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
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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).
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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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The Earthquake Sequence corroborated by the large variations in the amplitudes of the waterborne T-phases re- Figure 2 shows the broad distribution of corded in Hawaii (Eaton and others, 1961, p. epicenters, as located by the U.S. Coast and 137). Geodetic Survey (Talley and Cloud, 1962), and During 1960, the main shock was followed the regional vertical displacements associated by an aftershock train that included some 56 with the 1960 Chilean sequence. Most of the major earthquakes with magnitudes of 5.8 or epicenters and all the known deformation occur more (Duda, 1963, Table 1). The aftershock in an elongate belt more than 200 km wide that distribution and the phase shifts of radiated lies between, and parallel to, the Andean surface waves (as analyzed by Press and others, volcanic chain and the axis of the Peru-Chile 1961, and Benioff and others, 1961), suggest trench. This belt extends from the Arauco that the rupture extended southward from the Peninsula (37 y>° S.) roughly 1000 km south- epicenter of the main shock at a velocity of 3 to ward to the vicinity of the Taitao Peninsula 4 km/sec for a distance of between 960 and (47° S.). Unless otherwise specified, seismic 1280 km. The remarkably long duration (about data referred to in the following section are 7 minutes) of the T-phase of the main shock from Talley and Cloud (1962). reported in Hawaii by Eaton and others (1961, The secjuence began on May 21, 1960, at p. 136-137) is also consistent with the inferred lOh 02m 50s GMT, with a moderately severe fault length and rupture velocity. shock (Richter magnitude 7.5) on the Arauco Hypocentral depths have not been deter- Peninsula near the northern end of the affected mined for the initial and main shocks or for region. It was followed by a series of smaller most of the smaller shocks of this sequence. Of shocks, or aftershocks, in the same general area, those aftershocks for which hypocenters could at least eight of which were recorded at tele- be determined, all were shallower than 64 km seismic distances. except three, which were at depths of 92, 107, The culminating shock (magnitude about and 150 km. Because seismologic control for 8.5) occurred on May 22, 1960, at 19h llm 17s Chilean earthquakes is poor, large errors may GMT, roughly 33 hours after the initial shock. exist in all the epicentral locations and especial- Its epicenter was located by the Coast and ly in the focal depths indicated for the after- Geodetic Survey on the continental shelf shocks. According to Lomnitz (1969, written approximately 80 km offshore from the main- commun.), experience in Chile has shown that land coast and 140 km southwest of the the distribution of recording stations tends to epicenter of the initial shock. Precise location push epicenters toward the cast. If so, location of the epicenter was not possible, however, errors for the 1960 sequence were probably because first arrivals were masked by the large biased in the direction of putting the epicenters foreshocks that immediately preceded the main too far shoreward. Assuming that the zone in shock. From a study of the temporal and spatial which faulting occurred is approximately variations of the strain release, Duda (1963, p. delineated by the aftershock distribution 5537) concluded that the main earthquake (Benioff, 1951), the available data suggest the was actually a complex event consisting of two faulting was mainly confined to the crust and 8.3 magnitude shocks 40 seconds apart at 19h upper mantle along the continental margin, 10m 37s (epicenter 38 Y>° S., 74 y>° W.) and although there is a remote possibility that 19h llm 17s (epicenter 39° S., 74 1A° w-) portions of the adjacent deep ocean floor may which were preceded at 18h 55m 57s by a also have been included. smaller shock (M = 7.8; Duda) with epicenter s VERTICAL TECTONIC at 38° S., 73 }/z° \\. Lomnitz and Hax (1966) showed that there were 6 major foreshocks DISPLACEMENTS which came progressively closer to the main Vertical tectonic displacements associated epicenter and that the time interval between with the 1960 earthquake sequence affected foreshocks became progressively shorter prior an area of at least 130,000 sq km of southern to the mam shock. Based on a study of seis- Chile (Fig. 2). Figure 3 shows the 155 data mograms of the sequence, C. Lomnitz (1969, points along the coast, as well as lines of leveling written commun.) concluded that Duda's inland, where vertical displacements relative estimate of 8.3 for the magnitude of the shock to sea level have been determined. Also shown of 19:10:37 is high. The multiple nature of the on Figure 3 are highly generalized isobase fault rupture responsible for the earthquake is contours, or lines of equal vertical displacement
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Epicenter of May 2Jst initial shock and aftershocks (TaHey and Cloud, 1962)
Epicenter of May 22nd main shock and aftershocks (Taltey and Cloud, I%2)
Zone of uplift Dashed where inferred
Zone of subsidence Dished where interred 0 ----- ...... 0 Approximate zero isobase Dotted where interred
Peru-Chile Trench axis Dashed where sediment filled (Hayes, 1966)
Figure 2. Spatial distribution of epicenters and zones of land-level change associated with the 1960 Chile earthquake sequence relative to the Peru-Chile Trench axis and Andean volcanic chain.
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which are based upon these data. For data as determined from pre- and post-earthquake points in coastal areas, any vertical change, the levelings. In addition, the sense and relative method of determination, estimated accuracy, magnitude of vertical displacement in offshore and other pertinent information are given in areas could be inferred to some extent from the Table 1. The deviations between pre- and post- nature of the seismic sea waves (tsunami) quake levelings along a segment of the Inter- generated by these movements. american Highway are shown diagrammatically in Figure 4. Previous Data Previous published information on vertical Data Sources displacements in the earthquake-affected region Information on vertical displacements along include estimates and measurements obtained the coast was obtained in 1968 from (1) shortly after the earthquake at about 20 local- measurement of the differences in height be- ities. Most of these were taken at centers of tween the pre- and post-earthquake positions population along the mainland coast and on of the lower growth limit of terrestrial vegeta- Isla Chiloe (Alvarez, 1963; Andrade, 1960; tion, (2) measurements of the changes in Galli and Sanchez, 1963a, 1963b; Saint-Amand, position of the extreme pre- and post-earth- 1961; Sievers and others, 1963; Thomas and quake high-tide lines as noted by local residents, others, 1963; Watanabe and Karzulovic, 1960; (3) estimates of changes in the position of shore- Weischet, 1963; and Wright and Mella, 1963). line markers relative to tide levels as reported The majority of these observations on shore- by local residents, and (4) measurement of the line displacements were incidental to studies of difference between the extreme upper growth damage caused by the earthquake and the limits of pre- and post-earthquake mussels related seismic sea waves. In most cases, avail- (Mytilus). At a number of localities, more than able data are insufficient to evaluate the ac- one method was used to determine the change, curacy of the reported changes, and for a few or multiple measurements were made by a localities, there are inconsistencies in the sense single method. Wherever possible, data were and displacement reported by different authors. obtained at or near bedrock sites where the For these reasons, and because all the localities effects of subsidence due to surficial compaction were revisited during 1968, the earlier data and slumping of unconsolidated deposits were were not incorporated in Figure 3. With a few minimized. Measurements made in areas of minor exceptions, however, they are reason- unconsolidated deposits, where surficial sub- ably consistent with the findings of this study. sidence may have occurred, are so noted in Table 1. These data were supplemented inland Shoreline Changes from the coast with vertical displacements of Most of the determinations of land-level bench marks along the Interamerican Highway change along the coast (and all of them in
200 300 400 500 600 NORTH DISTANCE (KM) SOUTH Figure 4. Deviation in meters between pre-earthquake (1957-1959) and post-earthquake (1963-1964) levelings along the line Los Angeles to Puerto Montt (location shown on Fig. 3). Data courtesy of Instituto Geografico Militar (Chile) and Interamerican Geodetic Survey, U.S. Army Corps of Engineers.
