Surface Wave Detection with a Broad Band Accelerometer

Abstract

WE report preliminary results of a study to determine the capability of a broad band vertical accelerometer to detect surface waves from small earthquakes. The instrument uses a quartz fibre in torsion and capacitive position sensing¹. It was designed as a low noise, low drift broad band accelerometer, and has produced data on Earth normal mode excitation after earthquakes (5 to 40 cycle h⁻¹, ref. 2). For this study, the output was modified by an active filter to copy the response of Pomeroy et al.³ and Molnar et al.⁴. Such a response does not use the wide band aspects of this instrument and the shape was dictated primarily by the shape of the ground noise spectrum, which has a minimum in amplitude between periods of 20 and 30 s. This minimum may be inferred from the known decrease in noise amplitude with increasing period at shorter periods and seismic background noise data⁵. It is also shown in the data of Savino et al.⁶.