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Information message about the strong earthquake in the region of Nikobar Islands on July 24, 2005

On July 24, 2005 the strong earthquake has happened in the region of Nikobar Islands at 15:42 GMT with magnitude М=7.5. Epicenter of this earthquake was in 640 km to the North-West from catastrophic earthquake in Sumatra, which happened on December 26, 2004 (М=8.8) and in 840 km to the North-West from the other catastrophic earthquake in Sumatra, which happened on March 28, 2005 (М=8.5). Aftershock process in this region is still under way, epicenter of the earthquake on July 24, 2005 was located at the cross zone of aftershocks having North-Western spread direction.

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

Preliminary processing has been done with data from 33 stations, accurate processing - with data 41 stations.

Fig. 1 presents fragments of records for this earthquake obtained from digital stations Ala-Archa (delta 38 degrees), Borovoe (delta 48 degrees), Garni (delta 53 degrees), Arti (delta 55 degrees), Kislovodsk (delta 56 degrees). Fig. 2 presents fragments of records for this earthquake obtained from digital stations South-Sakhalinsk (delta 58 degrees), Yakutsk (delta 61 degrees), Obninsk (delta 64 degrees), Magadan (delta 68 degrees), Petropavlovsk (delta 70 degrees).

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	15-42-06.50	7.95	92.28	33	41	7.50/16	7.10/12	8.5-9	67	IPC GS RAS (Accurate)
2	15-42-06.7	7.97	92.27	33	33	7.5/12	7.1/9	8.5-9	67	IPC GS RAS (Preliminary)
3	15-42-07.6	7.99	92.16	33	350	7.2Мw			36	CSEM
4	15-42-05.67	7.93	92.15	10	146		6.7/60			NEIC (Preliminary)
5	15-42-05.66	7.93	92.14	10	158	7.5/114	6.8/66		36	NEIC (Accurate)
6	15-42-00.0	8.11	94.59	30		8.3				ORFEUS

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

ORFEUS - Observatories and Research Facilities for EUropean Seismology

Epicenter

This earthquake occurred near the western boundary of the aftershock zone of the devastating Sumatra-Andaman Islands earthquake of December 26, 2004. The aftershock zone of the December 26th earthquake continues to be active and forms a crescent-shaped region extending about 1200 km north of Sumatra, Indonesia. The recent event was a strike-slip earthquake, probably in the India plate below its shallowly dipping boundary with the India plate. The earthquake resulted from horizontal displacement of the ground across a nearly vertical fault plane. This is unlike the thrust-fault motion that produced the December 26th earthquake and that occurred on the interface of the eastward-subducting India plate and the overriding Burma plate. The recent event therefore occurred on a different fault than the fault that caused the Great Sumatra-Andaman Islands earthquake. However, the two earthquakes are almost certainly related. A great earthquake can trigger earthquake activity on faults that are distinct from the main-shock fault plane, and separated from it by tens or even hundreds of kilometers. A magnitude 7.2 earthquake is considered a major earthquake, but the size of this recent earthquake pales in comparison to the December 26th earthquake. A 7.2 strike-slip earthquake typically ruptures a 15 km by 80 km patch along a fault, whereas the December 26, 2004 earthquake ruptured a patch of fault about 1200 km long and 200 km wide. http://earthquake.usgs.gov/eqinthenews/2005/usaxay/.

Macroseismic manifestations

Insignificant underground strikes have been felt in Chennai and Visakhapatam (Tamilnad State, India). Thailand Authorities warned the population of six coastal provinces about the danger of tsunami. Later tsunami warning was called off. Adaman and Nikobar Islands, which bolong to India, suffered from tsunami in December last year. 178000 people in 11 countries became victims of that tsunami, including 10000 living on Nikabar and Andaman Islands.

Focal mechanism and seismic moment of the earthquake

Focal mechanism solutions of the earthquake 24.07.2005 have been calculated with TMC method in NEIC and HARVARD centers and are shown in figures 3, 4 and table 2. In accordance with data from different centers these solutions are almost the same. But they differe from the solution of the earthquake on December 26, 2004. The motion in origin for the earthquake 24.07.2005 has happened under the action of almost the same value compression and expansion strengths. Compression strengths were oriented along meridian, expansion strengths were longitudinally oriented. Both nodal planes are steep falling, NP1 plane has South-East - North-West spread direction, NP2 plane has North-East-South-West spread direction. The type of the motion - drift, for NP1 plane-right side drift, for NP2 plane- left side. The value of seismic moment Мо in accordance with NEIC made 6.5* 10** 19 n*m, the value of moment magnitude - Mw = 7.2. The value of seismic moment Мо in accordance with HARVARD made 8.8* 10** 19 n*m, the value of moment magnitude - Mw = 7.2.

Table 2

Axes of main strengths						Nodal planes						Center
T		P		N		NP1			NP2
Pl	Azm	Pl	Azm	Pl	Azm	Stk	Dp	Slip	Stk	Dp	Slip
15	78	4	347	75	244	122	77	172	214	82	13	NEIC
8	76	17	344	71	189	121	72	-173	29	83	-18	HARVARD

NEIC - National Earthquakes Information Center of US Geological Service

HARVARD - Harvard Geophysical Center (USA)

Figure 3

Figure 4