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LOCATION* VERTICAL CHANGE* LOCALITY Latitude Longitude DATA ESTIMATED NUMBER* (south) (west) Feet Meters SOURCE § ACCURACY" 52 41°58.0' 73°32.2' -5.3 -1.6 VV fair 53 42°08.5' 73°28.2' -5.0 -1.5 TT good 54 42°10.2' 73°24.5' -4.6 -1.4 VV fair 55 42°16.5' 73°21.0' -4.3* -1.3* VV fair 56 42°18.7' 73°16.0' -3.0 -0.9 VV fair 57 42°20.5' 73°08.0' -0.8 -0.2 TT poor 58 42°19.5' 72°48.2' +2.8 +0.8 VV good 59 42°24.5' 72°43.5' +2.9 +0.9 VV fair 60 42°29.8' 72°39.0' + 1.7 +0.5 VV good 61 42°31.2' 72°49.0' +3.0 + 0.9 VV good 62 42°39.5' 72°50.2' +3.3 + 1.0 VV good 63 42°38.0' 73°04.5' + 2.0 +0.6 VV good 64 42°37.0' 73°13.7' 0.0 0.0 VV fair 65 42°36.5' 73°17.2' -2.8 -0.8 VV poor 66 42°36.2' 73°20.0' -2.5 -0.8 VV fair 67 42°33.0' 73°26.2' -2.9 -0.9 VV good 68 42°37.0' 73°30.2' -3.7 -1.1 VV fair 69 42°35.0' 73°37.5' -4.5 -1.4 VV poor 70 42°36.0' 73°41.2' -5.0* -1.6* VV fair 71 42°37.0' 73°44.0' -4.8 -1.5 VV good 72 42°37.0' 73°49.7' -5.2 -1.6 VV fair 73 42°31.2' 73°48.0' -5.0 -1.6 VV fair 74* 42°37.7' 74°07.0' -3.3 -1.0 L f 75 42°53.1' 73°29.3' -3.1 -0.9 VV fair 76 42°56.3' 73°37.8' -4.5 -1.4 TT good 77* 42°55.0' 72°43.5' +3.3 + 1.0 L ? 78 43°08.0' 73°31.5' -3.0 -0.9 VV good 79 43°07.0' 73°34.5' -3.8* -1.2* VV poor 80 43°07.5' 73°37.7' -4.0* -1.2* VV fair 81 43°08.0' 73°38.0' -3.9 -1.2 VV good 82 43°09.0' 73°41.7' -4.5 -1.4 VV fair 83 43°08.7' 73°46.0' -4.8 -1.5 VV good 84 43°15.0' 73°42.7' -5.5 -1.7 VV good 85 43°20.2' 73°42.0' -4.8 -1.5 VV fair 86 43°21.0' 73°47.2' -4.8 -1.5 VV good 87 43°21.0' 74°03.0' -6.9 -2.1 VV good 88* 43°34.0' 74°50.0' + 12.0 +3.6 TT ? +8.5 +2.8 TT ? 89 43°49.0' 74°01.5' -5.6 _ 7 VV good 90 43°53.5' 74°00.5' -4.3 - .4 VV good 91 43°53.0' 73°52.5' -3.4 - .0 VV fair 92 43°56.5' 73°48.5' -3.1 - .9 VV fair 93 43°58.2' 73°48.5' -3.6 - .1 VV good 94 43°54.0' 73°45.7' -4.4 - .3 TT fair 95 43°54.0' 73°45.0' -3.5 -1.1 TT fair 96 43°59.2' 73°44.7' -3.3 -1.0 VV good 97 44°03.2' 73°43.0' -2.9 -0.9 VV good 98 44°05.5' 73°39.0' -2.2 -0.7 VV fair 99 44°08.0' 73°39.5' -2.2 -0.7 VV fair 100 44°07.5' 73°32.5' -1.3 -0.4 VV good 101 44°08.0' 73°29.0' 0.0 0.0 VV fair 102 44°09.2' 73°49.5' -3.3 -1.0 VV good 103 44°03.0' 73°08.2' -5.3 -1.7 VV good 104 44°07.0' 74°10.0' -5.0 -1.6 VV good 105 44°11.5' 74°19.7' -3.7 -1.2 VV fair 106 44°13.0' 74°15.7' -5.0* -1.6* VV good 107 44°14.7' 74°10.5' -5.1 -1.7 VV good 108 44°15.2' 74°07.5' -4.5 -1.5 VV fair 109 44°17.2' 74°06.7' -4.6 -1.5 VV good 110 44°17.2' 74°05.0' -4.2 -1.4 VV good 111 44°21.2' 74°01.2' -4.2 -1.4 VV fair 112 44°19.5' 74°00.2' -4.0 -1.3 VV good 113 44°20.7' 73°56.5' -3.6 -1.2 VV good
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 TABLE 1. COASTAL DATA POINTS FOR VERTICAL TECTONIC DISPLACEMENTS PRESUMABLY ASSOCIATED WITH THE MAY 1960 CHILE EARTHQUAKES
LOCATION* VERTICAL CHANGE* LOCALITY Latitude Longitude DATA ESTIMATED NUMBER* (south) (west) Feet Meters SOURCE § ACCURACY" 1 36°46.0' 73°09.0' + 1.0 +0.3 TT fair 2 37°04.2' 73°09.6' 0.0 0.0 L p 3 37°09.9' 73°11.7' 0.0 0.0 TT good 4 37°14.1' 73°25.4' 0.0 0.0 TT fair 5 37°11.9' 73°33.5' 0.0 0.0 TT fair 6 37°02.6' 73°31.4' 0.0 0.0 TT 3 7 37°35.9' 73°39.5' +4.0 +1.3 TT good 8 38°14.6' 73°14.7' +3.1 +0.9 VV good +3.3 + 1.0 TT good 9 38°20.2' 73°30.4' -0.8 -0.2 TT fair 10 38°23.3' 73°53.4' +3.3 +1.0 TT fair +2.9 +0.9 VV poor 11 38°24.4' 73°57.3' +5.9 +1.8 MM good 12 38°43.6' 73°25.7' -4.1' -1.2' TT fair -5.0* -1.6' TT fair 13 38°47.0' 73°24.0' -4.5 -1.4 TT poor 14 39°13.1' 73°12.8' -6.3* -2.0' TT ? 15 39°26.0' 73°12.4' -5.3' -1.6' VV fair 16 39°47,4' 73°12.5' -8.9' -2.7' TT fair 17 39°51.7' 73°25.4' -6.8 -2.1 VV fair -6.0 -1.8 TT fair 18 39°56.0' 73°35.0' -2.3 -0.7 TT fair 19 40°08.1' 73°39.7' -1.8 -0.5 TT poor 20 40°13.6' 73°43.1' -2.2 -0.7 VV fair 21 40°30.4' 73°49.2' -4.4 -1.3 TT fair 22 40°33.4' 73°45.6' -5.0 -1.6 TT good 23 40°41.2' 73°49.2' -4.1 -1.2 TT fair 24 41°17.6' 73°52.5' -3.5 -1.1 VV good -3.5 -1.1 TT good 25 41°37.0' 73°37.0' -5.2* -1.6' L ? -5.0' -1.5' VV fair 26 4T38.0' 73°36.0' -5.7« -1.7' VV fair 27 41°45.2' 73°43.0' -7.8 -2,4 TT fair 28 41°46.7' 73°25.2' -4.2 -1.3 TT fair 29 41°48.2' 73°22.2' -3.4 -1.0 TT good 30 41°46.7' 73°14.5' -2.3 -0.7 TT fair 31 41°48.0' 73°07.7' -1.3 -0.4 TT fair 32 41°46.7' 73°08.0' -3.2 -1.0 TT fair 33 4T45.2' 73°07.7' -2.0 -0.6 TT good 34 41°43.2' 73°04.2' -1.2 -0.4 TT poor 35 41°31.2' 73°02.7' 0.0 0.0 L ? 36 41°30.0' 73°49.0' + 1.6' +0.5' TT good 37 41°30.5' 72°48.0' +2.3 +0.7 TT good 38 41°32.2' 72°45.4' +2.6 +0.8 VV good 39 41°37.3' 72°40.3' +2.9 +0.9 VV good +3.7 +1.1 TT good 40 41°41.7' 72°38.6' +2.4 +0.7 TT fair 41 41°44.7' 72°34.2' +3.0 +0.9 TT good +2.4 +0.7 VV fair 42 41°43.5' 72°28.2' +2.8 +0.8 TT good 43 4T39.0' 72°23.5' 0.0 0.0 TT fair + 1.6 +0.5 VV poor 44 41°39.2' 72°18.0' + 1.0 +0.3 TT good 45 41°51.2' 73°59.7' -3.2 -1.0 VV good 46 41°51.5' 73°57.7' -5.9 -1.8 VV fair 47 41°52.2' 73°55.5' -4.8 -1.5 VV good 48 41°52.7' 73°53.2' -5.0 -1.5 VV fair 49 41°53.6' 73°51.0' -4.3 -1.3 VV poor 50 41°49.7' 73°38.0' -6.4 -1.9 TT fair -5.9 -1.8 VV fair 51 41°59.7' 73°31.7' -5.0 -1.5 TT good (locations shown on Pi. 1)
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LOCATION* VERTICAL CHANGE* LOCALITY Latitude Longitude DATA ESTIMATED NUMBER* (south) (west) Feet Meters SOURCES ACCURACY** 114 44°21.5' 73°52.2' -3.5 -1.1 VV good 115 44°22.5' 73°51.0' -3.5 -1.1 vv fair 116 44°24.0' 73°24.0' -2.4 -0.7 VV fair 117 44°25.5' 73°43.7' -1.3 -0.4 vv good 118 44°29.0' 73°37.2' 0.0 0.0 vv good 119 44°36.7' 73°36.2' 0.0 0.0 vv good 120 44°47.2' 73°36.5' 0.0 0.0 vv good 121 44°44.5' 73'49.5' -3.4 -1.0 vv fair 122 44°42.5' 74°15.5' -3.0 -0.9 vv fair 123 44°43.0' 74°20.0' -4.9 -1.5 vv good 124 44°38.5' 74°44.0' 0.0 0.0 vv fair 125 44°39.7' 74°37.5' 0.0 0.0 vv fair 126 44°38.5' 74°28.2' -2.7 -0.8 vv fair 127 44°38.5' 74°27.2' -3.8 -1.2 vv good 128 44°43.5' 74°25.7' -3.2 -1.0 vv fair 129 44°47.0' 74°23.5' -4.4 -1.3 vv good 130 44°49.7' 74°19.0' -3.2 -1,0 vv fair 131 44°52.2' 74°09.7' -3.0 -0.9 vv fair 132 44°53.5' 74°04.0' -2.5 -0.8 vv fair 133 44°56.0' 73°55.7' 0.0 0.0 vv fair 134 45°00.2' 73°48.5' +2.2 + 0.7 vv poor 135 45°45.1' 73°43.7' -2.0 -0.6 TT poor 136 45°01.0' 72°30.0' 0.0 0.0 vv fair 137 44°55.0' 75°02.2' + 18.6 +5.7 vv good 138 45°12.7' 74°30.7' -1.7 -0.5 vv good 139 45°19.2' 74°19.2' -2.0 -0.6 vv good 140 45°20.5' 74°19.0' -2.3 -0.7 vv good 141 45°23.0' 74°16.0' -2.4 -0.7 vv good 142 45°24.5' 74°02.7' -1.4 -0.4 vv fair 143 45°28.0' 73°58.5' -0.8 -0.2 vv fair 144 45°28.5' 73°52.5' 0.0 0.0 vv good 145 45°28.0' 73°49.0' -1.5 -0.5 vv poor 146 45°21.0' 73°49.2' 0.0 0.0 vv poor 147 45°17.0' 73°00.5' + 1.0 +0.3 TT fair 148 45°17.2' 73°36.5' 0.0 0.0 TT fair 149 45°17.2' 73°34.7' + 1.0 +0.3 TT good 150 45°14.5' 73°33.7' 0.0 0.0 TT good 151 45°12.0' 73°32.0' 0.0 0.0 TT fair 152 45°09.7' 73°31.5' +3.3 + 1.0 TT fair +2.3 + 0.7 L ? 153 45°18.0' 73°22.2' -1.5 -0.5 TT poor 154 45°17.5' 73°12.2' 0.0 0.0 TT fair 155 45°21.7' 72°05.5' 0.0 0.0 L * Locality not visited during this study. t Coordinates from 1:250,000 scale Carta Preliminar, Inst. Geografico Militar. * Locality on unconsolidated deposits where some surfkial subsidence may have occurred. § L, vertical estimated by local resident; TT, measured difference between pre- and post-earthquake position of extreme tides as indicated by local residents; \\i measured difference between lower growth limit of pre- and post- earthquake terrestrial vegetation; MM, measured difference between upper growth limit of pre- and post-earthquake mussels (Mytilus). ** Good, ±0.2 m; fair, ±0.4 m; poor, ±0.6 m. Query indicates no basis for evaluation. Comments: Locality No. 6, 7, 152 Average of three measurements. 20, 49, 52, 75, 85, 134 Probable minimum submergence. 60 Average of three measurements; range +1.6' to +1.9'. 68 Average of three measurements; range +3.4' to +4.0'. 74 Estimate of submergence by F. Junemann based on increase in level of an intertidal lake. 77 Estimate by Clodorimo Torres. 88(a) Measured in 1968 by Lt. Peter Hadida Sch., Chilean Navy. 88(b) Measured in 1960 by Lt. Oscar Boehnwald O., Chilean Navy.
