A TRIGGERING DIGITAL SEISMOGRAPH UTILIZING MICRO‐PROCESSOR WITH MEMORY FOR PRESERVING FIRST‐MOTION INFORMATION *

Most of the data recorded at seismological observatories are essentially noise, not signal. One way to achieve a higher information density is to trigger the seismograph, as in strong‐motion accelerographs. This approach loses the first motion—not important for earthquake‐engineering studies, but very important for studies of focal mechanism. To eliminate this defect, various cumbersome techniques have been used, such as closed‐loop tape‐recording.

Here we describe a seismograph that records only the signals and does not lose the first motion. Our procedure uses a microprocessor (MC6800) to monitor the energy level adaptively, uses a solid‐state memory like a large shift‐register, and has the microprocessor trigger recording from the solid‐state memory onto a tape cassette whenever a signal (as defined by energy level) is recognized by the microprocessor.

A field station consists of a crystal clock, seismometer, amplifier, analog‐to‐digital converter, Parallel Interface Adapter (PIA), MC6800 microprocessor, 8K solid‐state memory, Asynchronous Communications Interface Adapter (ACIA), cassette tape‐recorder, and battery. The duration of operation depends upon the frequency content of data, the sampling rate used, and the frequency of events. The design criteria used for the stations of the first field system are a 1 Hz to 10 Hz bandpass amplifier, 8‐bit digitization, 5 samples per cycle of highest frequency content, and quality tape cassettes. These requirements are appropriate for exploring for magma chambers in subduction zones—the motivation for developing this system. A playback facility uses the DEC LSI‐11 computer, which has an assembly language set very similar to that of the MC6800.

This equipment, or a slight modification thereof, would be suitable for recording strong‐motion earthquakes, for monitoring quarry blasts, or for any seismological effort in which the triggering criterion can be specified as an algorithm for programming into the microprocessor firmware. The resulting data are digital and only include significant signals, thus the data reduction facilities may properly concentrate on analysis and interpretation. Because the parameters of the system can be easily modified in firmware or software, the system is very flexible and can be considered the Universal Geophysical Recorder. An adaptation to recording transient EM phenomena is in progress. In this application, the source is controlled, but the use of the self‐triggering approach eliminates the need for a radio‐control system to activate the recorders, HIG Contribution no. 753.