Douglas J. Lalla | USGS Science

USGS2010

Seismic observations of Augustine Volcano, 1970-2007

Seismicity at Augustine Volcano in south-central Alaska was monitored continuously between 1970 and 2007. Seismic instrumentation on the volcano has varied from one to two short-period instruments in the early 1970s to a complex network comprising 8 to 10 short-period, 6 broadband, and 1 strong-motion instrument in 2006. Since seismic monitoring began, the volcano has erupted four times; a relatively minor eruption in 1971 and three major eruptions in 1976, 1986, and 2006. Each of the major eruptions was preceded by 9 to 10 months of escalating volcano-tectonic (VT) earthquake activity that began near sea level. The major eruptions are characterized seismically by explosive eruptions, rock avalanches, lahars, and periods of small repetitive low-frequency seismic events often called drumbeats that are associated with periods of lava effusion, and they all followed a similar pattern, beginning with an explosive onset that was followed by several months of discontinuous effusive activity. Earthquake hypocenters were observed to move upward from near sea level toward the volcano’s summit over a roughly 9-month period before the 1976 and 1986 eruptions. The 1976 eruption was preceded by a small number of earthquakes that ranged in depth from 2 to 5 km below sea level. Earthquakes in this depth range were also observed following the 2006 eruption. The evolution of earthquake hypocenters associated with the three major eruptions, in conjunction with other supporting geophysical and geological observations, suggests that the Augustine magmatic system consists of a deeper magma source area at about 3.5 to 5 km below sea level and a shallower system of cracks near sea level where volatiles and magma may temporally reside as they ascend to the surface. The strong similarity in seismicity and character of the 1976, 1986, and 2006 eruptions suggests that the processes responsible for magma generation, rise, and eruption at Augustine Volcano have been roughly constant since the early 1970s.

Alaska↗

USGS2010

A two-step procedure for calculating earthquake hypocenters at Augustine Volcano: Chapter 7 in The 2006 Eruption of Augustine Volcano, Alaska

This chapter describes a two-step technique for determining earthquake hypocenters at Augustine Volcano. The algorithm, which was originally developed in the mid-1970s, was designed both to overcome limitations in the standard earthquake-location programs available at the time and to take advantage of the detailed seismic-velocity information obtained at Augustine Volcano. Hypocenters are calculated on the basis of a two-dimensional (2D) ray-tracing procedure that accounts for in plane lateral discontinuities within the seismic velocity structure. This algorithm calculates the minimum P- and S-wave travel time between theoretical grid points embedded in the velocity structure to each station in the seismic network. Station corrections that account for the differences between the model and actual velocity structure are derived from a time-term analysis of the 1975 active-source seismic experiment. Each relocated hypocenter is assigned to the grid point with the lowest rms residual between observed and calculated arrival times. Statistical techniques are used to assess the effect of random errors in P-wave-arrival determination on hypocentral location. These tests suggest that the 2D ray-tracing procedure presented here is able to resolve earthquake hypocenter depths to within 0.25 km between the volcano's summit and sea level and within 0.5 km from sea level to depths of 2 km below sea level.

Alaska↗