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uninhabited areas on the southern insular shelf) Vertical tectonic displacements were deter- were measured directly by leveling between mined at some 53 inhabited coastal sites from the pre- and post-earthquake lower growth measured differences between the pre- and post- limits of terrestrial vegetation along the shore. earthquake tide levels relative to some reference The coastal region affected by tectonic move- feature indicated by local residents. Most ments in 1960 has an equable climate and high residents in the areas visited are farmers, fisher- rainfall so that shores are either covered by men, and shellfish gatherers who are usually dense natural vegetation or are under cultiva- well aware of any relative changes in the posi- tion down to the extreme lower growth limit of tion of sea level. Our usual procedure was to ask terrestrial vegetation. Only cliffs or unstable residents to indicate the pre- and post-earth- dune-swept shores have no vegetative cover. quake positions of the annual extreme high In sheltered localities where wave action is tides relative to fixed familiar references such negligible, the lower growth limit is sharply as roadways, piling, fences, breakwaters, docks, defined and corresponds almost exactly to piers, or reefs. The vertical difference between annual extreme high-tide levels. On more the two marks was then measured by leveling. exposed shores, the growth limit is commonly Wherever possible, an effort was made to obtain less distinct and is at the uppermost limit of replicate measurements. wave splash or runup. Thus, downward move- The major source of error in the measure- ment of the land relative to sea level resulted ments is the accuracy of the pre-earthquake in submergence of a fringe of nearshore vege- extreme high-tide levels indicated by the local tation and peaty soil (Fig. 5, A and B). Con- residents, as there was usually no uncertainty versely, emergence of the land was recorded by about the position of the corresponding tide encroachment of post-earthquake terrestrial levels in 1968. The estimated accuracy of these vegetation downward onto former intertidal measurements is considered good (+ 20 cm) areas (Fig. 6, A and B). where the pre-earthquake extreme high-tide Figures 5 and 6 show that in 1968 the pre- levels could be indicated with reasonable cer- earthquake lower growth limit was still clearly tainty relative to fixed markers in bedrock visible along many sheltered shorelines. In areas, fair (+40 cm) where it could be ap- contrast, erosion or changes in beach configura- proximately located, and poor (+60 cm) where tion along most shores exposed to strong wave there was doubt about its correct position or and current action had obliterated the evidence the reference markers were located in areas of to varying degrees. In subsided areas, the possible surficial subsidence. vertical displacement was measured from the One critical measurement of the vertical root level of dead vegetation exposed at low- change on remote Isla Guafo, which could not tide stages to the corresponding level for living be visited during the field study, was made on plants. Plafker's request by Lt. Peter Hadida of the The large diurnal tide range, particularly in Chilean Navy, who had personal knowledge of restricted waters of the Archipielago area, made the pre-earthquake tide levels there. observation of the pre-earthquake vegetation At 7 localities, estimates of land-level changes lines possible at low-tide stages even in localities were obtained from port authorities and local of maximum submergence. Along relatively residents. Measurements of these displacements exposed shores, the measured changes generally could not be made in 1968, either because of represent the minimum submergence, because high tides at the time of the visit or because the pre-earthquake lower limits of vegetation of post-earthquake changes in shoreline con- growth were commonly eroded shoreward. In figuration. The accuracy of these data is, there- areas of uplift, the change was measured from fore, not known. the lower limit of living post-earthquake The approximate amount of uplift at one vegetation to the lower limit of pre-earthquake locality along the sparsely vegetated rocky vegetation as defined by mature brush and trees. shore of Isla Mocha was determined from the The accuracy of these measurements is con- measured difference in height between the sidered to be good (+20 cm) where both the upper growth limit of living post-earthquake pre- and post-earthquake vegetation lines were mussels and that of the dead pre-earthquake sharply defined, fair (+ 40 cm) where only one mussels which were elevated above tide level limit was clearly defined, and poor (±60 cm) but still remained lodged in crevices in the rock. where both limits were vague. As the upper growth limits for both the pre-
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 Figure 5. Effects of shoreline submergence. A. Farmland near Valdivia permanently submerged by at least 2 m of tectonic subsidence and an unknown amount of surficial subsidence of unconsoli- dated deposits. B. Submerged vegetation along the north shore of Isla Chiloe. The extreme lower growth limit of terrestrial vegetation prior to the earthquake was approximately at the root level of dead trees in the foreground; the post-earthquake limit is above the gravel beach (at the feet of the man). Tectonic submergence at this locality, obtained by leveling between the pre- and post-earthquake vegetation lines, is at least 1.8 m.
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 Figure 6. Effects of tectonic uplift. A. Raised shore of Isla Guamblin showing old sea cliff (white), emergent former intertidal zone (gray band), and present intertidal zone (white band above water line). Area outlined is shown on Figure 6B. B. The measured height between the upper and lower limits of the gray band of post-earthquake vegetation (between the arrows) indicates roughly 5.7 m of tectonic uplift at this locality. The vegetation is growing on a poorly indurated Tertiary sandstone which has numerous fresh shells of limpets, mussels, and boring clams in growth position. The age of the oldest vegetation on this surface, and reports by sea-lion hunters, who occasionally visit this uninhabited island, suggest that the uplift occurred during the 1960 earthquake sequence.
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and post-earthquake mussels were clearly Subsidence occurred in a central zone 75 to defined, the measured change is probably ac- 110 km wide and at least 800 km long. The curate to within ±20 cm. zone extends in an east-west direction from the vicinity of the coast almost to the western Levelings margins of the Andes, the axis of maximum To determine the amount and distribution of subsidence being along or near the crest of the the vertical tectonic movements inland from coastal mountain belt (Fig. 3). The maximum the coast, the Chilean Institute Geografico measured subsidence was 2.7 m near Valdivia, Militar releveled about 700 km of first-order although this figure probably includes some level lines connecting the cities of Talcahuano, surficial compaction. Subsidences of as much Bulnes, and Puerto Montt (Fig. 3). The pre- as 2.4 and 2.1 m were measured at bedrock sites earthquake leveling was carried out in 1957- near the north and south ends of Isla Chiloe, 1959. These lines were compared to the relevel- respectively. The northern limit of the zone ing of 1963-1964 that was tied at its northern is not known. However, it must lie south of the end to the Talcahuano tide gage. The tide gage Talcahuano-Bulnes level line along which at the south end of the line in Puerto Montt there was either no change or less than 11 cm of was destroyed during the earthquake, and uplift between the pre- and post-earthquake several years of tidal observations will be surveys. The southern limit of the subsidence required there before the position of reference lies south of 45° S. latitude—the most southerly tidal planes can be accurately redetermined. point reached during this investigation. Rapidly Vertical shifts indicated by the leveling data diminishing amounts of subsidence observed are probably accurate to within a few tens of toward the southern end of the region studied, centimeters. and small displacements of only about 70 cm at the most southerly localities suggest that the Distribution of Observed Uplift and downwarp probably terminates somewhere Subsidence near 46° S. latitude. The known and inferred vertical deformation The downwarp is bordered on its seaward associated with the earthquake sequence in- side by a zone at least 850 km long in which volves an elongated asymmetrical synclinal uplift of as much as 5.7 m elevated segments of downwarp and two flanking upwarps (Figs. 3 the mainland coast and three offshore islands and 7). These three zones are separated by lines (Isla Mocha, Isla Guafo, and Isla Guamblin). of zero land-level change or zero isobases with- The largest measured uplift was at Isla Guamb- out any detectable abrupt vertical offsets. lin, an isolated island perched close to the edge
0 1 --2 • Archipelago de los Chonos ~ — j_ 3 WEST 20 40 60 80 100 KILOMETERS EAST
Figure 7. Profiles of land-level change along lines AA' and BB' shown on Figure 3.
