Maule Earthquake

Damage Assessment and Seismic Intensity Analysis of the 2010 (Mw 8.8) Maule Earthquake

Maximiliano Astroza,a) Sergio Ruiz,b), c) and Rodrigo Astrozad), e)

The MSK-64 seismic intensities inside the damage area of the 2010 Maule earthquake are estimated. With this purpose, field surveys of damage to typical single-family buildings located in 111 cities of the affected area were used. Cities located close to the north part of the earthquake rupture suffered higher damage, but most of this damage concerned adobe and unreinforced masonry houses. Minor and moderate damage was noted in modern low-rise engineered and nonengineered constructions, especially in confined masonry buildings. Despite the large length of the rupture, which reached more than 450 km, only one intensity value equal to IX was determined, and 21% of the values were greater than VII. The attenuation of seismic intensity was controlled by the distance to the main asperity more than to the hypocenter, which would be an important characteristic of the megathrust earthquakes, and it should therefore be considered in the seismic risk of large subduction environ- ments. [DOI: 10.1193/1.4000027]

INTRODUCTION The 2010 Maule earthquake is the fifth-largest instrumentally recorded earthquake; thus, the study of damage and seismic intensities associated with the earthquake is useful to under- stand the effects of historical great thrust earthquakes and the seismic hazard in other seismic zones, where this type of earthquake is expected and there are no strong ground motion records of large earthquakes, as in Northern Chile (Kelleher 1972) or the Cascadia subduction zone (Heaton and Hartzell 1987). The great Chilean interplate or thrust earthquakes are characterized by a few zones of highest slip that controlled their rupture (Ruiz et al. 2011), which will be called asperities, following the original idea of Kanamori and Stewart (1978). The rupture process of the Maule earthquake was characterized by more than one asperity (Lay et al. 2010, Delouis et al. 2010, Tong et al. 2011, Lorito et al. 2011); however, the asperity with higher slip

a) Department of Civil Engineering, Universidad de Chile, Blanco Encalada 2002, Santiago, Chile b) Department of Geology, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile c) Départament des Géosciences, Ecole Normale Superieure, 24 rue Lhomond, 75005 Paris, France d) Faculty of Engineering, Universidad de Los Andes, San Carlos de Apoquindo 2200, Santiago, Chile e) Ph.D. student, Department of Structural Engineering, University of California, San Diego, La Jolla, CA, 92093

(main asperity) is located to the north of the seismic gap, approximately in the same rupture area of the 1928 Talca earthquake (Ruiz et al. 2012). On the other hand, the 2010 Maule earthquake caused damages in a zone inhabited by more than the 50% of the Chile’s population (INE 2002). The prevailing building types in the cities facing the 2010 earthquake are old traditional adobe constructions (6.1%) and masonry houses (51.9%) built in the rural zone and the urban area of cities located in the Chilean Central Valley; this means that constructions are concentrated in a few vulnerability classes (Medvedev and Sponheuer 1969, Grünthal et al. 1998). Most of these buildings were located on alluvial and fluvial deposits and many of them were old and weak nonengineered structures. In this paper, we describe the damage in the most common type of low-rise buildings found in 111 cities visited after the earthquake, and we estimate the MSK-64 seismic intensity values in those places based on the observed damage due to ground shaking. These intensity values are an important tool to establish a link between historical and modern large earthquakes, for which it is not possible to make an instrumental correlation, allowing a comparison from different points of view, for instance, in the size of the boundary of the damaged region or the distribution of damages (local site conditions). With these results, a correlation between the distance to the main asperity and the I MSK-64 seismic intensity ( MSK‐64) is made, and a seismic intensity attenuation formula is proposed for the 2010 Maule earthquake. The formula is valid for the intensity range ≤ I ≤ V MSK‐64 IX, and it will be useful to compare the damage in Maule earthquake with previous large earthquakes in Chile (Mw ≈ 8.0) for which strong-motion records are not available.

