Investigations related to scientific deep drilling to study reservoir-triggered earthquakes at Koyna, India
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Artificial water reservoir-triggered earthquakes have continued at Koyna in the Deccan Traps province, India, since the impoundment of the Shivaji Sagar reservoir in 1962. Existing models, to comprehend the genesis of triggered earthquakes, suffer from lack of observations in the near field. To investigate further, scientific deep drilling and setting up a fault zone observatory at depth of 5–7 km is planned in the Koyna area. Prior to undertaking deep drilling, an exploratory phase of investigations has been launched to constrain subsurface geology, structure and heat flow regime in the area that provide critical inputs for the design of the deep borehole observatory. Two core boreholes drilled to depths of 1,522 and 1,196 m have penetrated the Deccan Traps and sampled the granitic basement in the region for the first time. Studies on cores provide new and direct information regarding the thickness of the Deccan Traps, the absence of infra-Trappean sediments and the nature of the underlying basement rocks. Temperatures estimated at a depth of 6 km in the area, made on the basis of heat flow and thermal properties data sets, do not exceed 150 °C. Low-elevation airborne gravity gradient and magnetic data sets covering 5,012 line km, together with high-quality magnetotelluric data at 100 stations, provide both regional information about the thickness of the Deccan Traps and the occurrence of localized density heterogeneities and anomalous conductive zones in the vicinity of the hypocentral zone. Acquisition of airborne LiDAR data to obtain a high-resolution topographic model of the region has been completed over an area of 1,064 km2 centred on the Koyna seismic zone. Seismometers have been deployed in the granitic basement inside two boreholes and are planned in another set of six boreholes to obtain accurate hypocentral locations and constrain the disposition of fault zones.
KeywordsKoyna Triggered seismicity Scientific drilling Earthquake Reservoir
Earthquakes triggered by filling of artificial water reservoirs have been reported at about 100 sites worldwide during the past seven decades (Gupta 2002, 2011). Damaging-triggered earthquakes exceeding magnitude six occurred at Hsingfengkiang (1962), Kariba (1963), Kremasta (1966) and Koyna (1967). The occurrence of reservoir-triggered seismicity (RTS) has been associated with a number of potential factors including the rate of loading, highest water level reached and the duration of retention of high water levels (Kaiser 1953; Gupta et al. 1972a, b). Other studies have investigated the role of reservoir loading (e.g., Gough and Gough 1970a, b; Bell and Nur 1978; Roeloffs 1988) and the influence of pore fluid pressure (Snow 1972; Talwani et al. 1999; Do Nascimento et al. 2004). While major advancements have been made towards elucidating the role of individual parameters in triggering earthquakes, an integrated model that explains the genesis of RTS is unavailable.
Since 2005, a network of ten digital seismograph stations has been operated by the Council for Scientific and Industrial Research (CSIR)–National Geophysical Research Institute (NGRI), Hyderabad, in the Koyna–Warna region and earthquake parameters are estimated in near real time. Earthquakes of M ~ 4 are found to be preceded by well-defined nucleation, and real-time identification of nucleation has led to short-term forecasts (Gupta et al. 2006, 2007, 2011a). Over the years, a considerable shift in the seismic activity to the south has been observed. From moment tensor inversion studies, it can be seen that focal mechanisms are governed by pure normal type in the Warna region, whereas a significant component of strike-slip dominates in the Koyna region. Good estimates of focal depths are obtained by waveform modelling (Shashidhar et al. 2011, 2013).
Although several studies have clearly established the association of continued triggered earthquakes at Koyna with the loading and unloading of the Koyna and Warna reservoirs, the triggering mechanism is little understood. The existing models to comprehend the genesis of RTS suffer from lack of observations in the near field of the earthquakes. A proposal for scientific deep drilling and setting up a deep borehole observatory to study critical parameters in the near field of earthquakes was discussed and agreed upon at the 2011 ICDP International Workshop held at Hyderabad and Koyna (Gupta et al. 2011b).
In this paper, we report (1) the first results obtained from the exploratory phase of investigations carried out during the past 3 years and (2) discuss the probable location, design and instrumentation plan of the proposed Koyna deep borehole in the light of the results obtained so far.
