Investigations of Earthquakes and Other Seismic Sources in Regions of Volcanism

Source properties of earthquakes in Hawaii and seismological aspects of explosive volcanic eruptions are examined in three chapters. In Chapter 1, source depths are estimated for all earthquakes larger than magnitude 6 on the island of Hawaii since 1940 by comparing relative amplitudes of short-period surface waves to body waves. Rayleigh wave excitation functions are calculated versus source depth, and the calculation is compared with observed data and calibrated using known depths of recent earthquakes. In general, results show that large earthquakes near the volcanic flanks and fault systems are shallow (≤ 20 km), but those near active volcanic centers can be deeper (~ 50 km). Two earthquakes with the largest depth estimates (40-55 km and 35-50 km) occurred under the active volcanoes Mauna Loa and Kilauea, preceding eruptions by three days and 14 months respectively. As a check on the data set, which consisted of Pasadena seismograms alone, M_s values assigned from many global amplitude readings were compared with those from Pasadena amplitudes for worldwide earthquakes. Global M_s values on the average are 0.05 magnitude units larger than M_s values from Pasadena amplitudes. In Chapter 2, the horizontal single-force source used to model seismic radiation from the Mt. St. Helens landslide is investigated as the source of the M_s = 7.1 Kalapana, Hawaii earthquake. The azimuthal radiation pattern of 100 s Love waves is two-lobed, consistent with a horizontal single-force source. The observed surface deformation is also more consistent with the single force than the conventional double-couple shear dislocation source. The single force is a crude representation of motion of a large slide mass that is partially decoupled from the Earth. The interpretation is that the bulk of seismic radiation from the Kalapana earthquake was produced by large-scale slumping of the south flank of Kilauea volcano. The peak amplitude f₀ of the force time function is estimated at 1 x 10²⁰ dyne from Love and Rayleigh surface waves. The peak acceleration inferred from the seismic force is 10 - 100 cm s⁻², comparable to that of gravity on a gently inclined plane. In Chapter 3, far-field seismograms were searched for signals associated with recent large volcanic eruptions to examine whether models of the volcano as a seismic source derived for Mt. St. Helens are applicable to other explosive volcanoes. The 1982 eruption of El Chich6n in Mexico produced Rayleigh waves and body waves that were marginally recorded at IDA and SRO stations less than 40° away; still, several characteristics of the eruption can be inferred from the seismic waves. Near-field seismograms of smaller eruptions at Mt. Asama, Japan, were found to be comparable in size to smaller secondary eruptions of Mt. St. Helens, and appear to have a more complicated source. Atmospheric pressure waves recorded on barographic instruments from several large explosive eruptions are compared and show differences in signal duration, amplitude, and characteristic period that are indicative of the overall size of the eruption.