Methods for Amplitude Calibration and Orientation Discrepancy Measurement: Comparing Co‐Located Sensors of Different Types in the Southern California Seismic Network

Abstract

Modern seismic networks commonly equip a station with multiple sensors, to extend the frequency band and the dynamic range of data recorded at the station. In addition, in our recent study we showed that comparison of data from co‐located seismometers and accelerometers is useful for detecting instrument malfunctions and monitoring data quality. In this study, we extend comparison of data from different co‐located sensors to two other applications: (1) amplitude calibration for data from vertical short‐period sensors with strong‐motion sensors as baseline and (2) measurement of orientation discrepancy between strong‐motion and broadband sensors. We perform systematic analyses of data recorded by the California Institute of Technology/U.S. Geological Survey Southern California Seismic Network. In the first application, we compare the amplitude of data from vertical short‐period sensors to that of data from co‐located strong‐motion sensors and measure the amplitude calibration factors for 93 short‐period sensors. Among them, 49 stations are measured at ∼1.0⁠, 42 measured at ∼0.6⁠, as well as two outlying stations: GFF at 0.3 and CHI at 1.3. These values are found to be related to the sensors’ sensitivity values. In the second application, we measure orientation discrepancy between 222 co‐located broadband and strong‐motion sensors. All the vertical orientation differences are found to be within 5°. However, the horizontal orientation differences of 22 stations are greater than 6°, among which four stations have reverse rotation or 180° from the expected orientation. These measurements have been communicated to network operators and fixes are being applied. This study, together with our previously developed data monitoring framework, demonstrates that comparison of different co‐located sensors is a simple and effective tool for a broad range of seismic data assessment and instrument calibration.