(34o53'59"N, 139o05'52")

On 13 May 1998, the Headquarters for Earthquake Research Promotion and the committee, that was established after the Kobe earthquake (January 1995) with the aim to promote surveys and researches related to earthquakes, released the monthly reports on earthquakes in April 1998. Seismic activity in the eastern part of the Izu Peninsula which had been elevated since 20 April 1998, started declining around 10 May. The daily counts peaked at >1000 on 21-22 April. The activity focused in the area of 8 km east-west, 3 km north-south, and 2-9 km deep, offshore of Kawanazaki (city of Ito). This site is located southeast of the site for the October 1996 activity and south of that for the March 1997 activity. Though recent activities in this area started with the maximum magnitude of earthquakes in the initial stage, the maximum earthquake (M=5.7) in this time occurred on 14-th day (3 May). The mechanism of the earthquake is SE-NW compression, similar to that in 4 March 1997 near here. Totally 10,800 earthquakes were counted during 20 April to 12 May, among them 207 were felt-earthquakes.

During this seismic event, strainmeter, tiltmeter, and GPS showed changes in various degrees. Change in water level in a well was also observed. Strainmeters and tiltmeters installed in the eastern part of the Izu Peninsula showed a slow deformation about 15 hr before an earthquake started, a clear deformation during 21-22 April, and a decline of deformation thereafter. GPS survey observed an extension of distance between Komuro and Hatsushima (12 km) as much as 8 cm, and the extension became stagnant on 9 May. Water level in a well (north of Omuroyama) changed largely in the early stage of the activity and around 3 May when the maximum earthquake occurred. These observations in Crystal deformation indicate an inflation of the seismically activated area, repeating the same phenomena as before.

The Headquarters for Earthquake Research Promotion and the committee presented possibilities that crustal deformation in this time had an intimate relation with magmatic activity in shallow levels, and that the present activity will decline.

Link to: figures (hypocenters and daily counts of earthquakes) in the site, Seismic activity off coast of Ito 1998/04/16 - 1998/04/15, presented by Earthquake Research Institute, University of Tokyo

Information contacts: the Headquarters for Earthquake Research Promotion, STA, hyarai@sta.go.jp. Prof. Katsuyuki ABE, Earthquake Information Center, ERI, U-Tokyo, abe@eri.u-tokyo.ac.jp. Volcano Research Center, ERI, U-Tokyo, nakada@eri.u-tokyo.ac.jp

(13 Mar. 1997)
National Committee of Earthquake Survey issued the general comment on earthquake swarms around Izu-Tobu Volcano Group in the 12 Mar. afternoon; this earthquake swarm activity probably came to an end, though small ups and downs of the activity may be associated. Swarms of earthquakes which started on 3 Mar. actively, had declined since around 10 Mar. The total number of earthquakes reached 9,080 at the Kamata JMA St. (ca. 10 km SW of swarm), including 441 earthquakes which could be felt. Four earthquakes larger than M=5.0 occurred; the maximum was M=5.7 on 4 Mar.

The epicenters of earthquakes were under the sea, close to the east coast of Ito City (Shiofukizaki) and 5-10 km deep in the earliest stage, and they extended eastward in about 5 km-long, EW-trending area, centered 2 km off the coast, and with the depth of 2-8 km. Small low frequency earthquakes were observed during 4-6 Mar. The active area was just NW of that of the earthquake activity in 1995. The degree of deformation during the activity was close to that in 1995.

The rate of ground deformation with GPS, tiltmeter, volumetrical strain meter had slowed these days. Total extension of the line between Hatsushima and Komuroyama (ca. 12 km), crossing above the swarm, reached 13 cm, whereas the accumulated strain at the Higashi-Izu JMA St. (ca. 20 km SWS of swam) was about 6x10^7 without steps due to large earthquakes. The deformation indicates that the quake swarm area had swelled, and this phenomenon is similar to that which had repeated in this area these tens of year.

The prefectural government and police took the residents in Ito City off the alert in the 12 May afternoon.

(12 Mar. 1997)
JMA report in the 11 Mar afternoon: The seismic activity and ground deformation had become fainter than before; being relatively quiet after 9 Mar. Total number of earthquakes reached about 9,000 including 440 of quakes which could be felt. No long-period seismic event had occurred since 5 Mar. JMA commented that the activity seems to approach to an end. The GPS data of Geographical Survey Institute showed 12 cm-extension of the distance between Hatsushima and Komuroyama (about 12 km) for 9 days.
Link site: Seismic activity at Izu-Tobu, provided informally by graduate students of ERI, U-tokyo, using the data of National Research Institute for Earth Science and Disaster Prevention, STA. Sorry! Visitors with "ac.jp" or "go.jp" address can take access. Hypocenters of EQ's, daily number of EQ's whose hypocenters could be determined, and the focal mechanism

(9 Mar. 1997)
The total number of earthquakes which had occurred since 3 Mar. in Izu-Tobu (counted at the JMA Kamata St.) reached about 8,400 including about 400 of quakes which could be felt (JMA report in the afternoon, 9 Mar.) . The accumulated strain with the volumetric strain meter was about 11x10^-7 at the JMA Higashi-Izu St. The quake with M=4.1 took place at 23:53 (JST) of 8 Mar. The earthquake activity had been relatively low in level since the early morning of 9 Mar. GPS data by Geographical Survey Institute (GSI), however, showed still continuous and steady ground deformation since 3 Mar. According to GSI, the distance between the Komuroyama (near eastern coast of the Izu Peninsula) and Hatsushima (offshore of Ito), crossing over the main earthquake swarm in the NEN-SWS direction, lengthened by 9 cm for the first 6 days. The Hydrographic Department, Maritime Safety Agency reported no discolored sea surface around above the earthquake swarms, based on their own aircraft survey in the 9 Mar. morning.

