Warna Reservoir Research Articles

Seasonal Variations in the b-Value of the Reservoir-Triggered Seismicity in the Koyna–Warna Region, Western India

—The analysis of the local earthquake catalog revealed fine features in the behavior of seasonal components of induced seismicity in the Koyna–Warna region, western India. The seasonal variations in seismicity associated with annual fluctuations in water levels in the Koyna and Warna reservoirs have local maxima during the year, corresponding to the known mechanisms of immediate and delayed responses of reservoir-triggered seismicity (RTS). The superposed epoch analysis combined with the stochastic modeling of random earthquake catalogs revealed regular changes in the b-value of the frequency–magnitude relationship of the earthquakes within the annual cycle of seasonal seismicity fluctuations. The minimum b-values fall in the intervals of the maxima of both the immediate and delayed seismic responses. The maximum b-values are observed during the rise and fall phases of water levels in the reservoirs. The pattern of changes in the seismicity together with the b-value behavior at the activation phase of the immediate RTS response is consistent with the scenario of gradual redistribution of the failure process from the lower to higher scales, characteristic of the activation of transient seismicity. A pilot laboratory experiment on cyclic initiation of fracture process by pore pressure in a granite sample extracted from a well in the Koyna–Warna region of reservoir-triggered seismicity demonstrated the patterns of changes in the acoustic regime similar to the seasonal features of the Koyna–Warna region seismicity.

Koyna earthquakes: a review of the mechanisms of reservoir-triggered seismicity and slip tendency analysis of subsurface faults

Koyna region in western India experienced more than 1,00,000 earthquakes of different magnitudes (M ~ 1.0–6.3) in the past five decades. Earthquakes in this region are believed to be triggered by a change in fluid pressure due to the percolation of the reservoir (Koyna and Warna reservoir) water into the subsurface. A drilling program was set up by the Ministry of Earth Sciences, India and International Continental Scientific Drilling Program (ICDP) to study the deep subsurface lithology, structure, thermal attributes, etc. as the area is covered by ~ 950 m of thick Deccan basalts. This paper reviews all the hypotheses proposed by earlier workers to explain the mechanism of reservoir trigger causing earthquakes and summarizes such theories to a simple generic model. Slip tendency analysis was further carried out based on the proposed model to explain the dependence of fault slip on fault geometry, rock mechanical properties, stress and fluid gradients. Finally, faults at various depths were characterized (favourably oriented, unfavourably oriented and severely misoriented) based on their potential to go into failure.

Assessment of heterogeneities of the shallow crust from geophysical logging in the Koyna-Warna region, Deccan Volcanic Province, India

Analysis and comparison of geophysical logs from seven exploratory boreholes of 1–1.5 km depth in the Koyna-Warna region of India illustrate variations in physical properties and structural characteristics of the basalts and underlying granite-gneiss basement in and around the seismogenic zone. In the north, south of the Koyna reservoir, densities are higher, gamma values are lower, fractures dip at high angles with low fracture permeabilities. To the south, around the Warna reservoir, densities are lower, gamma values higher, and resistivity values are low. In the southernmost borehole at Khadi, basement is highly fractured, and there are significant amounts of water outflow associated with the fracture system. The fracture density increases to the south, with fracture dips ranging from medium to low angles with permeability values two orders of magnitude higher than in the northern boreholes. These variations in physical and structural properties resulting in a highly heterogeneous crust, reflect the cumulative effect of episodes of tectonic activity over millions of years that this region has undergone.

One-Dimensional Velocity Model, Station Correction and Earthquake Relocation of Local Earthquakes in the Koyna–Warna Region, India

In this paper we investigated the feasibility of using the higher-frequency waveforms to characterize the seismicity in the Koyna–Warna region, globally well-known for its reservoir-triggered earthquakes. The local seismicity and source parameters have been extensively explored by many researchers using various types of data, e.g. analog data, broadband seismological data, and deep seismic sounding data. However, here we have utilized the seismological data recorded by 97 temporary 4.5-Hz geophones to locate the local seismicity in the region. We first investigated an appropriate 1-D P-wave velocity model through the inversion of P- and S-times. In order to compensate for the thick and laterally heterogeneous basalt, we incorporated the station corrections in the 1-D velocity inversion process. The station correction varies from + 0.09 to − 0.20 s for the P-waves, with clear demarcations of positive and negative station corrections corresponding to the northern and southern regions, respectively. The patterns of positive and negative station corrections suggested that the northern part is characterized by lower velocity, while slighter higher velocities present in the southern part of the study area are relative to the estimated 1-D velocity model. The earthquake locations are further improved by using the double-difference (hypoDD) approach, which enabled us to accurately map shallow subsurface seismogenic faults in the region. The data comprises 10,062 P-wave and 9049 S-wave catalog differential times from 348 earthquakes (1.0 ≤ ML ≤ 4.5). The precisely relocated earthquakes (ERH < 0.05 km ERZ < 0.5 km and rms < 0.02 s) form the three clusters of earthquakes in the NW–SE direction along the lineaments close to Warna Reservoir. The observed seismicity trends on the surface could be due to the vertical manifestations of faults or fractures. Their alignments are confirmed by the analysis of focal mechanism which exhibits a predominant normal type of faults with one nodal plane consistent with the main alignment of the seismic events.

