Koyna-Warna Region Research Articles

Изменение строения поверхностей базальта и гранита при трении

AbstractFriction-induced changes in the structure of the surface layer of basalt and granite samples extracted from a well in the triggered seismicity zone in the Koyna–Warna region, India, have been studied by infrared, Raman, and photoluminescence spectroscopy. It has been found that friction leads to a partial degradation of quartz, albite, and clinopyroxenes crystals. Instead of these crystals, a thin layer of a mineral with a low coefficient of friction—kaolinite—is formed on the surface.

Volcanology and Lava Flow Morpho-types from the Koyna-Warna Region, Western Deccan Traps, India

ABSTRACT Flow mapping and physical volcanology of 15 basaltic lavas exposed in three critical road pass sections (ghats) in the Koyna-Warna region of the western Deccan Traps is presented in this paper. Transitional lavas like rubbly pahoehoe are most common morpho-type exposed in these ghat sections. Sinking of rubbly breccia into flow interiors and formation of breccia-cored rosette are common in some lava flows. Few rubbly lavas exhibit slabby tendencies. The amount and nature of the associated rubble is variable and result from the mechanical fracturing and auto-brecciation of the upper vesicular crust in response to distinctive stages in the cooling, crystallization and emplacement history of individual lava flows. Occurrence of aa and pahoehoe morpho-types in the lava flow sequence is subordinate. Three prominent pahoehoe flows separated by red bole horizons are seen in the upper parts of the Kumbharli ghat. These are thick, P-type sheet pahoehoe. The pahoehoe lavas represent compound flow fields that grew by budding, endogenous lava transfer and inflation. Presence of pahoehoe lavas in the Koyna-Warna region hints at possible hitherto unrecorded southern extension of Bushe-like flow fields. This study reconfirms the existence of pahoehoe-slabby-rubbly-aa flow fields and transitions even in the upper echelons of the Deccan Trap stratigraphy. The study of morphology and internal structure of lava flows exposed at the ghat sections in the Koyna-Warna region could guide sub-surface core-logging that is critical in deciphering the physical volcanology and emplacement dynamics of basaltic lava flows penetrated by drill holes sunk under the scientific deep drilling programme.

Ground Electrical and Electromagnetic Studies in Koyna-Warna Region, India

ABSTRACT In this paper, we present results of the Ground Electrical and Electromagnetic (GEE) studies (MT, CSAMT, DRS) carried out in the Koyna-Warna zone since 1976 and discuss their relevance in understanding the seismicity of the Koyna-Warna seismic zone. Though there were not many GEE studies carried out in this region, the existing ones provided a vital information, in particular, on the subsurface crustal structure, the Deccan Trap thickness and its variation and also the nature of the basement in this region. The GEE studies rule out the presence of any subtrappean sediments in this region. An interesting feature that has been identified from MT studies is the presence of a well-defined crustal block structure, characterized by high resistive blocks interspersed with moderately conductive features. All these structural features clearly point out to the intensity of tectonic disturbance that the region was subjected to in the geological past. The conductive features are correlatable with some of the known seismogenic structural features, for e.g., the Konya Fault Zone, the west coast fault, the Donachiwada fault. The moderately resistive zone sitting over the high resistive blocks as seen both in MT& DRS models is inferred to be related to the generally fractured nature of the shallow crustal column. The conductive linear features bordering the resistive blocks represent fluid filled fracture/fault zones. It is inferred that because of the NE to NS oriented compressive stress regime in the Indian shield, due to the Himalayan collision tectonics, some of these structural features may become the locales of stress accumulation which may get released due to fluid filling of these zones under the influence of nearby reservoirs, resulting in triggering of seismicity. Results from different GEE studies conducted in the study area are found to be highly consistent with each other as well as with the Rasati bore hole data, thus bringing significant validity to the subsurface model derived. Further, the 3D modelling of the MT data acquired in the Koyna-Warna region together with airborne gravity gradient and magnetic studies carried out under the Deep drilling program would open up new gate ways to accomplish multi-parametric three dimensional modeling, that will provide still more detailed and relevant subsurface image of this important RTS zone.

