The Mw 8.8 Chile Earthquake of February 27, 2010

EERI Special Earthquake Report — June 2010 Learning from Earthquakes The Mw 8.8 Chile Earthquake of February 27, 2010 From March 6th to April 13th, 2010, mated to have experienced intensity ies of the gap, overlapping extensive a team organized by EERI investi- VII or stronger shaking, about 72% zones already ruptured in 1985 and gated the effects of the Chile earth- of the total population of the country, 1960. In the first month following the quake. The team was assisted lo- including five of Chile’s ten largest main shock, there were 1300 after- cally by professors and students of cities (USGS PAGER). shocks of Mw 4 or greater, with 19 in the Pontificia Universidad Católi- the range Mw 6.0-6.9. As of May 2010, the number of con- ca de Chile, the Universidad de firmed deaths stood at 521, with 56 Chile, and the Universidad Técni- persons still missing (Ministry of In- Tectonic Setting and ca Federico Santa María. GEER terior, 2010). The earthquake and Geologic Aspects (Geo-engineering Extreme Events tsunami destroyed over 81,000 dwell- Reconnaissance) contributed geo- South-central Chile is a seismically ing units and caused major damage to sciences, geology, and geotechni- active area with a convergence of another 109,000 (Ministry of Housing cal engineering findings. The Tech- nearly 70 mm/yr, almost twice that and Urban Development, 2010). Ac- nical Council on Lifeline Earthquake of the Cascadia subduction zone. cording to unconfirmed estimates, 50 Engineering (TCLEE) contributed a Large-magnitude earthquakes multi-story reinforced concrete build- report based on its reconnaissance struck along the 1500 km-long ings were severely damaged, and of April 10-17. A complete list of coastline in 1835, 1906, 1928, 1960, four collapsed partially or totally. The team members begins on page 19. 1985, and 2010 (Cisternas et al., earthquake caused damage to high- 2005). Tectonic deformation result- The research, publication, and dis- ways, railroads, ports, and airports ing from the February 27th quake tribution of this report were funded due to ground shaking and liquefac- played a substantial role in the ob- by the EERI Learning from Earth- tion. The earthquake was followed served damage. Ground shaking quakes project, under grant #CMMI- by a blackout that affected most of and surface effects were observed 0758529 from the National Science the population, with power outages Foundation. Additional support was affecting selected regions for provided by the Pacific Earthquake days. Estimates of economic Engineering Research Center, Fed- damage are around $30 billion. eral Highway Administration, Ameri- According to the USGS (2010), can Society of Civil Engineers, and the earthquake epicenter was the host organizations of participat- in a zone where the Nazca ing individuals. plate is being subducted down- ward and eastward beneath Introduction the South American plate. The On Saturday, February 27, 2010, earthquake occurred as thrust at 03:34 a.m. local time (06:34:14 faulting on the interface be- UTC), an Mw 8.8 earthquake struck tween the two plates, with the central south region of Chile, an epicenter at 35.909°S, affecting an area with a population 72.733°W (just off the coast exceeding eight million people, in- 105 km NNE of Concepción) cluding 6.1M, 0.8M, and 0.9M in the and a focal depth of 35 km. urban areas around Santiago, Val- The estimated dimensions of paraíso/Viña del Mar, and Concep- the rupture zone were 500 km ción, respectively. Figure 1 shows long by 100 km wide. The the location of the main shock and earthquake struck in an area aftershocks relative to major cities. that had been identified as a In the region of strongest ground seismic gap, with projected shaking, ground accelerations worst case potential to produce exceeded 0.05g for over 120 s. an earthquake of Mw 8.0-8.5 Coastal locations were affected by (Ruegg et al., 2009). The rup- Figure 1. Main shock and aftershocks of both ground shaking and tsunami. ture zone extended beyond the Mw 4 and larger between 2/27/10 and Over 12 million people were esti- northern and southern boundar- 3/26/10 (USGS). 