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

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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).

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).

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).

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).

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).

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).

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).

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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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.

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.

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. Please contact Willie Lee at.