U.S. Department of the Interior U.S. Geological Survey

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador

Open-File Report 01–395

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 1 Cover photograph San Miguel volcano viewed from the south. (Photograph by J.J. Major, U.S. Geological Survey).

2 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador Lahar-Hazard Zonation for San Miguel Volcano, El Salvador

By J.J. Major, S.P. Schilling, C.R Pullinger, C.D. Escobar, C.A. Chesner, and M.M. Howell

U.S. GEOLOGICAL SURVEY Open-File Report 01-395

Vancouver, Washington U.S.A. 2001

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 3 U.S. DEPARTMENT OF THE INTERIOR Gale Norton, Secretary

U.S. GEOLOGICAL SURVEY Charles G. Groat, Director

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

Scientist-in-Charge U.S. Geological Survey U.S. Geological Survey Information Services 1300 SE Cardinal Court, Bldg. 10 P.O. Box 25286 Vancouver, WA 98683 Denver, CO 80225 (360) 993-8900 (303) 202-4210 FAX: (360) 993-8980

4 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador CONTENTS

Introduction ...... 1 Debris Avalanche, Landslide, and Lahar ...... 2 Future Landslides and Lahars at San Miguel Volcano...... 4 Lahar-Hazard-Zonation Map ...... 4 Lahar Hazard Forecasts and Warnings ...... 5 Protecting Communities and Citizens from Lahar Hazards ...... 6 References ...... 6 Additional Suggested Reading ...... 7 End Notes ...... 7

PLATE [In pocket] I. Volcano-hazard zonation for San Miguel Volcano, El Salvador

FIGURES 1. Location of major cities and significant Quaternary volcanoes in El Salvador ...... 2

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 5 6 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador Lahar-Hazard Zonation for San Miguel Volcano, El Salvador

By J.J. Major, S.P. Schilling, C.R Pullinger1, C.D. Escobar1, C.A. Chesner2, and M.M. Howell

INTRODUCTION sized blocks that are dispersed into the atmosphere and fall to the ground). Little is known, however, San Miguel volcano, also known as about prehistoric eruptions of the volcano. Chaparrastique, is one of many volcanoes along Chemical analyses of prehistoric lava flows and the volcanic arc in El Salvador (figure 1). The thin tephra falls from San Miguel volcano indicate volcano, located in the eastern part of the country, that the volcano is composed dominantly of basalt rises to an altitude of about 2130 meters and (rock having silica content <53%), similar to the towers above the communities of San Miguel, El lava erupted by Hawaiian volcanoes. The chemi- Transito, San Rafael Oriente, and San Jorge. In cal composition of eruptive products and the lack addition to the larger communities that surround of evidence of large cataclysmic eruptions sug- the volcano, several smaller communities and gests that prehistoric eruptions probably were coffee plantations are located on or around the similar in nature to the historical eruptions. flanks of the volcano, and the PanAmerican and Unlike San Salvador and San Vicente volcanoes, coastal highways cross the lowermost northern San Miguel volcano does not appear to have had a and southern flanks of the volcano. The popula- history of violent explosive eruptions. tion density around San Miguel volcano coupled Volcanic eruptions are not the only geologic with the proximity of major transportation routes events that present hazards to local communities. increases the risk that even small volcano-related Landslides and associated debris flows (watery events, like landslides or eruptions, may have flows of mud, rock, and debris--also known as a significant impact on people and infrastructure. lahars when they occur on a volcano) that can San Miguel volcano is one of the most active occur during periods of no volcanic activity are volcanoes in El Salvador; it has erupted at least 29 also of concern. In 1998 at Casita volcano in times since 1699 [1] (numerals in brackets refer to Nicaragua, extremely heavy rainfall from Hurri- end notes in the report). Historical eruptions of cane Mitch triggered a landslide that moved down the volcano consisted mainly of relatively quies- slope and transformed into a rapidly moving cent emplacement of lava flows or minor debris flow that destroyed two villages and killed explosions that generated modest tephra falls more than 2000 people. Torrential rains at San (erupted fragments of microscopic ash to meter Miguel volcano in 1988, 1994, 1999 and 2000

