Tectonic Frame
Western Nicaragua, as well as the rest of the occident of Central America, is inside the so-called Circum-Pacific Fire Belt,
characterized by intense seismic and volcanic activity.
The Caribbean plate (a continental plate, upon which almost the entire Central America rests) is moving toward the east. On its western margin, it is being pushed by the Cocos plate (an oceanic plate), which moves toward the northeast at higher speed.
The result is the subduction of the Cocos plate under the Caribbean plate, and some of the consequences are the Middle-American Trench where the two plates get in contact, a graben in the continent and, inside this, the Volcanic Chain.
The Middle-American Trench is from 4 to 5 km deep [Shah et al., 1975] and extends, approximately 130 km off the Central American Pacific coast [Montero, 1990], from Mexico to Costa Rica.
On the other hand, the graben is a lengthened tectonic depression, parallel to the Pacific coast, and it extends to the south
until northern Panama, and to the north it continues up to central Guatemala
[Cluff & Carver, 1973].
The section of this depression across the Nicaraguan territory constitutes an asymmetric graben and it is known as Nicaraguan Depression. It is located some 200 km from the Middle-American Trench [Arce, 1973] and it crosses the occident of the country with direction NW, from the Gulf of Fonseca to the north, to near El Limón (Costa Rica) to the south. The two big sweet-water lakes, Lake Xolotlán and Lake Cocibolca, are included within the Depression, as most of the volcanic centres in Nicaragua.
The Central American Volcanic Chain is aligned parallel to the Middle-American Trench, and it extends from Mexico to Costa Rica. Parts of it are the volcanoes of the Marrabios mountain range, aligned along a NW-SE segmented line of some 300 kilometres long.
Managua is located in the southern shore of Lake Xolotlán, where the Volcanic Chain suffers a flexion toward the south.
Inside that area a structural depression is located: the Managua Graben. This is a micrograben inside the Nicaraguan Depression. It is approximately 15 km wide to the south and 26 km to the north, and has a longitude of at least 36 km [Woodward-Clyde, 1975].
The segmentation of the Volcanic Chain at this point is due to the change in the orientation of Cocos plate. The Managua Graben is considered to be a boundary area between two plate segments. The stresses caused by the plate interaction and shear stress in the boundary area of the plate segments, and also the influence that exerts the migration of the magma in the magmatic chambers inside the graben, produce the complex faulting within the Managua Graben.
The faults inside the Managua Graben have been active during millennia, and they produce seismic movements cyclically, at focal depths of 5 to 12 km [SOP, 1973]. Although these events are of small to moderate intensity, they are able to produce extensive damages, as during the earthquakes of 1931, of 1968 and of 1972.
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