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Jolly, A.D., 2000

Subsurface structure of the volcanoes in Katmai National Park, Alaska

Bibliographic Reference

Jolly, A.D., 2000, Subsurface structure of the volcanoes in Katmai National Park, Alaska: University of Alaska Fairbanks, Ph.D. dissertation, 169 p., illust., maps.


The three-dimensional velocity, attenuation, and b-value structure is mapped beneath the Katmai group volcanoes, in south-central Alaska. Data for these studies include 4,320 earthquakes recorded in the period July 26, 1995, to November 30, 1999, on a 5-18 station short-period seismograph array. The velocity structure is determined by inversion of P-wave travel times for 8,041 rays from 815 earthquakes. The inversion revealed the lowest velocities (3.6-5.0 km/s) centered beneath Novarupta, Trident, and Mageik volcanoes between the surface and 4 km below sea level and moderately lower velocities at 0-6 km depth between Martin volcano and Katmai caldera (4.5-6.0 km/s). Higher relative velocities (5.0-6.5 km/s) prevail outboard of the volcanic axis and at Griggs volcano. The attenuation structure is determined by inversion of the amplitude spectra roll off to obtain t* for 1,301 rays from 230 earthquakes in the magnitude range (0.8 < ML < 1.8). The inversion, which is well constrained in the depth range 0-6 km, reveals higher attenuation along the volcanic axis 1/Q = 0.008-0.018 (55 < Q < 125) and lower attenuation in non-volcanic regions of the study area 1/Q = 0.01-0.000 (100 < Q < [infinity]). The attenuation is greatest beneath Mageik, Trident, and Novarupta (1/Q = 0.018; Q = 55) between the surface and 6 km below sea level. Frequency-magnitude distributions are determined by mapping b-values for approximately 1,300 earthquakes larger than the magnitude of completeness (0.7 ML ). The analysis reveals high b-values at Mageik volcano (1.2-2.2), intermediate b-values at Martin (1.0-1.6) and Katmai caldera (1.2-1.4) and low b-values at Trident (0.6-1.2). Results point to the existence of a large region of partially molten rock centered beneath Mageik, Novarupta, and Trident volcanoes at 0-4 km depth. The localized nature of the high b-value zone at Mageik volcano suggests that the magma is discontinuous, occurring as several distinct bodies. The deeper high-attenuation anomaly might mark the now solidified but highly fractured plumbing system associated with the 1912 Novarupta eruption.

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