May 12, 2017
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion

Evaluation of Seismic Activities for April 2017

1 Major Seismic Activities

There was no remarkable activity.

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

* On April 30, there was a magnitude (M) 5.4 earthquake at a depth of approximately 55 km under the southern Tokachi region. The focal mechanism showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates.

(2) Tohoku Region

* On April 9, there was an M5.0 earthquake at a depth of approximately 45 km, offshore Fukushima prefecture. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the continental plates.

(3) Kanto-Chubu Region

* On April 12, there was an M4.6 earthquake at a depth of approximately 55 km under the southern Ibaraki prefecture. The focal mechanism had a compression axis in an N-S direction. This event occurred within the Philippine Sea plate.

* On April 20, there was an M4.5 earthquake at a depth of approximately 5 km in the northern Ibaraki prefecture. The focal mechanism showed a strike-slip fault type with a tension axis in an ENE-WSW direction. On the same day there was also an M4.2 earthquake in the vicinity of this event. These events occurred within the crust.

* In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.

(4) Kinki-Chugoku-Shikoku Region

There was no remarkable activity.

(5) Kyushu-Okinawa Region

* On April 29, there was an M5.6 earthquake offshore east of Osumi peninsula. The focal mechanism showed a normal fault type with a tension axis in a WNW-ESE direction.

Supplement

* On May 4, there was an M4.1 earthquake at a depth of approximately 10 km in the Kumamoto region, Kumamoto prefecture. The focal mechanism showed a normal fault type with a tension axis in an NNW-SSE direction. This event occurred within the crust.

* On May 9, there was an M6.4 earthquake under the ocean area near Miyakojima. The focal mechanism showed a strike-slip fault type with a tension axis in an NW-SE direction (preliminary).


Note: GNSS (Global Navigation Satellite System) is a general name of satellite positioning system such as GPS.



Supplementary Information to the Evaluation of Seismic Activities for April 2017

May 12, 2017
Earthquake Research Committee

1 Major Seismic Activities

The following are the numbers of earthquakes by magnitude (M) that occurred in Japan and the surrounding area during April 2017.

(reference)

The following events for the period from April 2016 to late March 2017 have been listed as "Major Seismic Activities" in monthly "Evaluation of Seismic Activities."

  – Kumamoto Earthquakes  M6.5 on April 14, 2016 and M7.3 on April 16, 2016
(Depth of approximately 10 km)
  – Southern Ibaraki Prefecture  M5.5 on May 16, 2016
(Depth of approximately 40 km)
  – Uchiura Bay  M5.3 on June 16, 2016
(Depth of approximately 10 km)
  – Northern Ibaraki Prefecture  M5.4 on July 27, 2016
(Depth of approximately 55 km)
  – Kumamoto Region, Kumamoto Prefecture  M5.2 on August 31, 2016
(Depth of approximately 15 km)
  – Ocean Area near Okinawa Main Island  M5.6 on September 26
(Depth of approximately 45 km)
  – Central Tottori Prefecture  M6.6 on October 21, 2016
(Depth of approximately 10 km)
  – Offshore Fukushima Prefecture  M7.4 on November 22, 2016
(Depth of approximately 10 km)
  – Northern Ibaraki Prefecture  M6.3 on December 28, 2016
(Depth of approximately 10 km)
  – Offshore Fukushima Prefecture  M5.7 on February 28, 2017
(Depth of approximately 50 km)

2 Nationwide Seismic Activities by Region

(1) Hokkaido Region

There is no supplementary information for Hokkaido region.

(2) Tohoku Region

There is no supplementary information for Tohoku region.

(3) Kanto-Chubu Region

– Since April 6, the seismic activity in Chuetsu region, Niigata prefecture has been rather high, and there were 11 earthquakes with seismic intensity 1 or over until May 8. The largest was an M3.9 earthquake on April 14.

–"In the GNSS data for the Tokai region, there has been no change of activity which is likely to be associated with the Tokai earthquake.":
(This evaluation is in agreement with the views presented at the regular meeting on April 24 of the Assessment Committee for Areas under Intensified Measures against Earthquake Disaster (reference below).)

(Reference)

Recent seismic and crustal activity in the Tokai region and its vicinity (Seismological and Volcanological Department, Japan Meteorological Agency, April 24, 2017)
"No change of activity which is likely to be associated with the Tokai earthquake has been observed until now. (Excerpted)"

(4) Kinki-Chugoku-Shikoku Region

There is no supplementary information for Kinki-Chugoku-Shikoku region.

(5) Kyushu-Okinawa Region

– In April, no earthquake with maximum seismic intensity 4 or over (0 in March) and 1 earthquake with maximum seismic intensity 3 (1 in March) occurred in the Kumamoto and Aso regions, Kumamoto prefecture. Note that 2 earthquakes with maximum seismic intensity 4 already occurred until 11th in May. A series of seismic activity of the 2016 Kumamoto Earthquakes in the Kumamoto and Aso regions still continues, decaying as a whole, and the seismic activity at present will continue for a while. According to the GNSS observation, postseismic deformation in the area centered on the series of the seismic activity is converging as a whole.


Reference 1
Earthquakes that are described in the "Evaluation of Seismic Activity" (Monthly Report of Seismic Activity in Japan) have the following features.

  1. Events of M6.0 and greater, or events that have maximum seismic intensity 4 or greater
  2. Onshore events of M4.5 and greater that have maximum seismic intensity 3 or greater
  3. Events under the ocean areas of M5.0 or greater that have maximum seismic intensity 3 or greater

Reference 2
Information that is described in the "Supplementary Information to the Evaluation" includes:

  1. Reference information related to seismic activity described in the "Evaluation of Seismic Activity"
  2. Recent earthquake activity over the past year related to the events described in "Major Seismic Activity" in the "Evaluation of Seismic Activity"
  3. Seismic activity that has been evaluated, but not included in the "Evaluation of Seismic Activity" because it is within normal bounds and not prominent