1 Servicio Nacionale de Estudios Terretoriales, Ave. Roosevelt y 55 Ave. Norte, Torre El Salvador (IPSFA) 2 Department of Geology/Geography, Eastern Illinois University, Charleston, IL 61920

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 7 triggered debris flows (lahars) on the northwestern DEBRIS AVALANCHE, LANDSLIDE, slopes of the volcano, and they traveled AND LAHAR downslope and damaged the main road that leads to San Jorge. Although modern landslides and The slopes of a volcano can become unstable, lahars at San Miguel thus far have caused only fail catastrophically, and generate a landslide. A minor loss of property and have been primarily a large rapidly moving landslide at a volcano is short-term inconvenience, destructive rainfall- and commonly called a debris avalanche. Slope earthquake-triggered landslides and debris flows instability at volcanoes can be caused by many on or near San Salvador volcano (figure 1) in factors. Large earthquakes, torrential rains, or September 1982 and January 2001, and at San steam explosions can trigger landslides perhaps as Vicente volcano in September 2001, demonstrate large as 10 million cubic meters or more. Land- that such mass movements in El Salvador can also slides triggered by these mechanisms are usually be lethal. relatively shallow, up to several tens of meters This report describes the hazards of landslides deep. On the contrary, magma rising upward and lahars in general, and discusses potential through a volcano can push aside older volcanic hazards from future landslides and lahars at San rock and deform and steepen the flanks of a Miguel volcano in particular. The report also volcano, or warm acidic ground water can circu- shows, in the accompanying lahar-hazard-zonation late through cracks and porous zones deep inside a map, which areas are likely to be at risk from volcano, alter strong rock to weak slippery clay, future landslides and lahars at San Miguel. A and gradually weaken the volcano so that it is report by Chesner (2000) focuses on a broader susceptible to large debris avalanches. Debris volcanic-hazard assessment of the volcano. avalanches triggered by these mechanisms can be

90ºW. 88ºW.

GUATAMALA EL SALVADOR HONDURAS

14ºN. V. Santa Ana Santa Ana Lake Coatepeque V. Izalco V. San Salvador

San Salvador Lake Ilopango San Vicente V. San Vicente San Miguel V. San Miguel Pa cific O V. Cochague ce an 040 km

Figure 1. Location of major cities and significant Quaternary volcanoes in El Salvador. Circles indicate major cities, triangles indicate major volcanoes. Lake Coatepeque and Lake Ilopango are large silicic calderas.

