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Information message about earthquake on April 8 1999

On April the 8-th 1999 at 5 pm Moscow time (1pm GMT) the strong deep earthquake has happened on the border of Russia with China (100 km to the North-West from Vladivostok city). It was felt in a number of towns of Primor'e with intensity of 3 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. Preliminary summary processing of this event has been done with the use of data from 15 stations, while amendment - with data from 29 stations including 16 stations of Russia, 8 of CIS and 5 foreign.

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 1, 2 below.

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	13-10-31.70	43	130	550	15		7.00/4			IPC GS RAS
2	13-10-32.60	43	130	550	29	6.5/9	7.0/7			IPC GS RAS (Amended)
3	13-10-38.00	44	130	580			7.2			IPC GS RAS (Sakhalin Center of RAS)
4	13-10-28.40	43	130	500	191		6.2-7.2			CSEM
5	13-10-34.00	43	130	560	125		6.5/55			NEIC
6	13-09-29.50	42	130	112	18		7.1/12			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

Macroseismic manifestations

Described here event happened at the long depth inside the Earth's mantle. Thus, despite of the significant magnitude (M more than 7.0), it's macroseismic effect on the surface was not so huge in intensity (up to 2-3 MSK). The area, where the event was felt, was wide enough. This earthquake was felt in Russia in the following places of Promor'e region: Habarovsk III; Komsomolsk on Amur, Zeia II-III, Vladivostok, Nahodka II, as well as in Japan's south of Hokkaido island and north of Honshu island with II-III MSK.

Historical records

According to the Seismological bulletin of CEME GS RAS the whole number of earthquakes have been detected in this border area between Russia and China with the source's depth more than 500 km in a period between 1955 and present time. Their parameters are presented in the table. Epicentral locations of these events, including one, which happened on April 9 1999 are shown in the figure below.

NN	Year	Month	Day	Time	Latitude N degr.	Longitude Е degr.	Depth	MPSP
1	1957	01	03	124828.0	43.95	130.61	570	7.4
2	1957	01	03	134333.0	43.87	130.60	560	5.6
3	1959	10	29	143025.0	42.86	131.06	535	6.7
4	1970	10	12	093337.0	42.80	131.10	560	5.8
5	1973	09	10	074334.0	42.70	131.10	580	6.4
6	1976	04	11	130150.0	43.00	131.00	550	5.0
7	1976	04	11	130337.0	42.70	131.10	550	5.5
8	1979	01	31	123625.1	42.83	131.25	560	6.2
9	1979	12	25	033652.6	43.24	131.19	530	5.8
10	1981	10	05	042814.7	42.91	131.24	550	5.0
11	1981	11	27	172145.4	42.97	131.24	550	6.3
12	1983	10	08	074526.9	44.18	130.80	572	5.9
13	1984	04	15	073411.9	42.88	131.21	547	5.8
14	1993	11	02	071450.2	42.82	131.20	550	4.8
15	1994	02	08	051826.9	43.37	130.74	564	4.8
16	1997	07	06	015147.7	42.83	131.47	549	5.0

MPSP-magnitude for Р wave (short period channel)

Figure 3

Focal mechanism and seismic moment of the earthquake

Focal mechanism of the earthquake 08.04.1999 (0=13 hours 10 minutes) has been calculated in CEME GS RAS by the first signs of Р-waves arrival at 61 stations, which are located in the interval of epicentral distances 1.3 - 96.2 degrees. In enough uniform spread between azimuths from 12 up to 359 degrees the speed of longitudinal waves at the depth of 550 km was assumed as equal to 9.85 km/sec (2). 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. Focal mechanism of earthquake 08.04.1999 is characterized almost by the pure thrust. One of it's nodal planes has close to meridian spread direction, while the other - close to latitude. In the upper table one can find solutions for the earthquake's source according to the data from Harvard's center USA, which have been done by cmt method(3).

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
49	82	14	189	37	291	75	16	-156	188	84	-76	IPC GS RAS
48	73	25	183	32	300	81	26	-160	189	81	-65	HARVARD

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

HARVARD - Harvard Geophysical Center (USA)

Figure 4

Literature

1. New Catalogue of strong earthquakes on a territory of USSR from most ancient times till 1975. M. "Science" 1977. 2. Bolt B. Inside the Earth // M. “Mir” 1984. P.177. 3. Dziewonski A., Chou T. and Woodhouse J. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity // J. Geophys. Res.1983. p.2825-2852.