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. Generally, any asymmetry in the transfer of high frequencies in the feedback velocimeter would produce a long-period disturbance of this type. Users of near-fault broadband velocigrams may numerically simulate the disturbances, without any knowledge of their physical nature, and subtract them from the records. The decontaminated records still may have a strange, bow-shaped form, related to the near-field ramp and the static displacement (of the order of 1 x 10(-5) m in this article). The effects studied in this article seem to have a general character, for apparently any feedback-controlled broadband velocimeter.