Mechanism of triggered seismicity at Koyna: An evaluation based on relocated earthquakes

Seismicity associated with the Koyna Reservoir, western Maharashtra, is unique because it is one of the few sites in the world where earthquakes of M ≥ 5.0 continue to occur three decades after the initial spurt of activity. This contrasts with most other seismogenic reservoirs where temporal patterns of seismicity are characterized by one or two peaks that taper-off with time. Persistence of moderate seismicity at Koyna is enigmatic, considering also the low seismic productivity of the peninsular shield. Temporal association of major earthquakes with annual reservoir loading has been observed for long. In this paper, we report a discernible difference between the spatial pattern of earthquakes associated with the reservoir loading and unloading. A conceptual model is proposed to explain the continued occurrence of earthquakes. SEISMICITY associated with the Koyna Reservoir is unique in many ways. Koyna is among the four reservoirs that have triggered earthquakes of magnitude ≥ 6.0 (ref. 1). Earthquakes of M ≥ 5.0 continue to occur here nearly three decades after the major burst of activity in 1967 (ref. 2). Peaks of seismic activity show remarkable temporal association with the increased rate of filling 3 . In the backdrop of low strain rate and low seismic productivity, characteristic of stable continental regions 4 , seismicity at Koyna, located in the Archean craton is rather unusual. Over 100,000 earthquakes of M ≥ 0.0; nearly 100 events of M ≥ 4.0 and 10 earthquakes of M ≥ 5 have been reported from this region 5 . Such frequency of earthquakes, especially those exceeding M 5.0, in a short span of time, is a rarity in the stable shield regions. As reported by McGarr and Simpson 6 , an event of M 6.3 and a sequence continuing for more than 30 years do not fit the usual mainshock‐aftershock decay pattern observed for tectonic earthquakes. They note that, ‘the reservoir and annual cycles in water level continue to interact with, and augment the tectonic stress field in a complex manner that has led to a persistent sequence of anomalous seismicity’ . In this paper we explore some important factors that may be conducive to the continued seismicity at Koyna. The adjectives ‘induced’ and ‘triggered’ have often been used liberally and interchangeably, to describe earthquakes stimulated by reservoirs. We will not discuss the physical basis of these attributes, but refer the reader to McGarr and Simpson 6 who prefer to use the adjective ‘triggered’, to describe earthquakes caused by artificial reservoirs. The recent report of the USCOLD (US Committee on Large Dams) 7 also recommends the same. We also favour the adjective ‘triggered’ and use it in this paper to describe the seismicity at Koyna. The ongoing seismicity at Koyna suggests that annual loading cycles in the reservoir that was impounded in 1962, continue to influence the overall stability of the region. Filling of the Warna Reservoir in 1985 is another important factor that must have affected the hydrogeological environment. Sustained increase in fluid pressure, complemented by repeated failure in the fault zone may have led to many complex processes that are not directly observable. However, we expect that the spatial and temporal patterns of earthquakes may reflect some of these processes. The quality of data, especially those prior to 1993, was not good enough for detailed interpretations. This limitation has been recognized for long and there have been several efforts to improve the quality of locations 8‐11