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Information message about a strong earthquake off the coast of Alaska on July 29, 2021.

An earthquake (devastating at the epicenter on the SHSI-17 scale) with M = 8.0 occurred on July 29, 2021 at 06:15 GMT (09:15 Moscow time) off the coast of Alaska.

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 11 min after registration to the Ministry of Emergency of the Russian Federation (EMERCOM).

Preliminary processing has been done with data from 13 stations, accurate processing - with data from 72.

The red marker on the map shows the epicenter of the earthquake, the origin of which was in the Pacific Ocean, S of the Alaska Peninsula, USA, at a depth of 15 km, 76 km SE of Perryville and 151 km E of Sand Point.

	Time in origin (GMT)	Latitude, degree	Longitude, degree	Depth, km	Stations number	Ms/N	mb/N	I0 (calculated)	GAP	Center
1	06:15:45.0	55.54	-158.11	15.0	72	8.0/46	7.3/55	10.5-11	70	GS RAS(Verified) Federal research center, Geophysical survey of Russian Academy of Sciences
2	06:15:56.6	55.78	-158.12	100.0	13		6.6/11	5.5-6		GS RAS(Preliminary) Federal research center, Geophysical survey of Russian Academy of Sciences
3	06:15:50.5	55.54	-158.01	43.4	873	Mw 8.2			16	CSEM European - Mediterranean seismological center
4	06:15:49.9	55.47	-157.92	46.7	82	Mw 8.2	7.0		66	NEIC National Earthquakes Information Center of US Geological Service

Fig. 1 presents fragments of records obtained from digital stations Kodiak Island (∆ 3.8º), College (∆ 10.6º), Petropavlovsk (∆ 25.1º), Seimchan (∆ 25.8º), Severo-Kurilsk (∆ 27.5º). Fig. 2 presents fragments of records obtained from digital stations Tiksi (∆ 33.6º), Kurilsk (∆ 35.1º), Yakutsk (∆ 36.2º), Yuzhno-Sakhalinsk (∆ 37º), Ussuriysk (∆ 44.6º).

Image 1

Image 2

GS RAS registered 23 aftershocks with М>=4.6 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
20213020	29/07/2021 06:23:28.0.0	55.7	-157.81	15.0	35		6.6/23	8.5-9	Alaska Peninsula
20213018	29/07/2021 06:32:34.0.0	55.43	-156.98	15.0	52		6.2/38	7.5-8	South of Alaska
20213022	29/07/2021 06:47:23.0.0	55.21	-157.82	15.0	39		5.5/18	6	Alaska Peninsula
20213019	29/07/2021 07:49:57.0.0	55.38	-157.48	15.0	39		5.0/23	4.5-5	Alaska Peninsula
20213021	29/07/2021 08:09:43.0.0	55.34	-157.42	15.0	32		5.0/21	4.5-5	Alaska Peninsula
20213023	29/07/2021 08:15:45.0.0	55.17	-157.15	15.0	27		4.8/21	4-4.5	Alaska Peninsula
20213024	29/07/2021 08:25:21.0.0	55.84	-156.23	15.0	24		4.8/8	4-4.5	South of Alaska
20213027	29/07/2021 09:28:21.0.0	55.63	-157.73	15.0	50		5.3/41	5.5	Alaska Peninsula
20213026	29/07/2021 09:53:40.0.0	55.4	-157.02	15.0	31		5.0/22	4.5-5	Alaska Peninsula
20213028	29/07/2021 10:26:54.0.0	55.02	-157.08	15.0	25		4.6/21	3.5-4	Alaska Peninsula
20213031	29/07/2021 11:08:46.0.0	55.6	-157.73	15.0	23		4.7/11	4-4.5	Alaska Peninsula
20213032	29/07/2021 12:42:43.0.0	55.35	-156.48	15.0	21		4.8/13	4-4.5	South of Alaska
20213033	29/07/2021 13:17:49.0.0	56.26	-156.43	15.0	30		5.1/20	5	Alaska Peninsula
20213035	29/07/2021 16:09:09.0.0	55.33	-157.18	15.0	57		5.6/36	6-6.5	Alaska Peninsula
20213037	29/07/2021 19:11:31.0.0	56.08	-157.09	15.0	25		4.9/19	4.5-5	Alaska Peninsula
20213038	29/07/2021 20:30:31.0.0	55.52	-157.02	15.0	48		5.5/26	6	Alaska Peninsula
20213039	29/07/2021 20:32:41.0.0	55.37	-157.02	15.0	54		5.9/36	6.5-7	Alaska Peninsula
20213040	29/07/2021 21:00:03.0.0	54.86	-157.71	15.0	36		5.0/25	4.5-5	South of Alaska
20213041	29/07/2021 21:39:13.0.0	55.11	-157.09	15.0	28		4.8/22	4-4.5	Alaska Peninsula
20213042	29/07/2021 22:18:28.0.0	55.5	-157.51	15.0	42		5.4/28	5.5-6	Alaska Peninsula
20213044	29/07/2021 23:38:13.0.0	55.16	-156.98	15.0	22		4.7/12	4-4.5	South of Alaska
20213049	30/07/2021 03:52:13.0.0	55.6	-157.21	15.0	63	5.4/34	5.9/40	6.5-7	Alaska Peninsula
20213050	30/07/2021 05:32:12.0.0	55.44	-159.1	15.0	34		5.1/21	5	Alaska Peninsula

