to supplementary reports
| 12 April 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan March 2000
There has been intense seismic activity accompanying the volcanic activity of Mt. Usuzan in Hokkaido, and a number of events of seismic intensity 5-lower have been recorded. Additional figure to supplementary reports
* Since 27 March, there has been intense seismic activity at Mt. Usuzan and the surrounding area related to volcanic activity. The major activity is at a depth of 10km or shallower, and the largest earthquake up to the end of March was an M4.3 event on 30 March. Its hypocenter was in the northwest part of the region of seismic activity, and the focal mechanism was the reverse fault type with a N-S or NE-SW compression axis. Including this event, up to the end of March seven earthquakes of seismic-intensity 5 lower have been recorded. GPS observation showed crustal shift related to volcanic activity. The current seismic activity and crustal shift are related to the volcanic activity of Mt. Usuzan.
(Seismic activity in April)
The frequency of seismic events in the vicinity of Mt. Usuzan had begun to decrease prior to the onset of eruption on 31 March, and on 1 April there occurred an M4.6 earthquake registering a seismic intensity of 5 lower, the largest event since the onset of volcanic activity. Variation in seismic activity due to volcanic activity has been limited to a 10km radius of Mt. Usuzan, and GPS observation has shown no prominent wide-area effects.
* An M5.3 earthquake occurred on 30 March east offshore of Hokkaido at a depth of about 50km. This earthquake occurred in the aftershock area of the 1994 East Offshore of Hokkaido Earthquake and is thought to be an aftershock of that event. to supplementary reports
An M5.0 earthquake occurred on 20 March in Sendai Bay at a depth of about 80km. In the vicinity of this hypocenter, M5-class events occurred in December 1997 and in January 1998. to supplementary reports
* An M4.3 earthquake occurred on 19 March in the Chuetsu region of Niigata Prefecture (Pref.) at a depth of about 15km. Also, some 10km westward, an M4.0 earthquake occurred on 25 March at a depth of about 15km. Some 10km to the south of these hypocenters there was seismic activity, including a maximum M5.4 event, between the end of 1990 and mid-1991.
* An M4.0 earthquake occurred on 24 March on the border of Fukui and Gifu Prefs. at a depth of about 10km. In a 30km radius of this event there has occurred in the last five years, on average, at least one M4-class earthquake per year.
* Earthquake activity in the subducting Philippine Plate in central Shizuoka Pref. continues at a level below the average. Moreover, the results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area showed no deviation from previous tendencies.
* Intense microearthquake activity in central Mie Pref. at a depth of about 10km that began in January 1999 had been diminishing, but in mid-February of this year activity intensified slightly and the area of activity expanded about 2km to the north, with a maximum M3.1 event on 26 March. to supplementary reports
There was no marked activity. to supplementary reports
There was no marked activity. to supplementary reports
A fairly deep-focus M7.6 earthquake occurred on 28 March some 600km south of Chichijima Island at a depth of about 120km. The focal mechanism showed a torsion axis in the direction of the subducting Pacific Plate. to supplementary reports
* A fairly deep-focus M4.5 earthquake occurred on 2 April in the foothill region of northern Miyazaki Pref.
* An M4.1 earthquake occurred on 7 April in the Joetsu area of Niigata Pref.
* An M 4.6 earthquake occurred on 10 April in southwestern Ibaraki Pref.
Corrigendum
Herewith we correct an error regarding the Kanto-Chubu Region contained in "Seismic Activity in Japan February 2000".
The M-value of the earthquake that occurred on 29 February offshore of Niigata Pref. at a depth of about 25km,Incorrect: 4.2, Corrected: 4.3 to supplementary reports
| April 12, 2000 |
| Earthquake Research Committee |
"There has been intense seismic activity accompanying the volcanic activity of Mt. Usuzan in Hokkaido, and a number of events of seismic-intensity 5 lower have been recorded":
Strong shaking was repeatedly recorded as a result of seismic activity deriving from the volcanic activity of Mt. Usuzan; in addition, in Japan or its environs, 42 earthquakes of M4.0 or greater were recorded during this month (42 in February; on average, over a 30-year period up to December 1999, 43 occurrences) and 3 events of M5.0 or greater (5 in February). Therefore, from the standpoint of the number of events of M4.0 or greater, the level of activity this month was the same as last month. Furthermore, the following were major areas of seismic activity since January of last year; however, none among these areas of seismic activity or their environs, except what is thought to have been an aftershock east offshore of Hokkaido, showed marked activity in March.
| - Sea near Tanegashima Island | 24 Jan. 1999 M6.2 (D=about 50km) |
| - Central Nagano Pref. | 28 Jan. 1999 M4.7 (D=about 10km) |
| - Off the southern coast of Akita Pref. | 26 Feb. 1999 M5.1 (D=about 20km) |
| - Sea near Niijima and Kozushima Islands | 14 Mar. 1999 M4.7 (D=10km or less) |
| 28 Mar. 1999 M5.0 (D=20km or less) | |
| - Southern Kushiro Administrative District | 13 May 1999 M6.4 (D=about 100km) |
| - Northern Wakayama Pref. | 21 Aug. 1999 M5.4 (D=about 70km) |
| - Taiwan | 21 Sep. 1999 M7.7 [per USGS] |
| - Central Inland Sea | 30 Oct. 1999 M4.5 (D=about 15km) |
| - In November 1999, M4.0 or greater earthquakeswere recorded in the Kumamoto District of Kumamoto Pref. (D=about 10km), offshore of Fukui Pref. (D=about 15km), and western Aichi Pref. (D=about 50km). | |
| - East offshore of Hokkaido | 28 Jan. 2000 M6.8 (D=about 60km) |
"Variation in seismic activity due to volcanic activity has been limited to a 10km radius of Mt. Usuzan, and GPS observation has shown no prominent wide-area effects":
Currently, seismic activity in a radius of 10km to 100km of Mt. Usuzan has shown no variation, nor has GPS observation between a 50km to 100km radius of Mt. Usuzan shown any variation. The chief known active fault zones surrounding Mt. Usuzan are the fault belt on the eastern edge of the Ishikawa Lowlands, lying to the east of Usuzan and the fault belt to the west in the Kuromatsunai Lowlands. Both are some 50km from Mt. Usuzan.