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of the continental shelf near the southern limit tsunami (as indicated by the aftershock area) of the area visited. The zone of uplift could to derive an average vertical displacement at possibly extend another 200 km southward to the wave source of from 5.7 to 10 m. This result the Taitao Peninsula (Fig. 2), where personnel must be regarded as a crude approximation be- of the Chilean Navy at a navigation lighthouse cause of the uncertainties involved in calculat- noticed apparent uplift of "a few centimeters" ing the potential energy of the tsunami wave after the earthquake (Subofficial Manuel from tide-gage records at stations distant from Barrios, January 1968, oral commun.). the source, equating source areas with the after- Although the extent of the uplift under the shock areas, and assuming that the bottom dis- ocean cannot be determined directly, analyses placements were instantaneous. Nevertheless, of the tsunami waves generated by deformation Hatori's results are significant because they of the sea floor during the earthquake suggest indicate that large submarine uplifts must have that significant uplift occurred there. The en- occurred over much of the submarine portion tire contiguous continental shelf and at least ol the aftershock area in order to generate the parts of the continental slope within the focal tsunami that was recorded throughout the region of the earthquake were probably in- Pacific Ocean basin. volved. The fact that the tsunami was recorded Slight uplift occurred locally in a zone that as an initial rise on almost all tide-gage records borders the downwarp on the east (Figs. 2 and throughout the Pacific Ocean basin (Berkman 3). Uplift of 1.1 m or less was measured, or was and Symons, 1964) indicates a positive wave reported by residents, in the coastal area east of resulting from upward motion of the sea bottom . Puerto Montt and along the shores of the Golfo at the source. de Ancud and the extreme northern part of the Hatori (1968, p. 358) constructed inverse- Golfo de Corcovado. In this area, the zone of wave-refraction diagrams based on the arrival uplift appears to be at least 60 km wide from times of the initial wave at three tide stations in east to west and is in the form of a gentle Chile north of the deformed region and at four anticlinal upwarp (Fig. 7, profile A-A'). places along the mainland coast where the Slight uplift may have occurred at a few initial arrival was observed by eyewitnesses. His localities east of the subsided zone near the wave-front reconstructions indicate that the southern limit of the area visited. Most notably, northern and eastern edges of the tsunami residents of the village of Puerto Aguirre (Fig. source along the mainland coast roughly cor- 3, loc. 152) report changes in pre- and post- respond to the limit of the uplifted area. How- earthquake extreme high tides that suggest 1 m ever, an exception is in the vicinity of the of emergence. However, if due to tectonic Arauco Peninsula where the wave source lies movements, uplift in that area must represent offshore within the zone of uplift in an area local displacements rather than regional warp- where the deformation occurred in part on the ing because, as shown on Figure 3, several previous day (see a following section). Compar- localities nearby were either unchanged or able data for the southern extent of the uplift reportedly underwent slight subsidence. An are unavailable because the outer coast is alternative possibility that cannot be discounted uninhabited in the Archipielago de los Chonos, with the available data is that the shoreline and the seaward limit of the tsunami source is, changes reported in the vicinity of Puerto of course, unknown. As inferred on Figure 2, at Aguirre result from local changes in tidal least those portions of the continental shelf and amplitudes rather than from tectonic move- slope that lie within the belt of highest after- ments. Presumably, the regional tectonic sub- shock activity are probably included. mergence that affected much of the Archipiel- Qualitative information as to the amount of ago de los Chonos could cause significant submarine uplift within the tsunami source changes in the tidal characteristics of restricted, area may be inferred from the waves that were shallow waterways such as the southern part of generated. Runup heights of 10 to 20 m at the Canal de Moraleda. numerous localities all along exposed shores It is not known whether changes in level between Isla Guafo and Isla Mocha suggest affected the uninhabited mainland coast along correspondingly large bottom displacements at the Canal de Moraleda and most of the Golfo the wave source. Hatori (1966, p. 1460-1461) de Corcovado because these areas were not used the tsunami energy (as calculated from visited during the field investigation; nor is the the waves recorded at tide gages) and an est- extent of the zone of slight uplift on the main- imated source area of 138,000 km2 for the land north of Puerto Montt known, because
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the zone lies east of the line of releveling along basin on the west (Fenner and Wenzel, 1942). the Interamerican Highway. A comparable fault contact that brings crystal- line rocks on the east against Tertiary sediment- Relationship to Geologic Features ary rocks on the west was observed during this The gross aspects of the geology of the investigation on Isla Stokes (Fig. 3, loc. 125) region affected by tectonic deformation in along the ocean side of the Archipielago de los 1960 are reasonably well known, although Chonos. Briiggen (1950, p. 202-205) has important details of the structure, stratigraphy, speculated that the reticulate shape and steep and Quaternary deformational history have west slope of the mainland coast from Isla yet to be resolved, and published detailed Chiloe north to the Arauco Peninsula indicate geologic maps are virtually nonexistent. The an offshore fault system characterized by down- following discussion is based largely on the to-the-west displacements. Major normal faults comprehensive syntheses of the geology of have been inferred by various workers, mainly Chile made by Briiggen (1950), and more on the basis of physiography, to bound all or recently, by Zell (1964). Saint-Amand (1961) parts of the grabenlike Central Valley, but prepared an excellent summary of the geology there appears to be no consensus regarding their of the earthquake-affected region and compiled locations. The linear arrangement of many a useful geologic map showing major known centers of volcanic and hot-spring activity in and suspected faults in the region. parts of the Andean volcanic chain has also been The geology of the earthquake-affected widely interpreted as suggestive of possible region may be conveniently subdivided into control by longitudinal faults. Few major the following three geomorphic provinces. (1) transverse faults have been delineated, and The Coastal Mountains, a subdued range of evidence has not been found for strike-slip mountains less than 1350 m high, underlain displacements on longitudinal faults, such as mainly by metamorphic rocks of Precambrian have been reported for the Atacama fault in or Paleozoic age and granitic plutons that are northern Chile (Saint-Amand, 1961, p. 26). considered to be of Cretaceous age. The pre- The profiles of land-level changes (Figs. 4 Tertiary rocks are locally overlain by slightly and 7) indicate that the vertical displacements to moderately deformed Tertiary marine and within the portion of the deformed region continental sedimentary rocks along the exposed on land were primarily due to broad western flank of the range and marine rocks on regional warping without any abrupt offsets the offshore islands. (2) The high Andes Moun- suggestive of significant dip-slip faulting. tains which are formed mainly of metamorphic Furthermore, there is no indication that the rocks of Precambrian (?) and Paleozoic age and isobase lines between the uplifted and subsided deformed Jurassic and Cretaceous bedded zones coincide with major physiographic sedimentary and volcanic rocks that are cut by features on land that might suggest block move- granitic plutons of Cretaceous or Tertiary age. ments of such features. The broad synclinal All these older rocks are in turn unconformably downwarp, for example, spans all of the Coastal overlain by a relatively undeformed, predomi- Mountains and a large part of the adjacent nantly andesitic, volcanic sequence of late Central Valley. Its eastern limit does not coin- Cenozoic age. A chain of imposing strato- cide with the eastern margin of the Central volcanoes up to 3760 m in elevation cap the Valley—inferred by some geologists to be a Andes. (3) The low-lying grabenlike Central major line of faulting—except at one point just Valley (and its drowned southern extension in east of Valdivia (Fig. 3). In the vicinity of the the Archipielago region) which contains a thick Arauco Peninsula and at Isla Stokes, the western fill of unconsolidated alluvial, fluvioglacial, and margin of the subsided zone locally is coincident volcanogenic deposits. with north-south-trending faults that juxtapose Few major faults have been positively Tertiary sediments on the west and pre- identified on land in the earthquake-affected Tertiary rocks on the east. However, both the region, and there is no evidence that any sur- absence of earthquake-related surface rupture face faults have been active in historic times. along the Arauco Peninsula fault (Saint- Of the numerous faults delineated on the Amand, 1961, p. 15) and the observed surface Arauco Peninsula, the most prominent is a warping opposite to the down-to-the-west major north-south-trending fault along which geologic offsets on the Arauco Peninsula and pre-Tertiary rocks of the Coastal Mountains Isla Stokes faults indicate that they played no have been uplifted relative to an early Tertiary significant role in the deformation.