On the Dynamics of the Seasonal Components of Induced Seismicity in the Koyna–Warna Region, Western India

The spatial and time dynamics are analyzed for the seasonal components of induced seismicity in the Koyna–Warna region of Western India. The peculiarities of the variations in these components are compared to the changes in the local tectonic regime inferred from the focal mechanism data of the earthquakes. Based on this, the hypotheses about the probable nature of the dynamics in the seasonal components of seismicity are suggested. It is noted that the variations in the seasonal seismic activity after the impoundment of the Koyna reservoir in the north are caused by the spatial migration of the induced seismicity and activation of the normal faults in the south. It is hypothesized that the process of fracture migration from the north to the south at this stage advanced the diffusion of the fluid from the Koyna reservoir, and as the water front reached the southern zone of normal faulting, this caused reactivation of the seasonal seismicity. An explanation is suggested for the stronger response of the seasonal activity in the region of Warna reservoir compared to the Koyna area: in contrast to Koyna, filling the Warna reservoir was geographically close to the area of activated seismicity. It is shown that the localization and sizes of the areas of the instantaneous and delayed components in the seasonal activity of the induced seismicity are determined by the localization and sizes of the areas of high stresses created by the increase in the pore pressure in highly permeable fault zones.

Role of reservoirs in sustained seismicity of Koyna-Warna region—a statistical analysis

Koyna-Warna region in western India is a globally recognized site of reservoir-triggered seismicity near the Koyna and Warna reservoirs. The region has been reported with several M > 5 earthquakes in the last five decades including M6.3 Koyna earthquake which is considered as the largest triggered earthquake worldwide. In the present study, a detailed statistical analysis has been done for long period earthquake catalogues during 1968–2004 of MERI and 2005–2012 of CSIR-NGRI to find out the spatio-temporal influence of the Koyna and Warna reservoirs impoundment on the seismicity of the region. Depending upon the earthquake clusters, we divided the region into three different zones and performed power spectrum and singular spectrum analysis (SSA) on them. For the time period 1983–1995, the earthquake zone near the Warna reservoir; for 1996–2004, the earthquake zone near the Koyna reservoir; and for 2005–2012, the earthquake zone near the Warna reservoir found to be influenced by the annual water level variations in the reservoirs that confirm the continuous role of both the reservoirs in the seismicity of the Koyna-Warna region.

Seismic hazard assessment of Koyna region, Peninsular India: using geospatial approach

BackgroundEarthquake-prone regions from the stable continental regions (like Peninsular India) warrant total seismic hazard estimation from possible sources — reservoir induced or tectonic. The Koyna region falls in the so-called stable continental region, though previously considered as aseismic because of its location in the Peninsular India, which was argued to be the continental Shield region; however, it is seismically active ever since the December 10, 1967, strong earthquake (M6.5) which killed around 200 people and damaged properties. The event was initially attributed to reservoir-induced seismicity. The present study aims at mapping the spatial distribution of hazard intensity in terms of peak ground acceleration and damage potential expressed as Modified Mercalli Intensity (MMI) levels owing to the scenario earthquakes (M4.0 to M6.9) from the linear sources and background seismicity in the Koyna region. For the seismic source model, geospatial integration approach is used. To delineate the source boundaries we perform spatial analysis of seismicity attributes and assigning weights based on the geological information. In addition, spatial buffering of thematic layers of faults and lineaments is applied to truncate the spatial uncertainties. Thematic attribute layers are integrated over the SRTM-DEM-derived geomorphic features and spatial seismicity pattern to define the possible linear seismic source zones. We employ ArcGIS® to perform geospatial analysis and to integrate thematic spatial information finally to prepare the seismic hazard and damage potential zones for the Koyna region.ResultsWe present the probabilistically estimated seismic hazard and damage potential estimates for the Koyna region. Spatial mapping of the hazard parameters is performed on Geographic Information System (GIS) platform. Along with the historical and instrumental earthquake data, we incorporate updated and relocated seismic events from circa 1618 to 2017 covering ~ 400 years period, for analyzing the mean annual rates of seismicity and earthquake recurrence intervals. The anticipated recurrence intervals of earthquakes ~ M5.5 is around 100 ± 10 years. The estimated peak ground acceleration (PGA) for 10% probability being exceeded in 50 years (i.e. 500 years of return period) is greater than 21% g is anticipated for the area located (~30 km radius) between the Koyna and Warna Reservoirs.ConclusionsThe hazard estimates for the 500 years return period earthquake scenarios (M4.0 to M6.9) are based on the bedrock level estimation. Estimated PGA-values are grouped into three categories that are relatively defined as high, moderate, and low hazard zones, respectively. Slightly damaging intensity (MMI-VII level) expected in 50 years has ~ 40% probability and PGA greater than 21% g for the area which is located between Koyna and Warna Reservoirs, which is identified as high hazard zone.

