Abstract: An earthquake detector is provided for mounting on a structural member of a building or the like, which is resistant to false alarms arising from door slamming, traffic, running, sonic booms, etc., and yet which is sensitive to earthquakes and especially to P-waves which often precede destructive S-waves. The apparatus includes a motion detector for generating an alarm signal, which is responsive only to vibrations below a frequency of about 14 Hz, to avoid false alarms from non-earthquake vibrations. The detector can include a cantilevered beam device with a weight on its free end, and which generates a signal as the weight moves up and down, and a filtering circuit that passes primarily signal components of a frequency between 3 and 14 Hz. The beam device preferably is highly damped so its response is only slightly higher at its resonance frequency than at other frequencies.

Abstract: An earthquake detector is provided, which can sound an alarm several seconds before the arrival of the major shock of an earthquake, and which can be easily adjusted for sensitivity. For detecting local earth shaking of at least a predetermined dangerous acceleration level that constitutes a significant danger, a detector is provided that responds to shaking motion of an acceleration level between one-half and one-eighth the predetermined dangerous acceleration level to detect P-waves that arrive before the destructive S-waves, and which generates an audible alarm upon such detection. In one detector, two mercury-type switches are mounted with their directions of sensitivity both horizontal but perpendicular, the switches being mounted on a circuit board that can be tilted to alter their sensitivities.