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Time Sequence of the Movements no detectable post-earthquake changes, which suggests the possibility that where such changes Most of the vertical tectonic displacements were observed, they were of local, rather than associated with the 1960 sequence occurred regional, extent. during the main shock of May 22, but some occurred at the time of the initial shock on May Previous History of Vertical Tectonic 21. Residents who were interviewed in 1968 Displacements along the mainland coast from the Arauco Historic records indicate that the 1960 Peninsula south to 39° S. (shown cross-hatched sequence was but the most recent event in a on Fig. 3) indicated that most, if not all, of the series of major earthquakes that have periodical- land-level changes took place during the initial ly resulted in notable tectonic displacements in shock, Isla Mocha does not seem to have been southern Chile. Notwithstanding the relative involved in these earlier displacements. That scarcity of instrumentally recorded earthquakes at least part of the sea floor was also deformed south of 37° S. since 1906 (Gutenberg and at this time is indicated by the fact that a small Richter, 1954, p. 42), the region affected by the tsunami was recorded along adjacent parts of 1960 earthquake has been one of high seismic the Chilean coast and at points as distant as activity at least since the arrival of the Hawaii (Eaton and others, 1961, p. 136). Eye- Spaniards in the middle of the 16th century. witness reports, and the enormous tsunami According to a compilation by Lomnitz (1969, generated during the main shock on the 22nd, written commun.), earthquakes with estimated suggest that much, if not all, of the vertical magnitudes over 7.5 occurred in the Chiloe- displacements throughout the remainder of the Valdivia sector in 1575, 1737, and 1837; major earthquake-affected region occurred at that earthquakes affected the sector centered on time. This sequence, and the pattern of seismic- Concepcion in the years 1570, 1575, 1751, and ity, suggest a multiple rupture in which the 1835. northern end of the fault zone slipped at the The earthquakes of 1835 and 1837 caused time of the initial shock, hung up for roughly significant vertical displacements of shorelines 33 hours, and then ruptured over the remainder at the northern and southern ends of the region of the area during the main shock. affected by tectonic movements in 1960. After As far as could be ascertained from discussions the great earthquake and tsunami of 1835, with coastal residents and government officials, which together destroyed Concepcion, uplift there were no discernible premonitory vertical of as much as 2.7 m was observed at various movements nor was there any appreciable post- coastal localities extending from Isla Mocha to earthquake recovery of vertical movements Concepcion (Darwin, 1851, p. 29-30). A major within the deformed region. Residents of Lebu earthquake just 2 years later in the Chiloe (Fig. 3, loc. 7) on the uplifted Arauco Pen- sector was accompanied by 2.4 m of uplift at insula reported slight post-earthquake sub- Isla "Lemus" (Lemu, Fig. 3, loc. 138) on the sidence on the order of a few tens of centi- seaward side of the Archipielago de los Chonos meters. A comparison of measurements made (Darwin, 1851, p. 27-28). Folklore of the at Lebu immediately after the earthquake Arauco Indians in the Lake Budi area, 100 km (Sievers and others, 1963, p. 1176) with data north of Valdivia on the mainland coast, obtained in 1968 suggests that any such records at least two large earthquakes that were recovery was probably small relative to the accompanied by submergence (Lomnitz, 1969, earthquake-related movements. At a few written commun.). localities within the subsided zone, residents Widespread submergence along the shores of feel reasonably certain that there was a slight the Archipielago from the vicinity of Puerto continual post-earthquake subsidence amount- Montt southward to the Taitao Peninsula was ing to as much as 20 percent of that which noted during a survey conducted in 1857. occurred during the earthquake. A particularly Admiral Francisco Vidal Gomez attributed this well-documented case of such subsidence is in subsidence to the 1837 earthquake (Briiggen, the Lake Budi area 100 km north of Valdivia 1950, p. 324). Briiggen (1950, p. 208-211) cites where Lomnitz (1969) reports about 2 m of numerous reports of submergence in this same submergence at the time of the earthquake and general region as early as 1675 which suggest an additional 20 cm in the interval 1960-1968. that the sinking was the result of both gradual The overwhelming majority of people inter- tectonic movements and sudden earthquake- viewed during this study, however, reported related displacements.
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At all places for which vertical displacements confidence, nor is it known to what extent they were reported, except at Isla Lemu, the earlier reflect tectonic movements or glacioeustatic movements apparently were in the same sense sea-level changes. as those in 1960. Along the segment of coast south of the Destructive tsunamis that accompanied the Arauco Peninsula, the post-glacial long-term earthquakes of 1562, 1570, 1751, 1835, and trend seems to be one of progressive sub- 1837 (Berninghausen, 1962) probably reflect mergence. This has resulted in drowned river large vertical displacements of the sea bottom mouths along the mainland coast north of Isla during these events. Although some smaller Chiloe and the occurrence of islands and an tsunamis may be attributable to submarine irregular ria coast along the southward exten- landslides or other causes, the energy require- sion ot the Coastal Mountains and Central ments for the larger ones favor a generative Valley in the Archipielago region. Although mechanism involving tectonic displacements of geomorphic features and historical records sug- the sea bottom on a regional scale. Such vertical gest that this submergence is recent, quantita- movements seemingly are characteristic of the tive data on its duration are unavailable. segment of the South American coast extending from Colombia to southern Chile along which 46 locally generated tsunamis have been re- HORIZONTAL DEFORMATION corded since 1562; at least one-half of these During the period 1950 to 1952, an arc of were destructive tsunamis that accompanied first-order triangulation with some supplement- major earthquakes (Berninghausen, 1962). ary second-order stations was extended along Since the early 16th century, a number of the Central Valley by the Chilean Instituto tsunamis have been recorded in Japan that Geografico Militar. The section of the arc originated along the Chilean coast (Takahasi from 38° S. to 41° S. was resurveyed by the and Hatori, 1961). Of these, the 1960 tsunami Instituto during the period 1966 to 1968. was by lar the highest and most destructive. Although many ol the original stations were Thus, the Japanese records suggest that the not recovered, enough angles were reobserved subsea tectonic movements in 1960 were prob- to permit a calculation ol the surface shear ably correspondingly greater than those strain along the triangulation arc. associated with any of the earlier earthquakes The method described by Frank (1966) was during a time interval of nearly 500 years. used to calculate the shear strains from the The geologic record of Quaternary vertical changes of the observed (unadjusted) angles movements in southern Chile is fragmentary during the interval between surveys (that is, and is based largely on detailed geomorphic 1951 to 1967). The calculated shear strains are studies made by Darwin (1851) at a few localities, Briiggen (1950), and Weischet (in Fuenzalida and others. 1965). Available data 72 = e12 + e2i suggest a complex history of Quaternary where 6,7 is the usual tensor strain and the co- vertical displacements in which different parts ordinate system is that shown in Figure 10. The of the region have undergone net uplift, net change in any angle is a linear function of the subsidence, or remained unchanged, relative local values of 71 and 72. Thus, the change in to present sea level. two neighboring angles is sufficient to specify Since Darwin's visit to Chile in 1835, it has 7i and 72. In this paper, we have used the been known that parts of the mainland coast, changes in 6 to 8 neighboring angles to deter- the outer coast of Isla Chiloe, and the offshore mine each local pair 71 and 72. The solution of islands are characterized by marine terraces as the 6 to 8 condition equations is done by least much as 200 m above present sea level. High squares, and the shear strains and their standard marine terraces are particularly well developed deviations are both determined. The groups of on the Arauco Peninsula and Isla Santa Maria angles used for each local determination are just north oi the peninsula. The presence ol designated by letters in Figure 8, and the marine terraces in areas affected both by uplift associated values for 71 and 72 are shown in and subsidence during the 1960 earthquake Table 2. The data in Table 2 are also presented implies net long-term emergence relative to sea in another way in the circled insets in Figure 8 level. Because there are no absolute ages for where the principal deviatonc strains, cal- any of these terraces, they cannot be cor- culated irom 71 and 72 (Frank, 1966), are related from place to place with any degree of shown. The true principal strains differ from
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the principal deviatoric strains only by the MECHANISM addition of a dilatation. Thus the principal axes for the true strain and the deviatoric strain are Steep Fault Models parallel, and Figure 8 gives a true picture of the As a result of analyses of Rayleigh-wave and directions of greatest horizontal tension and P-wave first motions, together with regional compression. (Notice the strain represented in geologic considerations, the 1960 Chilean earth- Figure 8 is the 1967 strain less the 1951 strain, quake sequence has been widely cited as result- that is, primarily the strain released by the ing from motion on one or more concealed 1960 earthquake. Thus the indicated east-west steeply dipping faults that roughly parallel the tension suggests thrust faulting and the ap- Chilean coast; the probable sense of slip on the parent left-lateral shear at A suggests right- causative fault was considered to be dextral lateral faulting.) strike-slip. What we intend to show in this brief review of previous work is that (1) the main lines of evidence used to infer a dextral strike slip were either ambiguous or have since been shown to be incorrect, and (2) a steep fault model is clearly incompatible with the data presented in this paper concerning the pattern of earthquake-related tectonic displace- ments. The initial phases of long-period (35 to 150 seconds) Rayleigh waves from aftershocks in Chile were tentatively interpreted by Aki (1960, p. 4172) as consistent with dextral strike- slip motion on a north-south-trending fault. A subsequent correction (Aki, 1962) of an error in calculating the instrumental phase shift in- dicated the motion would be sinistral rather than dextral. However, after reconsideration of the problem, Aki (1962) concluded that in view of the uncertainties in focal depth and phase velocity, it was not possible to define the focal mechanism from initial phases of Rayleigh waves using periods as short as those used in his study. It might be noted that the final fault models proposed in this paper, all of which have shallow focal depths, are consistent with the initial phases observed by Aki. Saint-Amand (1961) summarized the effects of the earthquakes, their time-spatial distribu- tion, and their broad geologic setting. On the basis of data available to him shortly after the event, he postulated (1961, p. 26-27) that the surface trace of the fault that produced the main shock lies between the zones of uplift and Figure 8. Part of the triangulation arc along the subsidence and trends roughly N. 9° E. for a Central Valley of Chile, showing the angles used to cal- distance of 1000 to 1200 km (approximately culate strain. The circled insets show the principal along the zero isobase shown in Fig. 1). Accord- deviatoric strains calculated for each section. The length ing to this interpretation, movement was at too of the double arrows indicates the magnitude of the great a depth to permit surface breakage where principal strains. The long rectangle on the left outlines the postulated fault intersected the continent the surface projection of the fault model (dip 36° E.) used for the dislocation calculations in Table 2. Only near its northern end. Aki's 1960 mechanism the northern half of the fault model is shown. Data solutions and analogies with the inferred style courtesy of Institute Geografico Militar (Chile) and of faulting along the Atacama fault in northern Interamerican Geodetic Survey, U.S. Army Corps of Chile were cited, which implied dextral strike- Engineers. slip movement on the primary fault, although
