1 EERI Special Earthquake Report — November 2005 Learning from Earthquakes Intensities and Damage Distribution in the June 2005 Tarapacá, Chile, Earthquake Editor’s Note: When a magnitude (M w ) 7.9 earthquake hit northern w w Chile on June 13 th , 2005, unrein- forced stone masonry and adobe dwellings were extensively dam- aged in ground motions ranging be- tween intensities IX-X in the epi- central area and VI near the coast. This report presents the findings of a University of Chile reconnais- sance team that visited approxi- mately 27 towns in the earthquake zone shortly after the event. It fo- cuses primarily on damage to dwell- ings and patrimonial structures and calculations of intensities. The team included Maximiliano Astroza, Ofelia Moroni, Ana Norambuena, and Rodrigo Astroza, all from the Department of Civil Engineering, University of Chile. They also pre- pared this report for EERI, the publication of which is funded by EERI’s Learning from Earthquakes Program, under National Science Foundation grant # CMS-0131895. Introduction The Tarapacá earthquake struck part of northern Chile on June 13, 2005, at 6:44 pm, local time. The U.S. National Earthquake Infor- mation Center (NEIC) reported a moment magnitude (M w ) of 7.9 and a body-wave magnitude (M b ) of 7. The Seismological Institute of the University of Chile located the epicenter 30 km southeast of the town of Tarapacá (latitude: -20º3’14”; longitude: -69º19’40”), at a depth of 114.9 km. The earth- quake produced relatively few large aftershocks in the days and weeks after the main event. Eleven people were killed, and there was damage to 9,350 dwellings (ONEMI 2005). The economic losses represent about US$40,000,000 for the Chil- ean government. Figure 1 shows the area affected, the epicenter, the strong ground motion recording sta- tions, and the major towns. The earthquake has been classified as an intraplate type, although the region is a highly active subduction zone where numerous large subduc- tion thrust earthquakes have struck over the last centuries. This is a des- ert zone where nitrate was produced until the middle of the 20th century. The port of Iquique, with 200,000 inhabitants, is the most populated city in the area affected by the earth- quake. The damage zone extended from Tana-Camiña gorge to Colonia Pintados. Intraplate earthquakes also affect other more populated regions such as Santiago, so it is important to analyze the performances of different types of construction. As an example, the M w 8.1 1939 Chillán earthquake was an intraplate event and killed approximately 30,000 people. Sociocultural Setting Throughout its history, the interior part of this region has been mostly inhabited by Aymara people who have left invaluable patrimonial con- structions and cultural artifacts. Nowadays, only 8% of the regional population lives in the interior towns—mostly children and older people—but many people come to this region for religious ceremonies once or twice per year. There has been a large migration to Iquique and its environs. Agriculture is the main economical activity of the region, on terraces at the bottom of the gorges. Onion, garlic, corn, carrots, and alfalfa are grown, plus some citrus and fruit trees including pears, pomegran- ates, olives, and quince. Figure 1. Overview map of area struck by the Tarapacá earthquake.
Transcript
1
EERI Special Earthquake Report — November 2005
Learning from Earthquakes
Intensities and Damage Distribution in the June 2005 Tarapacá, Chile, EarthquakeEditor’s Note: When a magnitude (Mw) 7.9 earthquake hit northern w) 7.9 earthquake hit northern w
Chile on June 13th, 2005, unrein-forced stone masonry and adobe dwellings were extensively dam-aged in ground motions ranging be-tween intensities IX-X in the epi-central area and VI near the coast.This report presents the fi ndings ofa University of Chile reconnais-sance team that visited approxi-mately 27 towns in the earthquake zone shortly after the event. It fo-cuses primarily on damage to dwell-ings and patrimonial structures and calculations of intensities. The team included Maximiliano Astroza, Ofelia Moroni, Ana Norambuena, and Rodrigo Astroza, all from the Department of Civil Engineering, University of Chile. They also pre-pared this report for EERI, the publication of which is funded by EERI’s Learning from Earthquakes Program, under National Science Foundation grant # CMS-0131895.
