| 10 May 2000 |
| Headquarters for Earthquake Research Promotion |
| Earthquake Research Committee |
Seismic Activity in Japan April 2000
There has been intense seismic activity accompanying the volcanic activity of Mt. Usuzan in Hokkaido, and on April 1 an event of a seismic intensity of 5 lower was recorded. to supplementary reports
*Since March 27, there has been intense seismic activity related to volcanic activity at Mt. Usuzan and the surrounding area. The frequency of seismic events in the vicinity of Mt. Usuzan had begun to decrease prior to the onset of eruption on March 31, and on April 1 there occurred an M4.6 earthquake registering a seismic intensity of 5 lower, the largest event since the onset of volcanic activity. Since April 4 there have been no events of M3.0 or greater. Variation in seismic activity due to the volcanic activity has been limited to a 10 km radius of Mt. Usuzan, and GPS observation has shown no prominent wide-area effects. to supplementary reports
*On April 13, an M4.8 earthquake occurred approximately 50 km off the coast of Iwate Prefecture at a depth of about 40 km, on April 26 two events of magnitudes 5.2 and 5.1 occurred off the coast of the Sanriku area (130 km offshore of Iwate Prefecture), and on April 30 there was another M5.1 earthquake at sea (180 km offshore of Iwate Prefecture). All of these events occurred within the aftershock region of the 1994 Sanriku Haruka Offshore M7.5 earthquake.
*On April 26, an M4.3 earthquake occurred at a depth of about 10 km in the Aizu region of Fukushima Prefecture. An M4.0 event occurred in the same vicinity in February 1999. to supplementary reports
*On April 7, in the Joetsu region of Niigata Prefecture, an M4.1 event occurred at a depth of approximately 15 km. This area has been the site of earthquakes of M4.0 and greater in the past.
*On April 10, in southwestern Ibaragi Prefecture an M4.6 event occurred at a depth of approximately 55 km. This earthquake occurred in conjunction with the subduction of the Philippine Sea plate and its focal mechanism was a reverse-fault type, on a northwest-southeast pressure axis. The earthquake mechanism was similar to those of other earthquakes that have occurred in this area in the past.
*Seismic activity in the subducting Philippine Sea Plate in central Shizuoka Pref. continues at a level below the average since last August. Moreover, the results of GPS observation in the Tokai (Shizuoka-Aichi-Gifu) area showed no deviation from previous tendencies.
*Intense microseismic activity in central Mie Prefecture at a depth of about 10 km that began in January 1999 had been diminishing, but activity intensified slightly beginning in February of this year in an area of activity expanded to the north, and this state has continued to the present. to supplementary reports
*On April 15, in southern Wakayama Prefecture an M4.8 event occurred at a depth of approximately 45 km. This earthquake occurred within the Philippine Sea Plate.
*On April 28, in northern Wakayama Prefecture an M4.2 event occurred at a depth of approximately 55 km. This earthquake occurred within the Philippine Sea Plate. to supplementary reports
*On April 2, in the northern foothill region of Miyazaki Prefecture a moderately deep-focus M4.5 event occurred at a depth of approximately 150 km. This earthquake occurred in the area of the tip of the subducting Philippine Sea Plate, and its focal mechanism showed a tension axis in the direction of the plate's subduction.
*Beginning around April 10, seismic activity of maximum M4.2 occurred in the sea surrounding Iriomotejima Island. In 1992, seismic activity of maximum M5.2 occurred in the same area.
*There was seismic activity of maximum M4.0 beginning around April 29, in central Oita Prefecture (in the area of Beppu Bay). In December of 1999, seismic activity of up to M3.3 occurred in the same area. to supplementary reports
| May 10, 2000 |
| Earthquake Research Committee |
"There has been intense seismic activity accompanying the volcanic activity of Mt. Usuzan in Hokkaido, and on April 1st an event of a seismic intensity of 5 lower was recorded.":
Strong deviations in seismic activity related to the volcanic activity of Mt. Usuzan were observed. There were 50 events of M4.0 or higher in Japan and the surrounding area . (There were 42 such events in March, 2000. The monthly average over the past 30 years is 43 events.) Of these events, six registered M5.0 or higher (as compared to three in March). When the number of M4.0 or higher events is considered, it can be seen that this month saw only slightly more activity than average.
The list below shows major seismic activity since February of last year. Among the events in this list, there were two M3 class seismic events (maximum seismic intensity of 4) on April 14th in the sea near Niijima and Kozushima Islands(the region where the events occured on March 14, 1997).
| Off the southern coast of Akita Prefecture | February 26, 1999, M5.1 (at a depth of about 20 km) |
| In the sea near Niijima and Kozushima Islands | March 14, 1999, M4.7 (at a depth of 10km or less) March 28, 1999, M5.0 (at a depth of 20km or less) |
| Central Southern Kushiro Administrative District | May 13, 1999, M6.4 (moderately deep-focusearthquake at a depth of about 100 km) |
| Northern Wakayama Prefecture | August 21, 1999, M5.4 (at a depth of about70 km) |
| Taiwan | September 21, 1999, M7.7 (source: US Geological Survey) |
| Central Inland Sea | October 30, 1999, M4.5 (at a depth of about 15 km) |
| Kumamoto Region of Kumamoto Prefecture (at a depth of about 10 km); off the coast of Fukui Prefecture (at a depth of about 15 km); and Western Aichi Prefecture (at a depth of about 50 km); events of greater than M4.0 on November, 1999 | |
| Off the east coast of Hokkaido | January 28, 2000, M6.8 (at a depth of about 60 km) |
| Hokkaido Iburi Administrative District (in the area around Mt. Usuzan) | March 30, 2000, seismic activity related to the volcanic activity, beginning with an M4.3 event (at a depth of about 10 km) |
"Variation in seismic activity due to the volcanic activity has been limited to a 10 km radius of Mt. Usuzan, and GPS observation has shown no prominent wide-area effects.":
To date, no changes in seismic activity within a 10-100 km radius of Mt. Usuzan have been seen, and no changes in the area 50 km and further from the volcano have been noted in GPS observation.