8 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador on the order of 100 million or more cubic meters in recur on the order of millennia to tens of millennia. size, and they commonly erode deeply and remove The size of a lahar is controlled by both the amount large segments of a volcano. A debris avalanche of water and the amount of loose sediment or can attain speeds in excess of 150 kilometers per volcanic debris available. Large debris avalanches hour; generally, the larger the avalanche, the faster or eruptions can dump tens to hundreds of millions and farther it can travel. Shallow, small-volume of cubic meters of sediment into channels and landslides typically travel only a few kilometers produce large lahars. Smaller landslides or erup- from their source, but massive, large-volume debris tions produce smaller lahars. avalanches can travel tens of kilometers from a Landslides and lahars can cause problems long volcano. Debris avalanches and smaller landslides after the event that formed them ends. Once destroy everything in their paths and can leave landslides and lahars fill stream channels with deposits tens of meters thick on valley floors. sediment, the streams begin to erode new paths, Debris-avalanche deposits have been found at and the new stream channels can be highly un- numerous volcanoes around the world, including stable and shift rapidly as sediment is eroded and volcanoes in El Salvador. At San Miguel volcano, moved farther down valley. Stream shifting can no deposits of a large debris avalanche have been cause rapid and dramatic bank erosion. Further- recognized yet, suggesting that large parts of the more, because stream channels are clogged with volcano have not collapsed catastrophically. sediment, they have less ability to convey water. Nevertheless, San Miguel volcano is a large, As a result, relatively small floods, which may steep-sided volcano, and the possibility of a future have previously passed unnoticed, can pose large debris avalanche, although slight, cannot be potentially significant threats to people living in dismissed. low-lying areas. People living in low-lying areas along river valleys are most susceptible to these Lahars, also called volcanic mudflows and secondary affects from landslides and lahars, but debris flows, are flowing masses of mud, rock, and areas on higher ground adjacent to river channels water that look much like rapidly flowing con- may be threatened by bank erosion. Examples crete. They are produced when water mobilizes from many volcanoes around the world show that large volumes of mud, rock, and volcanic debris. the effects of sediment deposition by landslides Commonly, landslides and debris avalanches will and lahars in stream channels can persist for years transform into lahars. Lahars, like floods, inun- to decades [2]. date floodplains and submerge structures in low-lying areas. They can travel many tens of Landslide and lahar deposits at San Miguel kilometers at speeds of tens of kilometers per volcano are found only locally. Aside from hour. Lahars can destroy or damage everything in deposits of rainfall-triggered lahars that occurred their paths through burial or impact. They follow within the past twenty years from tephra-covered river valleys and leave deposits of muddy sand slopes on the western side of the volcano, a few and gravel that commonly are a few meters thick. prehistoric (?) lahar deposits are found on the They are particularly hazardous because they can western flank of the volcano within about 4 km of travel far distances from a volcano and they affect the summit, and others are exposed locally at the stream valleys where human settlement is usually base of the volcano on other flanks. The limited greatest. In some instances, lahars can clog a distribution of lahars at San Miguel suggests that channel or block a tributary channel and impound eruptions of the volcano have not commonly a lake behind the blockage. Commonly, the generated landslides and lahars, nor have exten- impounded water will spill over the blockage and sive lahars formed during noneruptive intervals. generate a flood that moves down valley endanger- More likely, eruptions have blanketed the slopes ing people and property. of the volcano with tephra, and subsequent heavy rains have eroded that sediment or triggered small Like floods, lahars range greatly in size. The landslides that have mobilized into lahars. smallest lahars recur most frequently (perhaps every few years), whereas the largest typically

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 9 FUTURE LANDSLIDES AND LAHARS are similar to San Miguel to gain a general idea of AT SAN MIGUEL VOLCANO the possible hazards from landslides and lahars. This is a reasonable method because similar types Landslides and lahars, triggered by any of of landslides and lahars occur at many volcanoes, several mechanisms, can occur on any flank of the even though the exact types that occur, and their volcano. The direct effects of most landslides and relative frequencies and sizes, vary among volca- lahars at San Miguel will likely be confined to nic centers. within about 10 kilometers of the summit of the The accompanying lahar-hazard-zonation map volcano, although rare large lahars could travel (plate 1) shows areas that could be affected by farther. Such large events, perhaps involving more future landslides and lahars at San Miguel. The than about 10 million cubic meters of debris, have a low probability of occurrence and would likely location and size of an affected area will depend require a substantial tephra-producing eruption or on the location of a landslide or area of substantial a debris avalanche that removes a large part of the flank erosion, the volume and character of sedi- volcano. Both scenarios generally would require ment involved, the shape of the channel in which magma to rise to a high level in the volcano. the lahar occurs, and whether or not magmatic When magma rises into a volcano, it is accompa- intrusion was a key triggering process. nied by increased seismicity, visible deformation Potential areas of lahar inundation along major of the volcano, and other signs that provide channels that drain San Miguel volcano are warning of the onset of hazardous conditions. divided into nested zones on the basis of the size Future lahars likely will be triggered by of the lahar and their relative degree of hazard. erosion or landslides in the tephra mantle that Inundation boundaries are drawn on the basis of covers the slopes of the volcano, and probably will mathematical models that use calibrations from be smaller than about 1 million cubic meters in other volcanoes to predict the probable extent of size (see discussion in section on lahar-hazard lahars, and on our experience and judgement zonation map). Future eruptions of San Miguel derived from observations and understanding of likely will produce minor tephra falls that supply landslides and lahars at similar volcanoes. sources of new sediment for lahars. In general, Although we show sharp boundaries for the the western side of the volcano is more likely to be lahar-hazard zones, the limit of the hazard does affected by future lahars owing to tephra distribu- not change or end abruptly at these boundaries. tion. Although prevailing winds are seasonal, Rather, the hazard decreases gradually as distance easterly winds tend to dominate [3], and tephra from the volcano increases, and decreases rapidly thicknesses generally appear to be greater on the with increasing elevation above channel floors. western flank of the volcano than on other flanks. Areas immediately beyond outer hazard zones Although the direct effects of landslides and should not be regarded as hazard-free, because the lahars are likely to be confined within 10 kilome- limits of lahar hazard can be located only approxi- ters of the volcano, secondary effects related to mately, especially in areas of low relief. Many sediment erosion and transport along channels can uncertainties about the source, size, and mobility affect areas farther from the volcano and can of future lahars preclude locating the boundaries linger for several years. Such secondary effects of zero-hazard zones precisely. involve reworking and redistribution of sediment, Users of the hazard map in this report should bank erosion, loss of channel capacity, and en- be aware that the map does not show all areas hanced hazards of floods in low-lying areas. subject to hazardous landslides and lahars from San Miguel volcano. For this report, we defined LAHAR-HAZARD-ZONATION MAP zones of inundation from lahars of various vol- LAHAR-HAZARD-ZONATION MAP umes for prominent channels directed toward Because the details of the eruptive and land- populous areas. Other channels for which we slide history of San Miguel are poorly known, we have not modeled lahar inundation should not be rely on data from volcanoes around the world that considered as areas devoid of lahar hazard.

