H. V. S. Satyanarayana

Results from Local Earthquake Velocity Tomography: Implications toward the Source Process Involved in Generating the 2001 Bhuj Earthquake in the Lower Crust beneath Kachchh (India)

To comprehend the source processes of the 26 January 2001 Bhuj earthquake sequence of M W 7.7 and its influence on the seismic hazard of the Kachchh, we estimate various seismological parameters using reliable and accurate aftershock data. The estimated parameters led to several important findings including the delineation of an east-west-trending, south-dipping (≈45°) fault (North Wagad fault [NWF]), which touches the surface about 25 km north of Kachchh Mainland fault (KMF). The aftershock zone is confined to a 60-km-long and 40-km-wide region lying between the KMF to the south and NWF to the north, extending from 10 to 45 km depth. Focal mechanism solutions of the mainshock and 25 significant aftershocks of M W ≥3.0 obtained from waveform inversion of broadband data and local earthquake moment tensor inversion suggest that the region between the KMF and NWF is mainly characterized by reverse faults with east-west trend and southerly dip, matching with the geological faults in the region. The tomographic inversion technique is used to invert 5516 P -wave travel times and 4061 S - P travel-time differences from 600 aftershocks recorded at 8-18 stations. Tomographic results suggest a regional high-velocity body (characterized by high V p [7.0-8.5 km/sec], high V s [4.0-4.8 km/sec], and low σ [0.24-0.26]) with a head extending 60 km in north-south and 40 km in east-west at 10-40 km depths. This high-velocity anomaly is inferred to be a mafic pluton/rift pillow, which might have intruded during the rifting time (∼135 Ma). This crustal mafic pluton must be contributing significantly in accumulating large crustal stresses resulting in the generation of large earthquakes in this intraplate area. Another important result of our study is the detection of a low-velocity zone (low V p [6.5-7.0 km/sec], low V s [3.6-4.0 km/sec], large σ [0.26-0.265]) within the mafic body at the hypocentral depth of the mainshock (∼18-25 km), which is inferred to be a fluid-filled (trapped aqueous fluid resulting from metamorphism) fractured rock mass. The analysis of depth distribution of b -values suggests a high b -value zone between 15 and 25 km depths, which further supports this contention. Hence, the presence of fluids at the hypocenter might have facilitated the occurrence of the 2001 Bhuj earthquake within the inferred mafic body in the lower crust.

The deadliest stable continental region earthquake occurred near Bhuj on 26 January 2001

A large destructive earthquake occurred on 26 January 2001 in the region of Kutch, Gujarat, in Western India, with magnitude Mw 7.7. The earthquake caused very heavy damage and a large number of casualties with more than 20,000 deaths. A preliminary study of ground deformation, damage pattern and aftershock distribution is presented.

Aftershock activity and frequency-dependent low coda Qc in the epicentral region of the 1999 Chamoli earthquake of Mw 6.4

Abstract— On 28 March, 1999 (19:05:10.09, UT) a significant earthquake of Mw 6.4 occurred in the Garhwal Himalaya (30.555°N, 79.424°E). One hundred and ten well-recorded aftershocks show a WNW-ESE trending northeasterly dipping seismic zone extending from a depth of 2 to 20 km. As the main shock hypocenter occurred at the northern end of this seismic zone and aftershocks extended updip, it is inferred that the main-shock rupture nucleated on the detachment plane at a depth of 15 km and then propagated updip along a NE-dipping thrust plane. Further, the epicentral distribution of aftershocks defines a marked concentration near a zone where main central thrust (MCT) takes a significant turn towards the north, which might be acting as an asperity in response to the NNE compression due to the underthrusting of Himalayan orogenic process prevalent in the entire region. Presence of high seismicity including five earthquakes of magnitude exceeding 6 and twelve earthquakes of magnitude exceeding 5 in the 20th century is presumed to have caused a higher level of shallow crustal heterogeneity in the Garhwal Himalaya, a site lying in the central gap zone of the Himalayan frontal arc. Attenuation property of the medium around the epicentral area of the 1999 Chamoli earthquake, covering a circular area of 61,500 km2 with a radius of 140 km, is studied by estimating the coda Qc from 48 local earthquakes of magnitudes varying from 2.5–4.8. These earthquakes were recorded at nine 24-bit REFTEK digital stations; two of which were equipped with three-component CMG40T broadband seismometers and others with three-component L4-3D short-period seismometers. The estimated Qo values at different stations suggest on average a low value of the order of (30 ± 0.8), indicating an attenuating crust beneath the entire region. The frequency-dependent relation indicates a relatively low Qc at lower frequencies (1–3 Hz) that can be attributed to the loss of energy due to scattering on heterogeneities and/or the presence of faults and cracks. The large Qc at higher frequencies may be related to the propagation of backscattered body waves through deeper parts of the lithosphere where less heterogeneities are expected. An important observation is that the region north of MCT (more rigid highly metamorphosed crystalline rocks) is less attenuative in comparison to the region south of MCT (less rigid slightly metamorphosed rocks (sedimentary wedge)). The acceleration decays to 50% at 20 km distance and to 7% at 100 km. Hence, even 1g acceleration at the source may not cause significant damage beyond 100 km in this region.

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.

Delineation of Fractures through Acoustic Televiewer Log

Boreholes are the only sources for direct measurements of geological and geophysical characteristics of the shallow subsurface of the earth. Borehole imaging tool “Acoustic Televiewer (ATV)” is an advanced probe, which records 3D image of the bore hole wall and is used to obtain oriented images of bore hole and provides substantial information regarding lithology, structural information, detection of fractures and casing of the borehole. The images are highly sensitive to the presence of fractures, the delineation of which becomes accurate and reliable based on these acoustic images. Features identified on log-derived images can be correlated with core samples or can be used as substitute in the poor core recovery zones. In the present study, ATV log from a deep borehole drilled at Khadi Kolavan in Koyna-Warna region of Ratnagiri district, Maharashtra located west of the Western Ghat escarpment in the Deccan Volcanic province, India, is used for interpretation of structural characteristics of the formation. The same was correlated with other conventional logging methods such as resistivity and full waveform sonic (FWS) as well as core logging and rock quality designations (RQD) which generated new understanding of the basement and basalt cover in this region.

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.