January 12, 2005 |
Seismic Activity for December 2004
| * | On December 14, there was a M6.1 earthquake at a depth of approximately 10km in southern Rumoi subprefecture, Hokkaido. This event had a maximum seismic intensity 5 Upper, and caused damage. to supplementary reports |
| * | The aftershock activity of the earthquake (M7.1) that occurred off-shore of Kushiro on November 29, has been normally declining. The largest aftershock so far was the M6.9 event on December 6 (maximum seismic intensity 5 Upper). In addition, on December 22, there was a M5.7 aftershock at the eastern edge of the aftershock region. According to the GPS data, slight postseismic movements associated with the recent seismic activity were observed in eastern Hokkaido. | |
| * | On December 14, there was a M6.1 earthquake at a depth of approximately 10km in southern Rumoi subprefecture, Hokkaido. This event had a maximum seismic intensity 5 Upper in Hokkaido. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. It is thought that this is a shallow earthquake occurred within the crust. The seismic activity was a main shock - aftershock sequence, and it has been declining normally. According to the GPS data, crustal movements associated with this event were observed at the Obira observation station (Obira-cho, Rumoi-gun, Hokkaido) near the event, with a displacement of approximately 5cm in a NW direction and an uplift of approximately 3cm. These are consistent with the focal mechanism of the main shock. to supplementary reports | |
| * | On December 1, there was a M4.2 earthquake at a depth of approximately 75km under northern Miyagi prefecture. Also, on December 30, there was a M5.0 earthquake. Focal mechanisms of both events had compression axes in the direction of the subducting Pacific plate. It is thought that these events occurred within the Pacific plate. |
| * | On December 29, there was a M5.5 earthquake at a depth of approximately 40km off-shore of Miyagi prefecture. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates. to supplementary reports |
| * | The aftershock activity of the Niigata Chuetsu Earthquake on October 23 (the 2004 Niigata Chuetsu Earthquake) has been continuing to decline. The largest aftershock in December was a M5.0 event on December 28 (maximum seismic intensity 5 Lower) that occurred to the east of the main shock, within the aftershock region. to supplementary reports | |
| * | On December 1, there was a M4.0 earthquake at a depth of approximately 15km in southern Kyoto prefecture. to supplementary reports |
| * | Since December 12, shallow seismic activity occurred off-shore west of Kagoshima prefecture. There was a M5.1 earthquake on December 12, and a M5.3 earthquake on December 14. The focal mechanisms had reverse faults with a tension axis in a nearly NW-SE direction. The seismicity has been declining since December 15, and is mostly subsiding. to supplementary reports |
| * | On January 1, 2005, there was a M5.0 earthquake at a depth of approximately 90km off-shore of Ibaraki prefecture. |
| * | On January 2, 2005, there was a M4.2 earthquake at a depth of approximately 5km in southern Nagano prefecture. |
| * | On January 6, 2005, there was a M5.3 earthquake at a depth of approximately 60km off-shore east of Aomori prefecture. |
| * | On January 9, 2005, there was a M4.7 earthquake at a depth of approximately 15km in western Aichi prefecture. |
| * | On January 9, 2005, there was a M4.2 earthquake at a depth of approximately 10km in the Chuetsu region, Niigata prefecture. |
January 12, 2005
The following are the numbers of earthquakes by magnitude that occurred
in Japan and the surrounding area during December 2004.
| M4.0 or greater: 103 (126 events occurred in November) | |
| M5.0 or greater: 11 (27 events occurred in November) | |
| M6.0 or greater: 2 (21 events in total by the end of December 2004) |
(reference) Average numbers of earthquake occurrences over the past thirty years (1971 - 2000)
| M4.0 or greater: 46 / month | |
| M5.0 or greater: 8 / month | |
| M6.0 or greater: 1.3 / month, 16 / year |
The following regions have been noted as having significant seismic activity during the period from December 2003 to the end of November 2004.
