Edwin L. Harp

Real-Time Landslide Warning During Heavy Rainfall

A real-time system for issuing warnings of landslides during major storms is being developed for the San Francisco Bay region, California. The system is based on empirical and theoretical relations between rainfall and landslide initiation, geologic determination of areas susceptible to landslides, real-time monitoring of a regional network of telemetering rain gages, and National Weather Service precipitation forecasts. This system was used to issue warnings during the storms of 12 to 21 February 1986, which produced 800 millimeters of rainfall in the region. Although analysis after the storms suggests that modifications and additional development are needed, the system successfully predicted the times of major landslide events. It could be used as a prototype for systems in other landslide-prone regions.

Landslides Triggered by the 2002 Denali Fault, Alaska, Earthquake and the Inferred Nature of the Strong Shaking

The 2002 M7.9 Denali fault, Alaska, earthquake triggered thousands of landslides, primarily rock falls and rock slides, that ranged in volume from rock falls of a few cubic meters to rock avalanches having volumes as great as 15×106 m3. The pattern of landsliding was unusual; the number of slides was less than expected for an earthquake of this magnitude, and the landslides were concentrated in a narrow zone 30-km wide that straddled the fault rupture over its entire 300-km length. The large rock avalanches all clustered along the western third of the rupture zone where acceleration levels and ground-shaking frequencies are thought to have been the highest. Inferences about near-field strong shaking characteristics drawn from the interpretation of the landslide distribution are consistent with results of recent inversion modeling that indicate high-frequency energy generation was greatest in the western part of the fault rupture zone and decreased markedly to the east.

Pore pressure response during failure in soils.

Three experiments were performed on natural slopes to investigate variations of soil pore-water pressure during induced slope failure. Two sites in the Wasatch Range, Utah, and one site in the San Dimas Experimental Forest of southern California were forced to fail by artificial subsurface irrigation. The sites were instrumented with electronic piezometers and displacement meters to record induced pore pressures and movements of the slopes during failure. Piezometer records show a consistent trend of increasing pressure during the early stages of infiltration and abrupt decreases in pressure from 5 to 50 minutes before failure. Displacement meters failed to register the amount of movement, due to location and ineffectual coupling of meter pins to soil. Observations during the experiments indicate that fractures and macropores controlled the flow of water through the slope and that both water-flow paths and permeability within the slopes were not constant in space or time but changed continually during the course of the experiments.

Unusual July 10, 1996, rock fall at Happy Isles, Yosemite National Park, California

Effects of the July 10, 1996, rock fall at Happy Isles in Yosemite National Park, California, were unusual compared to most rock falls. Two main rock masses fell about 14 s apart from a 665-m-high cliff southeast of Glacier Point onto a talus slope above Happy Isles in the eastern part of Yosemite Valley. The two impacts were recorded by seismographs as much as 200 km away. Although the impact area of the rock falls was not particularly large, the falls generated an airblast and an abrasive dense sandy cloud that devastated a larger area downslope of the impact sites toward the Happy Isles Nature Center. Immediately downslope of the impacts, the airblast had velocities exceeding 110 m/s and toppled or snapped about 1000 trees. Even at distances of 0.5 km from impact, wind velocities snapped or toppled large trees, causing one fatality and several serious injuries beyond the Happy Isles Nature Center. A dense sandy cloud trailed the airblast and abraded fallen trunks and trees left standing. The Happy Isles rock fall is one of the few known worldwide to have generated an airblast and abrasive dense sandy cloud. The relatively high velocity of the rock fall at impact, estimated to be 110–120 m/s, influenced the severity and areal extent of the airblast at Happy Isles. Specific geologic and topographic conditions, typical of steep glaciated valleys and mountainous terrain, contributed to the rock-fall release and determined its travel path, resulting in a high velocity at impact that generated the devastating airblast and sandy cloud. The unusual effects of this rock fall emphasize the importance of considering collateral geologic hazards, such as airblasts from rock falls, in hazard assessment and planning development of mountainous areas.

The Borah Peak, Idaho Earthquake of October 28, 1983—Liquefaction

The most pervasive and damaging effects of liquefaction generated by the 1983 Borah Peak, Idaho earthquake occurred in the Big Lost River and Thousand Springs Valleys above Mackay Reservoir. Less severe effects occurred in the Big Lost River Valley south of Mackay Reservoir and in the Pahsimeroi Valley. Nearly all of the liquefaction effects developed in floodplain alluvium of late Holocene age. However, the sediment that liquefied beneath the alluvial fans on the east side of the Thousand Springs Valley was deposited in late Pleistocene time. The distance from the fault to the farthest effect of liquefaction was unusually short for an M S = 7.2 event. The distribution of liquefaction effects were consistent, however, with the distribution of MMI intensity and estimated peak ground motion parameters, both of which attenuated more rapidly than is generally expected for an earthquake of this type and magnitude.

The Borah Peak, Idaho earthquake of October 28, 1983: Landslides

The Borah Peak, Idaho earthquake caused several hundred landslides throughout an area of about 4,200 km2. The most numerous landslides were rock falls and rock slides, which occurred where slopes contained conspicuous, through-going, open fractures or were composed of weakly cemented rocks. The earthquake also produced several slumps and cracks in man-made fill, several soil liquefaction phenomena, a large debris flow, a large mud flow, and a few ground failures of other types. The most significant landslide damage was in Challis, where rock falls damaged at least 3 houses and 2 automobiles.