| 9 January 2002 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic activity in December 2001
On December 2nd, there was an intermediate-depth M6.4 earthquake with a maximum seismic intensity of 5 lower in southern interior Iwate Prefecture. And, on December 9th, there was an M5.8 earthquake at a depth of approximately 40 km, near Amami Oshima, which registered a maximum seismic intensity of 5 upper. There was attendant damage to both events.
In addition to these events, an M4.5 earthquake that occurred at a depth of approximately 25 km on December 8th in western Kanagawa Prefecture also registered a maximum seismic intensity of 5 lower. And, on December 18th, there was an M7.3 earthquake at a depth of approximately 10 km in Okinawa Prefecture in the sea near Yonagunijima, and a tsunami was observed in Okinawa Prefecture. to supplementary reports
There was no marked activity. to supplementary reports
*On December 2nd, there was an intermediate-deep M6.4 earthquake at a depth of approximately 120 km (see supplementary information, item 3) in southern interior Iwate Prefecture. This event registered a maximum seismic intensity of 5 lower, and there was attendant damage. Aftershock activity was low.
*On December 8th, there was an M4.5 earthquake at a depth of approximately 5 km near the border of Fukushima and Tochigi Prefectures (the Nakadori region of Fukushima Prefecture). There was an M4.1 earthquake in the same area on the 16th. to supplementary reports
*On December 8th, there was an M4.5 earthquake at a depth of approximately 25 km in western Kanagawa Prefecture, which registered a maximum seismic intensity of 5 lower. Aftershock activity was low.
*There were two transient periods of seismic activity in the area from Miyakejima to Niijima-Kodushima; the period from December 9 to 10 in the sea west of Niijima, activity included an M4.5 earthquake, and the period from December 28 to 29, which included an M3.3 earthquake in the sea northeast of Kodushima. The results of recent GPS observations of this area indicate that crustal movements have not entirely stopped, with changes continuing at a regular rate. However, there have been no new changes in response to the recent transient activity.
*On December 28th, there was an M4.2 earthquake at a depth of approximately 5 km near the border of Fukui and Shiga Prefectures (northern Shiga Prefecture), and there was attendant damage.
*Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but now appears to be returning to normal activity.
The slight changes of crustal movements centered in western Shizuoka Prefecture continue, which were seen in the results of GPS observation of the Tokai region. However, there is a sign of declining in the rate of the changes recently. to supplementary reports
There has been continuing microearthquake activity near the border of Wakayama and Nara Prefectures since late May 2001(see supplementary information). to supplementary reports
*On December 9th, there was an M5.8 earthquake at a depth of approximately 40 km near Amami Oshima (the sea near Amami Oshima), which registered a maximum seismic intensity of 5 upper. There was attendant damage. It is believed that this event occurred on the landward side within the plate. The focal mechanism had an east-west pressure axis. There was an M4.0 earthquake approximately 20 minutes before the M5.8 event. This seismic activity followed a foreshock-main shock-aftershock pattern. The subsequent aftershock activity declines with fluctuation.
*On December 18th, there was an M7.3 earthquake at a depth of approximately 10 km in the sea near Yonagunijima. A tsunami generated by this event was observed in Okinawa Prefecture. It is believed that this earthquake occurred at a shallow depth within the plate, landward of the Nansei Shoto trench. The focal mechanism was a normal fault type including a strike-slip component with an east-west extension axis. This seismic activity followed a foreshock-main shock-aftershock pattern. The subsequent aftershock activity declines with fluctuation, which is roughly the same as the average for Japan and the surrounding area. The largest aftershock, an M5.6 event, occurred on December 28. to supplementary reports
On January 4th 2002, there was an M4.0 earthquake at a depth of approximately 10 km near the border of Wakayama and Nara Prefectures. There has been continuing microearthquake activity in this area since late May 2001, including 1 to 2 M3.0 or greater events per month in the period up till December 2001. The activity increased in January 2002, with 15 M3.0 or greater events on January 4th. Based on a review of prior seismic activity in this area, it is believed that the present activity may continue with fluctuations in intensity for some time. to supplementary reports
| January 9, 2002 |
| Earthquake Research Committee |
There were 105 M4.0 or greater earthquakes in Japan and the surrounding area (as compared to 47 in November and a monthly average incidence of 46 in the thirty-year period up to the end of 2000). Of that number, 8 events were M5.0 or greater (as compared to 10 in November).
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 were two such events in December; there have been 11 such events in the period from January - December 2001. On July 3rd, there was a moderately deep M6.7 earthquake in the Mariana Islands (approximately 600 km south of Chichijima), which registered a maximum seismic intensity of 1 in Yokohama and other locations. If this event is included, the total number of M6.0 or greater earthquakes in 2001 is 12.
The following are evaluations of major seismic activity in the period from December 2000 to the end of November 2001.
| -Chuetsu region of Niigata Prefecture | 2 January 2001 M4.4 (at a depth of about 10 km) | |
| -Chuetsu region of Niigata Prefecture(about 40 km SSE of the 1/2/2001 event) | ||
| 4 January M5.1 (at a depth of about 10 km) | ||
| -Northern Hyogo Prefecture | 12 January 2001 M5.4 (at a depth of about 10 km) | |
| -Near Niijima-Kodushima | On February 13th 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 3rd 2001 M5.1 (at a depth of about 35 km) | ||
There is no supplementary information for the Hokkaido region.