SHAKING EFFECTS This earthquake caused widespread damage and was felt strongly throughout 500 km along the country, between 33.5°S and 38°S, and many towns and villages were affected. This study does not consider the damage in cities decimated by the tsunami that followed the earthquake, because it is very difficult to identify the damage associated only to the shaking effects. However, a field visit to the coast zone affected by the tsunami did not indicate very high damage levels due to the ground motion, and many traditional and newer single- family buildings are intact or with light or moderate damage. Light to moderate damage was also observed in other zones located in hard soil and rock conditions, despite the earthquake was felt most conspicuously in recent Quaternary deposits, mainly alluvial and fluvial, where shaking amplification have occurred historically. Different type of construction suffered very different degrees of damage during the earthquake. Structures built using weak materials (adobe) and old-fashioned buildings (unreinforced masonry buildings built before the 1940s) were more affected, and modern single-family, confined masonry dwellings suffered no damage, as was observed in many houses built in the last 30 years in the areas damaged by the Maule earthquake. The region affected by the earthquake was exposed to several major earthquakes in the past, including the 1985 Central Chile earthquake (Mw 8.0). However, the performance of DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S147 the buildings during past earthquakes was based on single-family dwellings of one and two- stories, built with adobe and masonry. In the 2010 Maule earthquake, reinforced masonry (RM) and confined masonry (CM) buildings of three and four stories were exposed to severe ground shaking for the first time. The construction of RM and CM apartment buildings in Santiago began in the 1970s and, in the 1990s, in other main urban centers located in the areas damaged by the 2010 Maule earthquake (e.g., Rancagua (34.16°S, 70.74°W), San Fernando (34.58°S, 70.99°W), Talca (35.42°S, 71.66°W), Constitución (35.34°S, 72.41°W), and Concepción (36.82°S, 73.03°W)). Most of the engineered masonry buildings of three and four stories remained undamaged; nevertheless, a few of them were severely damaged, and three three-story buildings collapsed. More details about the deficiencies of modern masonry construction are described by Astroza et al. (2012).

DAMAGE IN SINGLE-FAMILY BUILDINGS The majority of the building stock in the central-south region of Chile reveals that the dominant buildings are nonengineered adobe and brick masonry one-story structures (Morales and Sapaj 1996), and field observations suggest that the damage was concentrated in this type of construction. Typical buildings are described in turn below.

ADOBE BUILDINGS

Most of adobe buildings are old buildings rebuilt after the 1928 Talca (Mw 7.7) and 1939 Chillán (Mw 7.7) earthquakes, and they are single-story buildings constructed following traditional techniques and using locally available materials and labor based on experience without a technical support (Figure 1a and 1b). The walls are made of sun-dried clay brick in mud mortar, both stabilized with straw and made by hand. Foundation and plinth are built with brick or stone units bonded with mud or cement/lime mortar (Figure 2a). The roof is directly supported on the load-bearing walls (Figure 2b), and it is made with timber beams or timber truss covered by clay roofing tiles put over a thick mud isolation layer. Adobe houses have an overall height between 2.5 m and 4.0 m. The wall thickness ranges from 300 mm to 600 mm. Most walls are plastered either with cement/lime or mud mortar, with a thickness of about 40 mm. This plaster is bonded with an expanded thin steel perforated sheet to the wall. Wood lintels are put above the window and door openings (Figure 2c). Furthermore, there is no other reinforcement in the walls like vertical supporting columns or tie-beams running on the top of the walls. The partitions are built with a timber bracing frame structure infilled with adobe units and covered with a mud plaster (Figure 2d), without connections with the load-bearing adobe walls. The collapse of adobe buildings is due to a weak connection among the walls, and between the walls and the roof; the heavy weight of walls and roof; very low strength; and slender unreinforced walls. Also, the old age, low quality of construction and materials, and the lack of maintenance contribute to damage in some cases. The presence of buttresses in order to reduce the unsupported wall length can be proved as a good seismic reinforcement for this type of construction. Typical damages are: vertical cracks at the wall corners resulting in the separation of them (Figure 3a), diagonal cracks (Figure 3b), and collapse through out-of-plane failure S148 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 1. Photos of typical adobe structures in the affected area: (a) adobe house in Marchigue (34.40°S, 71.61°W) and (b) adobe house in Paredones (34.66°S, 71.89°W). mechanism (Figure 3c), with subsequent collapse of the roof due to the loss of the wall support (Figure 3d). A few adobe buildings survived the 2010 Maule earthquake with moderate damage level in moderate intensity area, IMSK < VII (e.g., Rancagua) and many buildings suffered partially or completely collapse in high intensity area, IMSK ≥ VII (e.g., Talca, Curepto (35.08°S, 72.02°W), and Constitución). DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S149

