Toward Understanding Subtle Instrumentation Effects Associated with Weak Seismic Events in the Near Field

Broadband observations of small earthquakes at short epicentral distances reveal a mixture of near-field effects and instrumental artifacts. We investigated these phenomena at a station equipped with an STS-2 and CMG-40T sensor situated almost above shallow M 3.0 to 3.8 events (peak ground acceleration 2 x 10(-1) m/sec(2)). The horizontal components were systematically accompanied by tiltlike disturbances, and the tilt obtained from the STS-2 records exceeded more than 10 times the values predicted by the source model. We also observed a so far uncommonly recognized type of disturbance, whose shape is the first derivative of the tiltlike disturbance. The most likely explanation seems to be clipping of high-frequency signal peaks within the sensor system. A computational model of a broadband feedback velocimeter as a linear dynamic system with saturation proved this interpretation on a qualitative level.