Strong Motion Data on a ADXL05 Sensor

The event, felt here in Redwood City California and located ~108km (67miles) from me, is the first earthquake recorded by my ADXL05 based strong motion sensor that went online 3/97. Besides the ADXL05 sensor (my *.LC6 event files), I have on loan from the Menlo Park USGS a Kinemetrics three component FBA-23A commercial strong motion sensor. I am currently monitoring the north-south (*.LC5 event files) and vertical (*.LC7 files) channels of the FBA-23A sensor. The single channel ADXL05 based sensor is also oriented north-south so that I could see how this sensor compares to the FBA-23A north-south channel.

Both north-south sensors are set for a maximum acceleration of +- 2 g. The data is collected using one of my 16-Bit A/D Cards so each A/D converter count is the equivalent to ~.0610 mg (4g / 65536 counts). As you can see from the seismograms below, both sensors produced about +- 100 counts, or around +- 6.1 mg, of acceleration. The main difference between the ADXL05 and the FBA-23A is the amount of background noise from the sensor itself. The ADXL05 sensor has about +-3 mg of internal noise and FBA-23A has less than 1 mg. As you can see from the seismograms below, the P wave is buried in the noise of the ADXL05 sensor but can be clearly seen on the FBA-23A sensor.