Figure 2. Photos of details in adobe houses located in the central-south part of Chile: (a) plinth built with brick, (b) support of roof, (c) wood lintel above the window, and (d) partition.

UNREINFORCED MASONRY BUILDINGS Low-rise unreinforced masonry buildings are mostly old construction built in the 1920s with burnt clay bricks in cement/sand mortar. They are located in the urban area of the main cities affected by the 2010 Maule earthquake, and some of them are part of Chile’s patrimonial heritage (Figure 4a and 4b). The wall thickness ranges from 300 mm to 450 mm. Building heights vary between 3 and 4 m for one-story buildings. In two- and three-story buildings, the floor is a timber diaphragm that works as a one-way slab where loads are transferred to the wood joist and then to the load-bearing walls. Usually, these buildings do not have reinforcement except for a RC lintel above the window and door openings. The roof is a corrugated iron sheet resting on a timber frame or truss. In some cases, there are RC tie-beams running on the top of the walls, where the roof is bore. The partitions are built with a timber-bracing frame infilled with adobe units without connections to the walls of the structural resistant system. Damages sustained in this type of construction are similar to adobe houses, due to lack of reinforcement in the wall corners and in the walls, and an inadequate wall-to-floor anchorage. These conditions would explain the vertical separation of the intersecting walls, in-plane diagonal shear cracking, out-of-plane wall collapse, flexural cracking of a free-standing S150 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 3. Typical damage in adobe structures and their corresponding grade of damage: (a) vertical cracks at wall corner (G3), (b) diagonal crack in wall (G3), (c) wall collapse through out-of- plane (G4), and (d) collapse of the roof (G5). parapet and gable walls and loss of a wall segment due to intersecting diagonal cracks. Out- of-plane damage is a very common damage pattern, especially in tall historical buildings as churches (Figure 5a) and in upper portions of buildings. In general, unreinforced masonry buildings sustained significant damage (Figure 5b), a confirmation of their fragility and that they should not be used in seismic areas. In the cities located within the areas damaged by the 2010 Maule earthquake, this sort of construction was not affected seriously by the 1985 Central Chile (Mw 8.0) and the 1960 Concepción (Mw 8.2) earthquakes, for example, Rancagua, San Fernando, Curicó (34.98°S, 71.22°W), Talca, Linares (35.85°S, 71.59°W), and Los Ángeles (37.47°S, 72.35°W).

CONFINED MASONRY BUILDINGS This type of construction is widespread over the central-south region of Chile. Typically, buildings are single-family, partially confined masonry dwellings of up to two stories high. They have had a good performance, including during the 1939 Chillán (Mw 7.7) earthquake and the 2010 Maule earthquake (Mw 8.8), where most of them did not experience any damage, with the exception of a few cases that suffered moderate damage. DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S151

Figure 4. Photos of unreinforced masonry structures (patrimonial heritage): (a) building of Talca Intendencia and (b) San Fernando governorship building.

