Information message about earthquake on March 21 1999

     On March the 21-st 1999 at 7 pm Moscow time (4 pm GMT) with interval of one minute at 7:16 pm and 19:17 pm two strong earthquakes have happened in region of Baikal lake, which were felt in a number of places of Irkutsk region with intensities from 2 to 6 MSK.
     Parameters of this earthquake have been determined by the Alert Service (AS) of Information Processing Center (IPC) in Geophysical Survey of GS RAS in Obninsk with the use of stationary data obtained from digital and analog seismic stations of Russia, countries of CIS and from abroad.
     Waveforms of this event, which were received by IPC in a close to real time mode from digital seismic stations on a territory of Russia, are shown in a figure below. Localization of the 2-nd earthquake has been executed with the use of some additional data from waveform processing by 14 stations IRIS, installed on SPYDER DMC IRIS (figure is given below here).

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:
Note: [1] - 1st earthquake, [2] - 2nd earthquake.
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 [1] 16-16-03.60 55 110  20  11  6.1/6 6.5-7.0 IPC GS RAS (Amended)
2 [1] 16-16-03.10 55 110        5.5 IPC GS RAS (Baikal Regional Center)
3 [2] 16-17-05.00 55 110  20  20  6.0/3   IPC GS RAS
4 [1] 16-16-02.80 55 111  33  11  6.10/6 6.5-7 IPC GS RAS
5 [1] 16-16-06.00 56 110  33  145   5.6-6.1   CSEM
6 [1] 16-16-02.30 55 110    102   5.7   NEIC
7 [2] 16-17-03.30 55 110    59   5.5   NEIC
8 [1] 16-16-02.60 55 110  2  23   5.2/17  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 GS RAS 11 aftershoks have happened after the main strike in the following 40 hours in earthquake's area with intensities M=3.8-4.8 and felt in nearby places with intensities from 2-3 to 4-5 MSK (table 2).
    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
    9903281999-03-22 16:13:15.055.8110.33313.4/ 1-2.5-3 Lake Baykal region, Russia
    9903271999-03-22 15:48:16.055.7109.93313.6/ 1-3 Lake Baykal region, Russia
    9903241999-03-22 12:29:08.055.7110.33314.8/ 1-4.5-5 Lake Baykal region, Russia
    9903231999-03-22 12:05:29.055.8110.33313.7/ 1-3-3.5 Lake Baykal region, Russia
    9903221999-03-22 08:46:33.055.43110.763353.2/ 1-2-2.5 Lake Baykal region, Russia
    9903211999-03-21 22:42:34.055.65110.633344.2/ 3-3.5-4 Lake Baykal region, Russia
    9903201999-03-21 21:14:19.055.71110.523364.3/ 2-4-4.5 Lake Baykal region, Russia
    9903191999-03-21 19:40:17.055.85110.54334-4.0/ 1-Lake Baykal region, Russia
    9903181999-03-21 18:15:37.055.81103314.1/ 1-3.5-4 Lake Baykal region, Russia
    9903171999-03-21 17:35:35.055.36111.213344.4/ 1-4-4.5 Lake Baykal region, Russia
    Macroseismic manifestations
         These earthquakes have been felt in the following places: Nizhni Angarsk, Kichera, verhnia Zimka with intensity 6 MSK, Severo Baikalsk with intensity 5-6 MSK, Irkutsk - with intensity 3 MSK, Bodaibo, Elantsi with intensity 2-3 MSK, Sludianka, Talaya, Tirgana with intensity 2 MSK
    Historical records
         These earthquakes happened in seismic active region of the north-east part o Baikal Rift zone, where the earthquakes with magnitude of 5.5 and calculated intensity of 8-9MSK in the epicenter took place in this century. Such earthquakes are: August 17 1902 with M=6.5 Io=9; Nizneangarsk August 6 1931 with M=5.9 Io=8; December 25 1937 with M=6.1 Io=8; September 23 1940 with M=5.7 and Muyakansk November 11 1962 with M=5.8 Io=(8). According to the data of Baikal Regional Center in the previous year 26 earthquakes with M=3-4 have happened in 100km zone around described event's source location, six of them have been felt in nearby places with intensity between 2 and 4-5 MSK. After February 18 of this year 15 strikes have happened here with М=3-4.5, most strong of them is - February 21 at 3:52am, felt in Nizny Angarsk, Kichera with intensity 3MSK, Bodaibo with intensity 2-3MSK, Severobailkalsk, Angoe with intensity 2MSK.

    Figure 2
    Focal mechanism and seismic moment of the earthquake
         For the first earthquake 21.03.1999 (0=16:16GMT) it has been calculated by CEME GS RAS by the first signs of Р-waves arrival at 35 stations, which are located in the interval of epicentral distances 8.3 - 87.7 degrees and enough uniform spread between azimuths from 5 up to 342 degrees. Parameters of the best solution are presented in the table, while position of nodal planes in projection of the lower hemisphere is shown in the figure. Mechanism of earthquake on 21.03.1999 is characterized by normal fault and strike-slip with subhorizontal orientation of expanding axis, directed in the cross of rift structures spread of Baikal lake. One of nodal planes had north-east spread direction, while the other - nearly latitude directed. Focal mechanism for the earthquake 21.03.1999 is like for the earthquake 11.11.1962. In areas of both sources of these events there is nodal plane with north-east spread direction, as well as it is noted domination of expanding strength, that is very specific for rift zones [2]. Seismic moment Мо of the earthquake has been determined from Р-wave spectra, which were registered by digital equipment IRIS on three seismic stations: Arti (ARU), Obninsk (OBN) and Kislovodsk (KIV), which are located at distances 28-43 degrees. Average value of Мо made 1.0 Е 18 n*m. The value of moment magnitude Mw was calculated from formula Kanamori [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
    015436244 54632135513694 55-44IPC GS RAS
  • IPC GS RAS - Informational Processing Center, Geophysical Survey, Russian Academy of Sciences

    • Figure 3
    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.


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