Mount Rainier was selected as a "Decade Volcano" by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI). Worldwide, IAVCEI selected 14 active volcanos near populated areas for special study.
Mount Rainier was selected as a Decade Volcano because of the hazard it poses to surrounding, highly populated areas, especially the Seattle-Tacoma metropolitan area. Mount Rainier has an extensive cover of snow and ice, which, if melted rapidly , could produce catastrophic floods and mudflows. The volcano has an extensive but poorly studied geological record of activity including lava flows, ash eruptions, avalanches, and mudflows. While it has had no significant historic eruptions, minor ones did occur in the mid 1800s, and dozens of lahars are known to have occurred in the last 5,000 years. Some of these lahar deposits are quite massive and extend into the populated Puget Sound lowlands. Because urban development of Mount Rainier's flanks and nearby valleys is still in early stages compared to many volcanos, we have a chance to mitigate the volcanic hazards through appropriate land use.
Seismicity within the volcano is not well understood, but most is likely of volcanic origin. Both high- and low-frequency events are located at shallow depth and occur at a rate of almost two dozen per month. The low-frequency events are usually poorly recorded, and most are probably seismic signals generated by glacier movement. The shallow high-frequency events are usually well recorded and fall in the magnitude range -0.5 to 3.0. Epicenters appear to align in a NE-SW trend across the summit, and three available fault-plane solutions agree in general with this trend. Because of the difficulty in determining accurate hypocenter depths within the mountain, interpretation of this alignment as representing a fault zone or potential zone of weakness is premature. Moran et al. (1995) showed that the allignment is likely an artifact due both to poor station distribution before 1989 as well as to use of an inappropriate velocity model. Thus additional seismic velocity structure studies are needed.
The subduction of the Juan de Fuca oceanic plate eastward under the North American continental plate is responsible for the Cascade Range. Mount Rainier is at the north end of a segment of this volcanic arc which consists of five large andesitic to dacitic strato-volcanos (including Mount St. Helens) intermixed by numerous minor basaltic centers. The north-south compressional regime generates numerous crustal earthquakes. Seismicity is concentrated in the central Puget Sound basin and along the west flank of the Cascades. A recent model for the region postulates a forearc basin thrust against the Cretaceous-age continental margin which now lies below more recent volcanics. It is seen as a a strong electrical conductor as determined by magnetotelluric surveys (SWCC in Map View) and may have controlling influence on the position of three major volcanos as well as current seismicity (Stanley et. al., 1987).
Hazards from Mount Rainier not only include those of a purely volcanic origin, but also those related to glacier outburst floods, rock-fall, and edifice collapse. Edifice failures, either spontaneous or triggered by large local earthquakes, represent a significant hazard which is hard to quantify. Seismic monitoring of Mount Rainier is part of the normal monitoring job of the Pacific Northwest Seismograph Network. Alarm mechanisms as part of seismic network operations can rapidly notify personnel of unusual or increasing seismicity which would likely precede renewed volcanic activity. It also can, and has, provided information about other hazardous events such as rock-fall and small debris flows. In its present configuration it is doubtful it could provide timely alarm information needed for the evacuation of nearby towns potentially impacted by a large spontaneous edifice collapse.
Because there is not much more than 200 years of written history in the Pacific Northwest most of what can be understood of the eruptive history of Mount Rainier and its potential hazards must come from geologic and geophysical studies. Several such studies are currently underway in response to the heightened interest generated by its choice as a Decade Volcano. A summary of work already done, and recommendations for future studies needed to improve our understanding of Mount Rainier and its hazards, was recently published by the U.S. Geodynamics Committee (1994).
Early work by Fiske et. al. (1963) on the geologic framework and early eruptive history of Mount Rainier, and studies by Crandell (1971) and Mullineaux (1974) on the more recent eruptions and lahars, provide the major geologic background on which recent and current work is based. Recent and ongoing geologic mapping, both on the volcano and in the region, is improving additional details. Previous geophysical studies, including magnetotelluric soundings, gravity and magnetic map interpretation and seismic exploration have helped to understand the tectonic history and setting of the region.
Major crustal refraction studies were run in 1991 and 1995 (Shown on
Map View)
and help define the 2-D crustal structure along those lines. The large velocity and structural variations observed indicate a simple 1-D layered velocity model is not a good approximation for the region. An on-going experiment to record natural earthquakes at a large number of sites on and around the volcano will be used in a tomographic inversion which should allow for a more three dimensional structural interpretation. This study, combined with refraction, electrical, gravity, and magnetics data, should help delineate major crustal features such as the SWCC and what role they play in the tectonics local to Mount Rainier.
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UW Dept. of Earth and Space Sciences ..... PNW EARTHQUAKES
