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Information message about the strong earthquake in the Pacific Ocean, on the east coast of Honshu Island, Japan, March 20, 2021.

An earthquake (strong at the epicenter on the SHSI-17 scale) with M = 6.7 occurred on March 20, 2021 at 09:09 GMT (12:09 Moscow time) in the Pacific Ocean, on the east coast of Honshu Island, Japan.

Parameters of the earthquake have been determined in the Alert Service (AS) of Information Processing Center (IPC) of Geophysical Survey of RAS in Obninsk with station data obtained from digital and analog seismic stations of Russia, countries of CIS and from abroad. Alert message on this earthquake has been delivered in 9 min after registration to the Ministry of Emergency of the Russian Federation (EMERCOM).

Preliminary processing has been done with data from 11 stations, accurate processing - with data from 61.

The red marker on the map is the epicenter of the earthquake, the origin was in the Pacific Ocean, on the east coast of Honshu Island, Japan, at a depth of 60 km, 79 km E-NE of Sendai, 264 km S-SE of Aumori and 360 km N-NE from Tokyo, capital of Japan.

	Time in origin (GMT)	Latitude, degree	Longitude, degree	Depth, km	Stations number	Ms/N	mb/N	I0 (calculated)	GAP	Center
1	09:09:45.0	38.56	141.69	60.0	61	6.7/19	7.0/27	6.5-7	152	GS RAS(Verified) Federal research center, Geophysical survey of Russian Academy of Sciences
2	09:09:55.8	39.25	141.21	100.0	11		6.3/9	5-5.5		GS RAS(Preliminary) Federal research center, Geophysical survey of Russian Academy of Sciences
3	09:09:46.5	38.52	141.55	60.0	217	Mw 7.0			36	CSEM European - Mediterranean seismological center
4	09:09:45.3	38.48	141.67	54.0	110	Mw 7.0/26			60	NEIC National Earthquakes Information Center of US Geological Service

Fig. 1 presents fragments of records obtained from digital stations Matsushiro (∆ 3.4º), Yuzhno-Kurilsk (∆ 6.3º), Kurilsk (∆ 8.1º), Yuzhno-Sakhalinsk (∆ 8.4º), Vladivostok (∆ 8.7º),. Fig. 2 presents fragments of records obtained from digital stations Kuldur (∆ 12.9º), Severo-Kurilsk (∆ 15.8º), Petropavlovsk (∆ 18.3º), Magadan (∆ 21.8º), Yakutsk (∆ 24.6º).

Image 1

Image 2

GS RAS registered 2 aftershocks with М>=4.5 within the day after the main shock.

N	Time in origin (GMT)	Latitude, degree	Longitude, degree	Depth, km	Stations number	Ms/N	mb/N	I0 (calculated)	Region
20211339	20/03/2021 16:41:18.0.0	38.26	141.76	60.0	20		4.5/8		Near east coast of Honshu, Japan
20211344	20/03/2021 21:56:37.0.0	37.69	141.8	60.0	31		4.8/20	2-2.5	Near east coast of Honshu, Japan

According to TASS seven people were injured in Miyagi prefecture, according to RIA Novosti two persons were injured in Iwate and Fukushima prefectures. The tsunami threat was announced in Japan, but then this warning was canceled. Shinkansen high-speed trains were stopped in several directions. 200 houses were left without electricity on the territory of Miyagi prefecture. The earthquake did not cause emergency situations at the nuclear facilities located in this region, including the Fukushima-1 nuclear power plant. The part of the support construction fell into the cooling pool with spent nuclear fuel (SNF) at the Onagawa NPP, but because of the small size of this part, there was no negative impact on the SNF assemblies. Cracks also appeared on the glass and on the ceiling in the corridor leading to the first and second units of NPP..
Fig 1 was taken from USGS NEIC, it presents USGS Community Internet Intensity Map made by 379 responss in 142 cities.

Image 1

Click here to view detailed macroseismic data (DyfitWeb service)

The focal mechanism solution of the origin for the earthquake of March 3 was calculated in GS RAS with signs of initial arrivals of P waves at 65 stations, including 19 stations which registered compression waves (plus signs) and 46 stations which registered tensile waves (minus signs). These stations are located in the interval of epicentral distances of 2.4-95.6º and in azymuth sector Az=5-350º. The focal mechanism solution in stereographic projection on the lower hemisphere and its elements are presented in the table. The table also contains focal mechanism solutions obtained at the USGS NEIC with W-phase Moment Tensor (Mww) technique and in Lamont-Dohert Earth Observatory of Columbia University, USA (LDEO GCMT) with СМТ (Centroid moment tensor) technique. Solutions from different centers are close. The earthquake occurred under the action of slight prevalence of compressive stresses oriented E-SE (AZ=107-114º). One nodal plane NP1 strikes to the south (STK=190-193º), another plane NP2 strikes N-NE (STK=21-28º). NP1 plane lies more flat (DP=20-24º) if compared with NP2 (DP=67-71º). The type of motion along NP1 – slip, along NP2 – reverse fault. The seismic moment according to USGS NEIC Mo=4.35*10**19 N*m, Mww=7. According to LDEO GCMT Mo=5.061*10**19 N*m, Mw=7.1.

Axes of main strengths			Nodal planes		Center
T	P	N	NP1	NP2	Center
PI: 64 Azm: 306	PI: 26 Azm: 114	PI: 5 Azm: 207	Stk: 193 Dp: 20 Slip: 76	Stk: 28 Dp: 71 Slip: 95	GS RAS Federal research center, Geophysical survey of Russian Academy of Sciences
PI: 68 Azm: 298	PI: 22 Azm: 107	PI: 4 Azm: 199	Stk: 190 Dp: 24 Slip: 81	Stk: 21 Dp: 67 Slip: 94	NEIC National Earthquakes Information Center of US Geological Service
PI: 65 Azm: 298	PI: 24 Azm: 109	PI: 3 Azm: 200	Stk: 191 Dp: 21 Slip: 81	Stk: 22 Dp: 69 Slip: 94	GCMT Lamont-Doherty Earth Observatory Global CMT Columbia University, USA

The earthquake happened 79 km north of another earthquake of February 13, 2021 on the east coast of Honshu Island with a magnitude of M = 7.0 and 93 km W-NW of the Great East Japan Tohoku Earthquake of March 11, 2011 off the east coast of Honshu Island with a magnitude of M = 8.8. Then after the earthquake and the tsunami that followed it killed more than 15 thousand people, more than 6 thousand were seriously injured. Thousands of buildings were destroyed. The height of the tsunami waves exceeded ten meters. The Fukushima-1 NPP was in an emergency state due to the failure of the cooling system, there was a strong radioactive contamination.