Results from the High‐Gain Long‐Period Seismograph Experiment*

Summary Eight high-gain long-period seismograph systems with peak analogue recording magnifications of up to 150 000 have been installed in different regions of the world. The particular passbands of these instruments are nearly optimum, especially the vertical components, in the sense that Earth noise is approximately pre-whitened by the instrumental response. A minimum in the level of Earth noise at periods near 30 s is observed on noise spectra from all of these sites which differ greatly in geographical location, geologic setting, and depth of overburden. At many of these stations surface waves with periods between 20 and 40 s are often observed from shallow focus earthquakes with magnitudes as low as mb= 4.1 at epicentral distances of 20° to 25°. Various filtering techniques have been applied to the data digitally recorded at these stations, especially polarization filtering of surface waves, and have enhanced the signal to noise ratio for surface waves from some earthquakes by as much as 6 db. These techniques should further lower the surface-wave detection thresholds.