Preface to the special issue on seismic array analysis and CEArray

Abstract

A seismic array is a set of seismographs being arranged in a regular geometric pattern to improve signalto-noise ratio (SNR). Seismic arrays at various length scales have been introduced since early 1960s and have been proved to be superior to single three-component stations for detecting and characterizing signals from earthquakes and explosions. The better detection capability of arrays is achieved with a so-called “beamforming” technique, which suppress noises while preserving signals, thus enhancing SNR. The “beamforming” technique also estimates the wave vector, i.e., backazimuth and apparent velocity of an incoming signal, which is crucial in identifying the signal, e.g., whether it is a P, S, local, regional or teleseismic wave. The Norwegian Seismic Array (NORSAR) was built in 1968, and was one of the earliest short-period seismograph arrays in the world. Doornbos and Husebye (1972) used it to measure apparent velocities of PKP precursors and found that scattering in the lowermost mantle is likely the cause of the enigmatic precursors. A key requirement for array data processing is the high signal coherency across the array. Based on this, a local or regional seismic network can be considered as a seismic array, as long as the recorded signals are highly coherent across the network, which is usually true in most cases. Benz and Vidale (1993) used approximately 700 short-period vertical-component seismic stations operated in California by the US Geological Survey and the California Institute of Technology as a largeaperture array and studied the underside reflections from the 410-km and 660-km seismic discontinuities of high frequency P waves. They found a very sharp 410-km which puts very strong constraints on the composition of the upper mantle and on the details of the