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A SEISMOLOGICAL STUDY OF THE BALUCHISTAN (QUETTA) EARTHQUAKE OF MAY 31, 1935. By K. R. RAMANATHAN, M.A., D.Sc., AND S. M. MUKHERJI, M.Sc., Colaba Observatory, Bombay. (With plates 25 to 28.)
A preliminary account of the earthquake from the geological and general points of view has been published by Mr. W. D. West1 in the Records of the Geological Survey of India. From the field evidence, Mr. West concluded that "in the case of the present earthquake there is no doubt about the position and extent of the epicentre, since severe damage was confined to a long narrow tract, away from which the intensity of the damage rapidly decreased. This tract extended from Baleli just north-west of Quetta through Dingar and Mastung to Mand-i-Haji and included the Shirinab Valley to the west of the Mastung-Kalat road. It is an area about 68 miles long and 16 miles wide. Within this area there were clearly places where the intensity was greater than elsewhere, notably Dingar and Mastung road and possibly Mand-i-Haji. Since it is well known that earthquakes are more severely felt on alluvium than on solid rock, it is possible that the length of the epicentral area as compared with its breadth has been enhanced to some extent by the fact that it is parallel to the valleys of the district." The surface crack extended from about 30°-3 N., 66°-9 E. to 29°-1 N., 66°-5 E., the centre of the region of maximum disturbance being 29°.7 N. and 66°-7 E. From the seismological evidence, the best position for the epicentre appears to be 29°.6 N., 66°5 E., slightly to the south-west of the above position, but well within the region of maximum intensity.
The materials available for study.
The following materials were available for the seismological study of the present earthquake.
1. The seismograms (horizontal components only) of the Indian observatories: Bombay, Agra, Calcutta, Hyderabad and Kodaikanal.
1 W. D. West, "Preliminary geological report of the Baluchistan (Quetta) Earthquake of May 31st, 1935." Rec. Geol. Surv. Ind., LXIX, Pt. 2, p. 203, (1936).
2. The seismograms of 15 foreign observatories: Batavia, Chiufeng, Medan, Peichiko, Tokyo, Göttingen, Ivigtut, Pulkovo, Scoresby Sound, Vienna, Adelaide, Melbourne, Sydney, Ottawa and Tacubaya. These seismograms had been obtained from the Directors of the respective observatories by Dr. S. C. Roy and were kindly placed at our disposal for purposes of study.
3. The data of travel-times of the principal phases recorded at 142 observatories, as measured at the observatories themselves and mostly collected at Oxford for the purpose of the International Seismological Summary. These were obtained from Miss Bellamy by the Director of the Geological Survey of India and kindly sent to us. Some data were also taken from observatory bulletins.
The position of the epicentre and the time of origin of the earthquake.
For determining the epicentral time and position of the earthquake, only the arrival-times of the P phase at different observatories were used, as this phase is in general the least subject to uncertainty. As a first approximation, the centre of the region of greatest disturbance in Mr. West's map of isoseismals was assumed to be the epicentre. The distances of the different observatories from the assumed epicentre were calculated from the geographical co-ordinates and using Jeffreys' and Bullen's table of traveltimes (published in 1934 in the International Seismological Summary for the year 1930), the times of arrival of P at the different places were calculated and compared with the observed times of arrival. A comparison of the mean residuals (observed minus calculated times) at observatories situated in different azimuths showed in what manner the hypothetical epicentre should be shifted in order to get a better fit and thus, by a process of successive approximation, the best position of the epicentre was determined and the corresponding epicentral time to calculated. The distribution of stations in different directions is markedly non-uniform, the directions best represented being north-west and north-east. Towards the south, the number of stations is few, and even among them, the times of first onset as recorded at the Indian stations were abnormally