Confined masonry buildings are built with shear masonry walls and reinforced concrete (RC) confining elements (tie-columns and tie-beams), where the walls are constructed first, followed by casting of concrete in RC tie-columns. Walls are built using a variety of masonry units (e.g., handmade solid clay unit and machine-made perforated and hollow clay bricks) in cement/sand mortar. The RC tie-columns are usually placed at wall intersections, and in many cases a RC tie-column is not provided at openings. The spacing between vertical RC confining elements is about 3 m to 3.5 m. A toothed interface between the walls and the tie-columns was observed in new and existing buildings, as shown in Figure 6a and 6b. The RC tie-beams are provided at the floor and roof levels. The floor system is planking on timber beams and the roof is a corrugated iron sheet on timber trusses, which is anchored S152 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 5. Typical damage in unreinforced masonry structures: (a) Putu church (35.22°S, 72.28°W) and (b) building in Curicó (34.98°S, 71.22°W). to RC tie-beams running at the perimeter on the top of the walls. Many buildings have small plan dimensions and they are attached between them (townhouse-style). Performance of single-family low-rise confined masonry is excellent, large majority of buildings were not damaged at all. The most common damage pattern observed in masonry walls were in-plane shear cracking (Figure 7a) and damage to the RC confining members, DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S153

Figure 6. Details of connection between walls and the tie-columns in confined masonry building. particularly RC tie-columns (Figure 7b). Out-of-plane wall damages were observed in parapet and gable walls (Figure 7c and 7d). The most severe damage in this type of buildings can be attributed to inadequate quality of mortar and construction process, deficiencies in detailing of reinforcement in RC confining elements, absence of reinforcement around the wall openings, and geotechnical and topo- graphics issues (Astroza et al. 2012).

SEISMIC VULNERABILITY Considering that the single-family buildings used to estimate the seismic intensity were built without the assistance of a professional (engineer or architect), there is no information about mechanical properties of them, but they represent the quality of the typical construction that had been affected by Chilean earthquakes in the last 100 years in this zone of the country, and consequently the observed response during previous events might be considered comparable. S154 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 7. Damage observed in confined masonry buildings: (a) in-plane shear cracking, (b) damage to the RC tie-column, (c) damage in parapet, and (d) damage in gable wall.

The building stock in the region can be grouped, according to ability to resist damage from earthquakes, into four vulnerability classes depending upon the wall materials and the reinforcement (Monge and Astroza 1989). The vulnerability assignment of the single-family buildings is conducted in terms of the MSK-64 macroseismic scale (Medvedev and Sponheuer 1969) and EMS-98 scale (Grünthal et al. 1998; Table 1); it is an attempt to cate- gorize, in a manageable way, the strength of structures, taking building type and other factors into account.

Table 1. Vulnerability class of single-family Chilean buildings

Vulnerability Class (VC) Basic type of structure (TS) VC-A Adobe. Masonry stone in mud mortar. VC-B Unreinforced masonry. Braced wooden frame infilled with adobe unit. VC-C Confined masonry. VC-D Confined masonry designed according to NCh2123 Chilean Code. DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S155

