Locating earthquakes in Pakistan region using Gauribidanur Array data: significance of Lg waves

This paper highlights the significance of Lg waves for determination of improved locations of earthquakes in Pakistan region, using the short period data of Gauribidanur Seismic Array (GBA), India. Lg waves travel with lower velocity as compared to P or S waves, resulting in a smaller ambiguity in epicentral distance estimation compared to P or S waves for a given uncertainty in travel time. Further, the fact that 1) Lg waves are usually the largest amplitude waves recorded at regional distances and 2) the onset of these waves, even for small events, can be picked up with better precision has motivated us to examine the feasibility of using them for locating earthquakes in Pakistan region. The analysis of Lg waves corresponding to 35 earthquakes reported by the United States Geological Survey (USGS) as having heir hypocenters in Pakistan region at a median distance of 2058 km and distributed azimuthally in the range of 317 deg - 350 deg from GBA, revealed that for different paths to GBA from this region, the Lg waves, in general, travel with velocity of 3.61± 0.08 km/s. However, it was observed that for the earthquakes near the boundary of Sulaiman Lobe, close to azimuth 335 deg with respect to GBA, the Lg wave velocity showed somewhat large scatter. When the events were divided and analyzed in two azimuthal groups viz. 317deg - 335deg and 335deg - 350deg, the observed scatter in Lg wave velocity in each group was found smaller. The near independence of Lg velocity with respect to azimuth within these two azimuthal groups, helped us to find following two relations between Lg-P travel times as observed at GBA (TLg-P) and epicentral distance (Δ) of the events at GBA, obtained from USGS reported locations (assumed as standard). Δ = -40.3797 + 0.3504 TLg-P - 0.000515 TLg-P, σ = 0.38 deg for (317 deg <Z<335 deg), (15.9 deg ≤Δ≤19.2 deg) and 260s ≤ TLg-P≤ 335s) and Δ deg = 7.99 + 0.0247 TLg-P + 0.000024 TLg-P, σ = 0.38 deg for (335 deg <Z<350 deg), (16.6 deg ≤Δ≤21.9 deg) and 275s ≤ TLg-P≤ 405s) These relations were subsequently used to determine the epicentral distances of ten new earthquakes originated in Pakistan region. The median error in distance for these events with respect to the USGS reported locations was found to be 20.5 km. When these events were located using the distance obtained from above equations, along with the azimuth obtained at GBA from P wave move-outs across the array, the median location error with respect to the USGS reported locations turned out as 50.5 km at a median distance of 2084 km. A large part of this location error was due to the error in azimuth estimation, which had a median value of 39 km. (author)