IntroductionThe Tarapacá earthquake struck part of northern Chile on June 13, 2005, at 6:44 pm, local time. The U.S. National Earthquake Infor-mation Center (NEIC) reported a moment magnitude (Mw) of 7.9 and a body-wave magnitude (Mb) of 7. The Seismological Institute of the University of Chile located the epicenter 30 km southeast of the town of Tarapacá (latitude: -20º3’14”; longitude: -69º19’40”), at a depth of 114.9 km. The earth-quake produced relatively few large aftershocks in the days and weeks after the main event. Eleven people were killed, and there was damage to 9,350 dwellings (ONEMI 2005). The economic losses represent about US$40,000,000 for the Chil-ean government. Figure 1 shows
the area affected, the epicenter, the strong ground motion recording sta-tions, and the major towns.
The earthquake has been classifi ed as an intraplate type, although the region is a highly active subductionzone where numerous large subduc-tion thrust earthquakes have struck over the last centuries. This is a des-ert zone where nitrate was produced until the middle of the 20th century. The port of Iquique, with 200,000 inhabitants, is the most populated city in the area affected by the earth-quake. The damage zone extended from Tana-Camiña gorge to Colonia Pintados. Intraplate earthquakes also affect other more populated regions such as Santiago, so it is important to analyze the performances of different types of construction. As an example, the Mw 8.1 1939 Chillán earthquake was an intraplate event and killed approximately 30,000 people.
Sociocultural Setting Throughout its history, the interior part of this region has been mostly inhabited by Aymara people who have left invaluable patrimonial con-structions and cultural artifacts.
Nowadays, only 8% of the regional population lives in the interior towns—mostly children and older people—but many people come to this region for religious ceremonies once or twice per year. There has been a large migration to Iquique and its environs.
Agriculture is the main economical activity of the region, on terraces at the bottom of the gorges. Onion, garlic, corn, carrots, and alfalfa are grown, plus some citrus and fruit trees including pears, pomegran-ates, olives, and quince.
Figure 1. Overview map of area struck by the Tarapacá earthquake.
2
EERI Special Earthquake Report — November 2005
GeologyLocated between 18º and 27º southlatitude in the North Chilean Andes, the region has four main morpho-structures with north-south orienta-tion: Main Cordillera, Precordillera, Intermediate Depression, and Coast-al Cordillera (see Figure 2). The most recent material in the region is quaternary-aged alluvial and fl uvial deposits. Most of the surveyed towns are located on top of this material, more than 1,500 m above sea level on the Precordillera and Intermediate Depression.
The Precordillera (1,500-3,600 m above sea level), which is almost en-tirely without vegetation, is a gentle west sloping surface crossed by E-Wtrending narrow deep valleys, whereoases have formed allowing moder-ate agricultural activities.
The Intermediate Depression is a N-S trending elongated basin with a mean altitude of 1,000 m. In the Intermediate Depression, three types of deposits can be found: fl uvial deposits at the bottom of deep val-
leys composed of silty sands and sands with gravel; alluvial deposits that comprise fi ne soils interbedded with thin coarse-grained soil layers; and Pica deposits in the southeast.
Strong Ground MotionsThe earthquake generated a numberof strong-motion records over a vari-ety of geologic site conditions. The hypocentral distanceDH and horizontal andvertical peak groundaccelerations (PGA)are indicated in Table 1.
The ratio of vertical tohorizontal PGA rangesfrom approximately 0.5 to 1.4 for this earth-quake. Hence, thecommonly assumedseismic code relation-ship (vertical PGA istwo-thirds of the hori-zontal PGA) is not ac-curate for these rec-ords.
The acceleration records obtained in Pica, the closest station to theepicenter (DH=125 Km), are shown in Figure 3. Notwithstanding thelarge peak acceleration, the dam-age in this town was minimal. Fig-ure 4 shows the undamaged build-ing where the recording instrument is located. Figure 5 shows the spec-tral acceleration responses of this record, for a damping ratio of 5%, indicating that rigid structures were likely the most damaged by this earthquake.
