34 Waves Generated by Landslides Into the Lake

Volcano and Earthquake Hazards in the Crater Lake Region, Oregon

Landslides May Cause Large Waves on Crater Lake

Waves Generated by Landslides Into the Lake

There are many examples of large waves caused by landslides. Those most relevant to the situation at Crater Lake have occurred in deep glacially-scoured bays and fjords where either a large mass of rock has fallen or slid into the water or where the submarine slope has failed. A spectacular example of a seismically related rockfall and ensuing wave is the July 9, 1958, event at Lituya Bay, Alaska, described by Miller (1960). Lituya Bay is an ice-scoured, nearly landlocked tidal inlet adjacent to the Fairweather Range and Fairweather fault in the Gulf of Alaska. It has a maximum depth of 220 m. In the 1958 event , 30×106 m3 of rock plunged into a 1.2-km-wide inlet of the bay from an elevation of up to ~900 m, causing water to surge over the opposite wall of the inlet to an elevation of 530 m, and generating a wave that moved down the bay 11 km to its mouth at a probable velocity of 160 to 210 km/hr. Miller (1960) presents evidence for several lesser events at Lituya Bay caused by a variety of phenomena. Waves generated by landslides in Norwegian fjords and lakes are described by Jørstad (1968). The catastrophic waves induced by a rock slide at Tafjord (8–9 km long x 1–1.5 km wide x 200–220 m deep), Norway, April 7, 1934, traveled at between 20 and 100 km/hr, were 1–16 m in height 3–11 km from the slide, and reached a maximum of 62 m in height 200 m from the slide. In this event, a total of 2-3×106 m3 of rock plus scree entered the water from a maximum elevation of 730 m. The Lituya Bay and Tafjord events are comparable in magnitude to a worst case scenario for Crater Lake. Rough calculations of minimum volumes of bathymetric features that may be landslides at Crater Lake are presented in table 6. The Chaski Bay slide (fig. 3) has a minimum volume of 93×106 m3. The minimum volume of rock in the block forming the prominent bench at Chaski Bay is 15×106 m3. A far greater volume of fractured and altered rock of the caldera wall above this feature might be presumed to be capable of failure. We stress that in order for such a slide to pose a significant wave-generation hazard, the slide mass would have to move rapidly into the lake.

The closed basin of Crater Lake caldera (Proximal Hazard Zone PA) is ~8 by 10 km at the rim and ~7 by 9 km at the shoreline. The maximum depth is 589 m, a large part of the lake is at least 450 m deep, and the high points on the rim are ~600 m above the lake. Postcaldera volcanoes form hills on the caldera floor, including the edifice capped by Wizard Island. Three-dimensional numerical models have been developed that simulate the effects of landslides entering bodies of water. Although the detailed propagation and character of waves induced by a landslide at Crater Lake, initiated subaerially or subaqueously, cannot be predicted directly from published numerical models of other landslides in fjords (e.g., Harbitz and others, 1993) or in bodies of water adjacent to volcanoes (e.g., Kienle and others, 1987), the numerical models lend credence to our concerns about Crater Lake. Initial waves likely would be followed by seiche effects caused by reflection of waves off of the caldera walls. Interference of waves could result in amplification.

The substantial depth of Crater Lake would cause a wave to travel at great speed. A common approach to determining the velocity of propagation is v = (g x h )1/2 where v= velocity, g = gravitational acceleration, and h = water depth. For h = 450 m, v = 66 m/s. For example, a wave initiated at Chaski Bay would reach the boat dock at Cleetwood Cove in about two minutes. The amplitude of the wave would diminish in the deep part of the lake but would increase on approach to the shore. Consequently, at the onset of shaking, perhaps as indicated by abundant, sudden rockfalls, it would be advisable for boats to head toward the center of the lake.

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