Detection and Location of Potential Sources of Background Low Frequency Surface Wave Energy

Earth's background free oscillations were reported recently by Nawa et al. (1998). While the observations are now well established, and the coupling of the solid earth with the atmosphere/ocean system is widely accepted as a likely source mechanism, detailed studies on sources, and in particular the spatial distribution of the sources, have been few. Ekström (2001) showed that coherent Rayleigh wave energy in the frequency range between 2 and 7 mHz is detectable in 94% of the 5 year period considered. Though most data of 5 year period are contaminated with surface wave energy generated from events, it implies that we can detect coherent Rayleigh wave energy from sources taking part in generating continuous free oscillation in the absence of significant earthquakes. However, his method does not provide enough resolution to determine spatial distribution of the sources. We propose an array based method as a modification of Ekström's method. This method is designed to detect low-level Rayleigh type surface wave energy by using one local array and provide rough estimate of back azimuth and arrival time of it. Testing of this method for small earthquake events shows the detection level of the method is lower than the documented background excitation levels (Ekström, 2001; Tanimoto and Um, 1999). Combining compatible detections from several arrays (3 or more) can help both identify sources of long period energy, and obtain an estimate of the source location. We have tested this method using vertical component recordings at BDSN (Berkeley Digital Seismic Network, USA), GRSN (German Regional Seismic Network) and F-NET (Japan) arrays. We will show and discuss the results of its application during intervals without significant earthquakes.