Also, the M4.6 earthquake that occurred on 1 April was on the southern verge of the area of seismic activity and its focal mechanism showed a NW-SE compression axis.
Moreover, it is customary for numerous earthquakes to occur at the time of volcanic activity, but M6.0 or greater events are rare. Regarding Usuzan, on the day before it erupted in 1910 there was an M5-class earthquake, but there is no record of an event exceeding M5.0 during the period following the eruption to the termination of activity. Also, during the year following the eruption in 1977, no earthquake of M5 or greater was observed within a 100km radius of Usuzan.
"An M5.3 earthquake occurred east offshore of Hokkaido at a depth of about 50km":
Aftershock activity of the M6.8 earthquake on 28 January to the east offshore of Hokkaido continues to steadily decrease. The recent event occurred some 50km northwest of this area of activity. Additional figure
"Sendai Bay at a depth of about 80km":
This earthquake occurred at the bottom plane of the double-planed seismic zone of the subducting Pacific Plate, and it is customary for the focal mechanism of such events to show a torsion axis in the direction of the subducting plate. Such was the case in this instance. Also, an M4.3 earthquake occurred in the near vicinity on 26 March.
Other seismic activity in the Tohoku area was as follows: an M3.8 earthquake occurred on 7 March in northern Iwate Pref. at a depth of about 5km.
"The Chuetsu region of Niigata Pref. at a depth of about 15km":
Most earthquakes in this area have a focal mechanism showing a NW-SE compression axis, and such was the case of the 19 March event; however the compression axis of the 25 March event lay in a NE-SW direction. Since 1926, M5.0 or greater earthquakes have occurred in this area (within a 30km radius), on average, once in ten years.
"Border of Fukui and Gifu Prefs. at a depth of 15km":
Results of GPS surveying show that the NW-SE direction is at maximum compression. Focal mechanisms of earthquakes in this area are known for showing a NW-SE or E-W compression axis, which comports with GPS survey results, and such was the case with the 24 March event. Also, this area corresponds to the southern fringe of the aftershock zone of the M7.0 Northern Mino Earthquake that occurred in August 1961.
"Earthquake activity in the subducting Philippine Plate in central Shizuoka Pref. continues at a level below the average":
The frequency of occurrence of earthquakes (M1.5 or greater) in the Philippine Plate in central Shizuoka Pref. had averaged some 6 events per month, but from August 1999 there have been 2 to 4 events per month; the lower-than-average condition continued in March.
(This coincides with the views (see reference below) expressed at the procedural meeting, on 28 February, of the Regional Assessment Committee for Strengthening Countermeasures against Earthquake Disasters.)
(Reference) Recent seismic and crustal activity in the Tokai (Shizuoka-Aichi-Gifu) area and vicinity (Meteorological Agency Earthquake and Volcano Office, 27 March 2000)
"In the Tokai area, seismic activity within both the crust and the subducting slab is at a low ebb, and, since last year, seismic activity in Suruga Bay and its western shores continues in a reduced state. Seismic activity in the Izu Peninsula as well has, since last year, shifted to a reduced state. Also, in regard to crustal change in the Tokai area, no marked variation has been observed.
"Intense microearthquake activity in central Mie Pref. at a depth of about 10km":
The escalation of microearthquake activity in central Mie Pref. that began in January 1999 had been diminishing since November 1999; however, from mid-February activity rose to September 1999 levels. Nevertheless, earthquake size was small, with only an M3.1 event exceeding the M3.0 level.
In the Kinki-Chugoku-Shikoku Region there was the following seismic activity.
- An M4.1 earthquake occurred on 18 March at a depth of about 15km in the vicinity of the southwestern verge (Awaji Island) of the main aftershock area of the 1995 Southern Hyogo Prefecture Earthquake. The focal mechanism was a strike-slip type showing a largely ENE-WSW compression axis and, thus, similar to that of the Southern Hyogo Prefecture Earthquake
- Microearthquake activity has continued since March 1999 in the central Inland Sea. Moreover, in early March, activity increased in prominence in an area 2 to 3km west of the previous main area of
activity, but has since abated.
In the Kyushu-Okinawa Region there were the following earthquakes:
- An M4.0 earthquake occurred on 29 March in the sea near Tanegashima Island at a depth of about 5km. In this vicinity in September 1996 there occurred an M5.7 earthquake.
- An M3.5 earthquake occurred on 4 March in northwestern Kagoshima Pref. at a depth of about 5km. Within a radius of 1km there was intense but temporary microearthquake activity that tailed off after about a day. The area of this activity is in the vicinity of the epicenter and in the middle of the aftershock area of the M6.5 earthquake that occurred at the end of March 1997.
"Earthquake some 600km south of Chichijima Island at a depth of about 120km":
The hypocenter was inside the subducting Pacific Plate. Earthquakes occurring in this area are known for having a focal mechanism of a type showing a torsion axis in the direction of the subducting Pacific Plate.
| 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. |
