A Preliminary Report of Two Earthquakes Recorded by both Broadband and Rotation Sensors

1. A Preliminary Report of Two Earthquakes Recorded by both Broadband and Rotation Sensors C. C. Liu1 , B. S. Huang1, and W. H. K. Lee2 1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan 2 U.S. Geological Survey, Menlo Park, CA 94025, USA

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5. Figure 4. Re-plotting the observed data for the September 26, 2005 earthquake on the same y-axis scale for the broadband velocity signals (top two frames) and for the rotation velocity signals (bottom 3 frames).

6. Figure 5. Integrated broadband and rotation signals for the September 26, 2005 earthquake. Same y-axis scale for the broadband velocity signals (top two frames) and for the rotation velocity signals (bottom 3 frames).

7. Figure 6. Power spectra for the recorded signals of the September 26, 2005 earthquake. Y-axis scale is different for each of the broadband velocity signals (top two frames) and for each of the rotation velocity signals (bottom 3 frames).

8. Figure 7. Power spectra for the displacement signals of the September 26, 2005 earthquake. Y-axis scale is different for each of the broadband displacement signals (top two frames) and for each of the angular rotation signals (bottom 3 frames).

9. Figure 8. Re-plotting the observed data for the January 8, 2006 earthquake on the same y-axis scale for the broadband velocity signals (top two frames) and for the rotation velocity signals (bottom 3 frames).

10. Figure 9. Integrated broadband and rotation signals for the January 8, 2006 earthquake on the same y-axis scale for the broadband velocity signals (top two frames) and for the rotation velocity signals (bottom 3 frames).

11. Figure 10. Power spectra for the recorded signals of the January 8, 2006 earthquake. Y-axis scale is different for each of the broadband velocity signals (top two frames) and for each of the rotation velocity signals (bottom 3 frames).

12. Figure 11. Power spectra for the displacement signals of the January 8, 2006 earthquake. Y-axis scale is different for each of the broadband displacement signals (top two frames) and for each of the angular rotation signals (bottom 3 frames).

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14. Conclusion • Since 1 μrad in-plane tilt will introduce about 1 μg on a horizontal accelerometer with active axis in that same plane, our observation indicates that the effect of angular rotation for an earthquake with moment magnitude of about 5 cannot be ignored in the double integration of the translational acceleration to obtain displacement. • Our preliminary observations from two earthquakes suggest that angular rotation is important event in the mid-field of an earthquake. This has important consequences in analyzing near-field strong-motion data recorded traditionally for translation motion only.

15. Conclusion (cont.) • Much work remains to be performed on our observed data. One important item that we have not done is calibrating the rotational transducer in the field. • At station HGSD, the deployment is not optimal: the broadband seismometer is located 100 m deep in a borehole, while the rotational transducer is at the surface. • At the same site (within 50 m or less), there is a downhole strainmeter that also recorded these two earthquakes, as well as a continuous GPS station. • We lacked a traditional accelerometer at the site, although several free-field accelerographs (operated by the Central Weather Bureau) are nearby. • We also need to examine the earthquake source parameters carefully.

16. Conclusion (cont.) • We are aware that several authors had published papers on their recorded rotation motions. However, we have not yet compared our results with theirs carefully. • Although a more detailed study of the observed data is underway, we release this draft to the Workshop participants for discussions. We solicit comments and suggestions. We have another R-1 transducer to be deployed soon, and will welcome suggestions for designing a better deployment.