There was the following seismic activity in the Tohoku region:
- On December 23rd, there was an M4.5 earthquake at a depth of approximately 100 km in Sendai Bay.
"On December 28th, there was an M4.2 earthquake at a depth of approximately 5 km near the border of Fukui and Shiga Prefectures (northern Shiga Prefecture), and there was attendant damage."
In this area, an M4.0 earthquake occurred on April 16th 2001 approximately 3 km northeast of the epicenter of the December 28th event, and there has been continuing microearthquake activity since August 2001, including a maximal M3.1 event, approximately 7 km to the north.
"Crustal seismic activity in Suruga Bay and the surrounding area had been at a slightly low level, but now appears to be returning to its previous level of activity."
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. This trend has continued in December.
"The slight changes in crustal movements centered in western Shizuoka Prefecture, which were seen in the results of GPS observation of the Tokai region continue, but the rate of the changes may be lower than before."
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. This trend has continued in December, although recently there has been some weakening of these changes.
(This evaluation is in agreement with the views (see reference below) expressed at the December 25th procedural meeting of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(reference) "Recent seismic and crustal activity in the Tokai Region" (Meteorological Agency, Department of Seismology and Volcanology, December 25 2001)
"Seismic activity within the slab had been high in the first half of this year in the Tokai region, but recent activity has been normal or slightly below average.
Seismic activity in Suruga Bay and the western coast had been at a moderately low level, but in the latter half of the year activity moved to a slightly high level.
Additionally, regarding crustal movement in the Tokai and surrounding regions, the results of observations by the National Geological Institute indicate some change over the long term since the beginning of the year, but there have been recent signs of weakening. There have been indications of direct linkage between crustal movement and seismic activity in the Tokai region."
There was the following additional seismic activity in the Kanto-Chubu region:
- On December 23rd, there was an M5.8 earthquake in the sea near Chichijima.
There is no supplementary information for the Kinki-Chugoku-Shikoku region.
"On December 18th, there was an M7.3 earthquake at a depth of approximately 10 km in the sea near Yonagunijima. ..."
The results of GPS observation of this area showed horizontal crustal movement associated with the event. North of the epicenter, this movement was southward, while east of the epicenter, it was to the east, which conforms with the focal mechanism of the event. In addition, approximately 20 seconds prior to the main shock, there was an M4.1 earthquake in about the same location.
There was the following additional seismic activity in Kyushu-Okinama Region:
- On December 21st, there was an M4.2 earthquake near Iriomotejima.
"On January 4th 2002, there was an M4.0 earthquake at a depth of approximately 10 km near the border of Wakayama and Nara Prefectures. ... Based on a review of prior seismic activity in this area, it is believed that the present activity may continue with fluctuations in intensity for some time."
The microearthquake activity at a depth of approximately 10 km (maximum magnitude to December 2001, M3.6) that has continued in the area of the border between Wakayama and Nara Prefectures since late May, has continued in January, centered to the south (on the Nara Prefecture side). The cluster of zones of activity since last May far extends 6 km to the north and south and 5 km to the east and west, with a gentle tilt to the west, and includes areas both of activity and inactivity. However, there has been no marked crustal movement observed in this region.
In recent years, there was an example of seismic activity including a maximal M4.0 earthquake in central Mie Prefecture, which began in late January 1999 and continued for about a year and a half.
3. Regarding the change in hypocenter depth of the M6.4 earthquake that occurred in the south of central interior Iwate Prefecture on December 2nd
In the report "Seismic activity in November 2001" the depth of the hypocenter of the M6.4 earthquake that occurred in the south of central interior Iwate Prefecture on December 2nd was reported as approximately 130 km, but has been amended to approximately 120 km.
(explanation) It has been confirmed from an analysis of seismic waves, that this event was two successive earthquakes that shared hypocenters in approximately the same location. The two earthquakes occurred approximately 0.6 seconds apart, and the results of a recalculation of the hypocenter depth taking this new data into account indicate that the hypocenter of the M6.4 event was at a depth of approximately 120 km.
Correction: In 2001 there were the following errors in counts of M6.0 or greater events in reports of "Major Seismic Activity", which are corrected below. Note that the July 3rd earthquake in the Mariana Islands is not included in these counts.
(March report, Supplement)"There have been two such events in the period from January to March 2001." The number should be amended to three.
(April report, Supplement)"There have been three such events in the period from January to April 2001." The number should be amended to four.
(May report, Supplement)"There have been four such events in the period from January to May 2001." The number should be amended to five.
(June report, Supplement)"There have been five such events in the period from January to May 2001." The number should be amended to six, and the period should be amended to "from January to June 2001".
(July report, Supplement)"There have been five such events in the period from January 2001 to the present." The number should be amended to six.
(August report, Supplement)"There have been seven such events in the period from January 2001 to the present." The number should be amended to eight.
(September report, Supplement)"There have been seven such events in the period from January 2001 to the present." The number should be amended to eight.
(October report, Supplement)"There have been eight such events in the period from January 2001 to the present." The number should be amended to nine.
(November report, Supplement)"There have been eight such events in the period from January 2001 to the present." The number should be amended to nine.
| 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. |