Measuring Mid- and Near-Field Rotational Ground Motions in Taiwan

Large rotational motions (about 200 micro-radians) excited by the 1999 Chi-Chi (Mw=7.6), Taiwan, earthquake were inferred from a dense accelerometer array 6 km from the northern end of the fault where large surface slips were observed (Huang, 2003). In December, 2000, C. C. Liu and B. S. Huang began measuring rotation motions using a PVC-5 transducer (0.2V/rad/sec) and a GyroChip sensor (1.43 V/rad/sec) at station HRLT near Hualien. However, no significant rotation motions were observed in the 5 years since, probably due to the low sensor sensitivity although many earthquakes occurred nearby. In July, 2004, Liu and Huang began measuring rotation motions using a far more sensitive triaxial sensor, R-1, made by eentec/PMD (50 V/rad/sec) at station HGSD near Cheng-Kung. At 18:50, September 26, 2005, an Mw=4.7 earthquake occurred 29 km south of station HGSD. The observed maximum rotation velocity for the EW, NS and Z component is 306, 499, and 1863 micro-rad/s, respectively. At 17:01, January 8, 2006, an Mw=4.6 earthquake occurred 36 km south of station HGSD. The observed maximum rotation velocity for the EW, NS and Z component is 98, 183, and 217 micro-rad/s, respectively. In both cases, the observed rotation waveforms (prominent frequency about 3 Hz) are far above the background noise (signal-to-noise ratio is usually greater than 100), and imply rotational motions of a few tens of micro- radians. However, the R-1 sensor has not been rigorously calibrated, and Liu and Huang are not sure of these measurements, which are orders of magnitude larger than a simple theoretical model by Bouchon and Aki (1982) would predict for these small earthquakes. To further investigate the question whether rotational ground motions are significant or not in the mid- and near- field of earthquakes, we are now planning a more careful experiment at two sites in Taiwan. Two R-1 and two RSB-20 (made by PMD) rotational velocity sensors are now being tested and calibrated by Robert Nigbor of UCLA and others before installation in Taiwan in 2007. At each site, a 32-channel 24-bit datalogger will be used to record continuously the R-1 and RSB-20 sensors, as well as 8 three-component linear accelerometers deployed adjacently (with spacing of about 100 meters).