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Information message about the strong earthquake Off W Coast of Northern Sumatra on April 11, 2012.

The strong earthquake with M=8.5 has occurred on April 11, 2012 at 8:38 GMT (12:38 Moscow time) Off W Coast of Northern Sumatra.

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

Preliminary processing has been done with data from 24 stations, accurate processing - with data 61 stations.

Fig. 1 presents fragments of records obtained from digital stations Diego Garsia (∆ 22.6º), Ala-Archa (∆ 43.4º), Taijon(∆ 46.5º), Ulaanbaatar (∆ 47º), Kungurtug (∆ 48.2º). Fig. 2 presents fragments of records obtained from digital stations Borovoe (∆ 53.9º), Garni (∆ 57.5º), Erimo (∆ 59.8º), Arti (∆ 60.5º), Obninsk (∆ 69.4º).

Figure 1

Figure 2

IPC GS RAS also has obtained information with parameters of this event from some seismological centers, these data are given in table 1:

Table 1

NN	Time in origin h-min-s (GMT)	Lat degr.	Lon degr.	Depth km	N s/st	Ms/ N	mb/ N	I0 calc	GAP	Center
1	08-38-34.5	2.35	92.92	25	61	8.5/32	7.3/34	10.5-11	57	IPC GS RAS (Accurate)
2	08-38-36.6	2.53	92.88	33	24	8.4/5	6.7/11	10-10.5	79	IPC GS RAS (Preliminary)
3	08-38-35.9	2.34	93.16	10	447	Mw 8.7			37	CSEM
4	08-38-37.4	2.31	93.06	22.9	395	Mw 8.6	7.4/197		18	NEIC (Accurate)
5	08-38-38.3	2.35	93.07	33	229		7.3/186			NEIC (Preliminary)

IPC GS RAS - Informational Processing Center, Geophysical Survey, Russian Academy of Sciences

CSEM - European - Mediterranean seismological center

NEIC - National Earthquakes Information Center of US Geological Service

Epicenter

Fig.3 presents with red star epicenter of this earthquake, which origin was located off western coast of Northern Sumatra, at the depth of 25 km, 450 km S-W of Banda-Aceh, Indonesia, 660 km W-S-W of Medan, Indonesia and 910 km W-N-W of Padang, Indonesia.

Figure 3

Aftershock

GS RAS registered 8 aftershocks within 2 hours after the main shock, the strongest of them has occurred at 10:43 with М=8.2, it's epicenter was located 163 km S-S-W.

N	Date & Time (GMT) yy-mm-dd hh:mm:sec	Lat degr.	Lon degr.	Depth km	N station	Ms/ N	mb/ N	Io calc.	Region
20121420	2012-04-11 09:27:57.0	1.22	91.82	33	29	-	5.9/ 17	5.5-6	North Indian Ocean
20121421	2012-04-11 09:48:24.0	1.64	92.43	33	24	-	5.3/ 15	4-4.5	Off w coast of Northern Sumatera
20121422	2012-04-11 09:51:41.0	2.66	90.43	33	21	-	5.5/ 16	4.5-5	Off w coast of Northern Sumatera
20121424	2012-04-11 10:01:18.0	2.45	90.42	33	37	-	5.5/ 22	4.5-5	Off w coast of Northern Sumatera
20121425	2012-04-11 10:08:29.0	2.59	90.19	33	26	-	5.4/ 15	4.5	Off w coast of Northern Sumatera
20121426	2012-04-11 10:21:15.0	2.88	92.37	33	33	-	5.5/ 22	4.5-5	Off w coast of Northern Sumatera
20121427	2012-04-11 10:36:55.0	0.92	92.14	33	20	-	5.4/ 13	4.5	Off w coast of Northern Sumatera
20121423	2012-04-11 10:43:07.0	0.85	92.5	15	62	8.2/ 29	7.0/ 33	11-11.5	Off w coast of Northern Sumatera

Macroseismic manifestations

RIA-Novosti reported about the broken bridge in the area of Aceh-Besar of most western Indonesian Province Aceh. In Banda-Aceh which is the capital of Aceh Province the underground shocks have damaged prison building, however there were no reports about missing prisoners. «Five persons were killed, including one in Banda-Aceh, three of them were killed in the region of Aceh-Besar and one in Lkhoksemava - this was reported by representative of Vakhudi rescue service. In accordance with this service, the disaster influenced most of all three Indonesian Provinces - Aceh, Western and Northern Sumatra, some residents of these provinces urgently need aid. To distribute this aid Indonesian authorities have already prepared five military ships. Indonesian center of tsunami notification has announced alert message for the first wave which reached the coast with height not exceeding 20 cm. Fig. 4 has been obtained from USGS NEIC , it presents the map of intensities composed with messages obtained from 1998 respondents living in 255 places.

Figure 4

Historical records

Epicenter of this earthquake was located 350 km W-S-W of the epicenter of catastrophic earthquake which occurred on December 26, 2004 with magnitude M=8.8 and 460 km W of the epicenter of the earthquake which occurred on March 28, 2005 with magnitude M=8.5. Figure 5 presents epicenters and focal mechanisms of these earthquakes and epicenters 11/04/2012: the main shock at 08:38 with М=8.5 and aftershock at 10:43 with М=8.2. The difference in focal mechanisms may explain the reason for the lack of large tsunami waves after the earthquake 11/04/2012.

Figure 5

Focal mechanism and seismic moment of the earthquake

Focal mechanism solution for the origin of the earthquake which occurred on April 11, 2012 at 08:38 was obtained in USGS NEIC with WPhase Moment Solution technique. It is shown at figure 6 in stereographic projection at the lower hemisphere, elements of this mechanism are presented in table 2. This earthquake has occurred under the action of S-E compression and N-E extension stresses of identical value. Both nodal planes are steeply dipping (DP=80–87º). One of them (NP1) has a sub-longitudinal strike (Stk=199º); another (NP2) has a sub-latitudinal strike (Stk=108º). The type of motion for both planes is strike-slip (right-lateral for NP2 and left-lateral for NP1). Seismic moment of this earthquake which occurred on April 11, 2012 at 08:38 was calculated in IPC GS RAS with Р-wave spectra obtained at four stations: «Talaya» (∆49.98º), «Arti» (∆60.52º), «Kislovodsk» (∆60.68º), «Obninsk» (∆ 69.43º), it makes Мо=1.01* 10**20 N*m. The average value of moment magnitude Mw was calculated with Kanamori formula, it makes Mw=8.0. Seismic moment in accordance with USGS NEIC makes Мо=9.0*10**21 N*m, Mw=8.6.

Table 2

Axes of main strengths

Nodal planes

Center

NP1

NP2

Azm

Stk

Slip

Stk

Slip

154

271

199

108

170

NEIC

NEIC - National Earthquakes Information Center of US Geological Service

Figure 6