According to National Center for Tsunami Prevention, tsunami threat was declared. Maximum tsunami wave height 0.21 m was registered at Old Harbor and 0.15 m at Sand Point.
Fig. 1 was obtained from USGS NEIC, it presents USGS Community Internet Intensity Map made by 268 responses in 47 cities.

Image 1

Click here to view detailed macroseismic data (DyfitWeb service)

The focal mechanism solution of the origin was calculated in GS RAS with signs of initial arrivals of P waves at 99 stations, including 84 stations which registered compression waves (plus signs) and 15 stations which registered tensile waves (minus signs). These stations are located in the interval of epicentral distances of 3.6-99.6º and in azimuth range Az=2-358º. 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 WW-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 compression stresses oriented S-SE (AZ=145-151º). One nodal plane NP1 strikes SW (STK=230-236º) according to GS RAS and LDEO GCMT and W-SW (STK=239º) according to USGS NEIC, another plane NP2 strikes E-NE (STK=54º-65º). NP1 plane lies more horizontally (DP=10-20º) if compared with NP2 (DP=54-65º). The type of motion along NP1 – thrust, along NP2 – reverse fault. The seismic moment according to USGS NEIC makes Mo=2.36 *10**21 N*m, Mww=8.2. According to LDEO GCMT Mo=2.96*10**21 N*m, Mw=8.2

Axes of main strengths			Nodal planes		Center
T	P	N	NP1	NP2	Center
PI: 64 Azm: 343	PI: 26 Azm: 151	PI: 5 Azm: 243	Stk: 230 Dp: 20 Slip: 76	Stk: 65 Dp: 71 Slip: 95	GS RAS Federal research center, Geophysical survey of Russian Academy of Sciences
PI: 59 Azm: 323	PI: 31 Azm: 145	PI: 1 Azm: 54	Stk: 239 Dp: 14 Slip: 95	Stk: 54 Dp: 76 Slip: 89	NEIC National Earthquakes Information Center of US Geological Service
PI: 55 Azm: 331	PI: 35 Azm: 150	PI: 1 Azm: 240	Stk: 236 Dp: 10 Slip: 86	Stk: 60 Dp: 80 Slip: 91	GCMT Lamont-Doherty Earth Observatory Global CMT Columbia University, USA

According to USGS NEIC, the earthquake occurred as a result of a shock-sliding fault near the subduction zone boundary between the Pacific and North American plates. At the site of this event, the Pacific plate is approaching the North American plate in the northwest at a speed of about 64 mm / year.. The boundary of the subsidence of the Pacific plate under the North American plate near the Alaska-Aleutian trench is located 125 km southeast of this earthquake. Strong earthquakes are common in the Alaska-Aleutian subduction zone. Since 1900, eight more earthquakes with a magnitude M> = 7 have occurred within a radius of 250 km from the event on July 29, 2021, including the earthquake on July 22, 2020 with a magnitude M = 7.9 (located 47 km southwest of the July 29 event 2021) and October 19, 2020 with a magnitude M = 7.7 (located 148 km west-south-west of the July 29, 2021 event). The largest of these was the M = 8.2 earthquake on November 10, 1938, which caused a small tsunami that was recorded both locally and in Hilo, Hawaii. The second largest earthquake recorded by modern seismic instruments also occurred on the Alaska-Aleutian Trench., The Great Alaska earthquake with M=9.2 of March 27, 1964, happened about 250 km from this event. The most severe damage was noted in and around Anchorage, where many buildings were destroyed. The earthquake triggered a tsunami that devastated many cities along the Gulf of Alaska and left severe damage in Port Alberney, Canada, along the west coast of the United States and Hawaii. The maximum wave height recorded in Valdez Bay was 67 meters. Tsunami waves have been obsrved in over 20 countries, including Peru, New Zealand, Papua New Guinea, Japan and Antarctica.