.103 N
Longitude: 142.86 E
Depth: 24.
Mw magnitude: 9.0
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Table1. Earthquake information. (ref. JMA source mechanism)
Fig.1. Event location. The epicenter is about 150km offshore of Miyagi prefecture.
2.観測点分布 (Station Distribution)
Fig.2. Station Distribution. Circles show 50, 100, and 150km from the epicenter. (However, most of the strong motion data are not available due to tsunami damage and electric power off.)
3.緊急地震速報発信状況 (JMA Earthquake Early Warning Report)
-------------------------------------------------------------------[01] 11/03/11-14:46:19 01 M4.3 38.2E 142.7E 010km (11/03/11-14:46.46)
[02] 11/03/11-14:46:19 03 M5.9 38.2E 142.7E 010km (11/03/11-14:46.47)
[03] 11/03/11-14:46:19 04 M6.8 38.2E 142.7E 010km (11/03/11-14:46.48)
[04] 11/03/11-14:46:19 5- M7.2 38.2E 142.7E 010km (11/03/11-14:46.49)
[05] 11/03/11-14:46:19 04 M6.3 38.2E 142.7E 010km (11/03/11-14:46.50)
[06] 11/03/11-14:46:19 04 M6.6 38.2E 142.7E 010km (11/03/11-14:46.51)
[07] 11/03/11-14:46:19 04 M6.6 38.2E 142.7E 010km (11/03/11-14:46.52)
[08] 11/03/11-14:46:17 04 M7.2 38.1E 142.9E 010km (11/03/11-14:46.56)
[09] 11/03/11-14:46:16 5- M7.6 38.1E 142.9E 010km (11/03/11-14:47:03)
[10] 11/03/11-14:46:16 5- M7.7 38.1E 142.9E 010km (11/03/11-14:47:11)
[11] 11/03/11-14:46:16 5- M7.7 38.1E 142.9E 010km (11/03/11-14:47:26)
[12] 11/03/11-14:46:17 5+ M7.9 38.1E 142.9E 010km (11/03/11-14:47:49)
[13] 11/03/11-14:46:17 5+ M8.0 38.1E 142.9E 010km (11/03/11-14:48:06)
[14] 11/03/11-14:46:17 6- M8.1 38.1E 142.9E 010km (11/03/11-14:48:25)
[15] 11/03/11-14:46:17 6- M8.1 38.1E 142.9E 010km (11/03/11-14:48:37)
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Table2. Earthquake early warning reports. A warning for public was issued about 8 seconds after the first P-arrival, which is 31 seconds after the origin time (4th report in red color).
Information ID, Estimated origin time, Maximum Intensity, JMA magnitude, Epicentral latitude, longitude, depth, and time to issue the report from the left.(ref. The Committee of Earthquake Observation and Research in the Kansai Area)
--> EEW reported by JMA.
Earthquakre early warning on TV (from youtube)
4.速報の猶予時間 (Warning Time)
Fig.3. Warning time. (Adapted and extended from JMA website)
5.観測記録 (Observed Seismic Intensity)
Fig.4. Observed JMA seismic intensity. (ref. JMA website) Intensity 7 was observed at one station in Kurihara city, Miyagi prefecture.
The table below shows a comparison of JMA and MM intensities.
Fig.5. Population exposure. A circle size shows the population at the town, and a circle color shows the seismic intensity at the town. 80,000 people experienced JMA intensity 7, 510,000 people experienced 6+, and 4,250,000 people experienced 6-.
6.予測震度 (Estimated Seismic Intensity)
Fig.6. Estimated JMA seismic intensity computed from the last warning. A black circle shows estimated fault dimension (current system assumes a point source). Compared with Fig.4, the estimated intensity was underestimated in Tohoku and Kanato region due to; 1) smaller estimated magnitude (Mj8.1 but actually Mj9.0), 2) smaller fault dimension (i.e. longer fault distance), and 3) no consideration of finite fault. People in Kanto region may not receive any waning since the criteria of warning is seismic intensity > 5-.
We used information on JMA website.
Referebce:
- Earthquakre early warning for other large earthquakes
- Masumi Yamada