Report on geological hazards in El Salvador

Add: 3-10-2017, 18:53   /   Views: 186

Introduction

El Salvador is the smallest country in Central America with its Capital in San Salvador. It covers a total area of 21,040km², with a population of 7.2 million as of 2009 resulting in an average density of equivalent 304 per km² and shares borders with the Pacific Ocean between Guatemala and Honduras (fig.1). This Country is placed on the most rugged topographic terrain which is made up of young volcanoes and eroded old ones. El Salvador has witnessed several destructive earthquakes this century alone due to its vulnerability to high seismic activities. This report gives an overview of the geotectonic situation giving rise to these hazards, the risk posed by these hazards and the mitigation measures to be applied to reduce human impact.

The Geology

The oldest formation in El Salvador from the Jurassic period (208 146 Ma) is found around Metapan, a city in the northern part of El Salvador, while the youngest at San Salvador is from Holocene period (after 0.011 Ma). The geology of El Salvador can be outlined as follows: The formation spreading over both the south coastal region and mountainous region north of San Salvador (Morazan group) are from the Holocene period, and they were formed during the Oligocene to the Miocene period. The middle region of the country consists of the Balsamo group formations from the Pleistocene period. Volcanic and igneous rocks are distributed nationwide in El Salvador. Basalts, andesites, rhyolites, lavas and pyroclastic rocks are found in mosaic. Alliviums are found spread only along the southern part of the coastal region. Granites and Limestones are found isolated in a small region at the northwest end of the country (http://jsce-int.org/).

History of Natural Disasters

El Salvador is situated in an intertropical Convergence zone (ITCZ), with a high seismic topographic terrain prone to hurricanes, earthquakes, Landslides and volcano eruption, (USGS 2004). In November 1998, Hurricane Mitch, (fig.4) killed 240 people, displaced 85,000 costing the country about $388 million in damage. On January 13th and then on February 13th 2001, a series of earthquakes and associated landslides killed 1159 people, destroyed 300,000 homes, costing the country about $1.6 billion in damages, (fig.6)(Crone, et al) making it the fifth largest earthquake in 50years. The last earthquake before this one was the upper crustal event that struck San Salvador in October in 1986, leaving 1500 dead and causing damages equivalent to 31% of GDP in the same year. Similar to these earthquakes are the destructive earthquakes that occurred in 1951 and 1965. Also in 1917 there was an eruption of the San Salvador volcano in which lava covered part of the northern zone, creating a Crater Lake inside the Boqueron when it evaporated and a cinder cone appeared, creating a rough area that remains unpopulated till date.

Volcanoes have been kinder in recent years, although landslides triggered in loose deposits on their slopes have had a terrible impact. Landslides are the results from Hurricanes and earthquakes. The mudslide triggered on the slopes of the San Salvador volcano by heavy rainfall in September 1982 buried 500 people and leaving 2400 homeless, (CEPRODE,2000). On October 1st 2005, the volcano Santa Ana (Ilamatopec) erupted. Due to early warnings by the SNET (Servicio Nacional de Estudios Territoriales) which was charged with the responsibility of mitigating natural hazards through hazard characterization, monitoring, and sensitization(Rose et al, 2004), evacuated the locals through the Civil Protection Service minimizing the extent of damage. The following day October 2nd Hurricane Stan struck the area causing a Lahar that swept from the upper flanks of the volcano, travelled seven kilometres, killing two people and destroyed several homes in its path which then terminated in Lake Coatepeque. There are three large cities in El Salvador, each located on the flanks of potential active volcanoes (San Salvador, Santa Ana and San Miguel), masquerading as beautiful lakes, but has once devastated El Salvador four times in the past 56,000 years, (Dull, 2004) and also in the centre of El Salvador is Ilopango Caldera which is another active volcano, (Mann et al). Most of these volcanoes are likely to be active again.

Tectonics Setting

Due to the extensive volcanic activities pyroclastic flows have blanketed the region with significant thickness of ignimbrites. The northern mountain of El Salvador is believed to be at the edge of the Central America Highland, associated with the Chortis Block (Rogers 2002). There appears to have been three tectonic events that have had the greatest effect on the morphology and structure of the northern mountain range of El Salvador.

The first and oldest event was the middle Miocene silicic ignimbrite flare-up which lasted roughly 10 million years; and ended abruptly 10 M.A. B. P. This extrusive event deposited up to 2 kilometers of ash on the southern borders of the Chortis Block, (Rogers 2002).

The second event was the North-South extension of the region sometime after the deposition of the Miocene aged ignimbrite. This extensional event may have been the result of orthogonal subduction of the Cocos plate underneath the Caribbean plate and thus causing a series of east to west trending halt grabens (Mann 2006).

The third event was the east-west extension of the Central America Highlands. This was caused by activity along the left-lateral Motagua translational fault in Guatemala. This extension is believed to be responsible for the development of the upper Rio Lempa watershed for the North-South system of faults, and the associated north-south trending drainages that are the locations for many of the Landslides in the region and the Landslides.

