Precise relative earthquake location using surface waves

Earthquake locations provide a fundamental tool for seismological investigations. While dense seismic networks can provide robust locations, accuracy and precision of these locations suffer outside dense networks. This is particularly true in offshore areas, where location analysis relies heavily on distant seismic observations. We present a method for estimating precise relative seismic source epicentroid locations using surface waves. Several reasons, including lower velocities and strength of the signal at distance, make use of surface waves for event location appealing. We focus on the Panama Fracture Zone region and relocate 81 strike‐slip earthquakes to produce tectonically consistent epicentroid locations. The resulting pattern of earthquakes more clearly delineates recently active regional structures than original body‐wave locations. The mean shift between the US Geological Survey National Earthquake Information Center epicenter and our epicentroids is about 14 km (the median is about 11 km), and typical origin time changes are generally less than ±2 s. We find that north of 6.5°N, the plate boundary motion is split across two roughly north‐south striking structures, the Panama and Balboa Fracture zones. For the last 36 years, slip along these two structures roughly matches slip along the Panama Fracture Zone to the south (from 4.5°N to 6.25°N), but the Balboa Fracture zone has roughly three times the moment than the northern Panama Fracture Zone. Our analyses show that observed Rayleigh‐wave signal‐to‐noise ratios for moderate‐to‐large shallow earthquakes are suitable for applying the procedure and that Rayleigh‐wave observations form a self‐consistent set of constraints on the relative location of earthquake centroids.