Attenuation and apparent source characteristics in the northern Indian oceanic crust surrounding Sri Lanka

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.