Since the formation of the Central America Range, the western portion has been continuously extending, forming terrain dominated by north-southwest running horsts and grabens. This is most likely the result of the distal effects of the left lateral strike-slip motion of the Motagua fault on the Caribbean plate (Rogers 2002). Many other large grabens have been formed in the sula graben in western Honduras. The headwaters of the Rio Lempa, location near the borders between El Salvador, Honduras and Guatemala is also within a graben. This graben runs through Honduras and terminates in El Salvador. Smaller faults can be observed running paralle to the larger graben throughout the region. These are believed to have formed under the same stress conditions.

The problems posed by earthquakes and Landslides are the continuous erosion of the road that wraps around the scarp of the landslip and other agricultural uses and also the lack of adequate repairs and strengthening of damage buildings an issue not addressed by code.

Characteristics of Storm-Induced Landslides

In comparison to the other three countries where pos-hurricane Landslide studies are being conducted, the impact of Landslides in El Salvador was modest and geographically limited. The abundant rainfall associated with the storm did cause dozens of Landslides in various parts of the country, but overall, the impact of these Landslides was far less serious than human losses and damage caused by flooding in El Salvador. In parts of Honduras, Nicaragua, and Guatemala, Landslides denuded almost entire slopes in some drainage basins (Harp et al, 2002; Bucknam et al, Cannon et al, 2001), in contrast, in El Salvador, the Landslides were widely scattered, and although they did cause serious problems locally, they did not create a major national disaster.

Virtually all of the Hurricane Mitch Landslides in El Salvador were shallow failures that involved the down slope movement of mainly unconsolidated surficial materials that are the product of deep tropical weathering. Most of the Landslides moved down slope as generally coherent masses and stopped at the base of a steep slope, but many of these masses became internally fragmented by the time they stopped moving. In some cases, the sediment in these Landslides contained enough water to become more fluid and evolve into debris flows, (Crone, et al).

Debris flow can be very damaging and deadly because they travel rapidly and can flow down the relatively low-gradient slopes of streams and valleys. In addition to Landslides caused by Hurricane, this includes both complex Landslides that have apparently been inactive for a long time and persistently active Landslide complex. Most of the inactive Landslides have distinct, but rounded head scarps hummocky topography and most appear to be earth slides and earth flows. Some involve large volumes of material and are deep-seated failures that include the unconsolidated surface soil and underlying bedrock. These pre-Mitch Landslides include both translational and rational Landslides. The most prominent active, pre-hurricane Landslide is the El Zomopera Landslide, near the Honduran border; a large retrogressive earth-slide debris-flow.

When Hurricane Mitch destroyed crucial health and transport infrastructure in El Salvador in 1998 , the health sector was set back by decades, (World Bank). The Climatic situation created disastrous impact where vulnerable populations faced:

  • Diseases
  • More Extreme Weather
  • Declining Agricultural Yields
  • Damage To Primary Industries and Fisheries
  • Natural Disasters
  • Water Shortage.
  • Mitigation Measures

    In the last decade, the relative lack of concerted efforts in hazards mitigation likely reflects the severity of more immediate demanding problems such as water quality, sanitation, various health issues, etc. The combined effect of Mitch and the 2001 earthquakes, however, has had an impact. In response to these, the Salvadorian government has taken the bold step in the past two years to establish a new government agency Servicio Nacional de Estudios Territoriales (SNET) in the Ministerio de Ambiente and Recursos Naturales(Lopez, et al). The new ministry aims to deal with the mitigation of natural hazards, and it will try to consolidate and strengthen infrastructure. Work on hazards characterization involves technical aspects such as monitoring of earthquakes, volcanoes, meteorology/hydrology and slopes. But education of the public about risk is badly needed. Geosciences education in Salvador is minimal; the only university degree program in the country is one in physics with an option in geophysics and training in earthquake-resistant structural design is also limited.

    Conclusion

    The people of E l Salvador lived in an extremely active geological setting and are exposed to multiple types of geological hazards. Destructive Landslides, floods, hurricanes, earthquakes and volcanic eruptions are natural events that cannot be prevented. They have occurred in the past, and similar events will certainly occur in the future. With adequate planning and advance preparation, the detrimental effects of these destructive events can be moderated.

    Considering the billions of dollars in losses that occurred in El Salvador since 1998, steps to reduce future losses are cost-effective investments. The development of Landslide susceptibility maps, flood and debris-flow inundation maps, and seismic and volcanic hazards maps will provide public officials with valuable tools to help guide future development and to help identify areas where existing development is exposed to natural hazards. Hurricanes Mitch and the two devastating earthquakes have created a new awareness in El Salvador of the need to prepare in advance rather respond after a disaster occurs. Scientific information such as the inventory contained in this report can be useful in planning and preparation can save the lives of Salvadorians and minimize the hardships that results from destructive natural events such as Hurricane Mitch and Strong earthquakes.

    Recommendations

    I recommend that the International Community, the World Bank, International Donors and the United Nations should provide scientific, technical and financial assistance to support the establishment of international centre for the study of regional and sub-regional institutions and networks devoted to addressing the problems caused by natural disasters mainly those associated with extreme weather events and earthquakes. I encourage international joint observation, research and the dissemination of scientific knowledge for effective disaster and risk reduction of sudden-impact disaster (e.g. storms, earthquakes, volcanic eruptions) and slow-onset disasters (e.g. sea level rise) and ensure wide dissemination of warning.