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TABLE 2. OBSERVED AND CALCULATED HORIZONTAL SHEAR
Shear Model V Model VI Section Component Observed 20 m Thrust 20 m Dextral (p strain) (/i strain) (AI strain) A 7i 7 ±3 22 1 72 -25 ±4 -13 -7 B 7i 26 ± 10 22 -3 72 -18 ± 15 -4 -10 C 71 56 ±15 23 2 72 -19 ±9 -0 -20 D 71 50 ±6 22 - 1 72 - 5 ±6 -0 -21 E 71 62 ±23 22 -1 72 30 ± 19 -0 -21 F 7i 36 ±4 27 -1 72 2 ±8 -0 -31 G 7i 39 ±4 27 -0 72 - 1 ±3 -0 -31 H 71 44 ±3 22 -0 72 - 1 ±3 -0 -21
Saint-Amand carefully avoided drawing con- of the well-defined plane, as determined by clusions from these data regarding the sense of the several nodal-plane solutions, is N. 10° E. slip. As previously noted, Aki's conclusions which agrees almost exactly with the over-all regarding the mechanism were partly based on fault direction suggested by Saint-Amand. ..." invalid assumptions. Furthermore, recent de- but that ". . .most of the solutions, interpreted tailed mapping at the north end of the Atacama in terms of faulting on a north-south plane, fault by Arabasz (1968) suggests that there has require either thrust or left-lateral movement. been no significant strike-slip offset along it The lateral sense does not agree with Saint- during late Cenozoic time. Amand." To explain the broad distribution of after- Theoretical analyses (Chinnery, 1961) and shocks and damage, Saint-Amand inferred that field studies in areas of strike-slip faulting movement on the concealed main fault was elsewhere, suggest that no reasonable combina- followed by slippage on a series of predominant- tion of strike-slip or even oblique-slip move- ly north-south-trending continental faults that ment on near-vertical faults could give the extend inland from the coast to the crest of the observed pattern of deformation associated Andes. This suggestion is difficult to reconcile with the 1960 Chilean earthquake sequence. with both the scarcity of aftershocks and ab- For this reason, together with the absence of sur- sence of surface displacement along the postu- face rupture and the apparent lack of agree- lated faults. Although some of the postulated ment between the various strike-slip inter- faults are in densely populated areas of the pretations of the first motions, we consider Arauco Peninsula and Chilean lakes region, predominantly strike-slip movement on a no evidence to support their existence could steeply dipping primary fault between the be found during the careful aerial and ground major zones of uplift and subsidence an unten- searches made by the numerous field parties able mechanism for this earthquake sequence. immediately after the event. Hodgson and Wickens (1965) evaluated Teleseismic Evidence body-wave first-motion data for 9 earthquakes Although many details of the focal mech- of the sequence by computer and found that, anism of the great Chilean earthquake of 1960 although none of the Chilean earthquakes are in doubt, there appears to be good evidence yielded a well-defined solution, most of them that the fault strike is about N. 10° E. and the suggested one plane striking approximately fault length about 1000 km. These figures were north-south and dipping steeply to the west. obtained mainly from the distribution of after- They report (p. 135) that "The mean strike shocks (Fig. 2) and have been confirmed by
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Ben-Menahem's directivity solution (Press and (1967) are not adequate to define the nodal others, 1961; Ben-Menahem, 1967), as well as planes usefully. The general impression ob- by the field deformation data presented in this tained from these solutions is that one nodal paper. What we wish to show in the following plane must dip steeply at an azimuth near 270° pages is that, given the strike of the fault plane, and that the other plane is virtually undeter- the teleseismic evidence suggests that the mined. Thus we conclude that the nodal-plane Chilean earthquake was probably associated solutions of Wickens and Hodgson are probably with right-lateral thrust faulting. consistent with the focal mechanism given as No nodal-plane solution is available for the model I in Table 3. main shock of the 1960 sequence because a Balakina (1962) reports two nodal planes, strong foreshock obscured the first motion from one dipping 70° at azimuth 245° and the other the main shock on most seismograms. Studies dipping 30° at azimuth 20°, for the foreshock of two earthquake sequences, the Alaska 1964 of magnitude 7.8 (origin time 18h 55m 57s series (Stauder and Bellinger, 1966) and the GMT, May 22, 1960, and focus apparently Rat Island 1965 series (Stauder, 1968), indicate within 50 km of the main shock), which oc- that aftershocks which originate in the same curred just 15 minutes before the main shock. general area as the main shock generally have The technique employed by Balakina employs virtually the same focal mechanism as the main not only the first motions of P waves but also shock. For this reason, it is appropriate to the first motions of both SH and SV waves. examine the nodal-plane solutions for foreshocks Unfortunately, Balakina does not discuss how and aftershocks in the Chilean sequence. well the two planes are defined by the observa- Wickens and Hodgson (1967) have studied tions. We note that the steeply dipping plane three foreshocks and three aftershocks. Their agrees reasonably well with the auxiliary plane best solution, which is for an aftershock of for model I, and we suppose the shallowly dip- magnitude 6.7 (origin time 02h Olm 08s GMT, ping plane is as poorly defined in Balakina's June 20, 1960, and focus apparently within 50 solution as it would be in an ordinary P-wave km of that tor the main shock), yields two nodal first-motion solution. If this is so, then the planes, one dipping 83° at azimuth 255° (S. 75° shallowly dipping plane may be replaced by W.), and the other dipping 72° at azimuth 165° one which remains orthogonal to the steeply (S. 15° E.). Hodgson and Wickens (1965, p. dipping plane but strikes in the same direction 134-135) state that the first plane is closely as the fault plane. This modified focal-mech- limited by the data, but the dip azimuth of the anism solution is given as model II in Table 3. second plane could vary from 77° (dip 7°) There is some important teleseismic evidence clockwise around to 165° (dip 73°). Inasmuch from the main shock which appears to be incon- as the plane dipping 83° at azimuth 255° ob- sistent with other observations. Although it is viously cannot coincide with a fault plane discussed here, we caution the reader that we striking N. 10° E., it appears reasonable to sug- are ultimately forced to reject this evidence. gest that the poorly defined plane is the fault Brune and others (1961) studied the ultra-long- plane and should strike N. 10° E. If so, then period Rayleigh waves (periods 200 to 700 that plane must dip 8° at azimuth 100°. seconds) recorded on the strain seismographs at The solutions for the other shocks in the Nafia (NNA), Peru; Ogdensburg (OGD), New Chilean series given by Wickens and Hodgson Jersey; and Isabella (ISA), California. The TABLE 3. ALTERNATIVE FAULT MODELS
Fault Slip (Fraction of Model Auxiliary Plane Fault Plane Total Slip) Dip Dip Az. Strike Dip Dip Az. Strike Dip Slip Strike Slip Thrust Dextral I (possible) 83° 255° 345° 8° 100° 10° .92 .42 II (possible) 70° 245° 335° 24° 100° 10° .84 .54 III (rejected 72° 221° 311° 36° 104° 14° .53 .86 IV (rejected) 61° 41° 311° 50° 104° 14° .62 .79 V (preferred) 54° 280° 10° 36° 100° 10° 1.0 0.0 VI (rejected) 90° 100° 36° 100° 10° 0.0 1.0
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initial phase at OGD and NNA (azimuths 0° the initial phases observed at the last two sta- and 354°, respectively, from Chile) was TT, but tions. Models III and IV are consistent with at ISA (azimuth 324° from Chile) the initial the observed initial phases, but they appear to phase was 0. It is shown in the Appendix that place ISA and perhaps OGD too close to the the observed initial phase, plus the requirement nodal lines in the amplitude pattern. The that the fault strike at about N. 10° E., requires general low amplitude of Rayleigh waves at that the ratio of strike-slip displacement to the OGD plus the unexplained absence of periods dip-slip displacement on the fault must not be near 500 seconds (Brune and others, 1961, p. much less than 2 cos 5, where 5 is the dip of the 2898) suggest that OGD may indeed be near a fault plane. It is also shown in the Appendix nodal line. The much larger amplitude of Ray- that the first P-motion from the main shock leigh waves at ISA requires it to be farther observed at NNA, plus the general observation from a nodal line than indicated by model III. that faulting must produce a region of major J. N. Brune (1969, written commun.) suggested uplift on the west and subsidence on the east, that the fit to the amplitude radiation pattern restricts the possible solutions to right-lateral could be improved and the fit to the initial thrust motion on a fault which strikes east of phase pattern preserved by rotating the radia- N. 14° E. The direction of dip is not specified. tion pattern for model III clockwise 10°, that To represent this solution, two models, HI and is, by changing the fault strike to N. 24° E. IV in Table 3, were chosen. The strike for these Moreover, the nodal line for long-period Love models was chosen as close to N. 10° E. as waves would now lie along azimuth 325°, close possible, and the dips were chosen so that the enough to NNA to account for the small models would reproduce with reasonable ac- amplitude Love waves observed there. Notice curacy the surface deformation shown in pro- that Brune agrees that the N. 10° E. strike file B-B' of Figure 4. Details of that selection defined by the deformation and aftershock are given in the following section. patterns is generally correct; what he pointed Figure 9 shows the Rayleigh-wavc radiation out was that the fault model which best explains patterns for the models in Table 3. Models I the long-period surface waves appears to strike and II are reasonably consistent with the ob- N.24° E. served amplitudes (relatively small at OGD and The apparent strike of N. 24° E. for the fault NNA and somewhat larger at ISA) of long- model based upon the initial phase is difficult period Rayleigh waves but inconsistent with to reconcile with the apparent strike of N. 10°
s s Fi:igurg e 9. Rayleigh wave radiation patterns for two of the fault models in Table 3. The inner figure represents thme amplamplitudi e radiation pattern and the outer figure represents the initial phase pattern. The radiation pattern for model I in Table 3 is quite similar to model II above. The radiation pattern for model IV is identical with that for model III above.
