During a one-month period in early 2001, El Salvador experienced two devastating earthquakes. On 13 January, a M-7.7 earthquake centered ∼40 km off the southern coast in the Pacific Ocean caused widespread damage and fatalities throughout much of the country. The earthquake triggered thousands of landslides that were broadly scattered across the southern half of the country. The most damaging landslide, a rapidly moving mass of ∼130,000 m 3 , occurred in the Las Colinas neighborhood of Santa Tecla, where ∼585 people were killed. Another large landslide (∼750,000 m 3 ) near the city of San Vicente blocked the Pan-American Highway for several weeks. One month later, on 13 February, a M-6.6 earthquake occurred ∼40 km east-southeast of San Salvador and triggered additional thousands of landslides in the area east of Lake Ilopango. The landslides were concentrated in a 2500 km 2 area and were particularly abundant in areas underlain by thick deposits of poorly consolidated, late Pleistocene and Holocene Tierra Blanca rhyolitic tephras erupted from Ilopango caldera. Most of the triggered landslides were relatively small, shallow failures, but two large landslides occurred that blocked the El Desagüe River and the Jiboa River. The two earthquakes triggered similar types of landslides, but the distribution of triggered landslides differed because of different earthquake source parameters. The large-magnitude, deep, offshore earthquake triggered broadly scattered landslides over a large region, whereas the shallow, moderate-magnitude earthquake centered within the country triggered a much smaller, denser concentration of landslides. These results are significant in the context of seismic-hazard mitigation for various earthquake scenarios.

Abstract The Doublespring Pass road site is an excellent location at which to examine the surface faulting and ground breakage that accompanied the 1983 Borah Peak earthquake. The site can be reached by traveling 23 mi (37 km) northwest from Mackay, Idaho, or 30 mi (48 km) southeast from Challis, Idaho (Fig. 1), on U.S. 93 to the Doublespring Pass road turnoff. The Doublespring Pass road heads northeast from the highway. The intersectionof the Doublespring Pass road with the highway is identified by a sign indicating the direction to the towns of May and Patterson. The turnoff is also identified at the intersection by a historical marker commemorating William E. Borah, after whom Borah Peak was named. The Doublespring Pass road crosses thefault scarps 2.5 mi (4 km) northeast of the intersection of U.S. 93 (Fig. 2). Doublespring Pass road is a wide, well-maintained gravel road that can be safely traveled by passenger car and bus to the site (although the road is impassable at times in the winter and early spring when snow covers the area). The site lies within Challis National Forest and is open to the public.