Exploratory phase investigations
Drilling and logging
Prior to undertaking the deep drilling, an exploratory phase of drilling of ten boreholes and related investigations is ongoing with the intent of constraining the subsurface geology, physical and mechanical properties of rocks, temperature and stress regimes. These data constitute critical inputs for designing the proposed deep borehole observatory at a depth of 5–7 km. The thickness of Deccan Traps and the nature of underlying rocks were uncertain as previous drilling in this region was limited to shallow exploration boreholes within the Deccan Traps. In the present study, each borehole is planned to go through the Deccan basalt pile and a few 100 m into the underlying basement rocks. The first two cored boreholes, Rasati (KBH-1) near Koyna and Udgiri (KBH-2) to the south of Warna, have been completed to depths of 1,522 and 1,196 m, respectively. The two boreholes broadly mark the northern and southern limits of the Koyna seismic zone (Fig. 3). Studies on cores and downhole measurements have brought out new and direct information about the thickness and properties of the Deccan Traps in the area and the nature of the underlying basement rocks, which had remained elusive so far (Rao et al. 2013; Roy et al. 2013a, b). Scientific investigations in the boreholes are expected to provide precise information about structure and physical properties of the subsurface rock volume in the vicinity of the Koyna seismic zone.
Heat flow studies
Heat flow, together with data on thermal properties of rocks, provides constraints for modelling the subsurface temperature regime in the Earth’s crust. Because scientific deep drilling to a depth of 5–7 km is proposed in the Koyna–Warna area, knowledge of temperature in the upper parts of the crust is critical for deploying appropriate drilling technology and for designing the deep borehole observatory.
Heat flow of 41 mW m−2 in the Koyna region was previously reported from measurements in two boreholes located in Koyna–Alore (Gupta and Gaur 1984). Each borehole, ~200 m deep, was drilled through lava flows comprising massive as well as vesicular basalt. Subsurface temperature models based on such data were poorly constrained due to lack of reliable heat flow determination from deep boreholes and thermal properties and radiogenic heat production measurements from the underlying basement rocks.
Airborne gravity gradiometry and magnetic studies
In the early 1970s, regional-scale gravity measurements were taken over the entire Deccan Trap region that provided first-order information about the nature of the gravity field and a possible interpretation in terms of the subsurface structure (Kailasam et al. 1972). In the late 1990s, particularly after the Latur earthquake of 1993, gravity measurements with better accuracy and smaller station spacing were taken in parts of the Deccan Trap region (Tiwari et al. 2001). However, these gravity measurements were taken along the main roads, and thus, there were very few observations in the Koyna–Warna seismic zone due to inaccessible terrain. These studies indicated a large wavelength gravity low encompassing Koyna–Warna earthquake region.
Airborne LiDAR studies
Airborne LiDAR and orthophoto data are being acquired in the Koyna–Warna region in order to generate a high-resolution and high-precision topographic model, which would be crucial to infer the surface geomorphology and active tectonics of the region. The topography in the area of LiDAR survey is 1,000 m on an average and the area is largely covered with vegetation.
LiDAR discrete return, waveform and intensity data as well as orthophoto images are being acquired in area of 1,064 km2 centred on the Koyna seismic zone. The aircraft uses a 125-kHz laser at line intervals of 250 m, hugging the terrain at about 600 m with flight speed of about 150 km h−1 to generate nominal point spacing (NPS) of 8–10 m−2. The processed data will reveal the details of the bare earth, leading to topographic details in terms of relief, steepness, relation with drainage patterns, which may provide information about structural conditions in the form of overhangs where unconsolidated materials have collapsed to form irregular ridges, moletracks, alluvials fans, doglegs across drainage, aligned vegetation or geothermal springs. Such features may expose conditions favourable to land uplift in a localized zone as well as response to hillslope erosion (Hunter et al. 2011). It is expected that mapping of the detailed surface geomorphology and inference of fine-scale structures and lineaments would in conjunction with the subsurface structure derived from drilling and other geophysical studies, help in understanding the stress field and seismo-tectonics of the region with respect to the reservoirs and their trigger effects on the seismicity in this region.
