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Designing Compact Causal Digital Filters for Low-Frequency Strainmeter Data

¹ Institute of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, California 92093-0225
 (D.C.A.)

² UNAVCO, Inc.
New Mexico Institute of Technology
Socorro, New Mexico 87801
 (K.H.)

For the strainmeter component of the Plate Boundary Observatory, filters are needed to produce low-frequency series (5-minute samples) from the higher- frequency (1 Hz) data generated by the instruments. We present design methods for finding filters that are efficient, causal, and compact. We use standard methods for generating symmetric finite impulse response filters, followed by root finding, selection of roots, and reconstruction of the weights, using procedures that make these processes numerically stable. The final filters show appropriate performance even in the presence of large teleseismic signals, but introduce unavoidable artifacts for strain data from large local earthquakes.