On the Systematic Long-Period Noise Reduction on Ocean Floor Broadband Seismic Sensors Collocated with Differential Pressure Gauges

Abstract

Vertical-component broadband seismic data collected on the ocean floor at regional distances from the shore and water depths on the order of 1-2 km are polluted by noise due to ocean infragravity (IG) waves in a period range (20-200 s) that is critical for regional structure studies and the determination of moment tensors for regional earthquakes. Using broadband seismic and differential pressure gauge data collected at the Monterey Ocean Bottom Broadband (MOBB) observatory, which has been operational since 2002, we identify the optimal length of time window to obtain the transfer function between the vertical seismic and pressure records. The maximum reduction in IG-wave-induced noise is obtained by using a one-day stack of transfer function inferred from data segment collected in the last 24 hours prior to data in which the noise removal method is applied. We show that a one-year stack of transfer function effectively reduces the noise level in the period band 50-100 s by 20 dB and by up to 15 dB at shorter periods. This resultant 20 dB noise reduction is comparable to that obtained from the one-day stack of transfer function. Our result suggests that the removal of noise induced by IG waves on MOBB vertical-component data can be done routinely without continuously calibrating the transfer function, also in near-real time, and is therefore also useful for routine regional moment tensor de- termination. This noise removal method has been implemented in the moment tensor determination system of the Northern California Seismic System.