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E. based on the pattern of aftershocks and sub- tion. The parameters which are adjusted in the sidence. Furthermore, it will be shown later least-squares procedure are ua, the dip-slip that the large component of right-lateral slip component of fault motion; W, the fault width; required for models III and IV is inconsistent 5, the dip of the fault; and x0, a parameter with the deformation of the triangulation arc in which measures the horizontal shift ot the fault the Central Valley. We are unable to explain surface parallel to itself. The last three param- why the initial phase pattern for the ultra-long- eters are shown in Figure lOb. The fit is run period Rayleigh waves is anomalous. The very for several values of h, and then the best over- complex crust and mantle structure near the all fit is selected. The fault length is not a continental margin could distort the pattern critical parameter in the adjustment and lor to some extent—particularly if the lithosphere this reason is not varied. A standard deviation plate is thrust hundreds of kilometers down into is calculated lor each of the adjustable param- the mantle at the continental margin, as is sug- eters on the assumption that the observational gested by the current theories of ocean-floor errors are normally distributed. spreading (Isacks and others, 1968). Another We have used this scheme to fit the BB' pro- deficiency in the theory is that only the far- file (Fig. 7) for an assumed fault length of 1000 field terms have been retained. For the very km. The parameter x0 is measured from the long wave lengths (about 3000 km at 500 approximate position of the edge of the conti- seconds) employed in this paper all of the nental shelf. The best fit was found to be a stations lie within a few wave lengths (3 wave fault which breaks the surface (h = 0) and dips lengths for ISA and OGD and 1 for NNA at 44.5° ± 7.0° E. The other fault parameters for 500 second period) of the source. Thus the this model arc u
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shown in Figure 11 (model A). The large stan- solution exists even for a buried reverse fault dard deviation in Ud, the dip-slip component of which dips to the west (Fig. 11, model C). motion, indicates that the fault model is not For the thrust fault models, the observed well determined. This may be attributed to the zone of major uplift results from relative sea- fact that data are not available over the region ward displacement of the overthrust block of maximum uplift. To define the solution more along a dipping plane, and the adjacent zone of precisely, it would be necessary to have vertical subsidence represents elastic extension and hori- deformation data which extends farther to the zontal attenuation of the crust immediately west. In the absence of such data, other fault behind the fault block. For the reverse fault models may be constructed which give satis- model, surface warping in these same two zones factory fits to the data. Two such models are would result from draping over the buried shown in Figure 8. We believe that model B fault. (dip 35.5° E., ud = 19.5 m, W = 60 km) is the The reverse fault model appears rather arti- most plausible model, as it provides the best fit ficial because (1) it seems unlikely that a fault with the teleseismic data, but the vertical dis- which extends to a depth of 100 km or more placement data alone are not adequate to ex- would stop 20 km short of the free surface, (2) clude other possibilities. Note that a satisfactory the broad areal distribution and predominantly
+20 r
SEA yflase of continental slope /Edge of continental shelf LEVEL
C:Ud=13.7m, 5=49.5,°W=105 knv;
B:Ud= 19.5m, 5=35.5°W=60km 50 t A:U,=35.4m, 6 =44.5° W=75 km QUJ
100 I 100 50 0 50 100 DISTANCE PERPENDICULAR TO ZERO ISOBASE (km) Figure 11. Three fault models which yield a satisfactory fit to the observed vertical tectonic deformation. The vertical displacements calculated for the models are shown in the upper part of the figure. The observed deforma- tion is indicated by small circles. The fault planes for each model are shown in the lower part of the figure.
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shallow focal region, as defined by aftershock has been incorporated into the results shown in distribution, does not support the inferred Figure 12. These results indicate that there is steep fault, and (3) the sense of displacement no difficulty in finding models which reproduce required on the inferred fault is exactly oppo- the observed distribution of major uplift, sub- site to that of known down-to-the-west faults sidence, and slight uplift, although none of in the vicinity of its projected surface trace. these models can closely duplicate both the For these reasons, we conclude that the reverse amplitudes and widths of the deformed zones. fault (shown in Fig. 11, model C) is not a Also, those models that provide the best fits reasonable model for the earthquake mecha- require a steep downward curvature of the nism, and it will not be considered further. fault plane. Such curvature is not indicated by The thrust fault models, on the other hand, the spatial distribution of aftershocks, although can satisfy most of the data, although the actual it is conceivable that slip on the curved portion faulting was undoubtedly far more complex oi the fault was accomplished by aseismic creep than the simple theoretical models (A and B) over an unspecified period of time before or shown in Figure 11. For instance, one might after the main shock. The available data do not expect fault imbrication near the toe of the permit firm conclusions regarding the origin of overthrust block and, perhaps, subsidiary slip- page on strike-slip or dip-slip faults within the upper plate. Furthermore, as discussed below, the displacement data may also suggest a down- ward curvature of the fault plane with depth. Gradual tapering in the width of the zone of subsidence from north to south within the de- formed region (Fig. 3) could reflect changes in the dip, width, or slip along the strike of the causative fault. In fitting these models, we have neglected the effects of strike-slip motion upon the vertical deformation. We have calculated the vertical deformation associated with strike- slip faults to check this approximation. It was found that the vertical deformation produced by strike-slip faulting is negligible except at the ends of the fault (x2 = ±L in Fig. 10). So far we have considered only the profile BB' (Fig. 7). In profile AA', the inverse tsunami wave refraction diagrams leave little doubt that the zone of major uplift does lie to the west, although the shape of the uplift is unknown since that portion of the deformed zone is beneath the sea. The uplift on the east end of AA' is anomalous and cannot be ex- plained by the simple plane fault models em- ployed so far. (A plane fault such as shown in Fig. 11, model B, actually produces a very minor uplift at some distance to the right. However, this uplift is very much smaller than that observed along AA'.) A simple modification that tends to produce the desired effect is the introduction of curva- ture into the fault surface. The effect of cur- Figure 12. Vertical tectonic deformation produced by an approximation to a curved fault surface. The vature can be approximated by considering a vertical deformation is given as a fraction of the slip on fault made up of several contiguous segments of the shallowest segment. In A and B, the slips on succes- differing dip, as shown in Figure 12. In such a sively deeper segments decrease as 1:1/2:1/3:1/4 . . .; model, one can also introduce the effect of a in C, D, E, and F, the slip on each segment is 4/7 that varying slip along the fault width; this effect on the preceding segment.
