The Earss Digital Seismograph : System Description and Field Trials

An earthquake detection and recording system known as EARSS has been developed for permanent seismograph stations and temporary field installations. It records three components of ground motion with a dynamic range of 120 dB. A frequency-domain algorithm detects earthquakes and initiates the recording of data on magnetic tape. Alternatively, EARSS can record data continuously, for preselected periods of time, or recording can be triggered by a time-domain phase picker. Up to 1500 earthquakes (25.5 Mbytes) can be recorded on each magnetic tape cartridge. The field version of EARSS supplies power to the tape drive only when data is being written to tape, thus reducing the normal power consumption of 12 watts (at 12 volts) to 2.5 watts. A field trial using a network of eight EARSS seismographs resulted in 1020 successful station-days of operation from a possible total of 1098 station-days (3 years). Of the 78 lost days of operation, 23 were due to power supply problems external to EARSS, and 52 were caused by a low-temperature failure of the recording system, which has since been corrected. A total of 442 Mbytes of data were recorded, of which about 250 Mbytes were useful data. INTRODUCTION An earthquake detection and recording system has been developed at Geophysics Division, New Zealand Department of Scientific and Industrial Research (now DSIR Geology and Geophysics) for permanent seismograph stations and temporary field installations. It is known as EARSS (an acronym formed from Equipment for the Automatic Recording of Seismic Signals), and it uses many of the ideas developed in the earlier SNARE seismograph network recording system (Gledhill and Randall, 1986). EARSS was developed because no commercially available digital seismograph simultaneously fulfills all the following requirements. 1. Reliable detection of earthquakes on a single station in the background of seismic noise. This is necessary because the large amount of data generated by digital data acquisition methods means that it is not possible to record a continuous record of ground motion; nor is a large number of false triggers acceptable. 2. Recording of a wide dynamic range of ground motion amplitude. 3. Accurate measurement of three components of ground motion. 4. A large data recording capacity. 5. Low power consumption so tha t remote field operation is possible. EARSS is a three-channel digital seismograph tha t employs gain-ranging techniques to acquire data with a dynamic range of 120 dB. The frequencydomain detection algorithm of Gledhill (1985) is used to identify earthquakes, which are then recorded on magnetic tape; up to 1500 earthquakes can be stored 1380 THE EARSS DIGITAL SEISMOGRAPH 1381 on each magnetic tape cartridge. The low-power version of EARSS supplies power to the tape drive only when data are being writ ten to tape, thus reducing the continuous power consumption to 2.5 watts. The instrument has been in use in prototype form since early 1987, and in the first 6 months of 1989 an extended field trail was conducted using both the permanent station version of EARSS and a low-power field version. This article describes the EARSS system and reports the results of the field trials.