Seismic risk in the Assam Gap, eastern Himalaya

Abstract

The seismicity data from 1825 to the present for the Assam (northeastern India) region show that seismicity rates there deviate from normal before and after major earthquakes. Along this 1,000-km-long section of a plate boundary, all shocks with magnitude M > 6.6 were preceded and sometimes followed by periods of significant seismic quiescence. No major earthquakes occurred without an associated seismic quiescence, and no such quiescence occurred at times other than before or after a major event. The most remarkable periods of quiescence lasted about 28 and 30 yr before the two great (M 8.7) Assam earthquakes of 1897 and 1950. Other periods of anomalously low seismicity preceded main shocks of magnitudes 6.7 (in 1950 and 1975), 7.8 (in 1869), and 7.7 (in 1947), with durations of 6, 8, 23, and 17 yr, respectively. These durations fit (with approximately the scatter of the original data) a published relation between precursor time and magnitude. Since these changes of seismicity rate were observed at the edges of and within the Assam gap, defined by the 1897 and 1950 great earthquakes, it is likely that a future major or great earthquake in this gap will be preceded by seismic quiescence. Whether a preparatory phase for an earthquake has begun in the Assam gap cannot be stated for certain because of the changing earthquake-detection capability in the area and because of poor location accuracy. INTRODUCTION root °f energy released by earthquakes in a given area (Beniofl, 1951; Brune, 1968)]. For this reason, seismic gaps can be defined The seismicity pattern in space and time was probably the successfully using the large earthquakes only. However, it may most important factor that led to the successful prediction of the be that the smaller background earthquakes hold the key to Haicheng earthquake (Anonymous, 1976). In that case, the mithe third element necessary for successful prediction: the time of gration of medium-magnitude events toward Haicheng alerted occurrence. This may be determinable once the relation of the local researchers to monitor the area, and the immediate foremain shocks and their precursory seismicity fluctuations shocks indicated that an earthquake warning should be issued. (Nersesov and others, 1973; Brady, 1974, 1976, 1977; Ishida and The idea of seismic gaps has been successfully used to idenKanamori, 1977; Evison, 1977; McNally, 1977) is understood, tify areas of increased earthquake risk and to estimate approxiIn addition to these long-term seismicity patterns, Jones and mate source dimensions of the expected ruptures (Fedotov, 1967; Molnar (1976) found that about 40% of large earthquakes have Mogi, 1969; Sykes, 1971; Kelleher and others, 1973). Seismic foreshocks that occur hours and days before the main event, gaps are sections of plate boundaries where no large earthquakes The area of interest in this paper is the Assam region (inset have occurred for a duration comparable with the recurrence in Figs. 1, 4). In this northeasternmost part of India, two great time. Even after numerous smalland medium-magnitude earthearthquakes of magnitude 8.7 have occurred in the past 80 yr, quakes occur in a gap, it must still be considered a place of high leaving a seismic gap between them, which we will call the stress concentration, because small earthquakes, unless associAssam gap. In this area the Himalaya are interpreted to form a ated with additional seismic creep, do not relieve a significant clearly defined arcuate zone of plate consumption, along which amount of tectonic strain compared to large ones [a fact that the Indian and Asian plates collide at a rate of about 5 cm/yr. can be checked by summing the seismic moments of the square Since the Assam gap is one of very few clearly defined thrust gaps on land, it is one of the few locations where the Earth's ________ behavior precursory to a great thrust earthquake may be moniPresent address: Geology and Geophysics Department, Roorkce tored in detail. As a first step we have examined the seismicity University, Roorkee, U.P. 247672, India. record of the Assam region for the past 150 yr. GEOLOGY, v. 6, p. 685-688 685 EPICENTER WITH MAGNITUDE MAJOR THRUSTS