EERI Special Earthquake Report — November 2001 Learning from Earthquakes Preliminary Observations on the Southern Peru Earthquake of June 23, 2001 This report is condensed from the Introduction Only one strong-motion record of post-earthquake surveys of numer- the main event has been recovered ous individuals and teams, most of On June 23, 2001, at 8:33PM UTC, from a station in Moquegua. Field which traveled to Peru with their 3:33 PM local time, a Mw=8.3 surveys and local reports indicate .own funding. They contributed their (USGS) earthquake struck near the that the duration of ground motion fi ndings to this report, one of a coast of south-central Peru along was relatively long (45-60 seconds), series in the Earthquake Engineer- the subduction zone between the with peak ground accelerations ing Research Institute’s Learning Nazca and South American plates, ranging from approximately 0.10g from Earthquakes (LFE) Program. at coordinates 16.2 S, 73.75 W. The near Arequipa to 0.30g near Mo- The publication and distribution of districts of Arequipa, Moquegua, quegua. this report are funded by the Nation- Tacna, and Ayacucho sustained al Science Foundation as part of losses. There were dozens of strong The National Institute for Civil De- EERI’s LFE Program, under Grant aftershocks in the region. The south- fense of Peru reports 77 dead, 68 #CMS 0131895. ern coastline was affected by a tsu- missing, 2,713 injured, 213,430 peo- nami following the main event; the ple affected, 33,570 houses dam- coastal towns of Ocoña and Cam- aged, and 25,399 houses destroyed. ana were particularly hard hit. Figure 1 Southern Peru earthquake and associated seismicity, June 23 -- July 23, 2001. Offshore epicenters extending from the main-shock epicenter southeast to Ilo probably occurred on or near the main-shock rupture surface. Boundaries of the 1868 rupture are as proposed by Dorbath et al. (1990). Inland shocks occurred on different faults than the main-shock and may have occurred independently of the main shock. (Dewey) Figure 1 Southern Peru earthquake and associated seismicity, June 23–July 23, 2001. Offshore epicenters extending from the main-shock epicenter southeast to Ilo probably occurred on or near the main-shock rupture surface. Boundaries of the 1868 rupture are as proposed by Dorbath et al. (1990). Inland shocks occurred on different faults than the main shock and may have occurred independently of the main shock. (Dewey) 1 EERI Special Earthquake Report — November 2001 The Earthquake and After- per year. The June 23rd earthquake shock Activity would have substantially reduced elastic strain in the 300-km zone James W. Dewey, U.S. Geological ruptured, but much of the remaining Survey, Denver, Colorado; Juan 700 km of the plate interface that Bariola, JBB S.A.C., Peru ruptured in 1868 and 1877 probably still remains in a condition of high The earthquake resulted from thrust elastic strain accumulated since the faulting on the boundary between late nineteenth century. the Nazca and South American plates. The locations of main-shock One strong-motion record was ob- hypocenter, early aftershocks (Fig- tained at Moquegua. Two other in- ure 1), and moment-tensor solutions struments in the epicentral area of the earthquake imply that fault- apparently malfunctioned. The rupture nucleated at a depth of ap- strong-motion site is situated ap- proximately 30 km beneath the proximately 175 km from the source coastline between Chala and Cam- of maximum slip identifi ed by Kiku- ana, and propagated updip toward chi and Yamanaka (2001), and 100 the Peru-Chile trench and to the km from nearest part of the main- southeast, terminating near Ilo. The shock fault-plane, offshore Ilo, that is causative fault-rupture had an along- suggested by the earthquake’s focal strike length of about 300 km and a mechanism and aftershock distribu- down-dip width of about 100 km. tion. The average fault displacement was The Universidad Nacional de Ing- several meters, but seismograms of enieria and the Centro Peruano the earthquake imply that slip varied Japones de Investigaciones Sis- greatly along the fault surface. For micas y Mitigacion de Desastres example, a moment-tensor analysis (CISMID) recovered and corrected conducted by Kikuchi and Yamanaka the ground acceleration time histo- (2001) shows maximum slip occur- ries (Figure 2) and the correspond- ring approximately 150 km south- ing acceleration response spectra east of the main-shock hypocenter. (Figure 3) from the Moquequa in- The duration of the overall main- strument recording in east-west, shock rupture was approximately 80 north-south, and vertical directions. seconds (Harvard Centroid Moment PGA values were .295, .220 and Tensor solution). .160g in the east-west, north-south and up directions, respectively. The earthquake struck part of an Larger accelerations were observed approximately 1000-km long section in the approximately normal-to-fault of the western South American plate direction comparatively to the paral- boundary that many seismologists lel-to-fault-direction. The