Regional estimation of Q from seismic coda observations by the Gauribidanur seismic array (southern India)

https://doi.org/10.1016/j.pepi.2004.03.004Get rights and content

Abstract

Attenuation properties of the lithosphere in southern India are estimated from 1219 vertical-component, short-period observations of microearthquake codas recorded by the Gauribidanur seismic array. The magnitudes of the earthquakes range from 0.3 to 3.7 and have focal depths less than 10 km. Coda-wave attenuation (Qc−1) is estimated by means of a single isotropic scattering method and a multiple lapse time window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers is used to estimate the contribution of intrinsic absorption (Qi−1) and scattering (Qs−1) to total attenuation (Qt−1). All the attenuation parameters are estimated, as a function of frequency for hypocentral distances up to 255 km. Results show a frequency dependent relation of the Qc−1 values in the range 1–10 Hz that fit the power law Q−1(f)=Q0−1(f/f0)ηAQ0−1 value of 0.014 and a decrease of f−1.2 have been found using data from the whole region. On the other hand, scattering attenuation is found to be greater than intrinsic absorption for all the frequency bands. A high value of the seismic albedo (which ranges from 0.68 to 1) is found which indicates that scattering is the dominant effect in the study region. Nevertheless, the attenuation parameters estimated are much lower than the obtained for other regions in the world. On the other hand, the observed energy at 0–15 s from the S-wave arrival time bends significantly downward with decreasing distance. In order to clarify this phenomenon, there is a need to take into account the vertical varying velocity structure in the theoretical model.

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

References (42)

  • J.N. Tripathi

    Small-scale structure of the lithosphere-astenosphere beneath the Gauribidanur seismic array deduced from amplitude and phase fluctuations

    J. Geodyn.

    (2001)
  • K. Aki et al.

    Origin of coda waves: source, attenuation and scattering effects

    J. Geophys. Res.

    (1975)
  • A. Akinci et al.

    Separation of scattering and intrinsic attenuation in southern Spain and western Anatolia (Turkey)

    Geophys. J. Int.

    (1995)
  • S.K. Arora

    A study of the earth’s crust near Gauribidanur, south India

    Bull. Seism. Soc. Am.

    (1971)
  • J.A. Canas et al.

    Intrinsic and scattering seismic wave attenuation in the Canary Islands

    J. Geophys. Res.

    (1998)
  • M. Fehler et al.

    Separation of scattering and intrinsic attenuation for the Kanto-Tokai region, Japan, using measurements of S-wave energy vs. hypocentral distance

    Geophys. J. Int.

    (1992)
  • A. Frankel et al.

    Energy-flux model of the seismic coda: separation of scattering and intrinsic attenuation

    Bull. Seism. Soc. Am.

    (1987)
  • Gangrade, B.K., Arora, S.K., 1996. Peninsular seismicity: a comparative study from regional earthquake data of two...
  • M. Hoshiba

    Separation of scattering attenuation and intrinsic absorption in Japan using the Multiple Lapse Time Window Analysis of full seismogram envelope

    J. Geophys. Res.

    (1993)
  • M. Hoshiba

    Simulation of coda wave envelope in depth dependent scattering and absorption structure

    Geophys. Res. Lett.

    (1994)
  • Hoshiba, M., Rietbrok, A., Scherbaum, F., Nakahara, H., Haberland, C., 2000. Scattering attenuation and intrinsic...
  • Cited by (24)

    • Attenuation of high-frequency P and S waves in Garhwal Himalaya, India

      2014, Tectonophysics
      Citation Excerpt :

      Information of wave attenuation (Q− 1) may be used for structure and tectonic interpretation (Aleqabi and Wysession, 2006; Frankel et al., 1990), for seismic hazard assessment by studying the ground-motion attenuation (Anderson et al., 1996), for understanding source processes (Abercrombie, 1997). Seismic waves attenuation characteristics have been widely studied using different techniques and data set in different regions of the world (Aki, 1980; Campillo and Plantet, 1991; Herrmann, 1980; Hough and Anderson, 1988; Masuda, 1988; Mitchell, 1995; Sato and Matsumura, 1980; Scherbaum and Sato, 1991; Takemura et al., 1991; Tripathi, 2001; Tripathi and Ugalde, 2004; Ugalde et al., 2006, 2007; Yoshimoto et al., 1993, 1998; Tripathi et al., 2010). Attenuation properties for direct P-wave (QP− 1) and S-wave (QS− 1) have been estimated using spectral analysis method (Anderson and Hough, 1984; Bianco et al., 1999; Castro et al., 1990; Giampiccolo et al., 2003; Tsujura, 1966) and time domain techniques (Bianco et al., 1999; Giampiccolo et al., 2003; Stewart, 1984; Wu and Lees, 1996) in different mediums and using rate of time decay wave amplitude (Aki, 1969; Kim et al., 2004; Ma'hood et al., 2009; Sato, 1977; Singh et al., 2012; Yoshimoto et al., 1993).

    View all citing articles on Scopus
    1

    On leave from Department of Earth and Planetary Sciences, Allahabad University, Allahabad, India.

    View full text