Apparatus for detecting earthquake generated P-waves comprising: means forming a housing adapted to be mounted to a supporting structure (28; 30) subject to movement by seismic forces; sensor means (16; 18; 20) affixed to said housing and operative to generate electrical signals proportional to motion experienced by said support structure, said sensor means (16; 18; 20) including multiple sensor elements (16; 18; 20) each of which is responsive to motion in a direction normal to one of a corresponding plurality of mutually orthogonal intersecting planes and operative to generate an electrical signal commensurate therewith, each said sensor element (16; 18; 20) includes a thin-film, cantilevered, piezo-electric structure having a proximal extremity rigidly affixed to said housing and an unsupported distal extremity; signal amplifying and filtering means responsive to said electrical signals and operative to amplify and pass signals having frequencies within the range of approximately 0.5 to 15 Hz; and signal processing means arranged to sample passed signals and to perform an automatic real-time offset zeroing function by averaging the passed signals over successive periods of time greater than approximately 10 seconds and taking an ongoing mean of the averaged passed signals as a zero point, but causing a sum of the passed signals to drop to a zero value after a few seconds of no signal, and to determine that a P-wave has been detected if (a) the averaged passed signal rises above a predetermined level, and/or (b) a power spectral density value rises above a predetermined trigger value within a predetermined window of time, and for generating an output signal commensurate therewith. means forming a housing adapted to be mounted to a supporting structure (28; 30) subject to movement by seismic forces; sensor means (16; 18; 20) affixed to said housing and operative to generate electrical signals proportional to motion experienced by said support structure, said sensor means (16; 18; 20) including multiple sensor elements (16; 18; 20) each of which is responsive to motion in a direction normal to one of a corresponding plurality of mutually orthogonal intersecting planes and operative to generate an electrical signal commensurate therewith, each said sensor element (16; 18; 20) includes a thin-film, cantilevered, piezo-electric structure having a proximal extremity rigidly affixed to said housing and an unsupported distal extremity; signal amplifying and filtering means responsive to said electrical signals and operative to amplify and pass signals having frequencies within the range of approximately 0.5 to 15 Hz; and signal processing means arranged to sample passed signals and to perform an automatic real-time offset zeroing function by averaging the passed signals over successive periods of time greater than approximately 10 seconds and taking an ongoing mean of the averaged passed signals as a zero point, but causing a sum of the passed signals to drop to a zero value after a few seconds of no signal, and to determine that a P-wave has been detected if (a) the averaged passed signal rises above a predetermined level, and/or (b) a power spectral density value rises above a predetermined trigger value within a predetermined window of time, and for generating an output signal commensurate therewith.