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Information message about earthquake on January 31 1999

On January the 31-st 1999 at 8:07 am Moscow time (5:07 am GMT) strong earthquake has happened in Dagestan, which was felt in a number of places with intensity 5-6 MSK.

Parameters of this earthquake have been determined in Alert Service (AS) of Information Processing Center (IPC) of Geophysical Survey of RAS in Obninsk with the use of stationary data, obtained from digital and analog seismic stations of Russia,countries of CIS and from abroad As it is seen the identity of data in determination of parameters of the earthquake by different centers is satisfactory, the biggest difference is in coordinates of epicenter, which makes 0.09 degr. for latitude and 0.29 degr. for longitude. The values for source's depth determined by IPC centers (AS GS RAS and SBNC (preliminary variant os seismic bulletin of Northern Caucasus)) and RSCD, are very close and make 19 - 20 km, according to NEIC and CSEM data the source was located within the Earth's crust, and only data of рphp mean that the source was deep in the Earth. We prefer determinations done by IPC and RSCD cause they had information from the nearest stations. Magnitudes MS from data of different centers are stable and make M=5.5-5.6.

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

Note: AB - Alarm Service, SBNC - summary bulletin of Northern Caucasus.

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	05-07-10.9	43	46	19						IPC GS RAS (SBNC)
2	05-07-11.5	43	46	25	35	5.6/16		6.5		IPC GS RAS (AS)
3	05-07-14.8	43	46		131	5.6/1	5.1/6			CSEM
4	05-07-14.1	43	46		50	5.5/50	5.4/25			NEIC
5	05-07-18.8	43	46	65	18		4.9/15			pIDC
6	05-07-08.5	43	46	20						DRIPC

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

pIDC - Prototype International Data Center

DRIPC - Regional Informational Processing Center of Dagestan Experimental-Methodical Seismological Department Geophysical Survey Russian Academy of Sciences

Aftershock

Processing of 33 aftershoks has been done by IPC GS RAS, for which the data were obtained from local network of Dagestan, points of survey of radiotelemetering system supervision for Kavminvodi test area (KVM), broadband digital seismic stations IRIS - KIV, base analog stations of the Northern Caucasus - Mahachkala and Piatigorsk, as well as reopened in January 1999 seismic station Grozny. Localisation of hypocenters was done by the program HYPO71 with the use of local speed cut of Earth's crust /3/. Catalogue, which includes parameters of the main strike and aftershoks is pesented in a table, locations of hypocenters are shown in the map. Here one can find hypocenter of the earthquake on May 14 1970 at 0=6:12 pm with M=6.6. It was the strongest and the most destructive earthquake in the last 29 years in Dagestan /3/ and in close location to it's source's region (to the north) there is epicenter of earthquake, which has happened on January 31 1999.

Macroseismic manifestations

This earthqake was felt in the following places of Dagestan: Buinaksk, Kizilurt - with intensity 5-6 MSK, Mahachkala - 5-5.5 MSK, Hasaviurte - 5 MSK, Babaurt - 3-4 MSK. Material damage from this earthquakeand it's aftershocks is according to the estimations nearly 35 million rubles.

Historical records

In the zone of this earthquake's epicenter (50 km around this point) according to historical data, which were published in /1/ and in Seismological Bulletin of CEME GS RAS (1975-1999) in the last 100 years 7 earthquakes with MS>=4.6 and intensity in epicenter from 5 to 8-9 MSK have happened. Parameters o fthese earthquakes are presented in table 2. All of them happened to the south and south-east from the epicenter of the earthquake on January 31-st 1999.

Table 2

Date Year mm dd	To h-min-sec (GMT	N, Degr.	E, Degr.	H, km	MS	Io
1953/02/27	20-30-04.0	42.90	46.80	30	4.9
1955/08/05	10-20-50.0	43.00	47.20	9	4.8	6
1956/08/27	15-37-24.0	42.90	47.10	14	4.6	5
1970/05/14	09-20-22.0	42.99	47.12	17	5.7	7
1970/05/14	18-12-24.0	43.00	47.09	13	6.6	8-9
1974/12/23	05-22-05.0	42.95	46.82	6	5.0	7
1975/01/09	23-09-42.8	43.09	47.10	4	5.2	8

Focal mechanism and seismic moment of the earthquake

Focal mechanism of the earthquake 31.01.1999 has been calculated in CEME GS RAS by the first signs of Р-waves arrival at 22 stations, which are located in the interval of epicentral distances 0.4 - 63.3 degrees with enough uniform distribution between azimuths from 49 to 334 degrees. Parameters of the best solution are presented in row 2 of table 3. Locations of nodal planes in projection of the lower hemisphere are shown in figure. Row 3 of table 3 - contains the solutions, obtained in Harvard Univercity by the method of centroid's moment tensor (CMT). Row 1 of the same table contains solutions for the focal mechanism of Dagestan earthquake, which happened of May the 14-th 1970. /2/. Focal mechanisms are characterized by the motion in source region of the thrust type. Spread directions of the cutting planes - from the north-west to the south-east coinside with available here system of cuts of the Main Caucasus thrust. The solutions of ocal mechanisms show that the fall angles of these cuts at the depth of hypocenters make 36-60 degrees. Elements of focal mechanism of earthquake on May 14 1970 are close to those for 31.01.1999 - nodal planes are oriented from the north-west to the south-east, planes falling is nder angles 43-50 degr., motion is presented by thrust too. According to /2/, plane N 2 is selected as an active plane of cut in the source region.

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
77	292	12	128	3	38	319	50	106	115	43	72	IPC GS RAS
70	155	16	300	11	33	143	37	118	289	58	71	IPC GS RAS
68	158	18	301	12	300	147	36	121	290	60	69	HARVARD

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

HARVARD - Harvard Geophysical Center (USA)

Literature

1. New Catalogue of strong earthquakes on a territory of USSR from most ancient times till 1975. M. "Science" 1977. 2. Balakina L.M., Zaharova A.I., Moskvina A.G., Chepkunas L.S. Natural relations of earthquake focal mechanisms with a geological structure of area // Physics of the Earth. 1996. N3. pp. 33-52. 3. Dagestan earthquake on May the 14-th 1970 V. 1. Seismology, geology, geophysics. M.: Science. 1980. 217 p.