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Study on related inland earthquakes

It is known that, in the aftermath of a giant earthquake, variations in stress fields in the Japanese Islands give rise to earthquakes of relatively large magnitudes at locations far removed from the source region of the giant earthquake. Here we focus on those that have occurred in inland areas and off coastal areas in the Japan Sea.

M > 6 earthquakes following the 11 March 2011 mainshock
Earthquakes of relatively large magnitudes have occurred in inland regions in the aftermath of the giant event. Five earthquakes of magnitudes 6 or greater, (1) to (5) as listed below, occurred beneath land and beneath coastal waters of the Japan Sea between immediately after the giant event and 19 March 2011. In Figure 1, focal mechanism solutions are plotted at the epicenters of these earthquakes. The epicentral locations and magnitudes are provisional data of the Japan Meteorological Agency (JMA). These earthquakes may have been triggered by large variations in inland stress fields due to the giant event off the Pacific Coast of the Tohoku District.

Figure 1. M > 6 inland earthquakes following the 11 March 2011 mainshock (as of 19 March)

Figure 1. M > 6 inland earthquakes following the 11 March 2011 mainshock (as of 19 March)
Epicenters in the mainshock source area (other than (1)-(5)) are from JMA preliminary data issued by 13 March.
The focal mechanism solutions of events (1)-(5) are from the Global CMT Catalog.
(1) 03:59, 12 March 2011: Northern Nagano Prefecture (M 6.2), depth 8 km
(2) 04:46, 12 March 2011: Off the Coast of Akita Prefecture (M 6.4), depth 10 km
(3) 22:31, 15 March 2011: Eastern Shizuoka Prefecture (M 6.2), depth 14 km
(4) 18:56, 19 March 2011: Northern Ibaraki Prefecture (M6.1), depth 20 km
(5) 07:12, 23 March 2011: Southeastern Fukushima Prefecture (M6.0), depth 10 km
* Analysis of seismicity near the Ibaraki-Fukushima Prefectural border

The hypocenters and focal mechanisms of these earthquakes are characterized as follows.


Information on the 03:59, 12 March, earthquake in northern Nagano Prefecture

The earthquake occurred in a region lying between the source areas of the 2004 Chuetsu earthquake (M 6.8) and the 1847 Zenkoji earthquake (M7.4) (Figure 2). Figure 3 shows focal mechanisms of microearthquakes occurring in the immediate vicinity of the event as determined by AIST's temporary observation in the aftermath of the 2004 Chuetsu earthquake. The event is of a reverse-fault type with a NE-SW compression axis, and is similar to earthquakes that occur in the neighborhood of the Tokamachi fault zone.

Figure 2. Characteristics of the location of occurrence of the 2011 northern Nagano Prefecture earthquake.

Figure 2. Characteristics of the location of occurrence of the 2011 northern Nagano Prefecture earthquake.
Modified from Imanishi et al. (2006).

Figure 3.Focal mechanism of the 2011 northern Nagano Prefecture earthquake and mechanism solutions for microearthquakes in its neighborhood. Modified from Imanishi et al. (2006).

Figure 3. Focal mechanism of the 2011 northern Nagano Prefecture earthquake and mechanism solutions for microearthquakes in its neighborhood. Modified from Imanishi et al. (2006).


Information on the 04:46, 12 March, earthquake off the coast of Akita Prefecture

The earthquake occurred near the mainshock hypocenter of the 1983 Nihon-kai Chubu (Central Japan Sea) earthquake (M7.7). The focal mechanism is of a strike-slip type with an ENE-WSW compression axis.


Information on the 22:31, 15 March, earthquake in eastern Shizuoka Prefecture

The earthquake occurred about 8 km to the SSE of the top of Mt. Fuji. The focal mechanism is of a strike-slip type with a NNW-SSE compression axis. Figure 4 shows geological structures near the epicenter, Figure 5 shows gravity anomalies, and Figure 6 illustrates the relationship with volcanic geology of Mt. Fuji.
(Related information has been submitted as materials to the 222nd meeting of the Earthquake Research Committee (in Japanese).)

