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The Guatemalan earthquake of February 4, 1976, with a surface-wave magnitude of 7.5, was generated by left-lateral slippage on the Motagua fault and was felt over an area of at least 100,000 km2. This earthquake claimed more than 22,700 lives and injured more than 76,000 people. The pre­ liminary estimate of losses is about $1,100,000,000. Ground breakage was observed and mapped for a distance of nearly 240 km along a segment of the fault. It is the longest surface rupture to have occurred in America since 1906. The inferred total length of faulting is nearly 300 km, postulated on the basis of small aftershocks and high damage concentration west of Guatemala City. Maximum horizontal slippage as measured on the fault, about 25 km north of Guatemala City, was 325 cm. The average horizontal displace­ ment is approximately 100 cm. Slippage also occurred on a number of secondary faults and caused damage to houses and other structures in the vicinity of Guatemala City. The February 4 earthquake caused extensive landsliding along the highway that leads from Guatemala City to the Pacific and Atlantic Oceans. Landslides also blocked many railroads and destroyed many communication routes in the highlands of Guatemala. A large landslide near Tecpan cov­ ered two small villages and dammed a river. Additional land­ slides were triggered by aftershock activity. Guatemala contains three major earthquake-generating zones. The first zone, which has the highest level of seismicity, is the Benioff zone that dips northeastward beneath Guate­ mala and is due to the Cocos plate being thrust beneath the Caribbean plate. The second lies at shallow depths along the chain of active volcanoes; the third is the fault system that bisects Guatemala from east to west. It is in this third zone that the February 4 earthquake occurred. The preliminary fault plane solution is consistent with al­ most pure sinistral slip on a nearly vertical fault striking about N. 65° E. This solution agrees with the geologically mapped fault trace. The seismic moment was determined to be 2.6xlOB7 dyne-cm from the amplitude of mantle Love waves (G-waves). A fault depth of 29 km was calculated from the seismic moment and the field observations of fault length and fault displacement. The earthquake was a low-stress-drop event, 3^Aff^18 bars. The energy release was l.lXlO23 ergs. Thousands of aftershocks followed the main earthquake. Epicenters for 78 small aftershocks determined from lo­ cally recorded data delineate the Motagua fault and several secondary faults. A number of epicenters in the eastern part of Guatemala follow the general trend of the inferred exten­ sion of the Motagua fault. A large number of events that occurred south of the Motagua fault and west of long 90.3°W. at depths of less than 14 km are associated with secondary faults, such as the north-south faults that bound the Guate­ mala City graben. The maximum Modified Mercalli intensity was IX in the Mixco area, in some sections of Guatemala City, and in Gualan. The Modified Mercalli intensity reached VI over an area of 33,000 km2. The concentration of high intensities ob­ served near the western end of the Motagua fault suggests the influence of a westward-propagating fault rupture. Communities and small towns that suffered 100 percent damage covered an area of 1,700 km2. At some localities, adobe structures near the causative fault (within 10 km) were essentially undamaged; at greater distances from the fault in the highlands, there was widespread collapse of adobe buildings. Modern earthquake-resistive structures in Guate­ mala City were damaged, including several hospitals. Several water tanks and corrugated-steel grain silos and numerous heavy parapets collapsed. The information compiled in this study will be useful in assessing and delineating earthquake hazards in Guatemala to reduce the potential for loss of life and property in future earthquakes. The information in this study is also important in evaluating the effects to be expected from large strikeslip earthquakes elsewhere, such as those that occur in coastal California. A more detailed 'study of the distribution of in­ tensities in Guatemala City could provide the basis for de­ lineating variations in the hazards due to ground shaking and ground failure throughout the city and could assist in develop­ ing land-use policies. INTRODUCTION The Republic of Guatemala in Central America suffered extensive loss of life and severe damage to its economy from the February 4, 1976, earthquake. A request by the Guatemalan government to the Organization of American States prompted the U.S. Geological Survey to send a team of scientists and engineers to Guatemala to investigate the cause of the earthquake, the geologic, seismologic, and en­ gineering effects, and the hazards resulting from the earthquake. U.S. Geological Survey Open-File Report 76-295, which was released in March 1976, provided a brief interim summary of the activities GUATEMALAN EARTHQUAKE OF FEBRUARY 4, 1976, A PRELIMINARY REPORT r i V ,^'Cuilapa ,-jUT| ApA 92 00 W 9100' 90° 00' 89° 00' 88° 00' FIGURE 1.