(8 Mar. 1997)
The JMA reported that the total number of earthquakes which occurred in the east Izu Peninsula by the 8 Mar. afternoon, was about 7,800 including about 350 which could be felt, at Kamata. The earthquakes with M's=4.3, 4.6 and 4.2 took place at 10:20 (JST), 16:33 and 21:35, 7 Mar. The activity of earthquakes had increased again since 7 Mar. A new earthquake swarm appeared about 10 km south of the main one in the 6 Mar. afternoon; both swarms are offshore of the peninsula. By the 8 Mar. morning, the accumulated strain at Higashi-Izu (JMA obs. St.) reached about 10x10^-7 (about 5x10^-7 without steps by big quakes). Earthquake Research Institute (ERI), University of Tokyo, had not observed low frequency earthquakes since the 6 Mar. morning. The main swarm of earthquakes which were determined precisely by ERI, locates in an area as small as 2 km across, about 5 km away from the east coast of Kawana-zaki (SE of Ito City). The depth ranges from 7 to 3 km. The Hydrographic Department, Maritime Safety Agency reported no discolored sea surface around above the earthquake swarms, based on their own aircraft survey in the 7 Mar. afternoon. The GPS data in the eastern part of the peninsula by Geographical Survey Institute indicate a continuous steady swelling of the region around the main earthquake swarm during 3-8 Mar.

(6 Mar. 1997)
According to JMA, the total number of earthquakes became over 4,500 at the Kamata seismic station in the 6 March afternoon; among them 137 earthquakes could be felt. The earthquake with M=4.4 occurred at 22:43 (JST) of 5 March. Total crustal strain accumulated at Higashi-Izu (eastern coast of Izu Peninsula) during this activity, was measured as about 8x10^-7 (about 4x10^-7 without steps by big quakes) with the volumetrical strain meter. Although the frequency of earthquakes has totally declined since the 5 March morning and the hypocenters have become more scattered than before, ground deformation has still continued.
Information contact: JMA.

(5 Mar 1997)
Japan Meteorological Agency (JMA) released the news about earthquakes in the Izu-Tobu Volcano Group on 4 and 5 March:
A swarm of earthquakes has occurred east of Izu Peninsula since 3 March 1997. The biggest earthquake with M=5.7 took place at 12:51 (JST), 4 March. By the 5 March noon, the number of earthquakes reached about 3,600 at the Kamata seismic station (S of Ito City). The activity focuses in the EW-trending zone 0-5 km offshore of Ito City (the eastern neck of the peninsula), about 3 km SE of Teishi Knoll which was formed in the 1989 eruption. The depths are smaller than 8 km, mostly around 5 km. Low-frequency earthquakes were observed four times since the 4 March morning.


The volcanic activity in the eastern part ("Tobu") of Izu Peninsula is characterized by densely distribution of volcanoes of both basalt-basaltic andesite and dacite. Cinder cones, lava flows, lava domes, and pyroclastic-flow plateau are distributed at there, and many submarine volcanoes are located on the sea floor offshore of the peninsula.

Ito is the largest city situated within Izu-Tobe Volcano Group, facing to the sea. The population is about 70 thousands. Many tourists are visiting hot springs and resorts in this city.

The 1989 eruption which is only one eruption historically recorded, occurred following the earthquake swarm similar to this time. The eruption occurred on the sea floor of about -80 m, offshore of Ito, issuing small amount of basalt; the scoria floated on the sea surface was coated by rhyolite (partially melted sediments?). The eruption started after relatively quiet 3 days following a week-long earthquake swarm; even during the quiet 3 days, ground deformation had continued. Volcanic tremors started 2 days before eruption, following LF events. Faculty of Science, Tohoku University (1991) re-determined the hypocenters of earthquakes of eruption precisely, and suggested that magma ascended from about 3 km depth at the rate of 0.3 km/day last 10 days.

Earthquake swarms had occurred frequently in this area since 1978, including that in 1989. Recently, for example, (1) during 11 September-20 October 1995, a swarm at Shiofukizaki (S shore of Ito) with max. M=4.8, 5 tremors and 7 low-frequency (LF) events; total about 9,400 EQ events at Kamata. (2) during 2-28 July 1996, a swarm near Ito City with max. M=2.4. (3) during 15 Oct.-5 Nov. a swarm at Shiofukizaki with max. M=4.1 and 5 LF events; 6,000 events at Kamata. (4) during 17-23 December, EQ's 5 km SE of Jogasaki (S of Shiofukizaki) with max. M=3.8. These swarms of earthquakes are interpreted to be due to frequent intrusion of magma or the related fluid into the shallow levels, based on the data on ground deformation associated with earthquake swarms.

The earthquake activity and the related ground deformation in this area have been monitored by JMA, National Research Institute for Earth Science and Disaster Prevention (Science and Technology Agency (STA)), Geographical Survey Institute, University of Tokyo, and so on. Since the Tokai area including this peninsula is the most essential region targeted for Earthquake Prediction Researches in Japan, geophysical monitoring network covers densely this area.

Information contact: JMA,
link to hypocenters of EQ's and magnitude-time (M-t) diagram w/Jpn notations and temporal change in frequency of EQ's w/Jpn notations in the homepage of Promotion of Research on Earthquake Prediction of STA.

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