1 EERI Special Earthquake Report — June 2010 resulted in drowned tidal flats and local areas of significant tsunami erosion. To the north, from about the town of Pichilemu to Valparaíso, there was little or no obvious uplift/ subsidence. The areas of coastal subsidence were exposed to substantial tsunami runup and scour, as well as wave damage, whereas areas of substantial uplift generally had relatively little damage from tsunami waves. However, in coastal areas close to the epicenter with only moderate uplift (e.g., Concep- ción and Dichato), there was substantial damage related to both strong ground motions and tsunami runup. Strong Motion The main shock of the earthquake was recorded by at least 15 strong motion instruments in the area Figure 2. Model of estimated surface deformation (after K. Wang, bounded by the cities of Santiago, 2010, personal communication), overlain by initial field estimates Viña del Mar, Angol, and Concep- of coastal uplift (GEER, 2010). ción. At the station nearest to the epicenter, Cauquenes city, the accel- over an area more than 100 km uplift affected harbor facilities (Figure erometer maximum 1g range was wide and 600 km long, from Val- 3), produced an emergent marine exceeded. Several of the recording paraíso in the north to Tirúa in the platform, and exposed the tidal habitat instruments are analog, so process- south. This is equivalent to the zone. In the central part of the rup- ing is slow and still underway. Some entire coastline of Washington and ture zone, coastal subsidence over a of the digital instruments have been Oregon. distance of about 50 km between the processed and reported in Boro- towns of Constitución and Bucalemu In south-central Chile, regional geo- schek et al. (2010) and National logic characteristics are largely con- trolled by long-term aseismic surface deformation, punctuated by sudden, coseismic coastal uplift and inland subsidence. These influence the pattern of ground motions and tsunami runup, and hence earthquake damage. The February 27 earthquake produced both uplift and subsidence along the coastline (Figure 2), and the variable pattern of deformation may have affected tsunami impacts on coastal communities. Reconnaissance-based estimates of deformation (GEER, 2010) support initial models of surface deformation. In the south, the Arauco Peninsula was uplifted and tilted gently eastward, with at least 2 m of coastal uplift on Isla Santa Maria and near the town of Lebu. The Figure 3. Fishing boats stranded within uplifted harbor of Lebu; uplift of 1.8 +/- 0.2 m in this area (photo: GEER, 2010). 2 EERI Special Earthquake Report — June 2010 Table 1. Preliminary Processed Records Maximum Accelerations of the damaged structures there. (from Boroschek et al., 2010 and DGF 2010) The cities of Viña del Mar and Station Maximum Maximum Talca, founded on marine and al- Horizontal Vertical luvial deposits, suffered extensive Acceleration Acceleration damage during the earthquake. (g) (g) Concepción is founded on a sedi- Santiago Universidad de Chile 0.17 0.14 Santiago Elevated Train Station Mirador 0.24 0.13 mentary valley, and the extensive Santiago CRS MAIPU 0.56 0.24 damage there was associated with Santiago Hosp. Tisne 0.30 0.28 site or basin effects. Among seven Santiago Hosp. Sotero de R’o 0.27 0.13 distinct zones in the city where Santiago Cerro Cal‡n 0.23 0.11 buildings or bridges collapsed cata- Santiago Campus Antumapu 0.27 0.17 El Roble Hill 0.19 0.11 strophically, six were parallel to the Vi–a del Mar (Marga Marga) 0.35 0.26 La Pólvora fault, which defines the Vi–a del Mar (Downtown) 0.33 0.19 northwest edge of the basin. Curico Hospital 0.47 0.20 Concepci—n Colegio San Pedro 0.65 0.58 Buildings. Liquefaction-induced Valdivia Hospital 0.14 0.05 ground deformations affected the seismic performance of several modern buildings. At a recently Seismological Service (2010); addi- pass bridges exhibited markedly dif- constructed hospital in Curanilahue, tional records from research and ferent performance: two collapsed with ten structurally isolated wings private institutions have not been while the other two had only minor ranging in height from one to six reported yet. Table 1 summarizes damage. Another example of local stories, individual wings underwent the known peak accelerations. The site effects in Santiago was the se- differential settlement and rotation records show two to three minutes vere structural damage of high-rise due to extensive liquefaction. Four of vibrations (Figure 4). Shaking buildings observed at Ciudad Empre- 8-story condominium buildings lo- higher than 0.05g lasted more than sarial, a recently constructed busi- cated in Concepción on a site filled 60 s in most of the records, and ness park founded on deep silty/clay with compacted sand were dam- more than 120 s in Concepción area sediments. Published data show that aged by liquefaction-induced per- records. the fundamental periods of soil pro- manent ground movement and by files in the area approximately match Elastic response spectra of several strong shaking. The Riesco building the fundamental resonant periods records are higher than elastic de- at this site underwent 30 cm of dif- sign demands from the Chilean seismic design code NCh433; however, displacement spectra demands are in general lower than those required in the National Base Isolation Build- ing Code NCh2745. Some records show important contributions to total signal energy from periods higher than 1 s. This behavior could be related to source or local soil condi- tions.

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