ALBERT

Description: Seismic arrays for detection of small earthquakes benefit from array processing aimed at reducing noise levels. We present a frequency-dependent multichannel Wiener filtering (MCWF) technique, which employs an adaptive least-squares method to remove coherent noise in seismic array data. The noise records on a number of reference channels are used to predict the noise on a primary channel, which can then be subtracted from the observed data. A sequence of aftershocks caused by the M (sub w) 6.1 21 February 2008 mainshock in Spitsbergen was recorded by the ARCES array in northern Norway. This aftershock sequence was filtered using the multichannel Wiener filters in both triggered and continuous modes. The Spitsbergen (SPITS) array, at a much closer distance to the source region, provides reliable reference information on the true number of detectable aftershocks. The conventional delay-and-sum beamforming combined with a band-pass filter could detect only 513 aftershocks with 181 false alarms, using a series of constraints comprised of signal-to-noise ratio, back azimuth, and slowness; the multichannel Wiener filtered results found 577 aftershocks with 165 false alarms using the same constraints. A complete automatic multichannel Wiener procedure is developed for event detection on continuous data. An appropriate signal-to-noise ratio threshold for aftershock detection of 2.7 is suggested. Compared to the beamforming method, the MCWF also reduces false alarms when detecting the same number of aftershocks.

Description: To overcome the potential contamination of the direct S waves by source-side anisotropy in shear-wave-splitting analysis, we describe a new approach that we call the reference station technique. The technique utilizes direct shear waves recorded at a station pair and depends on maximizing the correlation between the seismic traces at reference and target stations after correcting the reference station for known receiver-side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search. The algorithm also provides a delay time between both stations caused, for example, by isotropic heterogeneities. Synthetic tests demonstrate the stability of the estimated parameters, even where variability in near-surface properties (thickness and velocity of sediment layer) exists. We applied the reference station technique to data from seismic experiments at the northern margin of Tibet. Average splitting parameters obtained from the analysis of direct S-wave results are consistent with those obtained from previous SKS splitting measurements. Where differences exist, shear-wave fast polarization estimates resolved from direct S indicate a higher degree of internal consistency for closely spaced stations than those derived from SKS. This is probably due to the much larger number of direct S waves available for splitting measurements compared to SKS for the same observational period, resulting in higher quality measurements. We also demonstrate the ability of the technique to provide improved splitting measurements for temporary stations by following a bootstrap approach in which only a few stations with well-constrained SKS splitting parameters are used as seeds to determine the splitting parameters of a large array in an iterative manner. In addition, the S measurements sample the anisotropic layer with different angles of incidence and back azimuths, thus potentially providing additional constraints on more complicated anisotropic structures, and the interstation delay times could be used for tomographic studies to reduce the bias from anisotropic structure.