Regional estimation of Q from seismic coda observations by the Gauribidanur seismic array (southern India)
Introduction
The Gauribidanur Seismic Array (GBA; geographic coordinates of the array center point, 13°36′15″N, 77°26′10″E) was set up by the Bhabha Atomic Energy Research Centre and the United Kingdom Atomic Energy Authority in southern India and has been recognized as one of the most reputed in the world since its inception in 1965. It is a medium-aperture seismic array located about 90 km north of Bangalore with twenty short-period (T0=1 s) vertical-component seismometers arranged along two orthogonal arms (i.e., an L-shaped array) with a spacing of about 2.5 km (Fig. 1). Also, a broad band three component digital station operates at the intersection of the two arms (Mohan and Rai, 1992). The signals detected by the sensors are telemetered to the central laboratory for digital recording at a sampling interval of 0.05 s. The array lies in a relatively flat-lying area and the seismometer vaults are set in Archean rocks with unweathered gneiss lying within 2 m of the surface over most of the region (Mewat and Burch, 1974, Ram and Mereu, 1977).
The GBA is located in the Indian peninsula, on the western flank of the eastern Dharwar craton. The Dharwar craton is an Archaean domain which is one of the oldest geological provinces in southern India (Fig. 1). This block is divided by the 400 km long and 20–30 km wide, north–south trending Closepet granite body into the western and eastern parts. The western Dharwar craton (age 3.5–3.0 Ga) is made of old gneisses and greenstones with very few granites; on the other hand, the eastern Dharwar craton (age 3.0–2.6 Ga) is made of younger rocks with widespread N–S elongate plutons of late Archaean granites. The Closepet batholith (age 2.5 Ga) is the largest of these granitic intrusions and constitutes the boundary between the two parts (Moyen et al., 2003). The south Indian granulite terrain is composed of high-grade granulites of late Archaean metamorphism (2.6 Ga) and presents highland massifs with elevations reaching a maximum of 2.6 km. Another notable geological feature in southern India is the crescent shaped, Proterozoic, Cuddapah intra-cratonic basin (1600–1300 Ma) which consists of metamorphosed sandstones, shales, dolomites, quartzites and limestones (Singh and Mishra, 2002).
Using local earthquake data, Arora (1971) proposed a two-layered model for the Earth’s crust in the Gauribidanur region. He found a 16 km thick top granitic layer over a second layer 19 km thick above the mantle (i.e., with the Moho at 35 km depth). Observed velocities were found to be 5.67, 6.51 and 7.98 km/s for P phases, and 3.46, 3.96 and 4.61 km/s for the corresponding S phases.
The area near GBA belongs to the Indian shield, a type of region which is generally recognized as seismically stable. However, the region presents low to moderate intra-plate seismicity. According to Gangrade and Arora (1996), who performed a systematic investigation of the seismicity and seismotectonics of the peninsular Indian region, a slow and steady accumulation of seismic energy in this area could lead, occasionally, to earthquakes of moderate to significant magnitudes apart from the most frequently detected microearthquakes. Significant peninsular earthquakes have been occurred in the past, such as the induced Koyna earthquake (Mb=6.5; 10 December 1967), Ongole (Mb=5.8; 27 March 1967), Bhadrachalam (Mb=5.7; 13 April 1969), Hyderabad (Mb=4.5; 30 June 1983), Basta (Mb=4.9; 15 September 1983), Latur (Mb=6.3; 30 September 1993), and others. Gangrade and Arora (1996) concluded that all the past significant earthquakes occurred on fresh and unknown faults, not known to have ruptured before, and noticed the possibility that the tectonic lineaments which have not shown any seismic activity so far might do sometime in the future.
In this work, the attenuation properties of the medium will be investigated using microearthquakes that occurred in the region around GBA for which the array detection capability is maximum. The decay rate of the coda amplitudes (Qc−1) and the contribution of intrinsic absorption (Qi−1) and scattering (Qs−1) to total attenuation (Qt−1) will be estimated as a function of frequency. These parameters will help us to physically characterize the lithosphere of the region and will also be of great interest to the seismic hazard assessment in the area.
Section snippets
Methods of analysis and data used
The parameter Qc was defined by Aki and Chouet (1975) as a measure of the decay rate of coda envelopes within a given frequency band, which is independent of recording site and event location for a given region. They introduced a method for explaining phenomenologically the shape of the coda of local earthquakes as incoherent singly scattered S-waves from heterogeneities randomly distributed in a homogeneous medium. Later works on coda waves have refined the observations and have improved the
Data analysis and results
First, each seismogram was bandpass-filtered over the frequency bands 1–2 (1.5±0.5) Hz, 2–4 (3±1) Hz, 4–6 (5±1) Hz, and 6–10 (8±2) Hz. Then, the rms amplitudes Aobs(f|r,t) were calculated by using a 0.5 s spaced moving time window of length t±2 s for the frequency band centered at 1.5 Hz and t±1 s for the 3, 5, and 8 Hz centered frequencies. For each frequency band, the rms amplitudes for a noise window of 10 s before the P-wave arrival were also computed. Then, Qc−1 was estimated for each
Discussion and conclusions
Fig. 5 shows the surface projection of the ellipsoidal volume sampled by coda waves for the epicenters and stations used, thus showing the region that has been sampled using coda waves in this study. It can be observed that part of the Closepet granite intrusion as well as the eastern and western Dharwar cratons have been crossed by the scattered waves. Therefore, the medium properties inferred from this work will refer only to these regions.
The fit of the Qc−1 data to the frequency law Qc−1(f)=
Acknowledgements
We are very grateful to the Gauribidanur seismic array staff for providing the data used in this study. We also very much appreciate the constructive comments of two anonymous referees which have helped to improve the paper. J.N. Tripathi is thankful to the Department of Science and Technology, New Delhi, for financial support to get the data (HR/A-14/96). J.N. Tripathi was supported by a fellowship from the “Secretarı́a de Estado de Universidades” for the stay of foreign doctors and
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- 1
On leave from Department of Earth and Planetary Sciences, Allahabad University, Allahabad, India.