Deep Earthquakes Beneath Mount St. Helens: Evidence for Magmatic Gas Transport?

Abstract

Small-magnitude earthquakes began beneath Mount St. Helens 40 days before the eruption of 20 March 1982. Unlike earlier preeruption seismicity for this volcano, which had been limited to shallow events (less than 3 kilometers), many of these earthquakes were deep (between 5 and 11 kilometers). The location of these preeruptive events at such depth indicates that a larger volume of the volcanic system was affected prior to the 20 March eruption than prior to any of the earlier dome-building eruptions. The depth-time relation between the deep earthquakes and the explosive onset of the eruption is compatible with the upward migration of magmatic gas released from a separate deep reservoir.

The Cerro Galan ignimbrite

“…A major area of research concerning subaerial volcano morphology has been the recognition and investigation of sector collapse structures and attendant debris avalanche deposits on composite and shield volcanoes [ Ui, 1983;Siebert, 1983] Helens. These largely concern the relationship of tilt, uplift and seismicity, for example, during fissure eruptions at Kilauea [Davis, 1986;Dvorak, et al, , 1985 Other geophysical studies have been investigations of models for seismic tremor [Chouet, 1985] and geothermal noise [Kieffer, 1984], use of deep earthquakes at Mount St. Helens as a monitor for magmatic gas movements [Weaver, 1983], and a study of long range infrasonic acoustic signals from the E1 Chichon explosions [Mauk, 1983] Volcanic studies on other solar system bodies continued at a much lower level, mostly involving refinements of earlier studies made in the heyday of planetary geology. Katterfeld [1984] reviewed volcanism on Mercury, a topic which has been of some VOLCANIC ASPECTS OF MINERALIZATION…”

Tiltmeter analysis of Mount St. Helens, Skamania County, Washington

“…Including information from deformation surveys (Swanson and others, 1981), seismic evaluation (Scandone andMalone, 1983: Weaver, Zollweg and, modeling of inner-crater deformation patterns (Denlinger, 1984, personal communication), and inner-crater tiltmeter networks (Dzurisin, Westphal and Johnson, 1983) with the far field tiltmeter data, enough deformational evidence exists to argue effectively for the two chamber system.…”

Tiltmeter analysis of Mount St. Helens, Skamania County, Washington

“…This fault zone could be a primary controlling factor in the location of the magmatic conduit for Mount St. Helens (Weaver and others, 1983). The location of seismic activity between 1980 and 1983 at Mount St. Helens (see Figure 5) also suggests that a northeast trending fault may also be resultant of the volcanic activity and a controlling factor of future activity (Weaver and Malone, 1983).…”

“…(2) that two pressure sources can be inferred from geodetic data (SWanson, 1981), (3) that an ascertainable vertical zonation of hypocenter locations exists below Mount St. Helens (Weaver, Zollweg and Malone, 1983) and (4) that inner-crater tiltmeters recorded large deformations prior to dome building eruptions during 1981 and 1982 (Dzurisin, Westphal and Johnson, 1983), while far field tiltmeters indicated changs of less than 1 µrad.…”

“…This chamber was of sufficient volume to supply all the eruptions to date (March 1984) without suffering mineral depletion (Cashman and Taggert, 19831 Melson, 1983) or volume loss collapse. The second chamber in the system would be located at a shallow depth (approximately 1 km1 Weaver, Zollweg and Malone, 1983) vertically above the main chamber. Its existence resulted from a change in the stress fields allowing the formation of a relatively small (less than .007 km 3 ) "pulsating" chamber ( Figure 48).…”