Historic Earthquakes

1976 February 04 09:01 UTC
Magnitude 7.5


The most destructive earthquake since 1917 struck Guatemala on February 4. The magnitude 7.5 quake was centered about 160 km northeast of Guatemala City. The death toll has reached more than 23,000, and thousands have been injured. Damage was extensive. Most adobe type structures in the outlying areas of Guatemala City were completely destroyed, leaving thousands homeless. Transportation was impeded by the many landslides occurring in the area. Food and water supplies were severely reduced. Some of the areas were without electricity and communication for days. The main shock has been followed by thousands of aftershocks, some of the larger ones causing additional loss of life and damage.

Abridged from Earthquake Information Bulletin, July-August 1976, Volume 8, Number 4.

Guatemalan Quake Culprit Fault Identified

First reports from the U.S. Geological Survey (USGS) earthquake investigation team in Guatemala identified the Motagua fault as the main cause of the major (magnitude 7.5) earthquake of February 4, 1976.

The fault runs roughly east-west from a point about 15 miles (25 kilometers) north of Guatemala City eastward probably as far as Puerto Barrios near the Gulf of Honduras.

During an air reconnaissance of the Motagua fault zone, the USGS scientists observed fault breakage along a more than 100-mile (160-kilometer) stretch of the fault.

1976 Guatemala earthquake
Air view showing typical appearance of the Motagua fault rupture that caused the destructive February 4, 1976, earthquake.
The epicenter of the main shock - the point on the fault where the rupture began - has been pinpointed by the Survey's National Earthquake Information Service at latitude 15.27 North and longitude 89.25 West, about 12 miles (20 kilometers) west of Los Amates, south of Lake Izabal.

Dr. George Plafker, of the Survey's Menlo Park, California, field center, was one of several USGS scientists dispatched to Guatemala. He, together with Guatemalan geologists, identified the Motagua fault as the primary source of the energy that caused widespread destruction.

Plafker said, however, that the shorter Mixco fault, which is at right angles to the Motagua, also caused damage. The Mixco fault, which lies only 6 miles (10 kilometers) west of Guatemala City, was broken along at least 10 miles (16 kilometers) of its length. Although it is shorter than the Motagua fault, its closeness to Guatemala City also makes it dangerous. Scientists believe that the length of fault breakage is related to the energy released in an earthquake.

In terms of energy, the Guatemala earthquake released about 90 times the amount of energy released in the Managua, Nicaragua, earthquake of 1972, but only about 1/16th the energy of the San Francisco earthquake of 1906.

Plafker said that it was difficult to determine the extent of rupture along the Motagua fault. "It may extend farther east - perhaps to Puerto Barrios," he said, "but it passes into a swampy area where we could not see it. Nor could we see the fault rupture farther west, as landslides in the steep mountains obscured it. Because roads and railroads follow the fault valley, they have been blocked and are now unusable.

"The west end of the Motagua fault as well as the Mixco fault are volcanic terrain, dominated by intermittently active volcanic cones. There were no verified reports, however, of eruptions of any volcano," Plafker said.

Like California, Guatemala is known as "earthquake country." Both the Motagua and Mixco faults were already recognized as active. The Motagua fault separates the great North American and Caribbean tectonic plates of the Earth's crust. These plates and the few others that form the lands and seas are, according to geologic theory, "floating" on an underlying layer of plastic rock near the top of the mantle. When plates slide past each other as these two did in Guatemala, and as two plates do in California along the San Andreas, faults break and the Earth shakes.

Dr. Plafker was part of the initial USGS team sent to Guatemala immediately after the February 4 earthquake occurred. Other team members included Charles Knudson, also of the Survey's Menlo Park field center, and Alvaro Espinosa and Raul Husid of the Survey's Denver, Colorado, field center. Accompanying the team was Karl Steinbrugge, an earthquake engineer traveling on behalf of the USGS. He is a professor at the University of California at Berkeley.

A second USGS team dispatched to the Guatemalan region consisted of Manuel G. Bonilla, Menlo Park, and Charley Langer and Jean Whitcomb, Denver. These scientists, with Arturo Aburto, Director of the Managua, Nicaragua, Seismic Institute, aided the first U.S. team.

Aburto, Langer, and Whitcomb have been monitoring aftershocks with portable seismographs in an effort to pinpoint aftershock locations in relation to geologic structures and damaged towns and villages. Bonilla is a specialist in the geologic effects of earthquakes.

The U.S. scientists are cooperating with Instituto Central Americano de Investigacion y Tecnologia Industrial (ICAITI), a Central American research organization headed by Dr. Gabriel Dengo, and the Instituto Geográfico Naciónal represented by Drs. Oscar Salazar and Samuel Bonis. Both organizations are headquartered in Guatemala City.

Part of the Guatemala research is being funded by the Organization of American States.

Abridged from Earthquake Information Bulletin, May-June 1976, Volume 8, Number 3.