MSK INTENSITIES OF THE 2010 MAULE EARTHQUAKE The intensity values in 111 cities in the earthquake’s damage area are estimated from the field surveys considering the distribution of the observed damage in adobe and masonry single-family buildings of one and two stories. For this purpose, the buildings are classified in one of the vulnerability classes (VC) indicated in Table 1, and the grade of damage is assigned using the description of the MSK-64 macroseismic scale (Medvedev and Sponheuer 1969), adapted to the Chilean houses by Monge and Astroza (1989). The damage is classified by six grades as follows: Grade 0 (G0)-No damage; Grade 1 (G1)-Slight damage (fine cracks in plaster; falling of small pieces of plaster); Grade 2 (G2)- Moderate damage (fine cracks in walls; vertical cracks at wall intersections; horizontal cracks in chimneys, parapets and gables; spalling of fairly large pieces of plaster; falling of parts of chimneys; sliding of roof tiles); Grade 3 (G3)-Heavy damage (large and deep diagonal cracks in most walls; large and deep vertical cracks at wall intersections; some walls lean out-of- plumb; falling of chimneys, parapets and gable walls; falling of roof tiles); Grade 4 (G4)- Very heavy damage (partial or total collapse of a wall in the building; collapse of building partitions); Grade 5 (G5)-Collapse or destruction (Collapse of two or more walls in the building). The seismic intensity value (above MSK-64 intensity V) is calculated using the relation- ship between the intensity degree and the distribution of damage grades for each basic vulnerability class proposed by the MSK-64 scale, as adapted by Karnik et al. (1984), defining the number of houses damaged in the following terms: few (5%), many (50%), or most (75%; Table 2). Karnik et al. (1984) completed the description of the MSK-64 scale so that the total of 100% is reached for each intensity degree and vulnerability class. In order to include VC-D class in Table 2, we used the percentage proposed by Karnik et al. (1984) and the masonry buildings classification and distribution of the damage grade proposed by the European Macroseismic Scale (EMS-98; Grunthal et al. 1998). The damage field surveys were made in several blocks around the main square of each city (Astroza et al. 2010), taking care that the sample size was greater than 20 cases (Monge and Astroza 1989). The use of Table 2 reduces the ambiguities because the effects on a number of the most common type of buildings in a limited area are considered as sensors. The estimated intensity values are shown in the Table 3 and Figure 8. Seismic intensities were plotted as point data and as isoseismal map, which provides a good overall description of the extent of the damage. These isoseismal curves consider only the sites on hard soil (compact and old alluvial or fluvial deposits) and ignore damage due to the tsunami, land- slides, or liquefaction; in this way, the proposed isoseismal map represents the intensity values in a hard soil with shear-wave velocities between 500 m/s and 900 m/s, according to soil type II, in the new Chilean seismic code of buildings (INN 2010). From Figure 8, it is possible to appreciate that the VIII isoseismal curve of the 2010 Maule earthquake is related with the Northern asperity and the length of the rupture area is related with the length of the VII isoseismal curve, confirming that the damage is closely related with the asperity distance (Ruiz et al. 2011), rather than with the hypocenter distance. The isoseismal curves are elongated in the north–south direction, according to the earthquake slip distribution or the shape of the main asperities (Figure 8). S156 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Table 2. Damage grades (DG) and damage ratios (N, in %) for individual intensity degree of the MSK-64 scale (Modified fron Karnik et al. 1984)

VC-A VC-B VC-C VC-D Intensity degree DG N (%) DG N (%) DG N (%) DG N (%) G1 5 G0 100 G0 100 G0 100 V G0 95 G2 5 G1 5 G0 100 G0 100 VI G1 50 G0 95 G0 45 G45G250G150G15 G3 50 G1 35 G0 50 G0 95 VII G2 35 G0 15 G1 10 G5 5 G4 5 G3 5 G2 5 G4 50 G3 50 G2 50 G1 50 VIII G3 35 G2 35 G1 35 G0 45 G2 10 G1 10 G0 10 G5 50 G5 5 G4 5 G3 5 G4 35 G4 50 G3 50 G2 50 IX G3 15 G3 35 G2 35 G1 35 G2 10 G1 10 G0 10 G5 75 G5 50 G5 5 G4 5 G4 25 G4 35 G4 50 G3 50 X G3 15 G3 35 G2 35 G2 10 G1 10 G5 100 G5 75 G5 50 G5 5 XI G4 25 G4 50 G4 50 G3 35 G2 10 XII G5 100 G5 100 G5 100 G5 100

The highest intensity value is due to amplification effect of some Quaternary deposits. The highest MSK-64 intensity was IX in Constitución, located to the north of the epicenter on a silty sand soil. At locations where the MSK-64 intensity was VIII, almost all the adobe houses became uninhabitable (Damage grade 3 or more), and many of them (55%) totally or partially collapsed. Several cities, like Talca, Parral (36.15°S, 71.82°W), and Constitución, located in the epicentral zone, did not show a homogenous damage distribution, and the highest intensity was found on volcanic ash deposit at Talca and Parral and silty sand soil at Constitución DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S157

Table 3. Estimated MSK intensities (Astroza et al. 2010). The identification number of each city is shown in Figure 8