—Guatemala and its Departments (States), including its general location in Central America. Base map modified from Guatemala, Institute Geografico Nacional, 1974, 1: 500,000. of the earthquake investigation team. The open-file report and this publication represent the immediate response by our agency to the request of the Guate­ malan government. Guatemala has a long history of damaging earth­ quakes. Historic chronicles date earthquake occur­ rences in Guatemala from the period of the Spanish conquest. The chronicles indicate that the cities of Antiqua and Guatemala City have been badly dam­ aged by earthquakes more than 15 times since the early 16th century. The most damaging earthquakes to have occurred before the February 4 event were on December 25, 1917, and on January 3, 1918. These earthquakes and their aftershocks claimed numerous lives and partially destroyed Guatemala City. The February 4 earthquake caused extensive damage, as the following statistics from a few of the cities and Departments (States) show: 88,404 houses were destroyed and 434,934 people left home­ less in the Department of Guatemala, about onehalf that number in the Department of Chimaltenango (fig. 1), and all houses were destroyed in the towns of San Pedro Sacatepequez, El Jicaro, Sumpango, Tecpan, and Gualan. From the above, it is obvious that the principal hazard to life from earthquakes in the Republic of Guatemala is the collapse of manmade structures because of ground shaking. There are, in addition, other .serious earthquake hazards, especially land­ slides in the highlands and surface breakage along active faults. Landslides can interrupt lifelines, in particular, roads, railroads, and communication net­ works, and they can dam streams and cause lakes that are subject to sudden release upon failure of the dam. There is much to learn from this earthquake that is directly relevant to the problem of reducing earth­ quake hazards in the United States. The fault that produced the earthquake is the boundary between two crustal plates and is tectonically similar to the San Andreas fault in California and to the Fairweather fault in southeastern Alaska. Many of the modern buildings in Guatemala City are similar to structures in the United States and in particular to those in cities in coastal California. The different papers of this report present (1) the tectonic setting and the seismicity of the region; (2) the local seismic activity before the February 4 earthquake; (3) the main event and principal aftershocks as recorded at teleseismic distances; (4) the source parameters of the main event and possible future earthquakes on the fault that slipped on February 4; (5) estimation of strong ground INTRODUCTION motion due to the main event; (6) aftershocks from local data and their relation to primary and second­ ary faulting; (7) the geologic effects, including pri­ mary and secondary faults, landslides, and relation of faulting to damage; (8) the intensity of shaking and its distribution, casualties and damage, groundmotion parameters at intermediate distances, and source parameters from field observations; (9) damage and engineering implications and earth­ quake-resistant design code; and (10) design, con­ struction practice, and general observations on dam­ age to high-rise structures. The results from this collective investigation will aid in the reduction of earthquake hazards in Guate­ mala and in the United States, but the material presented in this report should be considered as preliminary. Some of the details may change, and the conclusions may be modified upon further scrutiny of existing data and upon analysis of new data gathered in later geologic field studies. ACKNOWLEDGMENTS We gratefully acknowledge: Colonel J. Guillermo Echeverria Vielman, Coordinator of the Comite Nacional de Emergencia; Mr. Jose Asturias, Scientific Coordinator of the Comite Nacional de Emergencia; Dr. Eduardo Gonzalez Reyez, Executive Secretary for Education, Science, and Culture of the Organi­ zation of American States; Dr. Gabriel Dengo, Di­ rector of the Institute Centroamericano de Investigacion y Tecnologia Industrial; and Ing. F. Hernandez Cruz, Director General, Institute Geografico Nacional, for their full support in our investigation. Mr. M. Freundel, Assistant Director USAID/ ROCAP, provided ground transportation and an ex­ cellent chauffeur. We wish to acknowledge the sup­ port given to us, in the questionnaire canvassing of Guatemala City, by Ing. M. A. Castillo Barajas, Director, and Ing. H. E. Molina, Assistant Director, of the Civil Engineering Department of the Universidad de San Carlos. Also, we are grateful to the university students who participated in this effort and to Dr. S. B. Bonis of the Institute Geografico Nacional, who also obtained a great number of intensity questionnaires for ou

The Guatemalan earthquake of February 4, 1976, a preliminary report