STUDIES RELATED TO SEISMICITY AND SEISMIC HAZARD IN GUATEMALA: SEISMIC SOURCES, PAST EVENTS AND MONITORING.

 

CAUSES OF THE SEISMICITY: THE TECTONIC ENVIRONMENT.

 

Guatemala is a seismically active area (Feldman, 1993). The sources of seismicity can be divided in two main environments (Molina et al 1996 (in Spanish)): the tectonic plate boundaries and the smaller scale regional and local faults (e. g. the faults in the volcanic highlands). Knowing the location and degree of activity of these faults becomes a fundamental element for understanding the seismicity and its associated hazard.

 

The tectonic plate boundaries consist of the Polochic-Motagua-Jocotan system, a right lateral strike slip fault system on the northern part of the country, and which represents the boundary between the North American and the Caribbean plates; and the subduction zone, south offshore in the Pacific Ocean, where the Cocos plate subducts under the Caribbean plate (Mann et al in prep., Anderson et al 1983).

 

The surface trace of the Plochic-Motagua_Jocotan fault system is better defined in the eastern part of the country and it becomes less clear towards the west (Burkart, 1978, Erdlac et al 1982, Dengo and Dengo, 1996 (in Spanish)), where the system vanishes under the Pleistocene volcanic cover, or diverts into different branches, however some of the major earthquakes that happened during the 19^th century seem to be related to these faults. The relative rates of strain accumulation and therefore the potential of each of these individual faults to generate earthquakes are also poorly known (Lyon_Caen et al in prep.).

 

The subduction zone is part of the Central American subduction plate boundary (Carr 1976) and is divided in different segments. Historically it has been the most active source of seismicity for Guatemala (Molina et al 1996, Benito et al N. D. (in Spanish)). The direction of the subduction is not perpendicular to the trench, resulting in a component that is parallel to it (DeMets, 2001). This, coupled with the stress transfer from the Polochi-Motagua-Jocotan system, has been suggested to be responsible for the grabens and faults in the Caribbean Plate wedge (Burkart, 1985; Guzman_Speciale_2001), and it could be related to some of the destructive earthquakes that take place in this second order fault system, as suggested for El Salvador (Martinez-Diaz, 2004).

 

The local faults in the volcanic highlands, although only representing a second order feature in tectonic terms, may represent a similar hazard than the Polochi-Motagua-Jocotan system or the subduction zone, given its shallow nature and its proximity to populated areas (Molina et al 1996 (in Spanish)). Guatemala City is an especially acute case, being situated within a tectonically active graben and having suffered the consecuences of major earthquakes in 1917 and 1918. The system of faults related to the graben in poorly known, but is acknowledged to be capable to generate destructive earthquakes (Bohnenberger, 1996 (in Spanish)).

 

 

 

PAST EVENTS:

 

Feldman (1993) compiled an extensive list of major (destructive) earthquakes encompassing the 15^th through the 19^th centuries. White et al (2004) analyzed the earthquakes related to subduction, over the past 5 centuries. Less research has been done on the earthquakes generated from the Polochic-Motagu-Jocotan system and the local faults, although Monzon (1996a, 1996b (in Spanish)) gives a summary of these events.

 

There have been around 70 important earthquakes during the XX century in Central America, 40 of them in local shallow faults on the volcanic arc, 20 in the subduction zone and 6 in the Chixoy – Motagua – Jocotan fault systems. For the entire Central American region, during the XX century the death toll due to earthquakes was of 17 000 due to local shallow fault earthquakes, 2 000 due to subduction zone earthquakes and 23 000 (the 1976 event) due to earthquakes generated by the Motagua – Polochic – Chixoy (Mozon, 1996; Molina et al 1996 (in Spanish)). With an average of 3 to 4 highly destructive earthquakes per century for Guatemala, the seismic risk in not a “very long term” problem. Beside that a bigger number of smaller and less damaging earthquakes, which usually affect smaller urban centers of areas of rural dispersed population areas also contribute to the seismic risk. The total number of earthquakes that cause some damage in Guatemala could be around 5 to 10 per century.

 

Historic seismicity has been attributed to all the above described sources and processes. Although the bigger events where form both subduction (e. g. 1902 and 1942) and transform fault (e. g. 1856, 1976) sourced, the smaller events form local fault systems have also been important because of the damage they’ve caused.  Some relatively small earthquakes, produced in local fault systems have produce very high intensities, like the 1917 and 1918 earthquakes, which where estimated to have a magnitude of 5.8 and produced intensities on the MMI scale of VIII. This earthquakes caused extensive damage in Guatemala City.

 

Some specific events have been studied in more detail, due to the availability of information and the impact they had. Peraldo and Montero (1996a and 1996b) studied the 1717 earthquake, based on historical documents. The 1976 earthquake has been studied in considerably detail, being the largest disaster related to natural hazards in Guatemala in recent times, and killing ~ 23 000 people. Bohnenberger and Barrillas-Cruz (1996 ) (in Spanish) summarize some of the papers that were published in relation to the earthquake.

 

SEISMIC MONITORING:

 

Monitoring of the seismic activity over long enough periods of time is crucial to characterize the seismogenic sources, to be able to estimate the potential that these sources have to generate earthquakes of different magnitudes. Besides the earthquake catalogs based on eye witnessed reports, covering back to 500 years, instrumental records catalogs usually go back to the 70’s.

Nowadays, different institutions and seismic networks routinely collect data on the seismicity in and near Guatemala. The Instituto Nacional de Sismología, Vulcanología, Meteorología e Hidrologia (INSIVUMEH (in Spanish)) manages the national seismic network, and keeps an online monthly database of earthquakes, as well as a near real time update of the last felt event.

 

Other regional networks that post earthquake information online in near real time include the Mexican Servicio Sismológico Nacional, the Salvadoran Servicio Nacional de Estudios Territoriales (SNET (in Spanish)), and the Nicaraguan Instituto Nacional de Estudios Territoriales (INETER (in Spanish)). World wide networks include the USGS Earthquake hazard program and the GEOFON Extended virtual network, from the GFZ Centrum in Potzdam, Germany.