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Determination of characteristics of the microearthquakes occurring in Lesser and Higher Himalaya India

Goswami Neetu, Gupta Subhash Chandra

Acta Geophysica

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2022

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Vol. 70, no 1

1--13

EN

The present study aims to estimate the frequency-dependent attenuation relations and source parameters of local earthquakes that occurred in the Garhwal Lesser and the Higher Himalaya around Uttarkashi town, India. The local earthquake data of 234 events recorded from a 12-station digital telemetered local seismological network in the region during 2008 to 2017 have been used for this purpose. The first data set of 126 events occurred around Uttarkashi in the Lesser Himalaya, the second data set of 73 events located northwest of Uttarkashi in the Higher Himalaya and the third data set of 35 events located about 35 km east of Uttarkashi in the vicinity of 1991 Uttarkashi earthquake (Ms~6.8) in the Higher Himalaya. The source parameters such as: seismic moment (Mo), source radius (r) and stress drop (∆σ), have been estimated from the displacement spectrum of the SH component of S-wave by applying Brune’s circular source model (Brune in J Geophys Res 75: 4997 5009, 1970). The attenuation characteristics are examined by estimating the quality factor (Qc) from the decay of coda waves of vertical component record using the Single Backscattering model (Aki and Chouet in J Geophys Res 80: 23 3322 3342, 1975). The Qc estimated at different values of frequency (1.5 to 24 Hz) have shown that Qc a function of frequency. Here, we obtain the following frequency-dependent attenuation relationships: Qc=110 f 0.99 (using first and second data sets) and Qc=142 f 0.87 (using third data set). The source parameters of 234 events show that more than 96% of events (227 events) have a low stress drop (less than 10 bars), five events have a stress drop between 10 and 100 bars and two events have exceptionally high stress drop of 271 and 532 bars. This shows that the region produces, by and large, low stress drop events. The variation in stress drops with depth demonstrates no clear increase or decrease in stress drop with focal depth. The Mo-fc scaling relations have been obtained for events with magnitudes two and above, Mo=2.7× 1016 fc −3.98 (for data set 1), Mo=1.4× 1014 fc −2.74 (for data set 2), and Mo=1.1× 1015 fc −3.28 (for data set 3).

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Source parameters of seismic events potentially associated with damage in block 33/34 of the Kiirunavaara mine (Sweden)

Nordström E., Dineva S., Nordlund E.

Acta Geophysica

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2017

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Vol. 65, no. 6

1229--1242

EN

Forty-six mining-induced seismic events with moment magnitude between −1.2 and 2.1 that possibly caused damage were studied. The events occurred between 2008 and 2013 at mining level 850–1350 m in the Kiirunavaara Mine (Sweden). Hypocenter locations were refined using from 6 to 130 sensors at distances of up to 1400 m. The source parameters of the events were re-estimated using spectral analysis with a standard Brune model (slope −2). The radiated energy for the studied events varied from 4.7 × 10 −1 to 3.8 × 10 7 J, the source radii from 4 to 110 m, the apparent stress from 6.2 × 10² to 1.1 × 10 6 Pa, energy ratio (Es/Ep) from 1.2 to 126, and apparent volume from 1.8 × 10³ to 1.1 × 10 7 m³. 90% of the events were located in the footwall, close to the ore contact. The events were classified as shear/fault slip (FS) or nonshear (NS) based on the Es/Ep ratio (>10 or <10). Out of 46 events 15 events were classified as NS located almost in the whole range between 840 and 1360 m, including many events below the production. The rest 31 FS events were concentrated mostly around the production levels and slightly below them. The relationships between some source parameters and seismic moment/moment magnitude showed dependence on the type of the source mechanism. The energy and the apparent stress were found to be three times larger for FS events than for NS events.

3

Preliminary Results of Anthropogenic Seismicity Monitoring in the Region of Song Tranh 2 Reservoir, Central Vietnam

Wiszniowski J., Giang N. V., Plesiewicz B., Lizurek G., Van D. Q., Khoi L. Q., Lasocki S.

Acta Geophysica

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2015

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Vol. 63, no. 3

843--862

EN

Song Tranh 2 hydropower plant and the reservoir containing backed up water are located in the Quang Nam province (Central Vietnam). The region experiences unusual seismic activity related to the reservoir impoundment, with earthquakes of magnitude up to 4.7. In result of cooperation between the Institute of Geophysics, Vietnam Academy of Sciences and Technology and the Institute of Geophysics, Polish Academy of Sciences a seismic network has been built to facilitate seismic monitoring of the Song Tranh 2 area. The network, operating since August 2013, consists of 10 seismic stations. Here we show that the network is sufficient for advanced data processing. The first results of monitoring of the earthquake activity in Song Tranh 2 area in the period between 2012 and 2014, especially the completeness of catalogs, study and comparisons between water level and the seismic activity suggest direct connection between reservoir exploitation and anthropogenic seismicity.