Co-seismic Gravity Changes in the Koyna-Warna Region: Implications of Mass Redistribution

ABSTRACT Koyna-Warna Region (KWR) is one of the known sites for reservoir triggered seismicity. The continued triggered seismicity over the five decades is restricted to a region of about 600-700 sq. km, which provides a unique opportunity to monitor geophysical anomalies likely to be associated with seismicity of the region. Present study confers temporal gravity changes recorded by gPhone and GRACE satellite and interprets observed changes in conjunction with seismological, geodetic (cGPS) observations and groundwater level measurements. GRACE data suggest that seasonal vertical deformation due to hydrological loading is ∼ 2 cm, which corroborates with continuous GPS observations. Seasonal hydrological loading of the region, which is in a phase of reservoir loading, might be influencing the critically stressed KWR leading to the seasonal seismicity of the region. The gPhone gravity data distinctly show co-seismic gravity signals for eight earthquakes of Mw > 2 and gravity anomalies show positive correlation on a logarithmic scale with earthquake released energy. To investigate the cause of gravity changes, an estimate is made for 14th April 2012 earthquake for Mw 4.8 using fault dislocation model. The recorded gravity changes of 189 μGal by gPhone located at a distance of 28 km from the hypocentre is much more than the estimate of ∼0.1 μGal calculated for Mw 4.8 Koyna earthquake. Therefore, it is inferred that co-seismic gravity signals for eight earthquakes are primarily caused due to redistribution of mass at shallow depth.

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.

Earthquake Focal Mechanism Studies in Koyna-Warna Region in the Last Five Decades — Current Understanding on Tectonics and Seismogenesis

ABSTRACT 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 the Warna 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.

In-situ Pore Pressure Variations in Koyna-Warna – A Promising Key to Understand Triggered Earthquakes

ABSTRACT The filling of the Koyna reservoir in western India and the associated triggered earthquakes have been well documented. Several studies have suggested that earthquakes are triggered on pre-existing faults in the region due to changes in pore pressure caused by pore pressure diffusion. To study in-situ pore pressure variations twenty-one borewells were drilled in the Koyna-Warna region under an Indo-German research program during 1995-1998. In most of these wells tidal signals are observed in well level variations indicating sensitivity to small strain changes in hydraulically connected, confined aquifers. Those signals, hence, are suitable to reflect variations in the stress field of local rock formations. More than a decade of well level monitoring has shown four types of earthquake related changes. The pre- and post-earthquake changes are mostly interpretative in nature and difficult to substantiate. The co-seismic and transient changes which are observed for local and large teleseismic events are well established. Wells connected to unconfined aquifers also showed changes related to seismicity in case of large magnitude earthquakes at closer distances. Some anomalous water level fluctuations are seen which are not associated with local or teleseismic earthquakes. These changes are coherent in nature and reflect aseismic regional volume strain.

Deccan Basalts in and around Koyna — Warna Region, Maharashtra: Some Reflections

ABSTRACT In this paper, an attempt is made to review the current status of chemo-and magnetostratigraphy of the Deccan Traps, particularly in the Western Province, and to interpret geomorphological and borehole data in relation to the earlier known structural data in the whole region, to that of the Koyna-Warna region, covering Deccan Traps and part the Archean basement. Koyna and Warna region comprises of chemically defined younger Formations of Wai Subgroup and Lonawala Subgroup displaying over-stepping arrangement from older Formations further north in the Nasik area and younger Formations down south. This is further corroborated by a single polarity change from reversed to normal, all the way to Belgaum. This is considered as a time marker around 65Ma (Chron C29r/C29n) matching with the K-T boundary. The Deccan Trap contact with the Archean basement in the 7 boreholes shows a plateau around 200m in the western section but a variation ranging up to 500m in the eastern side of the Warna region. These variations may be due to faulting or upwrap in this region. It is to be noted that the Koyna –Warna volcanic pile is similar to the thick sequences of the Mahabaleswar Ghat section further north in terms of their chemistry and magnetostratigraphy. A combination of surface and sub-surface data of the Koyna-Warna region may not show the presence of older volcanic formations such as Kalsubai Subgroup nor ages older than 65Ma. A few points are raised in this paper which may necessitate a detailed field, chemical, magnetic and geochronological study of both surface and drill cores in tandem with the available huge data base of the Deccan Traps, which may reveal surprises and perhaps provide clues to the origin of seismicity in this region and provide possible clues to a better understanding of the study area.