ratio of believed had high potential for great peak-vertical to peak-horizontal ac- earthquakes in upcoming decades celeration was 54%, slightly larger (Dorbath et al, 1990). Most of the than that in previous Peruvian plate-interface thrust zone between events. Chala, Peru (15.9 S), and Mejil- Figure 2 Acceleration time histories lones, Chile (23 S), had been seis- Aftershocks were predominantly of the east-west, north-south, and mically quiescent (defi ning a so- south of the main shock. Fault rup- vertical components of the corrected called “seismic gap”) since the great ture occurred following the same ground-motion recording in the city earthquakes in 1868 (Southern Pe- path shown by aftershocks — north of Moquegua. (The Universidad ru) and 1877 (Northern Chile). The to south directivity as has been com- Nacional de Ingenieria and the zone has been accumulating elastic mon in previous earthquakes in Peru Centro Peruano Japones de Inves- strain as the Nazca plate subducts (Wyss et al., 2000) — so energy- tigaciones Sismicas y Mitigacion de beneath the South American plate focusing effects may have caused Desastres [CISMID]). at a rate of approximately 78 mm larger intensities in towns south of 2 EERI Special Earthquake Report — November 2001 the epicenter such as Moquegua. Geotechnical Observations A. Rodriguez-Marek, Washington State University, Pullman; P. Repet- to, URS Corporation, Denver, Col- orado; J. Wartman, Drexel Univer- sity, Philadelphia, Pennsylvania; D. Baures, URS Corporation, Denver; E. Rondinel, Pontifi cia Universidad Católica del Perú; J. Williams, URS Corporation, Denver; J. Zegarra-Pel- lane, Pontifi cia Universidad Católica del Perú. The National Science Foundation sponsored the recon- naissance team, with additional sup- port from Washington State Univer- sity, Drexel University, the Catholic University of Peru, and URS Corpo- ration. Ground Failure: Considerable dam- age was done to highways in the regions of Tacna, Moquegua, and Arequipa as a result of landslides, rockfalls, fi ll densifi cation, and liq- uefaction. Observed ground failure was concentrated along Peru’s pri- mary north-south highway, the Pan- American Highway, and several of the other major roads connecting the cities of Atico, Camana, Arequi- pa, Ilo, Moquegua, and Tacna. Sev- eral instances of relatively minor ground failure were also observed in urban centers of the region. Fill densifi cation caused consider- able damage to bridge abutments and approaches. Roadway damage also occurred at the location of val- ley fi lls. The fi ll soils typically con- sisted of poorly graded coarse to fi ne sands and non-plastic silts. De- formations were largest along the edges of highway fi lls. Liquefaction was observed at three bridge sites located in alluvial val- leys. In one instance, liquefaction- Figure 3 Acceleration response spectra obtained from the corrected ground induced lateral spread severely motion along the east-west, north-south, and vertical components of the damaged a concrete bridge and recording in the city of Moquegua. (The Universidad Nacional de Ingenieria several residences located along the and the Centro Peruano Japones de Investigaciones Sismicas y Mitigacion banks of a river (Figure 4). At de Desastres [CISMID]) another site, liquefaction caused a 3 EERI Special Earthquake Report — November 2001 the ridge slopes varied, but were on the order of 35 to 60 degrees. Engineered Structures: Inspection of three large copper mines located in the region affected by the earth- quake indicated relatively minor damage to mining facilities. The only notable exception is the liquefaction fl ow of a heap-leach pad. Local au- thorities reported no damage to dams in the region. A concrete-face rockfi ll dam at a mine suffered minor damage due to fi ll settlement at a sacrifi cial portion of the crest. Figure 4 The offset in the corn rows (left to right near middle of photo) is Landslides due to localized displacement induced by lateral spread. (photo: geotechnical reconnaissance team) David K. Keefer, U.S. Geological Survey (USGS), Menlo Park, Califor- localized bearing-capacity failure of that public schools in Peru that were nia. This reconnaissance was car- a concrete column supporting a built in the same time period were ried out by personnel from the large and relatively new bridge. A generally constructed to a similar USGS (David Keefer, Robert Tilling, third site had about a meter of lateral design and using similar construc- and James Smith, who is currently spread, but no apparent damage to tion practices throughout the coun- on assignment to the USAID Offi ce structures. try. Observations of the performance of Foreign Disaster Assistance of school buildings in different loca- [OFDA]); the Instituto Geológico Landslides and slope stability fail- tions permit a comparative evalua- Minero y Metalúrgico del Perú ures occurred at localized cut and tion of the ground motions experi- (INGEMMET — Julio de la Cruz, fi ll sections along highways (Figure enced during the earthquake.