Feb 26, 2009 - The average values of seismic moment, fault radius and dislocation .... slices of rocks of the Indian plate have been thrusted southward and the older rocks .... Layer thickness. Vp. V~. Density. (km). (kin/s). (km/s). (g/cm 3). 2 ... the conceptual models of an earthquake source is a circular fault in which rupture.
PAGEOPH, Vol. 147, No. 4 (1996)
0033 4553/96/040657-i751.50 + 0.20/0 9 1996 Birkhhuser Verlag, Basel
Body-wave Wave-form Modelling and Source Parameters for the Indochina Border Earthquakes IMTIYAZ A. PARVEZ 1 and AVADH RAM ~
Abstract--Wave-form modelling of body waves has been done to study the seismic source parameters of three earthquakes which occurred on October 21, 1964 (Mb = 5.9), September 26, 1966 (Mb = 5.8) and March 14, 1967 (Mb = 5.8). These events occurred in the Indochina border region where a low-angle thrust fault accommodates motion between the underthrusting Indian plate and overlying Himalaya. The focal depths of all these earthquakes are between 12-37 kin. The total range in dip for the three events is 5~ ~ The T axes are NE-SW directed whereas the strikes of the northward dipping nodal planes are generally parallel to the local structural trend. The total source durations have been found to vary between 5-6 seconds. The average values of seismic moment, fault radius and dislocation are 1.0-11.0 x 1025 dyne-cm, 7.7-8.4 km and 9.4-47.4 cm, respectively whereas stress drop, apparent stress and strain energy are found to be 16-76 bars, 8.2-37.9 bars and 0.1-1.7 x 102~ergs, respectively. These earthquakes possibly resulted due to the tension caused by the bending of the lithospheric plate into a region of former subduction which is now a zone of thrusting and crustal shortening. Key words: Wave-form modelling, source parameters, Indochina earthquakes.
I. Introduction The H i m a l a y a s , flanked by the C h a m a n fault in the west a n d A r a k a n y o m a in the east, extend over a distance of a b o u t 2,400 k m a n d comprise an area of v a r y i n g degree o f seismicity. The H i m a l a y a n belt is believed to have been raised by crustal s h o r t e n i n g (GANSSER, 1966; DEWEY a n d BIRD, 1970; MOLNAR a n d TAPPONIER, 1975) following the collision of the I n d i a n a n d E u r a s i a n continents. The greater part of the H i m a l a y a n range consists of rocks of the P e n i n s u l a r shield (GANSSER, 1966). Interest in the collision process has been fueled by the inference that large-scale c o n t e m p o r a r y tectonics in C e n t r a l Asia (MOLNAR a n d TAPPONIER, 1975; TAPPONIER a n d MOLNAR, 1976) a n d in the I n d i a n P e n i n s u l a (CHANDRA, 1976) are a n o u t c o m e of the c o n t i n e n t a l collision. O b s e r v a t i o n s of n u m e r o u s faults a n d folds striking transverse a n d oblique to the general t r e n d o f the H i m a l a y a s a n d the parallelism o f these faults a n d folds to subsurface structures of the n o r t h I n d i a n plains, led VALDIYA (1976) to suggest a possible i n v o l v e m e n t of P e n i n s u l a r I n d i a in the tectonic f r a m e w o r k of the Himalayas.
1Department of Geophysics, Banaras Hindu University, Varanasi-221 005, India.
658
Imtiyaz A. Parvez and Avadh Ram
PAGEOPH,
The area has suffered numerous earthquakes, some of which were very damaging, claiming thousands of human lives. Several studies have been reported in the literature in which long period body wave-form data have been used to model a seismic source (LANGSTON and HELMBERGER, 1975; LANGSTON, 1977; CHUN6 and BRANTLEY, 1989; LIU and HERRMANN, 1991). In order to enhance the knowledge regarding earthquakes within stable continental regions and ultimately to increase our understanding of seismicity in the North-East (NE) Himalaya, we have studied the source parameters using the amplitude of direct P, SH and S V waves by wave-form modelling of three earthquakes which occurred in the Indochina border region (Fig. 1). Table 1 lists the location and magnitude parameters. The method of wave-form modelling consists of the computation of theoretical seismograms from the source models defined by its seismic orientation of the dislocation and the source time function. The comparison between theoretical and observed seismograms permits adjustment of values of the parameters that define the seismic
92~
93~
r. . . . . . . . .
29~ -
0
F
94~
g5~
96~
THRUST I I FAULT LINEAMENT EPICENTRES
.J 2r
/"
