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GEORGE PLAFKER J. C. SAVAGE U.S. Geological Survey, Menlo Par%, California 94025

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GEORGE PLAFKER J. C. SAVAGE U.S. Geological Survey, Menlo Par%, California 94025 GEORGE PLAFKER J. C. SAVAGE U.S. Geological Survey, Menlo Par%, California 94025 Mechanism of the Chilean Earthquakes of May 21 and 22, 1960 ABSTRACT The Chilean earthquake sequence of May 21-22, 17s GMT. During the latter event, sudden uplift 1960, was accompanied by linear zones of tectonic of adjacent portions of the continental shelf and warping, including both uplift and subsidence much or all of the continental slope apparently relative to sea level. The region involved is more generated the destructive tsunami that immediately than 200 km wide and about 1000 km long, and followed the main shock. lies along the continental margin between latitude Available data suggest that the primary fault or 37° and 48° S. Significant horizontal strains ac- zone of faulting along which displacement occurred companied the vertical movements in parts of the probably is a complex thrust fault roughly 1000 subsided zone for which triangulation data are km long and at least 60 km wide; it dips eastward at available. Displacements were initiated near the a moderate angle beneath the continental margin northern end of the deformed region during the and intersects the surface on the continental slope. opening earthquake of the sequence (Ms = 7.5) Dip slip required to satisfy the surface displace- on May 21 at 1 Oh 02m 50s GMT and were extended ments is at least 20 m and perhaps as large as 40 m. over the remainder of the region during the cul- There is some evidence that there was a minor minating shock (Ms = 8.5) on May 22 at 19h llm component of right-lateral slip on the fault plane. INTRODUCTION an understanding which is essential for the intelligent development of programs in earth- The sequence of major earthquakes that quake prediction, modification, and control in devastated much of central Chile on May 21 Chile and in other tectonically comparable and 22, 1960, was among the most notable regions. For geologists and geophysicists, it seismic events of this century. Within Chile, provided a unique opportunity to obtain data seismic shaking and destructive sea waves took on the present style of deformation along a more than 2000 lives and caused an estimated little-known segment of the seismically active $550 million in property damage. The disaster continental margin of South America and to area extended more than 800 km in a north- test these data against current hypotheses of south direction. Tectonic movements, involv- sea-floor spreading in the South Pacific Ocean. ing both uplift and subsidence relative to sea The purpose of this paper is to present new level, occurred over an even greater area of field data on the regional tectonic displace- southern Chile. The seismic sea waves, which ments that accompanied the Chilean earth- presumably were generated by crustal deforma- quakes, to review the pertinent seismologic tion within the epicentral region, spread across data, and to analyze the implications of these the Pacific Ocean, carrying disaster to distant data in relation to the earthquake mechanism. shores, most notably to Japan, Hawaii, and the Despite the intensive scientific and engineering Philippine Islands. Together, these areas studies made by investigators from several suffered an estimated $125 million in property countries shortly after the disaster, surprisingly damage and an additional 230 people were little has been published concerning the genera- killed (Saint-Amand, 1961; Talley and Cloud, tive mechanism of the earthquakes. There 1962, p. 36). were no surface fault displacements to provide This great seismic event holds special interest direct geologic evidence as to the orientation for scientists concerned with earthquakes and and sense of slip on the causative fault or faults, tsunamis because it has helped to provide an and the seismologic data were generally inade- understanding of the earthquake mechanism— quate to permit either reliable focal mechanism Geological Society of America Bulletin, v. 81, p. 1001-1030, 14 figs., April 1970 1001 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/4/1001/3442828/i0016-7606-81-4-1001.pdf by guest on 26 September 2021 1002 PLAFKER AND SAVAGE-CHILEAN EARTHQUAKES, MAY, 1960 solutions or precise delineation of the focal deep from latitude 4° N. to 40° S., and as a region; hence the lack of theories concerning well-defined negative gravity belt from 40° S. the generative mechanism. to at least 56° S. (Hayes, 'l966); (2) a sub- This study was initiated by the senior parallel discontinuous chain of active and dor- author after certain striking similarities were mant volcanoes that roughly follows the axis noted between the pattern of earthquake- of the Andean Cordillera (Gutenberg and related vertical displacements in Chile, as given Richter, 1954; Casertano, 1963); (3) a zone of