10 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador We used a mathematical technique calibrated topography flattens abruptly beyond the base of the with data from other volcanoes [4] to estimate edifice. As a result, lahars rapidly spill out of potential areas of inundation by lahars of various channels, spread, and stop. The most distant volumes. For each channel analyzed, we define hazard zones are associated with the deepest nested hazard zones that depict anticipated inunda- incised channels in which lahars remain confined. tion by hypothetical “design” lahars having Despite their relatively short runout distances, different volumes. We focus on estimating even the smallest lahars can cause fatalities and inundation areas of lahars likely to be triggered by damage infrastructure Although major communi- earthquakes or torrential rains, as these are the ties near the volcano are located 10 kilometers or most common causes of lahars at San Miguel more from the summit of the volcano, smaller volcano. We do not estimate areas of inundation communities, coffee plantations, and important from a large debris avalanche that might be transportation routes are located on the lower triggered by magmatic intrusion into the volcano. flanks of the volcano and lie within 10 kilometers Events of this type, particularly at basaltic volca- of the summit. The hazard zones of even the noes, are rare. The largest design lahar selected smallest lahars extend well into areas that are now for initiation within a single channel, 1 million settled or used for agriculture. cubic meters, reflects our estimate of the largest probable lahar that could be triggered by earth- quakes or torrential rains at San Miguel [4]. The LAHAR HAZARD FORECASTS AND intermediate (300,000 to 500,000 cubic meters) and smallest (100,000 cubic meters) design lahars WARNINGS are more typical volumes that might result from It is difficult, if not impossible, to predict the landslides triggered by such events. Landslides precise occurrence of landslides and lahars and lahars of these sizes have occurred historically triggered by earthquakes or torrential rains. at San Miguel volcano, as well as at San Salvador However, generally hazardous conditions that and San Vicente volcanoes, and lahars of up to favor formation of landslides and lahars can be 500,000 cubic meters are the most likely sizes to recognized. Forecasts for very heavy rainfall, occur again. which commonly trigger flood warnings, can serve Large lahars are less likely to occur than small as indicators of conditions favorable for landslides lahars. The annual probability of lahars of various and lahars. When San Miguel volcano erupts sizes is difficult to estimate, because ages and again, it is likely to disperse tephra fall on its extents of prehistoric lahars at San Miguel are not flanks. Subsequent erosion of that tephra can known. On the basis of examining the probabili- generate lahars similar to those that have occurred ties of occurrence of landslides and lahars of in past 20 years. In this case, the eruption of the various sizes at other major volcanoes within the volcano can serve as a warning that conditions are country [5] it is likely that landslides and lahars of favorable for lahar formation, and the distribution 1 million cubic meters or more in size have an of tephra fall can indicate which flanks are more annual probability of occurrence that is less than 1 likely to be affected. In the rare event of a large debris avalanche triggered by intrusion of magma in 10,000. Smaller landslides and lahars are much into the volcano, deformation of the volcano will more likely to occur; lahars of about 500,000 serve as a warning that conditions are hazardous. cubic meters or less may have an annual probabil- However, government officials and the public ity of about 1 in 100 to perhaps as great as 1 in 5 need realize that potentially lethal events can [6] occur in the lahar hazard zones with little or no In general, lahar hazard zones are within about warning. 10 kilometers of the summit crater. Local topogra- phy plays a large role in controlling lahar runout. Although landslides and lahars originate in and flow along steeply incised drainages on the flanks of the volcano, these channels shallow and the