Figure 4. Geological structures near the epicenter of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4)

Figure 4. Geological structures near the epicenter of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4) (Neotectonic Map "Tokyo" 1:500,000, Sugiyama et al. (1997))


Figure 5. Map of gravity anomalies near the epicenter of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4) (AIST, 2004)

Figure 5. Map of gravity anomalies near the epicenter of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4) (AIST, 2004)


Figure 6. Map of volcanic geology near the epicenter (red circle) of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4) (Geological Map of Mt. Fuji, 2002) and distribution of earthquakes beneath Mt. Fuji.

Figure 6. Map of volcanic geology near the epicenter (red circle) of the 15 March earthquake in eastern Shizuoka Prefecture (M 6.4) (Geological Map of Mt. Fuji, 2002) and distribution of earthquakes beneath Mt. Fuji.
Broken contours: Source areas of earthquakes observed between August 2000 and July 2001 (data according to NIED). Blue contour: Primary source area of low-frequency earthquakes / Green contour: Primary source area of other earthquakes.


Information on the 18:56, 19 March, earthquake in northern Ibaraki Prefecture

The Idosawa fault, the Tanakura tectonic line and the Takahagi fault exist near the hypocenter of the northern Ibaraki Prefecture event. The focal mechanism is of a normal-fault type, which is uncommon for this area, with a NE-SW tension axis.
The 19 March 2011 northern Ibaraki Prefecture earthquake and geological structures of the surroundings.

Figure 7. The 19 March 2011 northern Ibaraki Prefecture earthquake and geological structures of the surroundings.
(Geological Map of Japan 1:1,000,000, 3rd Edition, 2nd CD-ROM Version, GSJ AIST (2003))


Information on the 07:12, 23 March, earthquake in Southeastern Fukushima Prefecture

This earthquake occurred about 25 km to the northeast of the northern Ibaraki Prefecture event. The Idosawa fault is located nearby. The focal mechanism is of a normal-fault type, with the tension axis approximately in the east-west direction.
The 19 March and 23 March 2011 earthquakes and geological structures of the surroundings.

Figure 8. The 19 March and 23 March 2011 earthquakes and geological structures of the surroundings.
(Geological Map of Japan 1:1,000,000, 3rd Edition, 2nd CD-ROM Version, GSJ AIST (2003))



Analysis of seismicity near the Ibaraki-Fukushima Prefectural border

After the 11 March giant event, seismic activities became high in the areas where earthquakes (4) and (5) occurred. Figure 1 shows the epicenter distribution of all earthquakes, including small events, that occurred between 11 March and 23 March. Focal mechanism solutions according to NIED's F-net are also shown for events of relatively large magnitudes. To facilitate comparison of pre-11 March activities and the latest activities, Figure 2 plots the epicenters of earthquakes that occurred between 1990 and 2010. The activities near the epicenter of the 19 March event (M 6.1) are located in an area where seismicity used to be very low, including for small events. To compare the focal mechanism of the latest earthquakes with those of previous events, Figure 3 plots F-net focal mechanism solutions for earthquakes that occurred in February 2011 or earlier at depths smaller than 20 km in the surrounding areas. Greenish colors represent reverse-fault type focal mechanisms; reddish colors, strike-slip types; and bluish colors, normal-fault types. It appears that no great earthquakes had occurred in this region in latest years, and that no normal-fault type earthquakes had occurred in the surrounding areas.
Figure 1. Earthquakes near the Ibaraki-Fukushima Prefectural border.

Figure 1. Earthquakes near the Ibaraki-Fukushima Prefectural border. The hypocenters are due to the JMA integrated catalog until 23:59, 21 March, and due to Hi-net's automatically processed data from 22 March onward. The plots are only for earthquakes with depths smaller than 20 km. The focal mechanism solutions plotted are NIED's F-net solutions. Red circles, events with M greater than 5. Black squares, locations of the Fukushima Daiichi and Fukushima Daini nuclear power plants.


Figure 2. Hypocenter distribution for the period 1 January 1990 to 31 December 2010 (JMA integrated catalog).

Figure 2. Hypocenter distribution for the period 1 January 1990 to 31 December 2010 (JMA integrated catalog). The plots are only for earthquakes with depths smaller than 20 km. The seismicity is especially low near the epicenter of the 19 March event (M 6.1).


Figure 3. F-net focal mechanism solutions for earthquakes that occurred prior to the Off the Pacific Coast of Tohoku event at depths smaller than 20 km.

Figure 3. F-net focal mechanism solutions for earthquakes that occurred prior to the Off the Pacific Coast of Tohoku event at depths smaller than 20 km.

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