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Seismotectonic zones demarcation in the Shillong Plateau using the microearthquakes and radon emanation rate

Walia D., Lyngdoh A.C., Saxena A.

Acta Geophysica

2010

Vol. 58, no. 5

893-907

The Shillong Plateau signifies the intense tectonic processes that the region has experienced during the Tertiary Indo-Tibetan and Indo- Burman collisions. An attempt has been made to study the microearthquake and radon emanation rate to understand and identify the seismotectonic zones. The microearthquake data was recorded along a network of seven temporary seismic stations. The epicentral map prepared using the microearthquake data indicates sparse seismic activity over the Shillong Plateau. The alignment of earthquake epicenters does indicate definite pattern of the activity disposition and hence the active fault zones. The calculated b-value over the Shillong Plateau is low which indicates asperity and the fact that the 'stress' is being built-up. Accordingly, along the few identified active crustal structures, time integrated and continuous radon monitoring was made using the LR 115 and Barasol detectors. The sites with anomalous radon concentration are demarcated as active fault zones or sensitive seismotectonic zones and are being monitored continuously using the Barasol detectors. The anomalous radon concentration may not indicate the magnitude of impending earthquake but it certainly can be used to spatially locate the earthquake preparation zones.

Correlation of microearthquake occurrence and rock block tilts

Prochazkova D.

Acta Geophysica Polonica

2002

Vol. 50, nr 2

251-258

The paper describes the correlation between the microearthquake occurrence and the rock block tilt changes in the region of North Bohemian Brown Coal Mines in the period 1982 -1989.

Discrimination of microearthquakes and quarry blasts: a case study in northern Egypt

Badawy A.

Acta Geophysica Polonica

2001

Vol. 49, nr 3

331-341

In northern Egypt, both microearthquakes and quarry blasts are observed in complicated tectonic region. Therefore, reliable event identification, preferably based on a simple method that can be easily used in a routine processing, is essential for accurate and detailed seismotectonic studies. Amplitude spectra of microearthquakes and quarry blasts recorded by stations of the Egyptian National Seismograph Network (ENSN) have been computed. Most quarry blasts in northern Egypt are ripple-fired with time delays ranging from 20 to 25 ms; consequently, we seek an event discrimination method based on the recognition of ripple-firing patterns in the signal. For this end, we compared the spectra of complete seismogram signals of 55 microearthquakes and 150 quarry blasts at the same stations to eliminate wave-path and distance effects. The obtained results clearly indicate that the microearthquakes spectra seem to contain high frequency energy more than thee spectra of quarry blasts. We found a prominent spectral modulation in quarry blasts spectra but not in the spectra computed from microearthquake seismograms. The observed spectral modulation is mainly due to ripple-firing explosion and it is independent of time in the coda. Finally, the quarry blasts can be discriminated from non-ripple-firing events (e.g., earthquakes) on the basis of low frequency (less than 15-20 Hz) content and the spectral modulations.