D. Shashidhar

Investigations of continued reservoir triggered seismicity at Koyna, India

Abstract Koyna, located in the Deccan Volcanic Province in western India, is the most significant site of reservoir triggered seismicity (RTS) globally. The largest RTS event of M 6.3 occurred here on December 10, 1967. RTS at Koyna has continued. This includes 22 M≥5.0 and thousands of smaller events over the past 50 years. The annual loading and unloading cycles of the Koyna Reservoir and the nearby Warna Reservoir influence RTS. Koyna provides an excellent natural laboratory to comprehend the mechanism of RTS because earthquakes here occur in a small area, mostly at depths of 2–7 km, which are accessible for monitoring. A deep borehole laboratory is therefore planned to study earthquakes in the near-field to understand their genesis, especially in an RTS environment. Initially, several geophysical investigations were carried out to characterize the seismic zone, including 5000 line kilometres of airborne gravity gradiometry and magnetic surveys, high-quality magnetotelluric data from 100 stations, airborne LiDAR surveys over 1064 km2, drilling of 8 boreholes of approximately 1500 m depth and geophysical logging. To improve the earthquake locations a unique network of borehole seismometers was installed in six of these boreholes. These results, along with a pilot borehole drilling plan, are presented here.

The 14 April 2012 Koyna Earthquake of Mw 4.8: insights into active tectonics of the Koyna region

The 14 April 2012 earthquake of Mw 4.8 is the best monitored event in the Koyna region, a globally significant site of reservoir triggered seismicity in western India. Hence, investigation of this event assumes great importance, also considering its epicentral location close to that of the 1967 Koyna earthquake of M 6.3, the world’s largest reservoir triggered earthquake. Inversion of P-wave amplitude data along with the first motion polarities at 30 digital seismic stations provides a well-constrained strike-slip type focal mechanism solution, similar to that of the 1967 earthquake. The mechanism is further confirmed by moment tensor inversion of 3-component waveform data recorded at the three nearest broadband stations. The depth distribution of the aftershocks clearly delineates a NNE-SSW trending fault plane dipping about 78° to the WNW and coinciding with the trend of the Donachiwada fault, as well as the left-lateral fault plane of the focal mechanism solution obtained. The precise location, focal mechanism and the seismicity distribution from our dense network indicate that the activity in the Koyna region is mainly controlled by the NNE-SSW trending Donachiwada (D) fault zone rather than the Koyna River Fault Zone (KRFZ) on the west as suggested previously.

Recent seismicity patterns and microearthquake activity on an active intraplate fault system at Koyna-Warna, western India

Recent seismic activity of the Koyna region, western India is reported in this article. The Koyna region is considered to be one of the premier sites of reservoir triggered seismicity worldwide and it has been quite active since the initial impoundment of Shivaji Sagar Lake, north of Koyna Dam, during 1962 and nearby Warna Reservoir in 1985. Recently, a borehole seismic network consisting of 6 seismometers is deployed below the Deccan Traps to monitor the earthquakes in the study region, which recorded a large number of microearthquakes. A dense network of 23 surface broadband seismometers was also operative during the study period. A total of 2478 earthquakes of ML -0.8 to 3.7 recorded by the borehole seismic network, occurred during January 2016 to May 2017. Seismicity patterns in the recent time, which includes a few new zones of intense seismicity clusters in the vicinity of the borehole locations, are identified. Reservoir water levels are found to be strongly associated with the seismicity patterns. The seismicity is mostly concentrated in the vicinity of the Donachiwada fault and Warna during January–May, whereas it spreads during June–December. ‘b’ values have been estimated and found to be varying from 0.74 to 0.93. Seismicity continues to be present in the identified block where the pilot borehole has been drilled.

Earthquake focal mechanism studies in Koyna-Warna region in the last five decades — Current understanding on tectonics and seismogenesis

On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in theWarna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.

Seismic signal enhancement through statistical (Wiener) approach

This paper addresses the problem of improved earthquake parameter estimation of seismic signal in the presence of seismic noise recorded by Digital Seismic Recorder. A novel algorithm is investigated in two stages. First, the noisy earthquake signal power spectrum is estimated and the second step is to eliminate the estimated noise from the observed signal by spectral subtraction or Wiener filtering. This algorithm preserves the earthquake signal information and gives accurate earthquake parameters after enhancement process.