| 11 June 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in May 2003
On May 26, there was a M7.0 earthquake at a depth of approximately 70km off-shore of Miyagi prefecture. This event had a maximum seismic intensity 6 Lower, and caused some damage. to supplementary reports
There was no remarkable activity. to supplementary reports
*On May 26, there was a M7.0 earthquake at a depth of approximately 70km off-shore of Miyagi prefecture. This event had a maximum seismic intensity 6 Lower. (see the 112th Evaluation of the Earthquake Research Committee "Seismic Activity off-shore of Miyagi prefecture on May 26, 2003") The activity had a main shock-aftershock sequence, and has been declining. So far, the largest aftershocks (M4.9) occurred on May 27 (maximum seismic intensity 4) and on June 10 (maximum seismic intensity 3).
According to the GPS data, so far, no significant postseismic movement can be seen following the event.
It is estimated that there is a 10 percent chance of occurrence of M5.0 or greater aftershocks within 3 days. If an aftershock with magnitude of about 5.0 occurs, there will presumably be shaking with a maximum seismic intensity 4. Also it is estimated that aftershocks of M3.6 or greater (approximately corresponding to events with intensity greater than1) may occur about once a day until the end of June, and subsequently about once every two days until the end of July.
The Earthquake Research Committee has expected the repeat of the 1978 Off-shore Miyagi prefecture earthquake to occur in the near future, however, the recent event is not thought to be this earthquake. (see the 112th Evaluation of the Earthquake Research Committee) It is thought that the recent earthquake caused only a slight amount of stress change in the region of the 'expected off-shore Miyagi prefecture earthquake', which means there is no direct effect on the occurrence of the expected earthquake. to supplementary reports
*On May 6, there was a M4.3 earthquake at a depth of approximately 45km in southwestern Ibaraki prefecture. On May 31, there was also a M4.0 earthquake at a depth of approximately 55km.
*On May 10, there was a M4.5 earthquake at a depth of approximately 70km under northwestern Chiba prefecture.
*On May 12, there was a M5.2 earthquake at a depth of approximately 45km under the border between Chiba and Ibaraki prefectures. The focal mechanism had a tension axis in a NNW-SSE direction. This event occurred within the Philippine Sea plate. There was a M4.6 aftershock 2 minutes later.
*On May 17, there was a M5.1 earthquake at a depth of approximately 45km in northeastern Chiba prefecture.
*On May 18, there was a M4.5 earthquake at a depth of approximately 5km in southern Nagano prefecture. The 1984 Western Nagano Earthquake (M6.8) occurred in this vicinity, and since then stationary seismicity has been observed. The seismicity increased after the current event, but the activity had a main shock - aftershock sequence, and accordingly subsided.
*The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing, although the rates seem smaller compared to 2001. to supplementary reports
There was no remarkable activity. to supplementary reports
There was no remarkable activity. to supplementary reports
*On June 5, there was a M4.2 earthquake at a depth of approximately 10km in the Kaga region of Ishikawa prefecture.
*There was a M6.0 earthquake on June 9, and a M6.2 earthquake on June 10 near Taiwan.
| June 11, 2003 |
| Earthquake Research Committee |
99 earthquakes of M4.0 or greater occurred in Japan and the surrounding area during May (52 events occurred in April, and an average of 46 earthquakes per month have occurred over the past thirty years through the end of 2000). 59 of the events were part of the seismic activity and the aftershocks of the event that occurred off-shore of Miyagi prefecture on May 26. 3 of these earthquakes in May were M5.0 or greater. (4 events of M5.0 or greater occurred in April.)
Earthquakes of M6.0 or greater occurred, on average, 15 times a year during 1998 to 2002. (Over the past 30 years through the end of 2000, on average, M6.0 or greater events have occurred approximately 16 times a year) There was 1 event of magnitude equal to or over M6.0 in May 2003 and there has been a total of 1 M6.0 or greater event by the end of April this year.
The following regions have been noted as having significant seismic activity during the period from May 2002 to the end of April 2003.
| -Ocean area east of Aomori prefecture | M5.9 on Oct. 14, 2002 (depth of approximately 50km) | |
| -Off-shore of Miyagi prefecture | M6.1 on Nov. 3, 2002 (depth of approximately 45km) | |
| -Hyuganada | M5.7 on Nov. 4, 2002 (depth of approximately 35km) |
There is no supplementary information for the Hokkaido region.
"On May 26, there was a M7.0 earthquake at a depth of approximately 70km off-shore of Miyagi prefecture. This event had a maximum seismic intensity 6 Lower. c It is thought that the recent earthquake caused only a slight amount of stress change in the region of the 'expected off-shore Miyagi prefecture earthquake', which means there is no direct effect on the occurrence of that earthquake.":
Aftershocks occurred in a north-south direction over a length of 20km. The main shock is located in the southern part of the aftershock active region. The largest aftershocks were M4.9 events that occurred on May 27 and on June 10, both around the north edge of the aftershock active region.
In addition, the long term values for the probability of earthquake occurrence, that were evaluated by the Earthquake Research Committee in 2001, have become higher in time. The following is a comparison of the values of earthquake occurrence probabilities calculated for the beginning of January 1, 2001 and for the beginning of June 1, 2003. The rise in percentages is natural increase because the occurrence of the Off-shore Miyagi prefecture earthquake has not happened during the 2 years and 5 months from the last evaluation time.
| evaluation time | in 10 yrs | in 20 yrs | in 30 yrs | elapsed time since last earthquake |
| January 1, 2001 | 26% | 81% | 98% | approximately 60% |
| June 1, 2003 | 39% | 88% | 99% | approximately 67% |
"The long-term changes that have been observed since 2001 in the continuous GPS data in the Tokai region, still appear to be continuing, although the rates seem smaller compared to 2001".
The Pacific side of the Tokai-Chubu region maintains a steady rate with movement to the WNW, consistent with the subduction of the Philippine Sea plate toward the northwest. The results from GPS observations in the area centered on western Shizuoka prefecture, have been showing a trend of moderate change since about April 2001. The trends have continued through May 2003, but the changes are becoming smaller compared to 2001.
(This evaluation is in agreement with the views presented at the procedural meeting on May 23 of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disaster. (see reference below))
(reference) "Recent seismic and crustal activity in the Tokai region and its vicinity" (Japan Meteorological Agency, Department of Seismology and Volcanology, May 23, 2003)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.
Although the seismicity directly under Hamana lake is declining, on the whole, the seismic activity is at a normal level.
The long-term changes in crustal movements in the Tokai region and its vicinity caused by the slow slip on the plate boundary, has been continuing, although recently the rates seems smaller compared to 2001."
- On May 31, there was a M4.5 earthquake at a depth of approximately 65km in Iyonada.
There is no supplementary information for the Kyushu-Okinawa region.
| Reference 1 Earthquakes that are described in the "Evaluation of Seismic Activity"(Monthly Report of Seismic Activity in Japan) are events of M6.0 and greater, or events of M4.0 and greater (M5.0 or greater off-shore) that have maximum intensity of 3 or greater. Reference 2 Information that is described in the "Supplementary Information to the Evaluation" includes:
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