Types of Construction and DamageOne or two-story construction in thiszone can be classifi ed as unrein-
forced stone or rubble masonry fi xed with mud; adobe houses; quarry masonry fi xed with cement mortar; diagonally braced wooden frame fi lled with different materials; structural masonry, reinforced or confi ned; and wood construction.
Unreinforced stone masonry fi xed with mud: All of the old con-struction is of this type. The type of stone used is site-dependent. In the Pre-Cordillera region, quarry or rubble is used, while in places located close to Route A5 on the west side of Tamarugal Pampa (seeFigure 1), native saltpetre is used.Roofi ng materials have evolved from the original straw and mud type to zinc plates over wooden structures. Although most struc-tures have high wall density, large wall thicknesses, and small plan dimensions, their seismic cap-acity is very low, as evidenced byseveral collapsed structures ob-served in the epicentral zone. These failures buried people and the contents of the buildings.
Adobe houses: This type of con-struction is less frequent in this zone than in the central part of Chile. Adobe bricks are made with low plasticity, fi ne silts, andchopped straw. Although no rein-forcement is included, adobe per-forms better than the unreinforced stone masonry houses. Neverthe-less, fallen adobe walls were ob-served in the epicentral zone.
Quarry masonry fi xed with ce-ment mortar: In this type of con-struction, quarried stones are fi xed with cement mortar and are tied with reinforced concrete beams that support the roof. Its use is rather new in this zone, and damage tothis type of building was only moderate.
Braced wooded frame: Most of this construction is one or two-story and found in towns close to Route A5. These structures were built
Figure 3. Acceleration time history at Pica station (Boroschek et al. 2005)
Figure 4. Undamaged health center in Pica where ground motion record was obtained
since the end of 19th century, when oak pine from the nitrate mining installations was available. Void spaces between wooden timbers are fi lled with vertically aligned canes or stonescovered withmud or con-crete plaster.Generally, fi n-ishing and con-nections are ofgood qualityand, therefore,seismic perfor-mance is rathergood. Only amoderateamount of dam-age to stucco or fallen stones was observed. This kind of structure performed better in this earthquake than it did in the central Chile earth-quake of March 1985.
Structural masonry: Modern one or two-story public (healthcare cen-ters, schools) or private buildingsare structured with reinforced orconfi ned ceramic masonry or con-crete block walls. The behavior under seismic load was quite good, even in the epicentral zone, and some schools were used to shelter people whose houses had been damaged. However, there were exceptions: the Pica and Tarapacá schools, houses that were under construction, and some reinforced
Figure 6. Types of construction:(a) stone, (b) adobe, (c) quarry stone, (d) braced wooden frame, and (e) structural masonry.
(a) (b) (c)
(d) (e)
(a)◄
(b)▲
(c)►
(d)▼
Figure 7. Observed damage: (a) stone house at Iquinca, (b) adobe house at Usmagama, (c) structural masonry house at Pozo Almonte, and (d) braced wood frame at Huara.
masonry houses in Pozo Almonte. In the latter case, the wall density in one direction was quite low and the construction quality was very poor.
Signifi cant damage also was observed in nonen-gineered construction, where poor quality materi-als are used, detailed
5
EERI Special Earthquake Report — November 2005
reinforcement is inap-propriate, or openingsare excessive.
Wood construction:This type of construc-tion is made of pineoak brought to Chileby ships that came fornitrate at the end of the 19th and the be-ginning of the 20thcenturies. The architec-ture is reminiscent ofthe Anglo-Americanstyle. Examples in-clude the Iquique Ca-thedral, built in 1885;the Municipal Theater,built in 1889-1890; andthe buildings over Ba-quedano Street in Iqui-que. The good qualityof the wood and its fi n-ishing has helped tomaintain it in goodshape with little or nomaintenance. It hasgood seismic behavior.
Figure 6 shows examples of thesetypes of dwellings, while Figure 7shows observed damage in select-ed buildings due to the earthquake.
Patrimonial structures: Churches built in the 17th and 18th centuries represent the most valuable patri-monial structures in this region. They exist in nearly every town, re-gardless of how small the popula-tion is. Most of them have one nave with one or two bell towers joined tothe main structure, though the SanLorenzo of Tarapacá Church andthe Matilla Church have one iso-lated bell tower built of ashlar ma-sonry or lime and clay blocks, res-pectively. Unreinforced stone or adobe walls are the most common building materials. Original straw and mud roofs have been gradually replaced by zinc plates (Benavides et al. 1997).