The M4.6 event on April 1 occurred at the southern tip of the area of seismic activity, and its focal mechanism showed a NW-SE pressure axis.
Additionally, there was the following seismic activity in the Hokkaido region:
-In the period from April 4 to the end of the month, seismic activity of up to M3.5 at a depth of about 15 km occurred in the Kushiro Administrative District (about 10 km south of Mt. Meiakan-gake). There have been 0 to 6 events of M2 class per year in the period from 1995 to 1999 within a 20 km radius of this area.
-On April 12, in the sea off the coast of the Matsumae region (off the southwest coast of Hokkaido), there was an M4.3 earthquake at a depth of about 15 km. The area approximately 10 km to the east of the epicenter of this event saw swarm activity (maximum M4.2) in the period from October 1995 through the first half of 1996.
-On April 11, there was an M4.3 event off the northwest coast of Hokkaido at a depth of about 50 km.
-On April 26, there was an M5.0 event in the sea off Urup Island in the Chishima Archipelago.
"All of these events occurred within the aftershock region of the 1994 Sanriku Haruka Offshore M7.5 earthquake.":
The 1994 Sanriku Haruka Offshore earthquake occurred at the junction of the subducting Philippine Sea Plate and the Continental Plate, and its focal mechanism was a reverse fault-type along a NW-SE pressure axis. The four events reported all showed similar mechanisms.
"On April 26th, an M4.3 earthquake occurred at a depth of about 10 km in the Aizu region of Fukushima Prefecture.":
The focal mechanism was a reverse fault-type on a NW-SE pressure axis, which is the same as that of the February, 1999 event.
"On April 7, in the Joetsu region of Niigata Prefecture, an M4.1 event occurred at a depth of approximately 15 km.":
The focal mechanism was along a NW-SE pressure axis. This region includes the area of the border between Niigata and Nagano Prefectures, and lies in the northern part of the Shinanogawa fault zone.
"This area has been the site of earthquakes of M4.0 and greater in the past.":
Over the past 70 years, an average of one 4.0 or higher event per year has occurred within a 50 km radius of the event reported.
"Since August of last year, earthquake activity in the subducting Philippine Sea 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 Sea 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. Since the beginning of April there have been seven events, representing a moderate increase, but with only three events in the past twenty days, activity continues to be at a reduced frequency.
(This coincides with the views (see reference below) expressed at the procedural meeting, on 24 April, 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, 24 April 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.
In addition, the Kanto-Chubu region saw the following seismic events:
-On April 12, there was an M4.1 event at a depth of 55 km, and an M3.9 event at a depth of 50 km in the northeastern part of Chiba Prefecture. The focal mechanisms of both earthquakes were reverse fault-type on a northwest pressure axis, which is identical with the mechanisms of other events which have occurred in this are in the past. These earthquakes occur in conjunction with the subduction of the Pacific Plate.
-On April 14, two events of M3.3 and M3.5 occurred in the sea near Niijima and Kozushima Islands.
-On April 21, a deep-focus M5.7 earthquake occurred at a depth of 350 km in Wakasa Bay. The focal mechanism was a normal fault-type, with a north-south tension axis. In this same area, an M5.3 event occurred in January, 1996 and an M5.1 event occurred in January, 1997.
-Following the M4 class activity which occurred in mid-March, seismic activity in the Chuetsu Region of Niigata Prefecture has subsided since the latter half of April
"On April 15, in southern Wakayama Prefecture an M4.8 event occurred at a depth of approximately 45 km.":
The focal mechanism was a normal fault-type, on a north-south tension axis.
"On April 28, in northern Wakayama Prefecture an M4.2 event occurred at a depth of approximately 55 km.":
The focal mechanism was a strike-slip type, on a NW-SE pressure axis.
Additionally, the following events occurred in the Kinki-Chugoku-Shikoku region:
-On April 11, there was seismic activity of up to M4.0 at a depth of approximately 15 km in the Kii Suido channel.
-On April 13, there was an M4.0 event at a depth of 70 km in the Iyo Nada sea, and on April 30, there was an M4.0 event at a depth of 50 km in the Bungo Suido channel.
"There was seismic activity of maximum M4.0 beginning around April 29, in central Oita Prefecture (in the area of Beppu Bay).":
The seismic activity surrounding the event described began on April 29 and has continued on through the beginning of May. There was seismic activity including M3 class activity in this region in December 1999, but the area of activity this time occurred to the north of the December 1999 activity zone.
-On April 26, a moderately deep-focus M4.9 event occurred at a depth of 140 km in the East China Sea.
The following events were recorded:
-On April 22, a moderately deep-focus M5.4 event occurred at a depth of 160 km in the sea 600 km to the south of Chichijima Island. This was approximately the same site as the hypocenter of an M7.6 event that occurred on March 28.
| 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. |