V. M. Tiwari

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.

Water level estimation by remote sensing for the 2008 flooding of the Kosi River

Flood is a natural disaster which worsens when it is triggered by man-made constructions. This paper discusses one such flood event which occurred because of breach of a levee in the upper reach of the Kosi River in 2008, when floodwater spread over a large portion of the low-lying Ganga Plain of North Bihar, India. Here we have analysed a suite of space-based observations from radar altimetry, Moderate Resolution Imaging Spectroradiometer (MODIS) images, and Tropical Rainfall Measuring Mission (TRMM) precipitation data, together with in situ monthly precipitation data, with a main emphasis on the results from altimetry and MODIS data. A methodology to calculate water levels, using MODIS data and Envisat data together, is also discussed. Our analyses suggest a rise in water level of 1.0–1.4 m in the flooded region during the flood event and a maximum extent for the flooded area of around 2900 km2. Analyses of TRMM precipitation data do not indicate any influence of high precipitation in the upper catchment of the Kosi Basin on river water feeding into the plain area after breaching of dam. However, heavy and prolonged precipitation was found downstream of the dam over the flooded area during the flood period.

Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

The Andaman arc is the site of the giant mega-thrust earthquake of 2004 (Mw 9.3), one of the largest earthquakes that ever occurred globally (Lay et al., 2005). The earthquake originated at Bandah Aceh in the south, off the coast of northern Sumatra and ruptured a zone of about 1200 km cutting through the Andaman-Nicobar Islands. The focal mechanism given by the Harvard University indicated a thrust fault mechanism with a NW-SE trending plane. It is generally believed that this earthquake was caused by a sudden slip of the mega-thrust lock– up zone on the interface between the subducting Indo-Australian plate beneath the Burma plate (Sieh, 2005). Detailed marine seismic mapping across the subduction zone in the Sumatra region is, however, suggestive of a possible brittle failure at mantle depths (Singh et al., 2008). The Indian plate borders the Burmese plate along the Burma and Andaman arcs to the east. While the Indian plate obliquely subducts in the Andaman arc (Fitch, 1972; LeDain, 1984; Curray, 1979), it is believed to have a nearly strike-slip environment in the Burmese arc (LeDain, 1984; Kumar and Rao, 1995; Kumar et al., 1996; Vigny et al., 2003) with a possible cessation of subduction in the recent times (Rao and Kumar, 1999; Rao and Kalpna, 2005). The main tectonic features marking the India-Burma plate boundary are the Indo-Burman ranges in the north and the Andaman-Nicobar ridge to the south. To the east of the ridge lies the Andaman sea which is an active back-arc extensional basin (Curray, 2005) that was initiated about 4 million years ago (Raju et al., 2004), and exhibits transform faulting evidenced by strike-slip and normal fault earthquakes. The Burmese plate adjoins the Sunda plate to the east, along the NS trending Sagaing fault which is known to have a right lateral strike-slip motion. The West Andaman Fault and the Sumatran fault system are the major tectonic features towards southern Andaman, forming the continuity of the Sagaing fault across the Andaman-Nicobar ridge (Figure 1). On 10 August 2009, at 19:55:39 UTC an earthquake of magnitude 7.5 occurred to the north of the Andaman-Nicobar Islands off the coast of Diglipur Island at latitude 14.013o N and longitude 92.923o E (figure 1). The earthquake was felt not only in the Andaman and Nicobar Islands but also in several cities along the east coast of the Indian peninsula. This earthquake is considered significant due to its large size for a normal fault event in this

A 3D model of the Wathlingen salt dome in the Northwest German Basin from joint modeling of gravity, gravity gradient, and curvature

AbstractIn the past few decades, numerous attempts have been made on modeling of salt tectonics and deciphering the geometry of salt domes, which is a key challenge in petroleum exploration. We have derived a 3D density model of the Wathlingen salt dome, situated in the southern part of the Northwest German Basin from joint modeling of reprocessed torsion balance measurements. Gravity, gravity gradients Wzx and  Wzy, curvature derived from horizontal gravity gradients Wxy, and horizontal directive tendency are jointly modeled to decipher the geometric structure of the salt dome. The model was constrained by geologic and borehole information. We found that the Wathlingen salt dome is a mushroom-structured salt body, which is 14-km long, 4–8-km wide extending up to ∼4‐km depth. The top mushroom structure of the salt is horizontally spread up to ∼8  km. It would not have been possible to derive the complex 3D structure from modeling of gravity data alone.

Numerical simulation of present day tectonic stress across the Indian subcontinent

In situ measurements of maximum horizontal stress (SHmax) in the Indian subcontinent are limited and do not present regional trends of intraplate stress orientation. The observed orientations of SHmax vary considerably and often differ from the plate velocity direction. We have simulated orientation and magnitude of SHmax through finite element modeling incorporating heterogeneities in elastic property of the Indian continent and plain stress approximation to understand the variability of SHmax. Four different scenarios are tested in simulation: (1) homogeneous plate with fixed plate boundary (2) homogeneous plate with boundary forces (3) heterogeneous plate with fixed boundary (4) heterogeneous plate with boundary forces. The estimated orientation and magnitude of SHmax with a heterogeneous plate with boundary forces in the Himalayan region and an eastern plate boundary comprising the Indo-Burmese arc and Andaman subduction zone are consistent with measured maximum horizontal stress. This study suggests that plate boundary force varies along the northern Indian plate margin and also provides a constraint on the intraplate stress field in the Indian subcontinent.

Insights into the Lurking Structures and Related Intraplate Earthquakes in the Region of Bay of Bengal Using Gravity and Full Gravity Gradient Tensor

Comprehension of subsurface structures buried under thick sediments in the region of Bay of Bengal is vital as structural features are the key parameters that influence or are caused by the subsurface deformation and tectonic events like earthquakes. Here, we address this issue using the integrated analysis and interpretation of gravity and full gravity gradient tensor with few seismic profiles available in the poorly known region. A 2D model of the deep earth crust–mantle is constructed and interpreted with gravity gradients and seismic profiles, which made it possible to obtain a visual image of a deep seated fault below the basement associated with thick sediments strata. Gravity modelling along a NE–SW profile crossing the hypocentre of the earthquake of 21 May 2014 (Mw 6.0) in the northern Bay of Bengal suggests that the location of intraplate normal dip fault earthquake in the upper mantle is at the boundary of density anomalies, which is probably connected to the crustal fault. We also report an enhanced structural trend of two major ridges, the 85°E and the 90°E ridges hidden under the sedimentary cover from the computed full gravity gradients tensor components.

Co-seismic gravity changes in the Koyna-Warna region: Implications of mass redistribution

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.