Precise Relative Earthquake Depth Determination Using Array Processing Techniques

Abstract:

The mechanism for intermediate depth and deep earthquakes is still under debate. The temperatures and pressures are above the point where ordinary fractures ought to occur. Key to constraining this mechanism is the precise determination of hypocentral depth. It is well known that using depth phases allows for significant improvement in event depth determination, however routinely and systematically picking such phases for teleseismic or regional arrivals is problematic due to poor signal-to-noise ratios around the pP and sP phases. To overcome this limitation we have taken advantage of the additional information carried by seismic arrays. We have used beamforming and velocity spectral analysis techniques to precise measure pP-P and sP-P differential travel times. These techniques are further extended to achieve subsample accuracy and to allow for events where the signal-to-noise ratio is close to or even less than 1.0. The individual estimates obtained at different subarrays for a pair of earthquakes can be combined using a double-difference technique in order to precisely map seismicity in regions where it is tightly clustered. We illustrate these methods using data from the recent M 7.9 Alaska earthquake and its aftershocks, as well as data from the Bucaramanga nest in northern South America, arguably the densest and most active intermediate-depth earthquake nest in the world.