in published accounts, and those that ac- active seismicity, the lower limit of hypo- companied the 1964 Alaska earthquake (Plaf- centers generally deepening from beneath the ker, 1965, 1969). Field work was carried out trench to beneath the volcanic chain and con- in January and February 1968, during which tinent (Benioff, 1954; Gutenberg and Richter, period vertical movements of the land relative 1954); and (4) progressive thickening of the to sea level were determined from the displace- crust away from the ocean basin from about 11 ment of shoreline features at some 155 locali- km beneath the trench axis to between 55 and ties. Invaluable control on the amount and 70 km beneath the Andes (Fisher and Raitt, distribution of horizontal strain and vertical 1962;Lomnitz, 1962). tectonic movements inland from the coast was In marked contrast to the intense seismic provided by geodetic surveys oi the Institute activity along the western margin of South Geografico Militar of Chile. Dislocation model America, the adjacent continent and ocean studies of the vertical displacement data and basin are virtually aseismic, except for narrow horizontal strain, as well as analyses of the belts of shallow-focus earthquakes mainly along teleseismic evidence for the earthquake mecha- the crests of the East Pacific rise, the Galapagos nism were subsequently made by Savage. rift zone, and the Chile rise or West Chile ridge. Magnetic anomaly patterns across portions of SEISMICITY the East Pacific rise and the adjacent ocean floor suggest that during the last 10 m.y., the Regional Setting floor of the South Pacific Ocean has spread The May 1960 sequence of earthquakes in relatively eastward away from the rise. The Chile occurred at the southern end of a well- rates of spreading vary but average 4.7 to 6.0 defined belt of intensive seismic activity that cm/year in the segment between 48° and 27° S. follows the western margin of South America latitude, as indicated on Figure 1 (Pitman and for about 7000 km from Venezuela on the north others, 1968, p. 2082; Heirtzler and others, to the Chile rise on the south (Fig. 1). Seis- 1968, p. 2131). At the same time, the con- mologic data compiled by Gutenberg and tinental plate is believed to be spreading west- Richter (1954) for the period 1906-1944 show ward relative to the mid-Atlantic ridge in these that numerous large shallow and intermediate- same latitudes at roughly 2.0 cm/year (Heirtz- depth earthquakes occurred in the coastal ler and others, 1968, p. 2131). The spatial region to about 37° S. latitude in Chile, and distribution and focal mechanisms of earth- sporadic predominantly shallow earthquakes quakes that define the margins of this plate south of that latitude. Between the equator (Isacks and others, 1968) suggest that the and 30° S. latitude, infrequent shocks, with spreading is still continuing. Morgan (1968) epicenters located east of the Andes Mountains, and Isacks and others (1968) interpret the have occurred at depths as great as 650 km. tectonically active belt along the western Significantly, the area of the 1960 Chilean margin of South America as a sink zone along earthquake sequence was one of relative low which the spreading oceanic plate shears at an seismic activity during the period covered by oblique angle downward beneath the con- Gutenberg and Richter's study. However, it is tinental plate. The paleomagnetic data suggest an area in which several destructive earth- that the convergence along the interface may quakes have been recorded during post- be as high as 7 cm/year. The Chile rise ap- Columbian times. parently defines the southern margin of this Although it is an ocean-continent transition, moving plate. Magnetic anomaly patterns over this tectonically active segment of the Pacific the axial portion of the Chile rise suggest that rim has all the primary features of structural it has probably been a site of combined sea- arcs in ocean-island arc transition zones. These floor spreading and strike-slip faulting during features are (1) an oceanic trench (the Peru- the last 10 m.y. (Herron and Hayes, 1968, p. Chile trench), which extends as a topographic 134). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/4/1001/3442828/i0016-7606-81-4-1001.pdf by guest on 26 September 2021 o° - ^X.f ^ SgUTH .\ AMERICA **< 10° - 20' - PACIFIC OCEAN Major earthquakes (1906-1944) showing focal depth in kilometers 50° - Inferred direction and average spreading rate (cm/yr.) from of 1960 earthquake East Pacific Rise sequence —3 Submarine Contour (Thousands of fathoms) i i 100 Figure 1. Location of the 1960 Chile earthquake focal region relative to major tectonic features of South America and the southeast Pacific Ocean basin. Andean volcanic chain and epicenters of major earthquakes for period 1906-1944 after Gutenberg and Richter (1954); submarine topography from National Geographic Society Physical Map of the World (1967); spreading directions and rates on the East Pacific Rise from Heirtzler and others (1968).
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