Abstract

A data analysis of 171 broadband vertical‐component records of 62 shallow crustal distant events in the northern Indian Ocean region was undertaken to investigate the far‐source geometric attenuation rate, high‐frequency cutoff filter parameter (kappa), and apparent source characteristics. Event magnitude varied from 4.0 to 5.9 in the short‐period body‐wave magnitude (mb) scale, and hypocentral distance ranged between 2.3° (260 km) and 14.1° (1578 km) for the dataset. The far‐source geometric attenuation rate was estimated by fitting data to a predefined attenuation equation at a frequency range of 0.5–8.5 Hz, using the multiple linear regression method. A secondary regression in the frequency domain on constants resulting from the main (first) regression was performed to find kappa and the second hinge point of the trilinear geometric attenuation function. Source characteristics (stress drop, corner frequency, and moment magnitude) of selected events were determined using the Brune (1970, 1971) model.

The average far‐source geometric attenuation rate was determined to be R−0.5 (R is hypocentral distance) for the selected frequency range, but at low frequencies (below about 2 Hz) slightly lower rates than the average were observed. Kappa was found to be 0.041±0.009  s. The second hinge point of the geometric attenuation function was estimated to be 120±30  km, and the complete form of the function, after considering compliancy with actual records, was collated as  
graphic

Average static stress drops were found to be 9.5 and 16.0 MPa for the mb 4–5 and 5–6 magnitude bands, respectively. Corner frequencies approximately fell within 1.0–7.0 Hz for the dataset. Derived attenuation parameters were also validated using a ground‐motion comparison between recorded and stochastically simulated events.

Online Material: Tables of regression parameters, estimated source parameters, and stochastic simulation parameters, and a figure comparing the ground motion for simulated and recorded events.

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