The San Salvador Earthquake of October 10,1986—Geologic Aspects

Abstract

The San Salvador earthquake struck an area of upper Cenozoic volcanic rocks, consisting of basaltic to silicic flows and tuffs. Tuff deposits constitute the upper 30 m of section below San Salvador; thinner sections are in the hills surrounding the city. Tuffs were important in landslide development during the earthquake and may have amplified ground motion. Faults mapped in the area strike east-west, northwest-southeast, and less distinctly, northeast-southwest and north-south. Ringlike structures formed by volcanic subsidence are also present in the area. No evidence was seen of surface faulting associated with the October 10 main shock or an earthquake on October 13 on a separate fault 7 km west-northwest of the main shock. Numerous cracks were seen in the epicentral areas of both earthquakes, but these are easily explained as secondary ground failures. Both of the earthquakes occurred on unmapped faults. The main shock caused several hundred landslides in an area of at least 200 km². The most numerous landslides were soil slides and soil falls, which were especially common in stream banks and roadcuts. The earthquake also produced rock falls and slides, slumps, rapid soil flows, shattered ridge effects, and compaction. Landslides and related ground failures were triggered as much as 12 km from the epicenter and accounted for about 200 fatalities and at least 100 damaged homes.