| -Off-shore southeast of the Boso Peninsula (near the triple junction of the plates) | ||
| M6.7 on May 30, 2004 | ||
| -Off-shore of Iwate prefecture | M5.8 on August 10, 2004 (depth of approximately 50km) | |
| - Off-shore southeast of the Kii Peninsula (Off-shore of Tokaido) | ||
| M7.4 on September 5, 2004 | ||
| - Chuetsu region, Niigata prefecture (2004 Niigata Chuetsu Earthquake) | ||
| M6.8 on Oct. 23, 2004 (depth approximately 10km) | ||
| - Off-shore of Kushiro | M7.1 on November 29, 2004 (depth approximately 50km) | |
"On December 14, there was a M6.1 earthquake at a depth of approximately
10km in southern Rumoi subprefecture, Hokkaido. This event had a maximum
seismic intensity 5 Upper in Hokkaido. ...(snip)... These are consistent
with the focal mechanism of the main shock.":
The largest aftershock so far is a M4.8 event on December 14 that occurred to the west of the main shock.
"On December 1, there was a M4.2 earthquake at a depth of approximately
75km under northern Miyagi prefecture. Also, on December 30, there was
a M5.0 earthquake. Focal mechanisms of both events had a compression axes
in the direction of the subducting Pacific plate. It is thought that these
events occurred within the Pacific plate.":
These events are thought to be aftershocks of the earthquake off-shore of Miyagi prefecture on May 26, 2003 (M7.1).
"On December 29, there was a M5.5 earthquake at a depth of approximately
40km off-shore of Miyagi prefecture. The focal mechanism showed a reverse
fault with a compression axis in a WNW-ESE direction. This event occurred
at the boundary between the Pacific and the continental plates.":
This event occurred within the expected focal region of the Off-shore Miyagi Prefecture Earthquake. There were no notable changes observed in seismicity in the vicinity before and after the recent event.
- There were no remarkable changes, before and after the earthquake off-shore
southeast of the Kii Peninsula on September 5, in the trends of the long-term
crustal movements that have been observed since 2001 in the continuous
GPS data for the Tokai region.
(This evaluation is in agreement with the views presented at the procedural meeting on December 27 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster. (see reference below))
(reference) "Recent seismic and crustal activity in the Tokai region
and its vicinity" (Japan Meteorological Agency, Department of Seismology
and Volcanology, December 27, 2004)
"So far, there have been no changes of activity which are likely to be associated with the so-called Tokai earthquake.to the text
There is no remarkable seismic activity in general. The seismicity continues at a level lower than normal directly under Hamana lake. In other regions, seismic activity is generally at a normal level.
Crustal movements were widely observed in the Tokai region associated with the M7.4 earthquakes off-shore of Tokaido (off-shore southeast of the Kii Peninsula) on September 5. It seems that these are mainly step-like changes from the earthquakes. There were no remarkable changes before and after the earthquake in the trends of the crustal movements, that have been observed since the beginning of 2001. "
- In the period from December 2 through 8, there was seismic activity in northern Hyogo prefecture (near the border with the Tottori prefecture), with the largest earthquake a M3.4 event at a depth of approximately 10km.
- Since December 14, there was micro seismic activity at the depth of 15km in southwestern Hyogo prefecture, and activity is continuing intermittently. The largest earthquake so far is a M3.2 event on December 14.
- Since December 26, light seismic activity occurred at a depth of about 10km in the southern plains region of Miyazaki prefecture. Seismicity increased temporarily until December 27, however, it mostly subsided by the end of December. The largest earthquake was a M3.8 event on December 26 (maximum seismic intensity 3).
| Reference 1 Earthquakes that are described in the "Evaluation of Seismic Activity"(Monthly Report of Seismic Activity in Japan) are events of M6.0 and greater, or events of M4.0 and greater (M5.0 or greater off-shore) that have maximum intensity of 3 or greater. Reference 2 Information that is described in the "Supplementary Information to the Evaluation" includes:
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