MSK Intensity Cities ≥ VIII Peralillo (1), Pumanque (2), Licantén (3), Curepto (4), Constitución (5), Talca (6), Cauquenes (7), Parral (8) VII ½ Navidad (9), Doñihue (10), Coínco (11), Esperanza (12), Coltauco (13), Pichidegua (14), Población (15), Santa Cruz (16), Chépica (17), Lolol (18), Curicó (19), Gualleco (20), Batuco (21), Concepción (22) VII El Convento (23), San Enrique (24), La Boca (25), Matanza (26), Rengo (27), Litueche (28), Machalí (29), La Estrella (30), Peumo (31), San Vicente de Tagua Tagua (32), Las Pataguas (33), Teno (34), Rauco (35), Hualañe (36), Molina (37), Cumpeo (38), Pencahue (39), Bobadilla (40), Empedrado (41), Chanco (42), Retiro (43), Cobquecura (44), Quirihue (45), Ninhue (46), San Carlos (47), Quillón (48), Bulnes (49), Penco (50), Tomé (51), Florida (52), Lota (53), Santiago (106) VI ½ Pomaire (54), Talagante (55), El Monte (56), Melipilla (57), San Pedro (58), Rapel (59), Rancagua (60), San José (61), Marchigue (62), San Fernando (63), Paredones (64), Nancagua (65), La Huerta (66), Pelarco (67), San Clemente (68), San Javier (69), Villa Alegre (70), Linares (71), Longaví (72), Coelemu (73), San Nicolás (74), Chillán (75), Ñipas (76), Cabrero (77), Los Ángeles (78), Angol (79) VI San Antonio (80), Codegua (81), Las Cabras (82), Requínoa (83), Chimbarongo (84), Romeral (85), Yerbas Buenas (86), Trehuaco (87), San Ignacio (88), General Cruz (89), Pemuco (90), Monte Aguila (91), Yumbel (92), Yungay (93), Arauco (94), La Laja (95), Curanilahue (96), Nacimiento (97), Lebu (98), Los Álamos (99), Tres Pinos (100), Valparaíso (107), Viña del Mar (108) V Renaico (101), Cañete (102), Los Sauces (103), Contulmo (104), Purén (105), Los Vilos, Illapel, Salamanca

(Morales and Sapaj 1996). This pattern would be associated with the surface geological conditions (Astroza and Monge 1991). At Constitución, the situation was similar to that observed in Valparaíso (33.05°S, 71.60°W) after the 1906 Valparaíso (Saragoni and Carvajal, 1991) and 1985 Central Chile earthquakes (Acevedo et al. 1989), with high intensity values in the flat zones of the city (silty sand soil) and lower values at hill zones. Although in hill areas the seismic intensity was usually low, when the structures were located on the hilltop ridge, topographic effects were remarkable. At O’Higgins Hill, located in Constitución, the structures next to the ridge suffered important damage while structures located far away from the ridge did not suffer any damage (Figure 9a and 9b). A similar pattern was identified at Canal Beagle for the Chile Central 1985 earthquake (Çelebi 1987). S158 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 8. Intensities of each city and the isoseismal map inside the damage area of the 27 February 2010 earthquake. Roman numerals refer to the MSK scale and the Arabic numerals are associated with the cities in Table 3. The slip distributions of Maule 2010 earthquake modified from Tong et al. (2011) are also shown.

INTENSITY ATTENUATION A classical regression analysis was considered for fitting the intensity data collected in the 111 cities visited after the earthquake. We consider the intensity attenuation of the form:

IðΔÞ¼C þ C ⋅ R þ C ⋅ ðΔÞ EQ-TARGET;temp:intralink-;e1;50;162 1 2 3 log10 (1)

C C C where the constants 1, 2, and 3 can be associated to different stress drop, absorption and scattering, and geometric spreading respectively (Bakun and Joyner 1984). These constants are calculated by the least-square method considering the mean or median value of the DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S159