Evidence for Neoarchean Basement for the Deccan Volcanic Flows around Koyna-Warna Region, Western India: Zircon U-Pb Age and Hf-isotopic Results

ABSTRACT Deep boreholes around the Koyna-Warna seismic zone in the southwestern part of the Deccan Volcanic Province (DVP) enable a rare access to the basement beneath the thick (typically 1-2 km) lava pile, the nature of which has been enigmatic for long. Utilizing the drill core from borehole KBH-1 near Rasati village, we present here in situ Laser Ablation – Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) and LA-MC (multi collector)-ICPMS zircon U-Pb age and Hf-isotopic composition for two samples along with a brief description of the petrology and geochemistry of the representative lithounits of the basement section. The KBH-1 basement section comprises predominantly grey migmatite gneisses of granodiorite, tonalite and quartz monzodiorite composition apart from minor pink monzogranite. The grey gneisses show geochemical affinity to Neoarchean Tonalite-Trondhjemite-Granodiorite (TTG) suites and modern calc-alkalic granitoids. Zircons from a granodiorite and a monzogranite samples yield consistent U-Pb ages of 2710± 63 Ma and 2700±49 Ma (2σ errors). The initial 176Hf/177Hf values lie in a narrow range (0.281162 to 0.281283) corresponding to εHf(T) values of +3.7 to +8.0 indicating that the magmatic precursors of the KBH-1 gneisses represent juvenile magmatism around 2700 Ma. In terms of the composition and age, a correlation between the gneisses in the KBH-1 borehole and the Neoarchean basement gneisses of the Eastern Dharwar Craton (EDC) is emphasized arguing for the extension of the EDC to the Koyna-Warna region.

Sustained Seismicity near Koyna-Warna Reservoir: A Mish-mash of Physical Processes

ABSTRACT Earthquakes in Koyna-Warna region are triggered by the reservoirs but the reasons for sustained seismicity, in terms of magnitude and time, remain enigmatic. We critically review the proposed models/processes, which include fault interaction, flexure of Western Ghat escarpment, velocity heterogeneity, and earthquakes being considered as aftershocks. We suggest that each of these processes or models have limitations and are not capable of explaining all the features of seismicity individually. It is possible that all put together and some other unknown additional processes are at work and there is a mish-mash of several processes attending the region, causing continuing seismicity for past five decades.

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).

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.

Long-term Hydrochemical Earthquake Precursor Studies at the Koyna-Warna Reservoir Site in Western India

ABSTRACT Koyna-Warna region of western India is an active seismic zone due to the Reservoir Triggered Seismicity (RTS). Earthquake precursor studies are carried out monitoring hydrochemical and stable isotope signatures in the groundwater from 15 bore wells since January 2005, for more than 12 years (January 2005 to February 2017). Depth of these boreholes ranges from 100 to 250 m. Cyclic or temporal variation in hydrochemistry is observed in few sensitive wells in Koyna region. The Govare well in Koyna is found to be most sensitive and the observed hydrochemical cycle is closely associated with local earthquakes of M > 5. The earth-quakes M <5 occurring either in Warna cluster or close to the observation wells, did not generate hydrochemical precursory changes. The increase in hydrochemistry is hypothesized as mixing of two aquifer waters with different hydrochemistry. It is noted that a precursory hydrochemical cycle is observed during first quarter of 2015, but no earthquake M > 5.0 occurred till date. The cyclic changes in hydrochemistry, however, indicate on-going earthquake processes and an impending earthquake of M > 5 in the 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.

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.

