Earthquake extraction and correlation energy at Long Beach, California seismic survey

Discussion

A common observation is that the correlation energy is strongest in the southwest portion of the array at acausal (negative) time lags and strongest in the northeast portion of the array at causal (positive) time lags. This might be a consequence of the location of the array. Based on results from research utilizing data from ocean-bottom cables, the correlating energy is typically symmetric and circular about the virtual source location at both causal and acausal times. This is because energy typically reaches a given receiver in equal amounts from all azimuths in these deep-water environments, where the primary source of seismic energy is generated from the interaction of ocean currents with the ocean bottom. In other words, the principle of energy equipartition is more or less satisfied in deep-water environments. However, these results from Long Beach suggest that this principle is not being satisfied. The directionality of the Green's function from southwest to northeast suggests that the Pacific Ocean (which is south-southwest of the array) is a large source of directed, low-frequency energy. This causes the Green's function at causal and acausal times to be asymmetric.

The primary difference between the Green's function from each time window is the quality. The correlation energy from the January window is by far the most distinct, followed by the result from late-February and the result from mid-February. A look at the weather conditions in southern California may explain these results. At the time of the January time window, there was a large rainstorm that hit the region (http://www.wrh.noaa.gov/). During the late-February window, conditions were somewhat windy. In mid-February, conditions were pretty mild. Therefore, it appears that correlation results are best during stormy conditions, which happens to also be the case with seismic interferometry results from ocean-bottom cable data (de Ridder and Dellinger, 2011). These conditions may be best because during a storm the energy generated by the Pacific Ocean is much higher relative to any other noise source. Because the ocean is likely the dominant source of the directed, low-frequency energy that we see, then the more relative energy it has, the cleaner the resulting Green's function will be. With more correlation time, the results from February may be able to match the result from January.

Earthquake extraction and correlation energy at Long Beach, California seismic survey