Processing of regional seismic events using data from small-aperture arrays

Abstract A Regional ON-line Array Processing Package (RONAPP) has been developed and tested on data from a small-aperture seismic array (NORESS) at NORSAR, consisting of 12 sensors spaced with distances up to about 2 km. The processing package automatically detects and locates regional seismic events in the distance range from a few kilometers to the maximum propagation range of regional S -type waves ( Sn or Lg ). The package contains a conventional STA/LTA detector which works on a set of filtered beams deployed at typical phase velocities of Pn and Lg/Sn (8.0 and 4.5 km/sec). When a detection is found, data within a certain time window around the detection time are used for estimates of refined arrival time and signal frequency, and a wavenumber analysis is then performed following a proper prefiltering. This gives, in addition to arrival time and frequency, estimates of phase velocity, azimuth, and signal-to-noise ratio for each detection. Following each processed detection, an event location based on azimuth and S-P travel times is performed if the last detection is identified as an S -type wave (phase velocity below 6 km/sec), and a preceding P -wave (phase velocity above 6 km/sec) has been detected with about the same direction of approach. Appropriate procedures are developed to allow for several detections within the P and S coda, to keep track of “used” and “unused” detections, and to identify epicenter locations that override preliminary ones as the analysis progresses through the S wave train of the event. The processing package has been tested off-line and on-line on a number of data intervals for various experimental NORESS array configurations. The automatic processing package provides detection results (for regional events) that compare favorably with those from the full NORSAR array and location results that deviate from locations by the Fennoscandian network of seismological stations by typically 30 km, which is of the order of the location uncertainty of the network.