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.