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 11 PROTECTING COMMUNITIES AND of the volcano, all but that largest lahars will rise CITIZENS FROM LAHAR HAZARDS less than about 10 meters above river level. Landslides and lahars from San Miguel volcano Communities and citizens must plan ahead to will happen again, and the best way to cope with mitigate the effects of future landslides and lahars these events is through advance planning in order from San Miguel volcano. Long-term mitigation to mitigate their effects. efforts must include using information about lahar and other volcano hazards (see Chesner, 2000) when making decisions about land use and siting REFERENCES of critical facilities and development. Future development should avoid areas judged to have an Baum, R.L., Crone, A.J., Escobar, D., Harp, E.L., unacceptably high risk or be planned and designed Major, J.J., Martinez, M., Pullinger, C.R., and to reduce the level of risk. Smith, M.E., 2001, Assessment of landslide hazards resulting from the February 13, 2001, El Depending on the distance from the volcano, Salvador earthquake: U.S. Geological Survey the hazard zones depicted on the map are areas Open-File Report 01-119, 22 p. that will be affected within a few minutes to about Chesner, C.A., 2000, A preliminary hazard assess- one hour after the onset of a lahar. Beyond 10 ment of San Miguel Volcano, El Salvador. kilometers from the volcano’s summit escape may Unpublished report, Eastern Illinois University, be possible if people are given sufficient warning. Department of Geology and Geography, 28 p. Within 10 kilometers of the volcano lahars may happen too quickly to provide effective warning. Escobar, C.D., Mendez, I., and Ramirez, O.R., 1993, Therefore, citizens must learn to recognize for Estudio geologico preliminar del Volcan de San Miguel: Peligrosidad eruptiva, estado erosivo, y themselves hazardous conditions that favor sus consecuencias: Trabajo de Graduacion, formation of landslides and lahars. Facultad de Ingenieria y Arquitectura, Because landslides and lahars can occur Universidad Tecnologica, San Salvador, El without warning, suitable emergency plans for Salvador, 176 p. dealing with them should be made in advance. Iverson, R.M., Schilling, S.P., and Vallance, J.W., Although it is uncertain when landslides and 1998, Objective delineation of lahar-hazard lahars will occur again at San Miguel volcano, zones downstream from volcanoes: Geological public officials need to consider issues such as Society of America Bulletin, v. 110, p. 972-984. public education, communications, and evacua- Major, J.J., Pierson, T.C., Dinehart, R.L., and tions as part of a response plan. Emergency plans Costa, J.E., 2000, Sediment yield following already developed for floods may apply to some severe volcanic disturbance--a two decade extent, but may need modifications. For inhabit- perspective from Mount St. Helens: Geology, ants in low-lying areas a map showing the shortest v. 28, p. 819-822. route to high ground would be helpful. Major, J.J., Schilling, S.P., Sofield, D.J., Escobar, Knowledge and advance planning are the most C.D., and Pullinger, C.R., 2001a, Volcano important items for dealing with landslide and hazards in the San Salvador Region, El Salvador: lahar hazards. Especially important is a plan of U.S. Geological Survey Open-File Report action based on the knowledge of relatively safe 01-366, 17 p. areas around homes, schools, and workplaces. Major, J.J., Schilling, S.P., Pullinger, C.R., Lahars pose the biggest threat to people living or Escobar, C.D., and Howell, M.M., 2001b, recreating along channels that drain San Miguel Volcano-hazard zonation for San Vicente Volcano, volcano. The best strategy for avoiding a lahar is El Salvador: U.S. Geological Survey Open-File to move to the highest possible ground. A safe Report 01-367, 15 p. height above river channels depends on many Mercado, R., Rose, W.I., Najera, L., Matías, O., and factors including the size of the lahar, distance Girón, J., 1988, Volcanic ashfall hazards and from the volcano, and shape of the valley. For upper wind patterns in Guatemala, preliminary areas beyond about 8 kilometers from the summit report: Publication of Department of Geological