A borehole seismic network, the first of its kind, is being deployed to precisely locate earthquakes and refine the information about probable fault zones inferred on the basis of the existing network of surface seismic stations. The network would comprise eight sensors at depths of 1.2–1.5 km in the granitic basement, in boreholes surrounding the seismic cluster. By installing the sensors in the granitic basement, the noise due to the overlying basalt cover is effectively eliminated, yielding clean records that allow identification of smaller earthquakes with better precision. The seismometers are of Sunfall make with 4.5-Hz three-component sensors placed at the bottom of each borehole with the Reftek recording system on the surface connected through cables. Two borehole seismometers are already installed and are functional while six more are planned to be installed in the near future.
Koyna seismicity and probable sites for scientific deep drilling
Plan of deep borehole
Although several studies have clearly established the association of continued triggered earthquakes at Koyna with the precipitation driven loading and unloading of the Koyna and Warna reservoirs, the triggering mechanism is little understood. Existing geological, hydrological and geophysical studies in the region provide a good initial framework to study the regional tectonic setting, but lack critical inputs needed to explore the physical mechanisms that connect the reservoir water level changes to the occurrence of earthquakes. Our knowledge about the physical properties of rocks and fluids in the fault zones and how they affect the build-up of stress over extended periods of time is limited by the lack of data from the near-field region. To address this critical information gap, it is proposed to undertake scientific deep drilling at a carefully chosen site and set up an observatory at depth to study pre-seismic, co-seismic and post-seismic changes in physical, chemical and mechanical properties in the near field of earthquakes. By instrumenting the deep borehole for long-term monitoring of critical parameters such as seismicity, temperature, fluid/gas and pore pressure, it may be possible to obtain unprecedented new information on the temporal changes of those parameters associated with earthquake occurrences.
The deep borehole observatory will comprise a suite of instruments and sensors installed inside the borehole. Potential installations include (1) arrays of three-component seismometers and deformation sensors at different levels, (2) pore pressure transducers, (3) temperature sensors and (4) in-line gas/fluid testing equipment. Monitoring of several parameters will be continued for a number of years after completion of drilling and instrumentation. Analysis of these data sets and modelling studies would lead to comprehending the genesis of reservoir-triggered earthquakes.
The proposed drilling project addresses a key ICDP scientific theme “active faulting and earthquakes”. The project will generate new information and knowledge about artificial water reservoir-triggered seismicity within an intraplate setting but also complement the other active fault zone drilling projects including the SAFOD (USA), Nojima fault (Japan), NanTroSEIZE (Japan), Chelungpu fault (Taiwan), Gulf of Corinth (Greece) and the GONAF (Turkey) (Zoback et al. 2011; Ma et al. 2006; Ando 2001; Cornet et al. 2004; Bulut et al. 2014).
The persistent seismicity at Koyna during the past five decades makes it a natural laboratory to study the genesis of earthquakes in a stable continental region and to elucidate the role of artificial water reservoirs in triggering earthquake activity by setting up a deep borehole observatory. Prior to undertaking scientific deep drilling and experimentation, an exploratory phase of geophysical investigations and drilling is being carried out in the Koyna area. These studies have brought forth critical new information that would guide the design of the deep borehole observatory. It is now clear that the proposed deep borehole will go through about 1-km-thick Deccan Traps, which is directly underlain by granite–gneiss basement. The basement granitoids are highly fractured and crushed at multiple depths with evidences for shear movement, possibly as a consequence of intense seismic activity in the area. The upper few kilometres of the crust is characterized by moderate temperature regime, with temperatures unlikely to exceed 150 °C at a depth of 6 km. Analysis of broadband seismic data indicate the possibility of reaching the hypocentral region of the M ≥ 2 earthquakes within a depth of about 5–7 km. Based on the scientific work carried out so far, a tentative location for setting up of the deep borehole observatory has been identified. A first-order plan of scientific deep drilling and observational facilities to be developed has been also presented.
We are grateful to the Government of Maharashtra for facilitating the drilling, airborne and other investigations, to the Atomic Minerals Directorate (AMD), Hyderabad, for providing technical advice at various stages of drilling operations and geological core logging, to Mineral Exploration Corporation Ltd. (MECL), Nagpur, for providing geophysical logging and geological core logging services. Discussions with ICDP-OSG and ONGC Ltd. regarding scientific deep drilling were helpful. The study was funded by the Ministry of Earth Sciences, Government of India.
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