Information message on the earthquake February 25, 1999

     On February 25, 1999 at 21:58 Moscow time (18:58 GMT) the strong eathquake has happened in the region of Baikal lake, which was felt in a number of places with magnitude from 3 to 5-6 MSK.
     Parameters of this event were determined in the Alert Service (AS) of Information Processing Center (IPC) of Geophysical Survey of RAS in Obninsk with use of stationary data, obtained from digital and analog seismic stations of Russia, counties of CIS and from abroad. Waveforms of this event have arrived in IPC in a close to real time mode from digital seismic stations located on a territory of Russia, which are shown on a picture.

Figure 1
 
     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 18-58-34.0 51 105  33  13  6.1/7 6.5 IPC GS RAS (AS)
2 18-59-38.9 52 104  33  184   5.5-5.8   CSEM
3 18-58-29.2 51 104  10  74   5.8/42  NEIC
4 18-58-32.6 51 104  21  20   4.7   pIDC
  • 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
    • Aftershock
         According to the data of Alert Service of GS RAS 7 aftershocks with M=3.8-4.8 and felt in places around from 2-3 up to 4-5 MSK have occurred in 10 hours after the main strike in the zone of event.
    Macroseismic manifestations
         The earthquake was felt in the following places: Listvianka - 5-6 MSK, Irkutsk - 5 MSK, Sludyanka, Angarsk 4-5 MSK, Talaya, Arshane, Kabansk - 4 MSK, Ulan Ude - 3-4 MSK, Zakamensk 3 MSK.
    Historical records
         This earthquake has happened in seismic active region of Pribakalye in tectonic zone of south-west end of Primorie mountain ridge, where earthquakes with magnitudes more than 6.5 MSK and which are felt in places around with magnitudes up to 9-10 are known historically. Such earthquakes are: on August 1, 1779 with М=6.6 Io=9; on March 7, 1829 with М=7.2-7.5 Io=9-10; on August 17, 1846 with М=6.7 Io=9; on April 11, 1902 with М=6.6 Io=8-9. However in this century there were no such strong events, the strongest from known events has happened on March 22, 1981 with М=5.7 22. Seismic activity increase in this region was detected since January 6, 1999 by Alert Service from survey data of teleseismic network of GS RAS. Up to 10 earthquakes with MS=2.5-4.5 have been detected here since the beginning of the year up till the moment of this strong event, they were felt in places of Baikal Region with magnitudes from 2 to 4 MSK. Table.
    Focal mechanism and seismic moment of the earthquake
         Focal mechanism of event on 25.02.1999 has been calculated in CEME GS RAN with signs of initial arrivals of P-waves at 30 stations, which are located in the intervals of 0.6-89.7 degrees of epicentral distances with quite uniform distribution in azimuth from 13 to 307 degrees. Parameters of the best solution are given in a table and positions of nodal planes in lower hemisphere are shown on the figure below. Focal mechanism was determined by tectonic processes in the rift zone of Baikal lake and presents a strike-slip and thrust. Both nodal planes have north-east spread direction, strike slip component prevails for both of them. Compression axis is oriented in cross of rift structures spread directions, that is typical for the earthquakes of this zone [2]. Seismic moment Мо of this event has been determined from Р-wave spectra, registered by digital equipment IRIS at three seismic stations: Arti (ARU), Obninsk (OBN) and Kislovodsk (KIV) located at distances 27-41 degrees. Average value of Мо was equal to 1.0 Е 18 n*m. The value of moment magnitude Mw has been determined from Kanamori formula [3], Мw=6.0.
    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
    243211859 591811826-134245 72-71IPC GS RAS
  • IPC GS RAS - Informational Processing Center, Geophysical Survey, Russian Academy of Sciences

    • Figure 2
    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. Hanks Tc and Kanamori J. A moment magnitude scale. //Jornal Geophys. Res. 1979. V 84, p. 2348-2350.


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