Geological Survey Professional Paper 1547 Sedimentology, Behavior, and Hazards of Debris Flows at Mount Rainier Washington

Detectable volcanic activity may precede the largest cohesive and noncohesive lahars, but this is not a premise on which the planning process can rely. As demonstrated by the flow record over the last thousand years (a period including lahars that exemplify each of the planning cases), no flows correspond to the single known episode of major activity during that time (tables 1 through 4). A correlation with precursor events is suggested by the concentration of large flows, such as the Osceola Mudflow and Paradise Lahar, during the mid-Holocene when many tephra-producing events occurred. This association, noted by Crandell (1971) and Mullineaux (1974, p. 17), does not relate the flows directly to volcanism but is evidence of a linkage that may lack better definition only because of the confidence limits on radiocarbon dating.

The potential nonvolcanic causes of both the cohesive and noncohesive debris flows and of the relatively small debris avalanches at Mount Rainier include (1) regional, nonmagmatic seismicity; (2) edifice effects; and (3) several phreatic effects of the active hydrothermal system, including rapid ice or snow melting, steam eruptions, failure in response to increased pore pressure, and lubrication of potential slip surfaces such as those of previous deep-seated failures again buried by edifice construction. The first two effects are discussed further here.

Mount Rainier is the site of occasional small earthquakes, the largest two of which may have been due to a strike-slip fault on the south side of the volcano (Crosson and Frank, 1975; Crosson and Lin, 1975). Nevertheless, the general areal distribution of historical earthquakes, such as a cluster of seven earthquakes in 1987 with magnitudes of 0.8 to 2.1 at depths less than 5 km shows a clear association with the volcano (University of Washington Geophysics Program, written commun., 1988). These earthquakes may be the result of edifice effects. Like many subduction-related stratovolcanoes, Mount Rainier is noteworthy for the large mass of layered material at high altitude, leading to gravitational stresses such as those described as edifice effects in a study of Hawaiian volcanoes (Fiske and Jackson, 1972). Some microearthquake activity at Rainier was ascribed to these crustal-loading effects (Unger and Decker, 1970), a view later modified (Unger and Mills, 1972). Some low-frequency tremors recorded at Longmire may result from glacier or debris flow movement in the Tahoma Creek, Kautz Creek, or upper Nisqually River drainages.

The Rainier area is subject to large regional earthquakes (Gower, 1978). If the Cascadia subduction zone offshore is storing the elastic energy characteristic of other subduction zones, several great earthquakes are necessary to fill the seismic gap represented by the zone (Heaton and Hartzell, 1987). The deep, plate-boundary earthquakes of 1949 (M 7.1) and 1965 (M 6.5), both with epicenters on the east side of Puget Sound, caused local incidences of rockfalls, slope failures, and flood-plain liquefaction throughout much of western Washington (Schuster and Chleborad, 1989). As many as five great earthquakes have occurred in the last 3,100 years, the latest about 300 years ago, as suggested by the stratigraphy of buried wetlands in southwest Washington (Atwater, 1988). Major slope failures can occur, however, in response to minor seismic accelerations at times of high susceptibility; for example, the largest historic landslide in Canada was probably triggered by a small (M 3.1) seismic event (Evans, 1989).

Many similar volcanoes have experienced collapses yielding large debris avalanches; Mount Rainier is apparently unusual because the large collapses have transformed directly to lahars, but more detailed study may show this also has occurred at some other volcanoes. Major sectors of the mountain are composed of steep, outward-dipping lava flows (frontispiece) between which hydrothermal water has infused and altered material to clay-rich zones that are potential slip surfaces (fig. 22). Where exposed at the surface, these zones range from 0.2 to several meters in thickness. At depth, hydrothermal alteration is doubtless more intense.

Consequently, volcaniclastic flows beginning as large debris avalanches from Mount Rainier may have little correlation with the warning signs of an eruption. Only one of the modes of collapse described by Francis and Self (1987) requires precursory activity. Moreover, Siebert (1984) found a uniform relation between the height of origin and the runout distance of debris avalanches, regardless of whether the initial slope failure was induced by an explosion or merely by gravity, indicating that the energy from an explosive initiation does not increase the runout distance and thus does not increase the hazard.