SOME OBSERVATIONS OF LANDSLIDES TRIGGERED BY THE 29-APRIL-1991 RACHA EARTHQUAKE, REPUBLIC-OF-GEORGIA

On 29 April 1991 an Ms 7.0 earthquake occurred in the Racha region of the Great Caucasus Mountains in north-central Republic of Georgia. The earthquake occurred on a thrust fault striking roughly east-west and dipping about 20-degrees to 45-degrees northward; focal depth was 17 /- 2 km. We observed no surface fault rupture, but the earthquake caus ed extensive structural damage to the many unreinforced stone building s in the area, and at least 114 people were killed. Many landslides we re triggered in a 2500-km2 epicentral area, and they caused much of th e structural damage and at least half the fatalities. We observed the following six types of landslides (in order of decreasing abundance): rock falls, debris slides, slumps, earth slides, rock block slides, an d rock avalanches. The types of landslides triggered by the earthquake are controlled primarily by lithology and geologic structure. Enigmat ic landslide processes associated with this earthquake include (1) del ays of several days between earthquake shaking and significant landsli de movement, probably caused by changes in groundwater conditions; (2) small co-seismic displacement of landslides active at the time of the earthquake, a possible result of viscoplastic damping of the seismic shaking; and (3) somewhat unusual failure geometries related to local topography and geologic structure.