High Frequency Cut-Off of Observed Earthquake Spectrum and Source Parameters of Local Earthquakes in Himachal Himalaya

A dataset of 20 local earthquakes (3.4Mw5.2) occurred in the Himachal Himalaya recorded by Indian nation strong motion instrumentation network have been analyzed to infer the characteristics of high frequency attenuation of observed earthquake spectrum and source parameters of earthquakes of this region. In this study Brunes earthquake source model (Brune, 1970) that yield a fall-off of 2 beyond corner frequency has been considered to estimate the source parameters namely seismic moment, source radius and stress drop of earthquakes. High frequency attenuation of earthquake source spectrum have been modeled with two high-cut fall off functions - factor presented by Anderson and Hough (1984) and another arbitrary high-cut filter (Boore, 1983; Wen and Chen, 2012) that fits well for frequencies greater than fmax. The seismic moment for these earthquakes vary between 4.2x1013 Nm and 2.3x1016 Nm and their moment magnitudes ranges from 3.4 to 5.2. The source radii of these events lie in the range 226 m to 511 m. The stress drops for these earthquakes vary from 0.2 MPa to 13.3 MPa and found in agreement with the other region of Himalaya as well as for other tectonically active regions of the world. Both functions fmax and accounting this high frequency attenuation seems to represent the same phenomenon. The results obtained in this study infer that source process is the prime controlling factor for this attenuation of earthquake spectrum at high frequencies. The data used in the present study is of 20 earthquakes, which were recorded on different sites. Data sets of earthquakes having large magnitude range (3.0M8.0) and recorded on stations having different geological site conditions will obviously help this effect to confirm properly.