| 14 May 2003 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in April 2003
There was no remarkable activity. to supplementary reports
*On April 29, there was a M5.9 earthquake off-shore east of Hokkaido. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred within the aftershock area of the 1994 Off-shore East Hokkaido earthquake (M8.2). to supplementary reports
*On April 17, there was a M5.4 earthquake at a depth of approximately 40 km off-shore east of Aomori prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates. to supplementary reports
*On April 8, there was a M4.6 earthquake at a depth of approximately 50 km in southwestern Ibaraki prefecture.
*On April 1, there was a M4.1 earthquake at a depth of approximately 10km near the border between Nagano and Gifu prefectures. The aftershock activity has mostly 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
*On April 2, there was a M4.2 earthquake at a depth of approximately 10 km in eastern Shimane prefecture. There was a M6.1 earthquake in this vicinity on June 4, 1978. to supplementary reports
*On April 12, there was a M4.8 earthquake at a depth of approximately 10km in northwestern Kagoshima prefecture. This event occurred near the west end of the aftershock area of the earthquake (M6.5) that occurred on March 26, 1997. The focal mechanism showed a strike-slip fault with a tension axis in a NNW-SSE direction, which is similar to the event in 1997. The activity had a mainshock-aftershock sequence, and is gradually declining. to supplementary reports
*On May 6, there was a M4.3 earthquake in nearly the same location as the April 8 earthquake mentioned above in the Kanto-Chubu region (southwestern Ibaraki prefecture, depth of approximately 50km).
*On May 10, there was a M4.5 earthquake at a depth of approximately 70 km under northwestern Chiba prefecture. And on May 12, there were M5.2 and M4.6 earthquakes at a depth of approximately 50km under the border between Chiba and Ibaraki prefectures.
| May 14, 2003 |
| Earthquake Research Committee |
52 earthquakes of M4.0 or greater occurred in Japan and the surrounding area during April (48 events occurred in March, and an average of 46 earthquakes per month have occurred over the past thirty years through the end of 2000). 4 of these earthquakes in April were M5.0 or greater. (4 events of M5.0 or greater occurred in March.)
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 no event of magnitude equal to or over M6.0 in April 2003.
The following regions have been noted as having significant seismic activity during the period from April 2002 to the end of March 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 April 17, there was a M5.4 earthquake at a depth of approximately 40 km off-shore east of Aomori prefecture.":
There was a M6.2 earthquake in this vicinity on August 14, 2001.
"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 April 2003. But the changes are becoming smaller compared to 2001.
(This evaluation is in agreement with the views presented at the procedural meeting on April 28 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, April 28, 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.
In addition, since around 20:00 on April 8, there have been irregular changes in the volumetric strain data at the Mikkabi station of the Japan Meteorological Agency. The changes accelerated temporarily from around 7:00 on April 9. Subsequently, the changes repeatedly accelerated and decelerated several times, before returning to normal levels around April 17. Currently, the strains have returned to a normal state. It is thought that these changes of strain were not caused by slip on the plate boundary, but by some local movements, such as landslides or movement of groundwater near the strainmeter."
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
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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