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Detection of seismic quiescences before 1991 Uttarkashi (MW 6.8) and 1999 Chamoli MW (6.6) earthquakes and its implications for stress change sensor

Chingtham Prasanta, Sharma Babita

Acta Geophysica

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2022

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Vol. 70, no 2

509--523

EN

The statistically point process model known as epidemic-type aftershock sequence (ETAS) model is employed for systematically investigating the seismic quiescence or seismic anomalies around the focal regions of large/strong earthquakes for NW Himalaya. For this propose, the model predicted the expected occurrence rates of earthquakes by estimating the model parameters from the earthquake occurrences times using maximum likelihood method, has been used. Then the exhibited relative quiescence due to decreasing occurrence rates from the modeled ones can be identified by inspecting the abnormally downward deviated plot from the extended cumulative curve of the Residual Point Process (RPP) events. Examination of such RPP events in the whole time interval exhibits significant 1.5 years and 2.0 years of relative seismic quiescence before the strong 1991 Uttarkashi (MW 6.8) and 1999 Chamoli (MW 6.6) earthquakes, respectively. Considering the optimally oriented planes of Uttarkashi earthquake, the Coulomb stress changes (ΔCFS) have been investigated to check the rate of seismicity around the focal region of Chamoli earthquake. It has been found that ΔCFS of Uttarkashi earthquake exhibits stress shadow in or near the source zone of Chamoli earthquake and eventually decreases seismicity rates due to seismic quiescence in the source zone. On the other hand, the detected quiescence and activation relative to the predicted seismicity rate are consistent with the obtained Coulomb stress to depict the associated anomalies being sensitive enough to detect a slight stress change in the study region. Henceforth, the increased or decreased seismic activity due to seismic activation or quiescence is found to be consistent with the patterns of the Coulomb’s stress changes calculated on the ruptured fault planes of Uttarkashi earthquake. Hence, this ETAS model based on statistical technique can thus be incorporated with other sensitive geophysical instruments for identifying seismically quiet period not only in the seismic gaps, but also in its neighborhoods along the Himalayan range for mitigating seismic hazards due to impending great earthquakes.

2

Characteristics of seismic activity in the Western, Central and Eastern parts of the North Anatolian Fault Zone, Turkey: Temporal and spatial analysis

Öztürk S.

Acta Geophysica

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2011

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Vol. 59, no. 2

209-238

EN

Characteristics of seismic activity along the North Anatolian Fault Zone are analyzed between 1970 and 2010. Magnitude completeness changes between 2.7 and 2.9 in the North Anatolian Fault Zone. The frequency-magnitude distribution of earthquakes is well represented with a b-value typically close to 1. A clear decrease in temporal distribution of b-value is observed before the strong main shocks. Correlation dimension values are relatively large and the seismic activity is more clustered at larger scales in the North Anatolian Fault Zone. A statistical assessment is made in order to detect the current seismic quiescence anomalies in the beginning of 2010. Eight significant anomalous zones throughout the North Anatolian Fault Zone are detected. These are centered at: (1) 41.08°N-28.58°E (around Silivri), (2) 41.47°-29.51°E (in the Black Sea), (3) 40.69°N-29.78°E (including Izmit), (4) 40.26°N-26.46°E (around Gelibolu, Canakkale), (5) 40.59°N- 31.03°E (including Duzce fault), (6) 40.86°N-35.30°E (around Amasya), (7) 39.48°N-39.74°E (around Erzincan), and (8) 39.06°N-40.50°E (around Bingol).

3

Seismicity before and after the two great Sumatra earthquakes of 2004 and 2005

Gibowicz S., Dębski W.

Acta Geophysica

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2006

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Vol. 54, no. 4

343-360

EN

The Harvard CMT catalogue contains 481 shallow earthquakes that occurred between 1 January 1977 and 30 November 2005 within a broad region defined by the geographical latitude from 3°S to 14°N and by the longitude from 91°E to 102°E. There are 230 events that occurred before the great earthquake of 26 December 2004. Their surface distribution is not uniform and the source area of the 2004 great event appears as an area of seismic quiescence with a radius of about 100 km. There are 186 events that occurred between the two great earthquakes of 26 December 2004 and 28 March 2005. Practically all of them are located to the northwest from the great earthquake of 2005, that in turn was followed by 63 events, mostly located to the southeast. The cumulative seismic moment from earthquakes before the occurrence of the great event of 2004 increased rather regularly with time, with sudden increase about twenty years and two years before the occurrence of the great event. The seismic moment of earthquakes between the two great events increased rapidly during the first ten-fifteen days, then flattened out and increased slowly with time. After the great event of 2005 the seismic moment shows quiet increase during some 115 days, then sudden jump, followed by very small activity till the end of our observations. From the spatial distribution of seismic moment of earthquakes that occurred before the great event of 2004 it follows that its largest release appeared to the southeast from the great event, around the rupture area of the great earthquake of 2005. The largest release of seismic moment from earthquakes between the two great events is observed in the vicinity of the 2004 event and further up to the north. The seismic moment from earthquakes that occurred after the great event of 2005 was mostly released in its vicinity and further down to the south.