The behavior of seasonal variations in induced seismicity in the Koyna–Warna region, western India

Based on the earthquake catalog data for the Koyna–Warna region of induced seismicity in western India, the seasonal variations in seismic activity associated with annual fluctuations in the reservoir water level are analyzed over the time span of the entire history of seismological observations in this region. The regularities in the time changes in the structure of seasonal variations are revealed. The seasonal seismic activity is minimal in May–June when the reservoir level is lowest. During the remaining part of the year, the activity has three peaks: the fall peak in September, winter peak in November–December, and spring peak in February–March. The first mentioned peak, which falls in the phase of the water level reaching its maximal seasonal value is considered as the immediate response of the fluid saturated medium to the additional loading under the weight of reservoir water. The two subsequent maxima concur with the decline phase in the reservoir level and are interpreted as the delayed response associated with the changes in the properties of the medium due to water diffusion. It is shown that the intensities of the immediate and delayed responses to the seasonal water level variations both vary with time as does their ratio. The probable factors affecting the variations in the intensity of the seasonal components of the reservoir-induced seismicity are discussed.

3-D Modeling of Pore Pressure Diffusion Beneath Koyna and Warna Reservoirs, Western India

The mechanism of reservoir-triggered seismicity is well-understood and explains the earthquake occurrence at different reservoir sites. It can be attributed to the stresses due to water loading and to changes in fluid pressure in pores within the rock matrix. In the present study a 3-D fluid flow numerical model is used to investigate the pore pressure diffusion as a cause for continued seismicity in the Koyna–Warna region in western India. It is shown that reservoir water level fluctuations are sufficient to trigger earthquakes at the seismogenic depths in the region. Our numerical model suggests that a vertical fault with hydraulic conductivity in the range 2–6 m/day facilitates the diffusion of pressure at focal depths of earthquakes in the Koyna–Warna region. Also, for triggering of earthquakes a higher vertical conductivity is required for the Warna region than for the Koyna region. A lag of two months period is found between the maximum water level and the significant hydraulic head required to trigger earthquakes at the focal depth using the appropriate hydraulic conductivity for both the reservoirs.

Seismology in India

Recent seismological research in India can be mainly grouped into study of 1) seismogenesis and seismotectonics of the three regions viz., Himalaya, Andaman-Nicobar subduction zone and the Stable Continental region (SCR); 2) Strong motion study and 3) earthquake prediction. Last few years have witnessed significant change in monitoring of earthquakes through about hundred broadband seismographs in National Network and many more in clusters whose data are brought online to two centers at National Centre for Seismology, Delhi and INCOIS, Hyderabad. Extensive research on reservoir induced earthquakes has culminated into “Deep Crustal Drilling” (DCD) in the Koyna-Warna region down to focal depth to understand the nucleation and generation process of earthquakes.

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.

Understanding Conspicuous Gravity Low Over the Koyna–Warna Seismogenic Region (Maharashtra, India) and Earthquake Nucleation: A Paradigm Shift

The continued seismicity in Koyna–Warna region of the western part of Maharastra (India) and its relationship with subsurface structures, concealed below thick volcanic sequences, are studied in detail using gravity field along with newly available deep scientific drilling results. This seismically active zone is marked by a large conspicuous negative gravity anomaly, the causes of which are yet to be fully understood. Recent findings from the boreholes drilled in the Koyna (Gupta et al. in Int J Earth Sci 104:1511–1522, 2015) and Killari seismic zones, both of which penetrated the thick Deccan volcanic cover and the underlying Archean crystalline basement, have motivated us to revisit the Bouguer gravity field over this region, using a newly developed finite element method of regional-residual separation. Our study reveals the presence of two thick low-density/low-velocity crustal zones below the Koyna–Warna region, the shallower one between 5 and 13 km depth and the deeper one between 35 and 43 km depth just above the Moho. Both of these zones appear to contain mantle-metasomatised and fractionated magmatic material, respectively. Interestingly, the hypocenters of all M ≥ 5 Koyna earthquakes occur within the upper low-velocity/low-density zone. We also suggest high-order crustal exhumation below this region, which led to the removal of the entire sedimentary and granitic upper crustal column. This process has brought denser mid-crustal lithological facies close to the surface. Quaternary uplifting and movement of fault blocks along the old as well as newly created fault planes seem to be still continuing. A paleo-rift may have existed beneath this region below which Moho temperatures (~600 °C) and mantle heat flow (~31 mW/m2) are still high.