Three Dimensional Velocity Structure of the Koyna-Warna Region using Local Earthquake Tomography

ABSTRACT A dense seismic network (∼100 stations) was operated in the Koyna-Warna region from January 2010 to May 2010, that allow us to collect 400 high-quality local earthquake data of magnitude less than 4. In this region, the fault structure and tectonic setting that accommodate the induced seismicity is not well understood. To investigate the seismotectonics of the region, we have inverted 7826 P- and 7047 S-P arrival times for 3-D Vp and Vp/Vs tomographic models along with hypocenters parameters in the region. Although, Dixit et al. (2014) have performed 3-D local earthquake tomography with double-difference tomography code using catalog differential time data. In this paper, Simulps14 code on the same data set is applied. For better approach P arrival time and S-P travel times are inverted directly for Vp, Vp/Vs variations and earthquake locations. High Vp ∼5.9 to 6.5 and low to high Vp/Vs ∼1.69-1.74 imaged in the hypocenter region. These features interpreted as a fluid bearing rock mass under high pore pressure. It is also observed that below the trap basement form a local topography depression between the Koyna and Warna Reservoirs. To the South of the Warna reservoir, intense seismic activity defines a major cluster of ∼ 5 km width at 3 to7 km deep, located under the trap, where the basement is deepening. Such regions are inferred to be associated with the seismically active faults zones. The obtain velocity anomalies are reliable down to a depth of 10 km. This is also confirmed by the analysis of three resolution parameters viz. Hit count, Derivative Weight sum (DWS) and Resolution Diagonal Elements (RDE).

Lineament Fabric from Airborne LiDAR and its Influence on Triggered Earthquakes in the Koyna-Warna Region, Western India

ABSTRACT We present the results of the first airborne LiDAR survey flown in the Koyna-Warna region and examine the relationship between the lineament fabric and the ongoing seismicity in the region. Our studies reveal that earthquakes of M≥4.0 for the period 1968 to 2016 are strongly correlated with a 10 km wide N-S fracture zone, which not only represents the surface expression of seismically active basement faults, but also act as conduits for water percolation between the Koyna and Warna reservoirs. A decreasing trend in the annual distribution of earthquakes was observed from 1985. A new burst of seismic activity in 1993 followed the impoudment of the Warna reservoir. We report a change in annual seismicity pattern, where seismicity peaks during September and December in the pre-Warna period, with a new peak emerging during March-April subsequent to the impoundment of Warna reservoir. A model is proposed to explain the seismicity along dominant N-S lineaments and the impact of Warna reservoir impounding which altered the hydrogeologic regime in the region.

Reservoir Loading Parameters at Koyna and their Correspondence with Reservoir Triggered Seismicity

ABSTRACT Several water reservoir loading parameters such as the rate of loading, highest water levels reached, duration of retention of high water levels and whether the previous water maxima has been exceeded or not, influence reservoir triggered seismicity (RTS) in the vicinity of Koyna and Warna reservoirs. Here we present an update in the change of these parameters and their relation with RTS in the Koyna-Warna region.

Migration of Seismicity and Pore Pressure Diffusion for Five Decades Long Koyna–Warna Sequence and Precursory Nucleation Process for Earthquake Forecasting

ABSTRACT The continued reservoir-triggered seismicity for five decades in Koyna area has been attributed to southward migration of seismicity (during 1967-1992 near and south of Koyna dam and from 1993 onwards mostly near the new Warna reservoir). Spread of seismicity in the vicinity of reservoirs is attributed to pore-pressure diffusion. Moderate size Koyna – Warna earthquakes are found to nucleate at shallow depth (≤ 3 km) due to pore pressure caused by water level fluctuation of reservoir(s). The nucleation zone deepens along the critically stressed permeable fault zone to cause the occurrence of mainshock at the base of seismogenic layer (i.e. 5-10 km). The clustering of foreshocks up to 500 hr prior to several moderate size Koyna earthquakes of magnitude Mw 4-5 has been detected and used for quantifying the nucleation process. A static stress transfer by means of cascade model from one foreshock to next for the generation of foreshocks has been proposed for nucleation model. The nucleation process can be considered as an immediate earthquake precursor for the Koyna-Warna region.

