Measuring Rotation Motions in the Free Field of Taiwan W. H. K. Lee U. S. Geological Survey, Menlo Park, CA 94025, USA C. C. Liu Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan C. F. Wu Central Weather Bureau, Taipei, Taiwan

“ Perfectly general motion would also involve rotations about three perpendicular axes, and three more instruments for these. Theory indicates, and observation confirms, that such rotations are negligible”. C. F. Richter (1958). Elementary Seismology, W.H. Freeman & Co., p However, the 50+ near-field strong- motion records of the 1999 Chi-Chi (Taiwan) earthquake indicate that the ground motions near the 100 km rupture are complex. However, the 50+ near-field strong- motion records of the 1999 Chi-Chi (Taiwan) earthquake indicate that the ground motions near the 100 km rupture are complex.

C. C. Liu of the Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan began recording earthquakes with triaxial rotation transducers at Station HRLT near Hualien on December 12, Data logger: 12-channel, 19-bit, K2 by Kinemetrics, with FBA, PVC-5 rotation transducer (0.2V /rad/sec), and GyroChip rotation transducer (1.43 V/rad/sec). Trigger recording at 200 samples/sec. No useful rotation data were obtained after 5 years of observations.

C. C. Liu of the Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan began recording earthquakes with a far more sensitive triaxial rotation transducer (R-1 made by eentec/PMD) at Station HGSD near Cheng-Kung on July 1, The sensitivity of R-1 is 50 V/rad/sec, almost 30 times higher than GyroChip (1.43 V/rad/sec), and 250 times higher than PVC-5 (0.2V /rad/sec). At least 2 earthquakes have recorded excellent rotation data to be presented later in this Workshop (see paper by Liu, Huang, and Lee).

C. C. Liu and assistant at the HGSD station

Model R-1 triaxial rotational transducer (by eentec/PMD) as installed at HGSD station

Housing of the Quanterra Q330 datalogger with the R-1 rotational transducer covered. The box in the right covers the borehole with the Guralp CMG-3TD broadband seismometer installed at 100m down

We plan to deploy another triaxial rotation transducer (R-1 by eentec/PMD) in an experiment that we would like suggestions from the Workshop participants. The Central Weather Bureau (CWB), which has the seismic monitoring responsibility in the Taiwan region, will deploy triaxial rotation transducers in two more sites later this year. Again, suggestions from the Workshop participants will be appreciated. If rotation motions proved to be important, then we will recommend that CWB includes rotation transducers in their annual purchase of new accelerographs (about 50 units per year).