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.

Volcanic debris flows (lahars) in El Salvador pose a significant risk to tens of thousands of people as well as to property and important infrastructure. Major cities and nearly a third of the country's population are located near San Salvador, San Vicente, and San Miguel volcanoes. Debris flows traveling as little as 4 km from source at these volcanoes put hundreds to thousands of lives, property, and infrastructure at risk. We used a statistically based model that relates debris-flow volume to cross-sectional and planimetric inundation areas to evaluate spatial patterns of inundation from a suite of debris flows ranging in volume from 100,000 m 3 to as large as 100 million m 3 and examined prehistoric deposits and a limited number of historical events at these volcanoes to estimate probable frequencies of recurrence. Our analyses show that zones of greatest debris-flow hazard generally are focused within 10 km of the summits of the volcanoes. For typical debris-flow velocities (3–10 m/s), these hazard areas can be inundated within a few minutes to a few tens of minutes after the onset of a debris flow. Our analyses of debris-flow recurrence at these volcanoes suggest that debris flows with volumes of 100,000 m 3 to as large as 500,000 m 3 have probable return periods broadly in the range of ∼10 to 100 yr. Debris flows having volumes less than 100,000 m 3 probably recur more frequently, especially at San Miguel volcano. Despite the limited extents of the hazard zones portrayed in our analyses, even the smallest debris flows could be devastating. Urban and agricultural expansions have encroached onto the flanks of the volcanoes, and debris-flow–hazard zones extend well into areas that are settled densely or used for agriculture. Therefore, people living, working, or recreating along channels that drain the volcanoes must learn to recognize potentially hazardous conditions, be aware of the extents of debris-flow–hazard zones, and be prepared to evacuate to safer ground when hazardous conditions develop.

San Miguel volcano in eastern El Salvador is a classic composite cone, symmetrical and concave upwards. Its summit crater exceeds 344 m in depth and consists of several nested craters with nearly vertical walls. The inner crater has grown in depth and size since it was first described in 1924. The dominant eruptive product at San Miguel has been lava flows. Spatter and scoria cones commonly occur at flank vents that erupted historic lava flows. Lava flows erupted from flank vents on several occasions between 1699 and 1867, traveling as far as 8 km from their vents. During this time interval, the elevation of flank vents increased. All subsequent activity has been minor Strombolian and ash eruptions in the summit crater. Occasionally, scoria fall deposits, pyroclastic flow deposits, and phreatomagmatic ashes have been produced, but no substantial explosive event has been positively linked to San Miguel. Few lahar deposits have been identified as a result of the preponderance of lavas exposed on the upper and middle slopes. Geochemical analyses of lavas, tephras, and block and ash flow deposits erupted from San Miguel indicate that the majority of activity has been mafic in character, ranging between 51 and 53 wt% SiO 2 . Historic flank lavas plot at the mafic end of the chemical range and are basaltic. The most evolved flank lavas are basaltic andesites and comprise two chemically distinct subsets, distinguished mostly by the presence or absence of phenocrystic magnetite and their V and Al 2 O 3 contents. They occur only on the eastern flank of the volcano and appear to represent some of the oldest exposed lavas. A stratigraphic sequence of 22 crater lavas has the most restricted compositional range and exhibits two chemical trends. These trends may represent rapid-fire eruptions, with little to no intervening reposes, from two distinct batches of magma. Overall, the development of San Miguel volcano is fairly simple and consists of two evolutionary stages. The first stage consisted of eruption of lavas from a central vent and growth of the symmetrical cone. Shallowing of the subvolcanic magma chamber may have been associated with this stage. This was followed by a significant change in the subvolcanic plumbing system characterized by flank eruptions and onset of the second growth stage. Magma draining laterally from the magma chamber to the flank vents led to the collapse of the summit region. Modification of the summit crater is an active process that continues today.

Central America is one of the most tectonically active areas in the world, and its high seismicity rate demands the establishment of seismic instrumentation to monitor earthquake occurrence. The first seismological equipment installed in the region dates from 1882. In El Salvador, seismological observations began in the late nineteenth century when 15 Wiechert seismographs arrived in the country. A network of 12 short-period telemetric seismic stations became fully operational in El Salvador in late 1984; this network currently consists of 18 seismographs. Earthquake monitoring in the other countries of the region by permanent networks has operated since the 1970s. In the 1990s, all the countries in the region acquired SEISLOG data acquisition systems and new equipment, including broadband stations, which were donated to improve their seismic networks. In 1998, the Central America Seismological Center (CASC) was established in Costa Rica, marking a new stage in seismological observation in the region. The goal of this center is to locate regional earthquakes and compile seismic databases to be used in the estimation of seismic hazard. This paper summarizes the progress on seismic monitoring in El Salvador and reviews the scientific achievements of the CASC after three years of operation.