| 13 March 2002 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in February 2002
On February 12, there was an M5.5 earthquake at a depth of approximately 50 km in the sea off Ibaraki Prefecture. The event registered a maximal seismic intensity of 5 lower, and there was attendant damage. to supplementary reports
There was no marked activity. to supplementary reports
*On February 13, there was an M4.5 earthquake at a depth of approximately 10 km in northern Miyagi Prefecture. The focal mechanism was a reverse fault type with a NNW-SSE pressure axis.
*On February 14, there was an M5.0 earthquake at a depth of 60 km in the sea east of Aomori Prefecture. to supplementary reports
*On February 2, there was an M4.3 earthquake at a depth of approximately 10 km in the Chuetsu region of Niigata Prefecture.
*On February 5, there was an M4.4 earthquake at a depth of approximately 70 km in southwestern Ibaraki Prefecture.
*On February 11, there was an M5.0 earthquake at a depth of approximately 40 km in the sea off Ibaraki Prefecture (in the sea approximately 20 km east of Inubozaki).
*On February 12, there was an M5.5 earthquake at a depth of approximately 50 km in the sea off Ibaraki Prefecture (in the sea approximately 30 km east of Hitachi). The event registered a maximal seismic intensity of 5 lower and there was attendant damage. The focal mechanism was a reverse fault type with a WNW-ESE pressure axis, and same as the focal mechanism of past activity in this neighborhood. It is believed that this event occurred near the juncture of the subducting Pacific plate and the continental plate. Aftershock activity had nearly ceased by the end of the month.
*Low levels of seismic activity and crustal movement continue in the area from Miyakejima to Niijima-Kodushima.
*Crustal seismic activity in Suruga Bay and the surrounding area is at a usual level or slightly higher than that.
The slight changes of crustal movements centered in western Shizuoka Prefecture, which were seen in the results of GPS observation of the Tokai region, still appear to continue. to supplementary reports
*There has been continued microearthquake activity at a depth of approximately 10 km near the border of Wakayama and Nara Prefectures since May 2001, and the activity level increased in January. The level of activity since then has gradually declined, and the active zone has not increased in size. to supplementary reports
There was no marked activity. to supplementary reports
Other Regions
*On February 2, there was a deep M6.0 earthquake at a depth of approximately 410 km near Vladivostok. to supplementary reports
*On March 6, there was an M4.5 earthquake at a depth of approximately 15 km in eastern Shimane Prefecture.
*On March 7, there was a deep M6.0 earthquake at a depth of approximately 480 km in the southern Sea of Okhotsk.
*On March 11, there was an M4.1 earthquake at a depth of approximately 10 km in northern Tokushima Prefecture.
| March 13, 2002 |
| Earthquake Research Committee |
There were 34 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 63 in January and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 6 events were M5.0 or greater (as compared to 6 in January).
The average number of M6.0 or greater earthquakes in the period from 1998 to 2000 was 16 per year (the 30-year average up to 2000 was also approximately 16 per year). There was 1 such event in February 2002 (there were none in January).
The following are evaluations of major seismic activity in the period from February 2001 to the end of January 2002.
| -Near Niijima-Kozushima | February 13 2001, an M3.9 earthquake (at a depth of about 10 km) | |
| -Akinada "2001 Geiyo earthquake" | ||
| March 24 2001, M6.7 (at a depth of about 50 km) | ||
| -Central Shizuoka Prefecture | ||
| April 3 2001 M5.1 (at a depth of about 35 km) | ||
| -Southern interior Iwate Prefecture | ||
| December 2 2001 M6.4 (at a depth of about120 km) | ||
| -Western Kanagawa Prefecture | December 8 2001 M4.5 (at a depth of about25 km) | |
| -Amami Oshima | December 9 2001 M5.8 (at a depth of about40 km) | |
| -Sea near Yonagunijima | December 18 2001 M7.3 (at a depth of about10 km) | |
There is no supplementary information for the Hokkaido region.
There is no supplementary information for the Tohoku region.
"Crustal seismic activity in Suruga Bay and the surrounding area is at a usual level or slightly higher than that.":
A long-term review of activity shows a somewhat reduced incidence of activity since around 1996, with an even greater reduction beginning around October 2000. But in the past half year, there seems to have been a measurable increase in seismic activity within the crust, representing a return to the activity level seen prior to October 2000. But in the past half year, there seems to have a measurable increase in seismic activity, and the level has become as same as before October 2000.
In addition, seismic activity within the subducting Philippine Sea plate (slab) is in slightly lower than normal.
"The slight changes of crustal movements centered in western Shizuoka Prefecture, which were seen in the results of GPS observation of the Tokai region, still appear to continue.":
The results of GPS observations in an area centered in western Shizuoka continued to show a trend toward moderate change, which began in April 2001, involving movement at a fixed rate to the WNW in conjunction with the subduction of the Philippine Sea Plate to the northwest, on the Pacific side of the Tokai-Chubu region. There appeared to be some slight trend toward change in this movement since April 2001, which has continued through February 2002. However, there are no signs that the changes are accelerating.
(This evaluation is in agreement with the views (see reference below) expressed at the February 25 procedural meeting of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disaster)
(reference) "Recent seismic and crustal activity in the Tokai Region" (Meteorological Agency, Department of Seismology and Volcanology, February 25 2002)
"Seismic activity within the slab has been slightly low recently in the Tokai region.
Crustal seismic activity in Suruga Bay and the western coast is now occurring at a normal or slightly higher than normal level.
Additionally, regarding crustal movement in the Tokai and surrounding regions, the results of observations by the Geographical Survey Institute indicate some change over the long term since last year. At present, the activity appears to be continuing as before. There have been no indications of direct linkage with Tokai earthquake."
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
There was the following activity in the Kyushu-Okinawa Region:
*Aftershocks of the earthquake (M5.8) that occurred on Amami Oshima on December 9, while decreasing, continue.
*Aftershocks to the earthquake (M7.3) that occurred near Yonagunijima on December 18, while decreasing, continue.
Supplement
"On March 6, there was an M4.5 earthquake at a depth of approximately 15 km in eastern Shimane Prefecture.":
This event was an aftershock to the 2000 Western Tottori Earthquake.(M7.3)
There was the following additional activity since the beginning of March:
*Microseismic activity in northern Tochigi Prefecture since the end of February. The maximal event to date was an M3.5 event on March 10.
Correction
There was an error in the citation information in section 2.3 (Seismic activity by region: Kanto-Chubu Region) of the supplementary explanations to the report on Seismic Activity in January 2002, issued on February 13, which should be corrected as follows:
Page 3, line 39
Incorrect: Although the slight changes ... fluctuated and showed some indications of lessening, since that time they appear to have continued as before.
Correct: Although the slight changes ... fluctuated and showed some indications of lessening, since that time the activity appears to have continued as before.
| Reference Material 1 Standards to select earthquakes which are to be described in "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) Events of M6.0 or greater. Or, M4.0 or greater (M5.0 or greater at sea) and having a maximum intensity of 3 or greater. Reference Material 2 Standards to describe additional explanation in "Supplementary repot of the evaluation." 1 Reference items relating to seismic activity described in "Evaluation of Seismic Activity". 2 Activity related to seismic activity (of the past 12 months or so) described in "Major Seismic Activity in Evaluation of Seismic Activity." 3 Activity that have been evaluated but not described in "Evaluation of Seismic Activity" because of being within the bounds of normal activity or because the activity is not prominent. |