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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS003 Oral Presentation 1853
Detection of precursory changes in source parameters: A probable shortterm
forecasting model for the small- to moderate- size intraplate
earthquakes in the Kachchh seismic zone, Gujarat, India
Dr. Prantik Mandal
Seismology NGRI, HYDERABAD, INDIA IASPEI
R.K.Chadha, Mr. C. Styamurty, Mr. I.P. Raju, Mr. N. Kumar
Precursory changes in corner frequency as well as stress drops have been noticed about 2.5-3.0 days
prior to nine main earthquakes (Mw 3.8-5.6) of the 2001 Mw 7.7 Bhuj earthquake sequence, which
occurred in the Kachchh seismic zone during 2002-2006. The studied sequences suggest 2-8 numbers
of foreshocks, which have occurred within a circular area of 10-15 km from the epicenter of the
mainshocks during 80-160 hours period prior to the mainshocks. Since August 2002, the seismic activity
in Kachchh has been monitored by a close digital network consisting of 5-8 three-component digital
seismographs and 10-20 three-component digital accelerographs, which enabled us to relocate the
considered nine earthquake sequences using the joint hypocentral determination (JHD) technique
resulting in a reliable and accurate hypocentral parameters estimates (error in the epicentral location <
500 meter, error in the focal depth estimation < 1000 meter). The focal depths are varying from 2 to 35
km. The estimated corner frequency (fc), source radius (r), stress drop (Ds) and moment magnitude
(Mw) for foreshocks of nine studied sequences are ranging from 3.21 to 9.81 Hz, 128.4 to 487.5 m,
3.21 to 12.39 MPa, and 2.3 to 4.2, respectively. For all the nine earthquake sequences, we observed a
precursory decrease of 66-82 % in the corner frequency and a precursory increase of 65-98 % in the
stress drops during the 60-70 hours period prior to the mainshocks. We also noticed an increase of 30-
60 % in the fault length during the above-mentioned precursory period. The foreshocks for the studied
nine Kachchh earthquake sequences are found to be of Mogi's type-II classification, which follow
multiple rupture model of foreshock generation. Following dilantancy model, it can be inferred that the
observed an increase of 30-60% in fault length associated with a decrease in corner frequency and
increase in stress drop over most of the precursory period can be attributed to the presence of fluids at
the focal depths (15-30 km depth range), which may be considered as the probable precursors for
predicting moderate size Kachchh events with a lead time of 2.5-3.0 days.
Keywords: foreshocks, intraplate earthquakes, source parameters