Figure 9. Buildings of O’Higgins Hill–Constitución: (a) Collapse of building next to the ridge (first-floor collapse), (b) building located far away from the ridge (without damage). distance Δ for each intensity level. The variablepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiΔ can be the hypocentral distance or the distance to the asperity centroid, computed as R2 þ h2, where R is the distance between the site and the point on the earth’s surface vertically above the hypocenter, or the distance between the site and the point on the earth’s surface vertically above the asperity centroid respectively, and h is the depth of the hypocenter or the asperity centroid respectively. Using 945 values of Mercalli Modified Intensity (MMI) from 73 events (especially interplate or thrust) with M ≥ 5.5 and focal depth less than 120 km whose stroke Chile between 1906 and 1977, Barrientos (1980) established the following intensity attenuation formula for Chilean earthquakes: I ðΔÞ¼ ⋅ M − ⋅ ðΔÞ − ⋅ Δ þ EQ-TARGET;temp:intralink-;e2;71;316 MMI 1.3844 w 3.7355 log10 0.0006 3.8461 (2) The database considered by Barrientos (1980) includes 13 events with M > 7.0, which represents a high number of large earthquakes (including the 1906 Valparaíso Mw 8.5, 1928 Talca Mw 7.7, 1939 Chillán Mw 7.7 and 1960 Valdivia Mw 9.5). However, only 25 events > have intensity MMI V. In this way, the Maule earthquake (Mw 8.8) allows, for the first time, the study of the intensity attenuation of a complete database for a megathrust earthquake in the Chilean subduction. Furthermore, it is important to note that for the Maule earthquake, we are considering only values of intensity equal or higher than V, because few of the visited cities showed intensities lower than V, and such data are clearly incomplete; consequently, to consider them in the attenuation would induce statistical bias. Figure 10 shows the intensities estimated for the 2010 Maule earthquake versus the hypocentral distance and Barrientos’s (1980) formula for Mw ¼ 8.8. Two datasets are shown, where the black triangles are the intensities corresponding to locations to the south of the epicenter and the gray circles are the intensities corresponding to locations to the north of the epicenter (36.29°S, 73.24°W). Clearly, the Barrientos’s hypocentral attenuation formula does not reflect the intensity attenuation for the 2010 Maule earthquake appropriately. S160 M. ASTROZA, S. RUIZ, AND R. ASTROZA

Figure 10. MSK-64 intensity versus hypocentral distance plot for the 2010 Maule earthquake. Two sets of data are shown: Black triangles are the intensities corresponding to locations to the south of the hypocenter, and the gray circles are the intensities corresponding to locations to the north of the hypocenter.

However, the rupture process of this event shows one main asperity where the highest slips were concentrated. The centroid of main asperity is located to more than 100 km of the hypocenter. Coordinates and depths of the centroid of the main asperity inferred from Tong et al. (2011) and the hypocenter are summarized in Table 4. Considering that the most important high frequency energy was released from the one ≥ main asperity (Ruiz et al. 2012), it is natural to expect that the highest intensity (IMSK VIII) is closer to this asperity rather than to the hypocenter (Figure 8), which would imply that the greater damage produced by the 2010 Maule earthquake on the structures that were considered to evaluate the intensity (low-rise, rigid structures) were controlled by this asperity. Figure 11 shows the intensities estimated for the Maule earthquake versus the distance to the main asperity centroid (ΔA) without accounting for site effects explicitly. From this data, a regression analysis is carried out in order to obtain the intensity attenuation curve. Also, the confidence bounds of the mean values for a coefficient of 95%, equal to approximately two

Table 4. Coordinates and depths of the main asperity centroid (inferred from Tong et al. (2011) and hypocenter of SSN (2011)

Latitude Longitude Depth (km) Centroid Main Asperity 34.8°S 72.6°W 20 Hypocenter 36.29°S 73.24°W 30.1 DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S161 times the standard deviation if we consider normally distributed errors, are computed. From the regression analysis, the formula describing the intensity attenuation for the 2010 Maule earthquake is as follows:

I ðΔ Þ¼ − ⋅ ðΔ Þþ ⋅ Δ EQ-TARGET;temp:intralink-;e3;71;612 Maule−2010 A 19.781 5.927 log10 A 0.00087 A (3)

