Subsurface structure of the volcanoes in Katmai National Park, Alaska

Abstract

The three-dimensional velocity, attenuation and b-value structure is mapped beneath the Katmai group volcanoes, located in south-central Alaska. Data for these studies include 4320 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 8041 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 1301 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 ~1300 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.