These churches have been struck
Figure 8. (a) Camiña Church prior to ▲ and after ► the earthquake; and (b) Usmagama Church prior to ▼ and after ► the earthquake.
with a nave of 6.3 x 45 m, built in the 18th century, was affected by a fi re in 1955. Its reconstruction included a 17-m-long concrete beam resting on the adobe walls and a wooden roof (Figure 9). The only retrofi t tech-nique that worked well was the res-toration of some bell towers with confi ned concrete blocks, as shown for the Guarasiña church in Figure 10.
Different architectural styles and material solutions can be found in churches built near Route A5. They are wood-framed structures fi lled with different types of materials, and their seismic performance was much better. Figure 11 shows two of them (Pica and La Tirana). Church San Andres de Pica was built in 1877, with a wooden frame struc-ture fi lled with cane and mud; the nave is 19.25 x 44.25 m. La Tirana is one of the most famous sanctuar-
by earthquakes severaltimes and, unfortunate-ly, their maintenancehas not been ideal. Ret-rofi tting or repairing of subcompo-nents have followed different strate-gies, from using the same original material to introducing reinforced con-crete beams or columns. Unfortu-nately, none of these strategies has worked well and there has been extensive damage.
All the churches were damaged in theJune 13 event. Even in the towns least affected by the earthquake, where all other structures emerged unscathed, the churches often had some degree of damage. Figure 8 shows some emblematic churches (Camiña, and Usmagama) both prior to and after the earthquake. The Camiña church has a nave 7.5 m wide x 50 m long, adobe walls and an 18th century portal of quarry stone. Usmagama church, built in the 18th century, had an Andean baroque style façade and was retrofi t-ted with reinforced concrete columns. The San Lorenzo of Tarapacá Church,
6
EERI Special Earthquake Report — November 2005
and 5 (both based on Monge and As-troza 1989). In previous earthquakes, determination of the intensity values has been based only on the perfor-mance of adobe houses. However, in this case, there were more struc-tures with walls of unreinforced stone masonry fi xed with mud. These struc-tures were heavily damaged, and the intensity had to be verifi ed through comparisons with other types of con-struction, such as adobe and rein-forced masonry houses.
Figure 12 summarizes estimated in-tensities for every visited town. Much of the damage was concentrated be-tween Tarapacá and Mocha in the east-west direction, and as far as
ies in Chile; every July 16th about half a million people come there to venerate la Virgen del Carmen.
Seismic IntensitiesIntensity values are determined based on the statistical distributionof damage in different types of structures, which are classifi ed according to vulnerability (see Table 2). The European Scale of Intensi-ties (MSK) is used, which is based on the damage distribution for each vulnerability class as shown in Table 3.
Damage grades for different types of building are given in Tables 4
Figure 9. San Lorenzo de Tarapacá Church.Figure 10. Guarasina Church after restoration of the bell tower.
Figure 11. Wooden church structures: (a) Pica and (b) La Tirana.
Baquedano in the south. The loca-tion of Usmagama and Limacsiña on alluvial hillside deposits could explain the higher intensity values. Geotechnical engineers noted land-slides further north in the damaged zone, indicating that the rupture propagated from the epicenter to the north.
Superimposed in Figure 13 are the estimated intensities (I) and inten-sity average attenuation curves de-termined for the 1939 Chillán earth-quake (Mw = 8.1 [Reyes, 2003]) and the 1997 Punitaqui earthquake (Mw = 7.1 [Pardo et al. 2002]) as functions of the hypocentral dis-tance DH (Astroza et al. 2005). Both
7
EERI Special Earthquake Report — November 2005
Table 2. Vulnerability class
Table 3. Intensity degree based on damage distribution according to MSK (Monge and Astroza 1989)
Table 4. Description of damage in stone, adobe or masonry buildings
Table 5. Description of damage for buildings with braced wooden frame
earthquakes had the same rupture mechanism: intraplate motion at an intermediate depth. The damage caused by the present earthquake lies between that caused by the other two.
