![]() ![]() ![]() Seismotectonics of the region has been the subject of several studies (e.g., Tapponnier et al. The NER, India comprises distinct geological units, like the Himalayan frontal arc to the north, the highly folded Indo-Myanmar mountain ranges or Burmese arc to the east, the Brahmaputra river alluvium in the Assam valley and the Shillong-Mikir plateau is sandwiched between these two arcs, and thick sediments of the Bengal basin to the south. The long-period spectral amplitude of the source spectrum can be related to M 0 (Brune 1970), from which M W can be computed (Hanks and Kanamori 1979). We attempt to obtain the frequency-independent, long-period spectral level Ω 0 below the corner frequency of a displacement spectrum (Brune 1970, 1971). In this paper, we aim to improve the scaling between M W and M L, M W and M D, M 0 and M L, and M 0 and M D for 162 earthquakes in Shillong-Mikir plateau and its adjoining region of northeast India by extending the M W estimates to lower magnitudes using the spectral analysis of P-waves from vertical component seismograms. ( 2012) examined relationship between M W and M L for the northeastern region of India and they observed with a magnitude discrepancy 0.04 units. Sitaram and Bora ( 2007) reported an empirical relation between M L and M D for northeast India and they observed that M L starts to saturate above magnitude 6.5. Although the size of the events occurring in northeast India region is designated by M L in The India Meteorological Department (IMD) catalog, its consistency with moment magnitude ( M W) is not examined or reported. Several studies have been made to examine region-specific relation between M W and M L (Ristau et al. ( 2007), Sitaram and Bora ( 2007), and Al-Arifi and Al-Humidan (2012) derived an empirical relationship between the total signal duration of local earthquakes and magnitude for the area in Italy, NW Saudi Arabia, and northeast India, respectively.Ĭonversion of magnitude scales for small earthquakes plays an important role for seismic hazard calculations. A partially satisfactory theoretical basis for this correlation has been sought in terms of the properties of coda waves (Suteau and Whitcomb 1979 Lee and Stewart 1981). ( 1972) established an empirical formula for estimating the magnitude of local earthquakes using signal durations. ![]() This correlation has been consistently observed in different parts of the world. Since the moment can be estimated from the recording of all suitable seismographs, the only limitation for M 0 is the difficulty in locating and properly processing on scale recording from seismograph with adequate long-period response.Įstimating the duration magnitude ( M D) from signal duration is an empirical relation between the magnitudes of the earthquake and the duration of its recorded signal. Because the frequency distribution of radiated seismic energy changes with earthquake size (e.g., Aki 1967), magnitude scales suffer severe intrinsic limitations, such as saturation (Kanamori 1977 Hanks and Kanamori 1979 Hutton and Boore 1987) and discrepancies between the scales (Gutenberg and Richter 1956). The magnitude by definition quantifies the energy radiated over a particular fixed frequency band, M 0 is estimated seismically from the amplitude of far-field “long-period” seismic radiation (Aki 1966). This enables the wide acceptance of M W as a stable scale, for larger as well as small to moderate magnitude earthquakes (Lay and Wallace 1995). The moment magnitude scale ( M W), as defined by Kanamori ( 1977), has an advantage of not getting saturated for larger earthquakes, unlike the Richter amplitude-based scales (e.g., Hanks and Kanamori 1979 Howell 1981 Ottemoller and Havskov 2003). Based on M 0, moment magnitude M W has been defined by Hanks and Kanamori ( 1979). From then on, seismic moment was considered as a new parameter to specify the size of an earthquake. ( 1969) also suggested that the far-field static-strain field is proportional to M 0. Aki ( 1966) first measured the value of M 0 of the 1964 Niigata, Japan, earthquake. Following Richter, multitude of magnitude scales, each defined in terms of amplitudes recorded on a particular type of seismograph (i.e., over a particular limited spectral band) have been introduced.įrom the study of source mechanism by an elastic dislocation theory, Aki ( 1966, 1967) stated that the amplitude of a very long-period wave is proportional to the seismic moment, M 0, of an earthquake. The Richter local magnitude M L scale (Richter 1935) for an earthquake is still widely used in different parts of the world. The earthquake magnitude is regarded as the most directly measurable and simple parameter to specify quantitatively the size of an earthquake. ![]()
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