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Earthquakes Recorded with UTIG's Broadband Ocean Bottom Seismograph Jay Pulliam 1 (512-471-2696; jay@ig.utexas.edu) Yosio Nakamura 1 (512-471-0428; yosio@ig.utexas.edu) 1Institute for Geophysics, The University of Texas at Austin 4412 Spicewood Springs Road, Austin, TX 78751, United States From July 15-August 12, 1999 a test was conducted of a PMD Scientific's broadband three-component seismometer installed in UTIG's Ocean Bottom Seismograph (BBOBS) in the Gulf of Mexico (27\deg 18.36'N, 94\deg 23.49'E). PMD's use of a "molecular electronic transducer" produces a rugged, versatile seismometer with very low power requirements. The unit we tested has a passband that is flat to velocity between 0.03-50 Hz. These characteristics recommend PMD's seismometer for ocean bottom applications. Our goals were to incorporate the seismometer into our existing OBS and record earthquakes and noise samples for comparison to data recorded at the permanent GSN/USNSN station HKT, located at Hockley, Texas, 323 km NNW from the BBOBS deployment site. HKT is installed at 475 m below ground in a working salt mine and, despite occasional blasts related to mining activities, has extremely quiet noise characteristics in the frequency band of interest for earthquake recordings. More than 25 earthquakes were recorded during the test deployment, ranging in distance from 18\deg to 145\deg. Body-wave detections for P waves on the vertical component range from an mb=4.9 event at a distance of 64\deg to an mb=6.6 event at 96.6\deg to an mb=5.7 event at 145\deg, for which core phases appear clearly. Noise levels on the horizontals varied from 1.8 to 2.1 times the noise on the vertical but these levels do not appear to have compromised shear wave detection significantly, due to the larger amplitudes of shear phases. Rayleigh waves appear distinctly for the shallow larger events; Love waves are clearly recognizable on the horizontals for at least the ten largest events. One event, located on the California/Nevada border (mb=5.6, delta=19.6\deg), produced a large Lg phase at the continental HKT station that does not appear on the BBOBS, which was located 263 km offshore in 1475 m of water on the mid-slope of the northern Gulf of Mexico. Previous tests conducted onshore allow us to constrain the BBOBS instrument response and thereby characterize the site response of the ocean bottom setting in the July/August test. |
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