Conclusions� Stone or adobe houses must beavoided in regions where earth-quakes may strike. Confi ned ma-sonry, when it is well constructed, is the best material to resist damage in an earthquake for low-cost dwellings.
� Seismic design codes for mason-ry buildings should suggest mini-mum wall density per unit fl oor. In previous earthquakes, this param-eter has been related to expected damage levels, and it has been shown that a minimum wall den-sity per unit fl oor does control the damage level (Astroza et al. 1993).
� Vulnerability increases dramati-cally when the houses are located on alluvial deposits that are near retaining walls, as happened with houses built on hillsides.
� Generally, interior walls built with light construction materials uncon-nected to the lateral load-resistant structure do not have adequate support and may collapse, even if the main structure does not, thereby increasing the risk of injury to the inhabitants.
8
EERI Special Earthquake Report — November 2005
del Altiplano, Caseríos y villorrios ariqueños, Facultad de Arquitectura y Urbanismo, Universidad de Chile.
Boroschek, R., D. Comte, P. Soto, R. León, 2005. Registro Estación Pica-Terremoto Norte Chile 13 de Junio de 2005 M = 7,9, Informe Preliminar, Red de Acelerógrafos Zona Norte, Departamento de In-geniería Civil y Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile.
Harambour, S., 1990. Geología pre-cenozoica de la cordillera de los Andes entre las quebradas Aroma y Juan Morales I Región, Memoria para optar al título de Geólogo, De-partamento de Geología y Geofís-ica, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile.
INN, 1997. NCh2123.Of97 Norma Chilena: Albañilería confi nada—Re-quisitos de diseño y cálculo. In-stituto Nacional de Normalización, Santiago, Chile.
Monge, J., Astroza, M., 1989. Meto-dología para determinar el grado deIntensidad a partir de los daños, Proc. 5th Jornadas Chilenas de Sis-mología e Ingeniería Antisísmica, Santiago, Chile, August, 7-11. Vol I.
ONEMI, 2005. Onemi: Terremoto deja al menos 6018 personas dam-nifi cadas, El Mercurio on Line, Miér-coles 15 de Junio, www.emol.com.
Pardo M., D. Comte, T. Monfret, R.Boroschek, M. Astroza, 2002. TheOctober 15,1997 Punitaqui Earth-quake (Mw=7.1): A destructive event within the subducting Nazca plate in Central Chile, Tectono-physics, V. 345/1-4 , pp 199-210.
Reyes, M. 2003. El terremoto de Chillán de 1939 y el terremoto deTalca de 1928, Memoria para optaral título de Ingeniero Civil, Departa-mento de Ingeniería Civil, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile.
AcknowledgmentsThis research was partially fi nanced by the University of Chile, Iniciativa Científi ca Milenio Núcleo en Sis-motectónica y Peligro Sísmico and Fondecyt Grant Nº1030554. The authors thank the Chilean Army for logistic collaboration of Baquedano Regimiento staff, and Professor Ricardo Herrera for his comments.
References Astroza, M., M.O. Moroni, M. Kup-fer, 1993. Califi cación sísmica de edifi cios de albañilería de ladrillo
confi nada con elementos de hormi-gón armado, Proc. XXVI Jornadas Sudamericanas de Ingeniería Estruc-tural, Montevideo, Uruguay, Noviem-bre, Vol. 1, 327-338
Astroza, M., Sandoval, M., and Kau-sel, E., 2005. Estudio comparativo de los efectos de los sismos chilenos de subducción del tipo intraplaca de profundidad intermedia, Proc. 9th Jornadas Chilenas de Sismología e Ingeniería Antisísmica, Concepción, Chile, November, 16-19.
Benavides, J., Márquez de la Plata, R., Rodríguez, L., 1997. Arquitectura
Figure 12. Values of intensity degree
Figure 13.Comparison between intensities of 2005 Tarapacá Earthquake and attenua-tion relation-ships from 1939 Chillán Earthquake and 1997 Punitaqui Earthquake.
