HAZARDS AND HISTORY OF VOLCAN SAN SALVADOR AND HAZARD COMUNICATION IN EL SALVADOR

Abstract

The capital city, San Salvador (pop. 2 million) sits in the shadows of a volcano with an impressive array of hazards. The volcanoes around San Salvador have not exhibited any volcanic hazard for four generations. Overpopulation has caused people to build higher on the flanks of the volcano, with little comprehension of the risks. Earthquakes (1986) and mudslides (1992) have been the main focus of environmental hazard concern. Historic activity of Volcán San Salvador includes summit pyroclastic eruptions, and crater collapse, as well as phreatomagmatic explosion craters, cinder cones, and lava flows along N40W striking faults on its flanks. The city is within the hazard zones of both the summit (1000 meters above and 7 kilometers away from the city's center) and the flank faults (traversing through a portion of the city itself), it is important for people to understand the possible hazards.

The goals of the work are; to collaborate with Salvadorian officials, to figure out the eruptive history of the volcano and its associated fissures, and to begin communication about the volcanic risks that Salvadorians face.

This poster presents the progress of collaboration with Centro de Investigaciones Geotecnicas, the Comision Ejecutiva Hidroelectrica del Rio Lempa, and Universidad de El Salvador. Together, we forged a working hypothesis of the historic activity of the region. A detailed geologic map was compiled. Samples were collected for analysis. And most importantly there is an agreement to take the information gathered from the work completed to publish and disseminate it to the public and officials within El Salvador.

Photo of the inside of El Boqueron (the mouth) the younger crater of Volcán San Salvador.

San Salvador's eruptive history: stratigraphy and deposits

The volcano begins, with the construction of primary edifice. Lavas are found ponded against a graben fault. During construction there are plinian eruptions of Ilopango (Tierra Blanca 4) and Coatepeque. The first caldera collapse occurs producing a plinian andesitic ash fall deposit. A second edifice begins construction (Boquerón) while two more Plinian eruptions of Ilopango (Tierra Blanca 3 and 2) occur. Eruptive activity moves to the flanks with a combination of lava flows, cinder cones and hydromagmatic explosions deposits. A Ppinian eruption of Ilopango (Tierra Blanca Joven) blankets field area. Hydrated pyroclastic flow, `Talpetate" caps the western flanks of volcano, and causes multiple secondary landslides. Eruptions continue on the flanks, but there were no hydromagmatic eruptions after Talpetate. (Table 1)

Evidence for a second caldera collapse

Williams and Meyer-Abich (1955) first proposed that Volcán San Salvador had two caldera collapses. A Landsat satellite can produce an image with a scale large enough to see the structures that they proposed (Figure 2b).

Landsat image show two rims, Boquerón inside of a less distinct older San Salvador caldera.
Jabalí and Picacho are remnants of the older caldera rim
Older lavas are more deeply eroded. Boquerón lavas breach the caldera rim to the North and Southeast.

Field observations support the visual evidence.
Differences in topography and morphology; older lavas are more deeply eroded than Boquerón lavas
Different stratigraphic sequences on El Picacho and near Nueva San Salvador.
An unconformity at base of El Picacho.

Figure 4. Cartoon of the structure of Volcán San Salvador in cross section.

Flanking activity focused on two commonly observed fault zones

N80W Trend

All activity along this trend has been hydromagmatic, resulting in explosion craters.
Activity over time has migrated further away from the Boquerón edifice. (Table 2)
Last eruption that formed Laguna de Chamico occurred between 260 and 1100 AD.
Could this fault zone include a graben that trapped groundwater facilitating hydromagmatic explosion?

N40W Trend

All of the eruptions along trend have been monogenetic.
Fault runs through the Boquerón crater.
Last eruption south of the edifice was more than 1700 years ago
The most recent activity is on the northwestern side of the volcano, furthest from the city of San Salvador and Nueva San Salvador (Santa Tecla).
Fault scarps of N40W trend can be found all over the central graben valley of El Salvador.

Absence of hydromagmatic explosions in recent past

Hydromagmatic explosions may occur when magma interacts with groundwater. Rapid conversion of water to steam, causes a rapid increase in pressure. The explosion is the result from release of pressure. There are eleven hydromagmatic explosion craters in the field area.(Figure 5). The explosion were powerful enough to rip up the country rock (lithic clasts make up 20-80 percent of the deposits).

There has not been a hydromagmatic explosion in this area for over 900 years (Table 1). In fact there seems to be a shift from hydromagmatic eruption to lava extrusions on the flanks of San Salvador. The last ten magmatic events have been relatively dry; lava flow, cinder and splatter cones. (Table 1) The best hypothesis for the absence of hydromagmatic explosions in recent past is that the Talpetate deposit acts as an impermeable cap that reduces recharge of the groundwater aquifers. This would decrease the volume of water that interacts with the magma and prevents the explosive hydromagmatic eruptions. Talpetate is an indurated fine grain volcanic ash, that is appears to be impermeable, although no laboratory test for permeability have been completed. (Figure 6). It varies in thickness but was deposited over the majority of the active flanks.