From Figure 11, it can be noted that the data fit very well if we consider the distance to the northern asperity centroid. Consequently, this asperity would have controlled the damage produced by the earthquake on the structures if it took into account the estimation of the intensity values (low-rise rigid structures), which would be the result of the high-frequency pulses generated in this asperity (Ruiz et al. 2012). Formulas that use the shortest distance to the surface projection of the fault plane gave a low correlation, due to the higher release of energy was concentrated in the Northern zone of the fault rupture (see Figure 8). From Figure 11, it is also clear that the average trend of the intensity decay is well repro- duced by our relations and since the uncertainties have been estimated, the error in a new intensity estimate using the proposed relations is approximately of 0.5 intensity units, which is basically due to the nature of intensity assessment and other effects, for example, site conditions. Comparing Figures 10 and 11, it can be noted that the data are better grouped together, reducing the scatter considerably if we consider the distance to the main asperity instead of the hypocentral distance. This characteristic is very clear when the intensity value is equal to V. This implies that for the study of the seismic hazard of megathrust earthquakes having an extensive area of rupture, as in the 2010 Maule earthquake and the Valdivia earthquake of

Figure 11. Intensity versus distance to the main asperity centroid plot, comparing the estimated intensities with the Equation 3 for the 2010 Maule earthquake data. S162 M. ASTROZA, S. RUIZ, AND R. ASTROZA

22 May 1960 (Astroza and Lazo 2010), the intensity attenuation should be defined as a func- tion of the distance to asperities, instead of the hypocentral distance or the nearest distance to the rupture area, as had been assumed so far. This results in an important conclusion about megathrust earthquakes by giving a guide- line for future work related to intensity attenuation curves for thrust earthquakes of large magnitude.

DISCUSSION AND CONCLUSIONS Despite the large rupture length of more than 450 km, only one intensity value equal to IX was determined, and 21% of the values are greater than VII. The 2010 Maule earthquake low- intensity values are in agreement with previous studies of other big interplate earthquakes, such as the 1906 Valparaíso earthquake (Saragoni and Carvajal 1991), the 1960 Valdivia earthquake (Astroza and Lazo 2010), and the 2004 M 9.0 Sumatra-Andaman earthquake (Martin 2005). The most important damage occurred in the VI and VII Chilean Regions (34°S to 36°S), but most of this damage concerned traditional construction (adobe houses) and old masonry buildings (unreinforced masonry buildings). Minor damage, such as small cracks in walls, and moderate damage were noted in modern engineered and nonengineered buildings (confined masonry houses and buildings). In these cases, the damage may be attributed mainly to poor-quality construction and surface geological conditions, despite the fact that the most of these buildings do not have seismic design. The 2010 Maule mega-earthquake confirms the importance of the main asperities in the damage distribution, since the best fitting of the intensity attenuation formula is obtained considering the distance to the main asperity, instead of hypocentral or to the nearest fault distance. The characteristics of the 2010 Maule earthquakes should be considered in the seismic risk of a large subduction environment, such as the Northern Chile or the Cascadia zone, where megathrust earthquakes have not occurred in the last century. Macroseismic intensity is a valuable tool for supplementing seismic hazard assessments when a complete data set is available and there are not many strong motion records available, because of its direct relation to the damage produced by earthquakes. Finally, to investigate historical mega-earthquakes effects, it is required to establish a comparison between available seismic intensity data and the macroseismic data of the 2010 Maule earthquake (Mw 8.8). This is the only way we can fully exploit the wealth of historical information available.

ACKNOWLEDGMENTS The authors would like to thank Professors Maria Ofelia Moroni, Leonardo Massone, and Elizabeth Parra and the graduate students Francisco Cabezas and Felipe Cordero who col- laborated in the damage inspection. The field trip was funded by the Department of Civil Engineering of the Universidad de Chile. Sergio Ruiz has been partially funded by the Millennium Nucleus Program “Montessus de Ballore–IERC,” Ministry of Planning and Cooperation (Mideplan), Chile. DAMAGE ASSESSMENT AND SEISMIC INTENSITY ANALYSIS S163

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