Outline of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) —Earthquake Early Warning and observed seismic intensity—
The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) that occurred on March 11, 2011, caused strong ground motion around northeastern Japan. Before the strong ground motion hit cities, the Japan Meteorological Agency (JMA) issued Earthquake Early Warning (EEW) announcements to the general public of the Tohoku district and then the warning was automatically broadcast through TV, radios and cellular phone mails. The EEW was earlier than the S wave arrival and more than 15 s earlier than the strong ground motion (intensity 5-lower or greater on the JMA scale) everywhere in the district. Seismic intensity 7 was observed for only the second time since JMA introduced instrument-based observation for intensity measurements in 1996. Intensities of 6-upper and 6-lower were widely observed at many stations in the Tohoku and Kanto districts, over an area of approximately 400 km × 100 km. The duration of strong ground motions was quite long. For the Tokyo region, JMA EEW expected intensities of 4, which was an underestimation of the observed intensity (5-upper). This underestimation can probably be attributed to the large extent of the fault rupture.
Key wordsThe 2011 off the Pacific coast of Tohoku Earthquake strong ground motion seismic intensity Earthquake Early Warning
The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) occurred off the pacific coast of Japan on March 11, 2011, and caused a huge tsunami that killed more than 12,000 people, and left more than 12,000 people missing (Fire Disaster Management Agency, Japan, report of April 11, 2011). Strong ground motion was also recorded across a wide area of northeastern Japan. Just before the strong ground motion hit cities in the Tohoku district (northeastern Honshu Island), the Japan Meteorological Agency (JMA) issued the Earthquake Early Warning (EEW) announcements to the general public in the district. Seismic intensity of 7 (JMA scale) was observed, for only the second time since JMA introduced instrument-based intensity observations in 1996 (Hoshiba et al., 2010). Seismic intensities of 6-upper and 6-lower were also widely observed in the Tohoku and Kanto (central eastern Honshu) districts.
This paper outlines the warnings of strong ground motion (that is, EEW) and observations of the seismic intensity for the Mw 9.0 earthquake.
2. Observation of Seismic Intensity and Operation of JMA EEW
In Japan, the JMA intensity scale is widely used to measure seismic intensity. Since 1996 this scale has been based on instrumental measurements in which not only the amplitude but also the frequency and duration of the shaking are considered (JMA, 1996; Hoshiba et al., 2010). The 10-degree JMA intensity scale rounds off the instrumental intensity value to the integer. Intensities of 5 and 6 are divided into two degrees, namely 5-lower, 5-upper, 6-lower and 6-upper, respectively. Intensity 1 corresponds to ground motion that people can barely detect, and 7 is the upper limit. At present, seismic intensity is measured at more than 4,000 places throughout Japan by JMA, municipalities, local governments and the National Research Institute for Earth Science and Disaster Prevention (NIED). When an earthquake occurs, the intensity data are transmitted to JMA and summarized. The summary is broadcast through multiple media starting within two minutes after the earthquake, and it is updated as the data increase.
EEW aims at mitigating an earthquake disaster by giving people enough time to take appropriate safety measures in advance of strong shaking. It has been operational nationwide in Japan by JMA since October, 2007. For JMA EEW, the hypocenter is determined by a combination of several techniques (Hoshiba et al., 2008), using approximately 1,100 stations from the JMA network and the Hi-net network of NIED; magnitude is mainly from maximum displacement amplitudes (Kamigaichi et al., 2009). The JMA EEWs are divided into two grades: “forecast” and “warning” (Hoshiba et al., 2008; Kamigaichi et al., 2009; Doi, 2010). The EEW “forecast” is issued to advanced users when events are estimated to be M 3.5 or larger, or when the expected seismic intensity is 3 or greater. In the “forecast”, the regions are particularly specified where seismic intensity 4 or greater is expected. When intensity is expected to be 5-lower or greater at any observation station of the seismic intensity networks, “warning” is issued to the general public in regions where intensity 4 or greater is expected. The “warning” is broadcast in various ways, such as by TV, radio and cellular phone mails. JMA EEWs are updated as available data increases with elapsed time. Accordingly EEWs are issued repeatedly with improving accuracy. The “warning” is updated when the seismic intensity is expected to be 5-lower or greater in regions where the intensity was estimated to be less than 4 in the first “warning”: that is, where new regions are subject to shaking above the threshold. Even when the update causes the expected intensity to fall below 5-lower at any stations, the “warning” is not canceled so as to avoid confusion if the expected intensity rises again. In the updated “warning”, the newly added regions are described. At present operation, an update of the “warning” is given only when the elapsed time is less than 60 s from the first trigger, to avoid too late a warning due to the fluctuation of gradually increasing amplitude of later phases (e.g., M 5.8 events of May 8, 2008; JMA, 2008).
3. EEW during the Mw 9.0 Earthquake
4. Observed Seismic Intensity of the Mw 9.0
The EEW system expected intensity of 4 in the Tokyo region in the twelfth to fifteenth (final) issues (Fig. 2). This was an underestimation. Actual observations reached 5-upper, which is greater than the criterion of the EEW “warning”. The underestimation can probably be attributed to the large extent of the later fault rupture. For the northern part of Ibaraki prefecture (around IYASAT), where intensity expected in the first warning (fourth “forecast”) was less than 4, the expected intensity rose to 5-lower by the fourteenth “forecast”, but it was too late to update the “warning”, because it was issued 105 s after the trigger, which is later than the 60 s criteria at which upgrades are stopped.
5. Summary and Remarks
The 2011 Tohoku Earthquake (Mw 9.0) generated widespread strong ground motion, and seismic intensities of 6-lower and 6-upper were recorded in the Tohoku and Kanto districts over an area of approximately 400 km × 100 km. The durations of strong ground motion were very long.
The JMA EEW system issued one “warning” to the general public in the Tohoku district before the start of strong ground motion. It was earlier than the S wave arrival and 15 s earlier than strong ground motion (intensity 5-lower or greater) at the closest station to the epicenter.
After the mainshock, the EEW system did not work well for several hours because of high background noise from the coda waves of the mainshock and active aftershocks, and because of power failure and wiring disconnections. For several days, when earthquakes sometimes occurred simultaneously over the wide source region, the system became confused, and did not always determine the location and magnitude correctly. In 19 days from the mainshock to March 29, 2011, JMA appropriately issued EEW “warning” for 15 of the 22 events for which seismic intensity 5-lower or greater was actually observed. On the other hand, during the same time, 45 EEW “warnings” were issued, but actual observed intensities did not exceed 2 at any observation stations in 11 of the 45 events (JMA, 2011d).
The authors thank Dr. Stuart Weinstein, an anonymous reviewer, and Prof. K. Yomogida (editor) for their comments. Seismic intensity data were provided by JMA as well as NIED and local governments and municipalities. Waveforms were obtained from K-NET and KiK-net of NIED, and the unified hypocenter catalog and CMT catalog of JMA were used. We thank all of these entities for their effort in maintaining these observations and providing the data during very trying times. The JMA EEW uses a combination of several techniques developed by joint research with the Japan Railway Technical Research Institute, and also by NIED. It also uses real-time data from Hi-net of NIED in addition to JMA’s own network for hypocenter determination. Figures were made using Generic Mapping Tools (Wessel and Smith, 1995).
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