12 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador Engineering and Sciences, Michigan Technologi- [3] Upper-level wind patterns in Guatemala cal University: Houghton, MI, 34 p. between 3000 and 15,000 meters altitude are Portig, W.H., 1976, The climate of Central America, strongly seasonal (Mercado et al., 1988). in Schwerdtfeger, W., ed., World Survey of Similar wind patterns are likely in El Salvador. Climatology, Climates of Central and South From January to March, westerly winds America, v. 12: Elsevier, New York, p. 405-478. dominate. April and May are transitional Rymer, M.J., and White, R.A., 1989, Hazards in El months in which westerly winds give way to Salvador from earthquake-induced landslides, in more northerly and easterly winds. June Brabb, E.E., and Harrod, B.L., eds., Landslides: through October are characterized by easterly Extent and Economic Significance. Balkema, winds, and November and December are Rotterdam, p. 105-109. transitional months during which westerly winds gradually become dominant. The strong Simkin, T., and Siebert, L., 1994, Volcanoes of the seasonality of these winds will influence areas World, 2nd edition. Geosciences Press, Inc., Tucson, 349 p. affected by tephra falls. Erupted tephras will likely fall eastward of the volcano from January Williams, H., and Meyer-Abich, H., 1955, Volcanism through March, potentially cover broad regions in the southern part of El Salvador: University of to the east, south, and west in April and May, California Publications in Geological Sciences, affect areas west of the volcano from June v. 32, 64 p. through October, and possibly areas west, north, and east of the volcano in November and ADDITIONAL SUGGESTED READING December. Surface winds may also affect tephra distributions, and their patterns are Blong, R.J., 1984, Volcanic hazards. Academic diurnal as well as seasonal (Portig, 1976). Press, Orlando, 424 p. Therefore, all sectors around San Miguel Sigurdsson, H., Houghton, B., McNutt, S.R., Rymer, volcano can be affected by tephra fall, but H., and Stix, J., eds., 2000, Encyclopedia of overall the western flank is more likely to be Volcanoes. Academic Press, San Diego, CA., affected owing to the slightly higher probability 1417 p. of easterly winds. Tilling, R.I., ed., 1989, Volcanic hazards: Short [4] Lahar hazard zones were constructed by course in geology, v. 1, American Geophysical modeling lahar volumes of 100,000; 300,000; Union, Washington, D.C., 123 p. 500,000; and 1 million cubic meters. Using mathematical and digital cartographic tech- niques (Iverson et al., 1998), these volumes END NOTES were used to compute the estimated extent of [1] The geologic data upon which this report is inundation down stream from a source area. based come largely from Williams and Meyer- Although undated lahar deposits have been Abich (1955); Escobar et al. (1993); Simkin identified at San Miguel volcano and four and Siebert (1994), Chesner (2000); communi- lahars have descended the flanks of the volcano cations with personnel at Centro de within the past 13 years, the volumes of these Investigaciones Geotécnicas, San Salvador; and events are poorly known. Regionally, earth- our own reconnaissance investigations. quake- and rainfall-triggered landslides and [2] Analyses of limited data from volcanoes around lahars have had volumes as large as 10 million the world indicate that sediment yields from cubic meters, but most have had volumes of a river channels filled with volcanic debris by an few hundred to a few hundreds of thousands of eruption can remain higher than typical back- cubic meters (Rymer and White, 1989; Baum et ground levels for years to decades after an al., 2001; E.L. Harp and A.J. Crone, U.S. eruption. In some cases sediment yields can Geological Survey, personal communication). remain 10 to 100 times greater than typical At Casita volcano in Nicaragua, extremely background levels for more than two decades heavy rainfall from Hurricane Mitch triggered a (Major et al., 2000). landslide of about 1.5 million cubic meters in

Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 13 volume, but as it moved down slope it trans- million cubic meters, but most have had volumes formed into a lahar that scoured its channel and of less than a few to a few tens of thousands of its volume enlarged to more than 3 million cubic meters. Thus, earthquake-induced cubic meters (K.M. Scott, U.S. Geological landslides of small to moderate volume occur in Survey, personal communication). On the basis El Salvador about once every 12 years. At San of these data, we select a landslide and associ- Miguel volcano, known rainfall-triggered ated lahar of 1 million cubic meters to be a landslides and lahars have occurred four times probable maximum size likely to be triggered in within the past 13 years, which suggest that any single channel at San Miguel volcano by annual probabilities of occurrence of such earthquakes or torrential rainfalls. events may be as great as 1 in 3. These modern [5] Probabilities of occurrence of lahars having lahars did not reach major communities, but volumes of 1 million cubic meters or more at they did damage the main road leading from the San Salvador and San Vicente volcanoes are PanAmerican highway to San Jorge (see plate given in Major et al. (2001a, b). 1). The volumes of these lahars are not known, but their extents suggest volumes on the order of [6] We estimate possible annual probabilities of 100,000 cubic meters or more. Although the landslides and lahars having volumes of estimated probabilities determined above are 500,000 cubic meters or less at San Miguel highly generalized, we suggest that the annual volcano as follows. Historical earthquake- probability of landslides and lahars ≤500,000 induced landslides have occurred throughout El cubic meters in size at San Miguel volcano is Salvador at least a dozen times from 1857 to about 1 in 100 (comparable to San Salvador 2001 (Rymer and White, 1989; Baum et al., volcano; see Major et al., 2001a) to perhaps as 2001). Volumes of these landslides have great as 1 in 5. ranged from a few hundred to more than 10

14 Lahar-Hazard Zonation for San Miguel Volcano, El Salvador Lahar-Hazard Zonation for San Miguel Volcano, El Salvador 15

Major,others and

—

LAHAR-HAZARD ZONA LAHAR-HAZARD LAHAR-HAZARD ZONA LAHAR-HAZARD

LAHAR-HAZARD ZONA LAHAR-HAZARD

LAHAR-HAZARD ZONA LAHAR-HAZARD LAHAR-HAZARD ZONA LAHAR-HAZARD

TION FOR SAN MIGUEL VOLCANO, EL SAL EL VOLCANO, MIGUEL SAN FOR TION TION FOR SAN MIGUEL VOLCANO, EL SAL EL VOLCANO, MIGUEL SAN FOR TION

TION FOR SAN MIGUEL VOLCANO, EL SAL EL VOLCANO, MIGUEL SAN FOR TION

TION FOR SAN MIGUEL VOLCANO, EL SAL EL VOLCANO, MIGUEL SAN FOR TION TION FOR SAN MIGUEL VOLCANO, EL SAL EL VOLCANO, MIGUEL SAN FOR TION

V V

V

V V

ADOR ADOR

ADOR

ADOR ADOR

—U.S. Geological Survey — Survey Geological —U.S.

Open-File Report 01-395 Report Open-File

16 PrintedLahar on recycled-Hazard paper Zonation for San Miguel Volcano, El Salvador