2-D VELOCITY STRUCTURE ACROSS NEWBERRY CALDERA DERIVED FROM AMBIENT SEISMIC NOISE

Newberry volcano in central Oregon is a broad shield volcano characterized by a 6-8 km-wide caldera and numerous vents that have been active during the past 10,000 years. In September 2008, we operated 81 short-period three-component seismic stations across Newberry for approximately 20 days to record passive and active sources. Seventy-five of the stations were deployed in a NE-SW line, approximately 38 km long and centered on the caldera. Stations within and adjacent to the caldera were 300 m apart, while stations at the two ends of the line were 800 m apart. An additional line of 6 stations was deployed between the intracaldera lakes, north of and parallel to the main line. We employed a cross-correlation technique to derive Green functions between station pairs. Despite the relatively short recording time, we found that ambient noise cross-correlation produced coherent Green functions across the array for many station pairs. We used a low-frequency band, centered on 0.2 Hz, for station pairs separated by 5 km or more. A second band, centered on 1.5 Hz, was used for stations separated by less than about 4 km and produced consistent results for the closely-spaced intracaldera stations. Both bands are characterized by a dramatic changes in velocity across the caldera. The asymmetry in the low-frequency band cross correlations is consistent with dominant noise derived from the coast. The higher degree of symmetry in the higher frequency band, however, suggests noise sources within the caldera, or stronger scattering at these frequencies. Scattering is consistent with strong attenuation from natural and artificial ballistic sources observed at stations within the caldera. Group velocity estimates are complicated by the sharp relief, but vary from 1.4-2.7 km/s at 1-2 Hz within the caldera. Lower-frequency group velocities vary from as low as 0.9 km/s for some station pairs within the caldera to 2.6 km/s outside the caldera. The surface wave velocity measurements are inverted to map the velocity structure across and adjacent to the caldera.