Recent Seismicity Patterns and Microearthquake Activity on an Active Intraplate Fault System at Koyna-Warna, Western India

ABSTRACT 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.

Reservoir induced seismicity in the Koyna–Warna region, India: Overview of the recent results and hypotheses

The state of the art in the geological and geophysical study of the region of Koyna and Warna water reservoirs is reviewed. The probable geodynamical factors of induced seismicity are discussed. The detailed geophysical surveys, satellite geodetic data, and time history of the seismicity in the region reveal a complicated pattern of the structure and recent geodynamics of the region. The existing data suggest that the induced seismicity is here most likely to be caused by the regional (intraplate) stresses driving the displacements along the orthogonal network of the faults whose strength has dropped and continues decreasing due to the reservoir impoundment and operation processes. The evolution of the seismicity which started immediately after the rapid filling of the Koyna reservoir in the region of the dam, then rapidly expanded southwards and eventually became concentrated in the region of the subsequently constructed Warna reservoir shows that seismic events can be initiated by a number of factors whose contributions may vary with time. The key ones among them include reservoir loading and its seasonal variations; water saturation of the faults which guide the propagation of the front of fracture, increased permeability, and, probably, mineral transformations (hydrolysis) under the water level fluctuations in the reservoirs; and displacement of the front of the high pore pressure down to the main source zone of the earthquakes at a depth of 6–8 km. Based on the analysis presented in the paper, we outline the directions of the future research aimed at studying the nature and dynamics of induced seismicity in the region of large water reservoirs.

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 km 2 , 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.

Periodic variation of stress field in the Koyna–Warna reservoir triggered seismic zone inferred from focal mechanism studies

The Koyna–Warna region in western India is globally recognized as the premier site of reservoir triggered seismicity (RTS) associated with the Koyna and Warna reservoirs. The region is characterized by continuous seismic activity observed since several decades, including the world's largest triggered earthquake of M6.3 which occurred in Koyna in 1967. While the role of reservoirs in triggering earthquakes has been widely discussed, the actual tectonic mechanism controlling earthquake genesis in this region is hardly understood. The Koyna–Warna region is exclusively governed by earthquakes of strike–slip and normal fault mechanism distinct from the thrust faulting seen in other active zones in the Indian region. In the present study, a comprehensive catalog of 50 focal mechanism solutions of earthquakes that occurred during the last 45years in the Koyna–Warna region is developed, both from previous literature and from moment tensor inversion studies by the authors using broadband data from a local seismic network operating since 2005. The seismicity and fault plane data have enabled precise delineation of trends of the major causative faults, which are further accentuated using the double-difference technique. Stress inversion of the focal mechanism data has provided the best fitting principal compressive and tensile stress field of the region, which in conjunction with the deciphered fault zones provides a feasible model of seismogenesis in this region. Based on the observed temporal variation of faulting mechanism a model of alternating cycles of predominantly strike–slip and normal faulting is proposed, which is attributed to a periodic peaking and relaxation respectively of the horizontal compressive stress field in this region due to the Indian plate collision with Eurasia.

Borehole Seismic Network at Koyna, India

The Koyna region located near the west coast of India has been continuously having artificial water‐reservoir‐triggered earthquakes since the impoundment of Koyna Dam in 1962. To improve the accuracy of locations of these triggered earthquakes, a unique network of eight borehole seismometers surrounding the seismicity was designed. Five of these have been installed at depths varying from 1134 to 1522 m, well below the Deccan basalt cover. As it is difficult to install the borehole seismometers with desired orientation of the horizontal components, amplitudes of P waves were used to decipher the true orientations after installation. Analysis of data for the period March–June 2015 demonstrated that there is a remarkable increase in the number of earthquakes located with the help of borehole seismometers. Using the existing network of 21 broadband surface seismometers, 184 earthquakes of M L≥0.5 were located; however, with the borehole seismometers, the number of the located events increased to 888 such events. The data also indicate an increase in seismic activity corresponding to the period of increase in the rate of emptying of the Koyna and Warna reservoirs.