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the slight uplift, and it is entirely possible that end of the fault (45° S.). It is quite comparable, the uplift may result from some other factor however, with the 35-m thrust inferred (model rather than fault curvature. A, Figure 11) from the vertical deformation for To estimate the magnitude of the slip on the a fault with a somewhat steeper dip. The incon- fault surface from the horizontal deformation sistency does not appear serious, as each esti- data, we have employed dislocation theory mate of thrust is subject to a considerable un- (Savage and Hastie, 1969) to calculate the certainty because of the obvious inadequacies values of 71 and 72 at sections A through H for of the simple dislocation model (Volterra a fault similar to model B in Figure 11 (length dislocation in a uniform elastic half space) upon 1000 km, dip 36° E., width 60 km). The surface which the estimate is based. projection of part of the fault model is outlined in Figure 8. The calculated strain components Conclusions 71 and 72 are shown for two different slip sys- The spatial distribution of seismicity and the tems in Table 2. Model V corresponds to a pattern of surface deformation suggest that the 20-m thrust on the fault plane and model VI May 1960 Chilean earthquake sequence re- corresponds to 20-m right-lateral slip. Notice sulted from a complex rupture on a major that 71 is associated almost entirely with the thrust fault or zone of thrusting roughly 1000 dip-slip motion and 72 with the strike-slip km long that dips at a moderate angle from the motion. By adding the strains for models V and continental slope beneath the continental mar- VI in the proper proportion, the strains for any gin. The dislocation analysis of surface deforma- slip on the model fault can easily be calculated tion indicates that the most likely fault surface v VI (that is, 7; = (ud7i + us7; )/20, where ud is similar to that shown as model V in Table 3 and us are the dip-slip and strike-slip motions). and model B of Figure 11. The exact parameters The average shear strains (obtained by a single are not well defined, but it appears likely that least-squares fit to the changes in all 43 angles) the dip was of the order of 35° E., and the dip- for the sections C, D, E, F, G, and H in Figure slip component of movement required to 8 are 71 = 42 +4 and 72 = —3 + 3 micro- satisfy the surface deformation was at least strain. From the calculated strains for models 20 m and perhaps as large as 40 m. The anom- V and VI, those values suggest 40 m of thrust alous slight uplift on the landward side of the and no significant lateral slip on the model deformed region may reflect downward curva- fault. ture in the plane perpendicular to the fault The relatively large negative values of 72 at strike. the northern end of the lault (sections A and B There is some evidence that at least near the in Table 2) suggest that an appreciable com- north end of the fault there was a component of ponent of right-lateral slip may have occurred right-lateral slip. This is suggested by the there. However, part of the negative value of nodal-plane solutions (models I and II in Table 72 can be accounted for by the end effects 3) for an aftershock and a foreshock which oc- associated with thrust faulting (for example, curred at the north end of the fault and also by note the relatively large negative value of 72 at the horizontal deformation observed at the section A for model V, a pure thrust fault). The north end of the fault (sections A and B in nodal-plane solutions for the foreshock and Figure 8). The horizontal deformation near the aftershock (models I and II in Table 3) which center of the fault (40° S.), however, clearly occurred at the northern end of the area sug- indicates that the lateral slip over that section gest a right-lateral component of slip which is was small. The vertical deformation data, of about half of the thrust component. Thus there course, neither confirm nor contradict the is support for some right-lateral slip at least existence of lateral slip. near the north end of the fault. On the other The initial phases of the ultra-long-period hand, the apparent left-lateral slip (72 > 0) Rayleigh waves observed at Kana, Peru, and indicated at E and F in Table 2 is probably Ogdensburg, New Jersey, are inconsistent with spurious because the indicated values of 72 are the fault model proposed in the preceding less than twice the associated standard devia- paragraphs. The large component of right- tions. lateral slip required by the observed initial The apparent thrust (40 m) near the center phase (see models III and IV in Table 3) is in- of the fault (40° S.) is appreciably greater than consistent with the horizontal deformation the 20-m thrust inferred (model B, Figure 11) indicated by the triangulation data. We have from the vertical deformation near the south no ready explanation of why the observed
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initial phases of the ultra-long-period Rayleigh since the early part of the 16th century. Avail- waves at Nana and Ogdensburg are inconsistent able data suggest, however, that the tectonic with the other observations. We suspect the deformation associated with the earlier events difficulty may be connected with interference was nowhere near as extensive as that which from the near-field terms, terms which were occurred in 1960. neglected in the simple theory used to interpret the initial phases. ACKNOWLEDGMENTS It should be noted that our interpretation of Field work in Chile was financed by a gener- the earthquake sequence is broadly compatible ous Award in Seismology from the Harry Oscar with the conclusion that the Chilean sector of Wood Fund of the Carnegie Institution of the Pacific rim marks the site along which the Washington and was carried out by Plafker oceanic crust and lithosphere are being rela- while on leave from the U.S. Geological Survey. tively underthrust beneath the continental Special thanks are due Clarence R. Allen who margin (Isacks and others, 1968). Although our encouraged the undertaking of this study. estimate of 20 to 40 m of thrust appears surpris- Anibal Gajardo of the University of Chile gave ingly large, it is not excessive if compared to able assistance in the field. Hans Augusto the measured horizontal thrust component on Grosse of the Departamento de Obras Publicas the order of 20 m that accompanied the 1964 (Department of Public Works) and Beatriz Alaska earthquake (Plafker, 1969). The rate of Levi of the University of Chile expedited the convergence of lithosphere at the Chilean coast logistic arrangements. Personnel of the Inter- may be estimated from rates of sea-floor spread- american Geodetic Survey and Institute ing determined from magnetic anomalies at the Geografico Militar kindly made unpublished oceanic rises. The maximum convergence rate releveling and triangulation data available to appears to be about 7 cm/yr (5 cm/yr from us. Thanks also go to the Chilean people whose the East Pacific rise and 2 cm/yr from the mid- hospitality and co-operation in connection with Atlantic ridge). At this rate, it would take this investigation were overwhelming, whether roughly 300 to 600 years to accumulate strain extended through military and public service equivalent to 20 to 40 m of thrust. Historic or privately. We are indebted to Clarence records indicate that the recurrence time of Allen, fames N. Brune, ferry P. Eaton, Cinna destructive earthquakes in this general sector Lomnitz, and Ben M. Page for their critical of Chile has been somewhat less than 100 years reviews of the manuscript.
APPENDIX Surface-Wave Fault-Plane Solution plane clockwise (as viewed from above) from the xi axis. This is the same notation as that In discussing the initial phase of surface employed by Haskell (1963). waves, it is convenient to employ a right- Haskell (1963) has calculated the Rayleigh handed co-ordinate system in which the xs-axis wave radiation pattern for an arbitrary fault in is vertical and directed downward, the x2-axis a uniform half-space. That treatment is appli- horizontal and directed along the dip azimuth cable to faulting in the earth if the wave lengths of the fault plane, and the xi-axis is horizontal studied are so long that the earthquake focus and directed along the fault strike in the sense may be considered to be on the surface. Thus required for a right-handed system. The com- for long-period waves, the initial phase (Brune ponents of the fault slip on the hanging wall of convention) should be given by the fault are fj, f2, and f3. We define the angle 17 by cos 77 + cos (2d + 77) (2) 2 2 I/2 sin r, = ft/ft + f2 ) , cos i = f /(f,2 + f 2)1/2 . (1) where PH implies that the phase of the follow- 2 2 ing quantity is to be taken and 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 APPENDIX 1027 a value appropriate to the upper mantle. The values of the Poisson ratio from 0.25 to 0.30. phase in the shaded areas is given by PH (cos TJ) Thus the choice of tr is not critical. and in the open areas by PH ( — cos TJ). The Other evidence must now be introduced to initial phase radiation pattern is then given by decide which of these two models is correct. the phase along a horizontal line at the appro- The most obvious evidence concerns the surface priate value of tan TJ. deformation in the vicinity of the fault. It is Brune and others (1961) have calculated the clear that there was extensive uplift to the initial phases of long-period Rayleigh waves re- west and subsidence to the east. The second corded at Nana (NNA), Peru; Ogdensburg piece of evidence concerns the first P-motion (OGD), New Jersey; and Isabella (ISA), Cali- from the main shock as observed at NNA fornia. The initial phase at the two former (A = 26°) on the long-period strain instrument. stations (NNA and OGD at azimuths of 354° This P-motion was clearly compressive as may and 0°, respectively, from Chile) was close to TT, be seen in Figure 14. It will be recalled from the but the initial phase at ISA (azimuth 324° from geometry of nodal-plane solutions that the Chile) was close to 0. Figure 13 shows that strike of the auxiliary plane must be orthogonal these values of initial phase are not consistent to the trend (that is, horizontal projection) of with a fault plane which strikes N. 10° E. the slip on the hanging wall of the fault plane. (that is, THRUST NORMAL Figure 14. Recording of Chilean earthquake on the N. 51° W. strain instrument at Nana, Peru (A = 26°). Figure 13. Graphic representation of initial-phase The P arrival from the main shock is beneath the down- radiation pattern for a surface fault in a uniform half- ward-directed arrow. The horizontal scale (time) is 1 space of Poisson ratio 0.286. The angle ij is defined in minute per smalldivision. For periods less than about 10 the insert where fi is the strike-slip motion and f2 is the minutes, the vertical scale is proportional to the time horizontal component of the dip-slip motion. The integral of strain. Compressive strain is represented by a shaded areas have the phase of COST; and the open areas downward deflection. The signal preceding the P the phase of —cost). arrival is from the foreshock at 18:55:57 GMT. 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 1028 PLAFKER AND SAVAGE—CHILEAN EARTHQUAKES, MAY, 1960 the strike of the auxiliary plane should be near III in Table 3), we have arbitrarily chosen a dip N. 50° W. of 36°. It should be noted, however, that the Let us now consider the left-lateral normal relative amounts of dip slip and strike slip in fault model found above. The observed surface the fault displacement are not particularly deformation at the fault requires that if the sensitive to the dip angle. For example, the faulting is normal, the fault plane must dip to motion would be 0.58 parts thrust and 0.82 the east. For left-lateral normal motion, the parts right lateral for dip 45°, and 0.45 parts auxiliary plane must dip to the southwest, and thrust and 0.90 parts right lateral for dip 0°. the first motion at NNA should be dilatation, A right-lateral thrust solution is also possible not compression as observed. Thus the left- for a fault dipping to the west. In that case, the lateral normal-fault hypothesis is inconsistent auxiliary plane dips northeast, and the first with the observations. P-motion at NNA will be compressive only if Now let us consider the right-lateral thrust- that dip exceeds 59°. The condition that the fault model. Assume first that the fault dips to fault plane must be orthogonal to the auxiliary the east. The auxiliary plane must then dip to plane then requires that the fault plane dip the southwest. For this configuration, the first must be less than 53°. Moreover, the fault P-motion at NNA should be compressive as plane dip must be greater than about 40° to was observed. In order to reproduce the ob- account satisfactorily for the observed surface served surface deformation, the dip of the fault deformation. Thus, to represent this solution plane should probably be restricted to values (model IV in Table 3), we have chosen a dip of 60° or less. To represent this solution (model of 50°. REFERENCES CITED Aki, Keiiti, 1960, Further study of the mechanism Ben-Menahem, Ari, 1967, Source studies from of circum-Pacific earthquakes from Rayleigh isolated seismic signals, p. 85-108 in Proceed- waves: Jour. Geophys. Research, v. 65, p. ings of the VESIAC conference on the current 4165-4172. status and future prognosis for understanding 1962, Revision of some results obtained in the the source mechanism of shallow seismic events study of the source function of Rayleigh in the 3 to 5 magnitude range: Michigan Univ., waves: Jour. Geophys. 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