Co-Integration Models for Koyna and Warna Reservoirs, India

Koyna region, a seismically active region, has many time series observations such as seismicity, reservoir water levels, and many bore well water levels. One of these series is used to predict others since these parameters are interlinked. If these series were stationary, we used correlation analysis. However, it is seen that maximum of these time series are nonstationary. In this case, co-integration method is used that is extracted from econometrics and forecast is possible. We have applied this methodology to study time series of reservoir water levels of this region and we find them to be co-integrated. Therefore, forecast of water levels for one of the reservoir is done from the other as these will never drift apart too much. The outcomes demonstrate that a joint modelling of both data sets based on underlying physics resolves to be sparingly useful for understanding predictability issues in reservoir induced seismicity.

Annual Periodicity in the Seismicity and Water Levels of the Koyna and Warna Reservoirs, Western India: A Singular Spectrum Analysis

Abstract Association of earthquakes with dams and reservoirs has been reported in several cases worldwide. The Koyna region in western India is one such famous example where earthquakes continue to occur even after 50 years of reservoir impoundment and after the 1967 Koyna earthquake ( M 6.3), the largest reported earthquake associated with a reservoir. We analyzed the seismicity data from January 2005 to June 2012 in the Koyna–Warna region as recorded by a local seismic network. We performed a singular spectrum analysis, which shows that water level series has dominance of variance for annual periodicity. For the earthquake time series, a particular zone of earthquakes located close to the Warna reservoir shows predominance of annual frequency, suggesting a compelling relation between seismicity of this zone and reservoir water level changes. The lower‐order periodicities in the earthquake time series suggest that additional processes (other than reservoir water level changes, which are not well understood and hence need further investigations) also contribute to earthquake activity throughout the year.

Low deformation rate in the Koyna–Warna region, a reservoir triggered earthquake site in west-central stable India

We analyse nine years of GPS measurements of crustal deformation from the Koyna–Warna region within the stable India plate. The Koyna–Warna region experienced a strong earthquake on 10 December 1967 (M 6.3) that is considered to have been induced by the impoundment of the Koyna reservoir and the continuing earthquake activity in the region is considered to be associated with the Koyna and Warna reservoirs. The earthquakes occur in a very small region of 30×10km2 in two well defined seismic zones, the NNE–SSW trending Koyna Seismic zone, and the NNW–SSE trending Warna Seismic Zone. These zones are characterised by predominantly left-lateral strike slip motion and normal motion, respectively. In 2003, we initiated campaign-mode GPS measurements in the region. Analysis of the GPS data collected over nine years indicate low to moderate deformation rate (<2±0.5mm/year) at a few sites within and close to the fault zones and no resolvable deformation elsewhere. This has been seen in many intra-plate seismic regions of the world with varying causative mechanism for the deformation. In the Koyna Warna region, the observed surface displacement rates of up to 2mm/year near the fault zones are consistent with a fault slip rate of about 7mm/year, and with the inferred sense of motion on the faults. The inferred fault slip rate is consistent with the total moment release during earthquakes of past six years in the Koyna Warna region which may imply that the ongoing earthquake activity causes the deformation in the region.

Seismicity, faulting, and structure of the Koyna‐Warna seismic region, Western India from local earthquake tomography and hypocenter locations

AbstractAlthough seismicity near Koyna Reservoir (India) has persisted for ~50 years and includes the largest induced earthquake (M 6.3) reported worldwide, the seismotectonic framework of the area is not well understood. We recorded ~1800 earthquakes from 6 January 2010 to 28 May 2010 and located a subset of 343 of the highest‐quality earthquakes using the tomoDD code of Zhang and Thurber (2003) to better understand the framework. We also inverted first arrivals for 3‐D Vp, Vs, and Vp/Vs and Poisson's ratio tomography models of the upper 12 km of the crust. Epicenters for the recorded earthquakes are located south of the Koyna River, including a high‐density cluster that coincides with a shallow depth (&lt;1.5 km) zone of relatively high Vp and low Vs (also high Vp/Vs and Poisson's ratios) near Warna Reservoir. This anomalous zone, which extends near vertically to at least 8 km depth and laterally northward at least 15 km, is likely a water‐saturated zone of faults under high pore pressures. Because many of the earthquakes occur on the periphery of the fault zone, rather than near its center, the observed seismicity‐velocity correlations are consistent with the concept that many of the earthquakes nucleate in fractures adjacent to the main fault zone due to high pore pressure. We interpret our velocity images as showing a series of northwest trending faults locally near the central part of Warna Reservoir and a major northward trending fault zone north of Warna Reservoir.