M8.3 Chile Earthquake

First Posting | Summary
Posting Date: September 17, 2015, 1:30:00 PM

On Wednesday, September 16, 2015, at 7:54 p.m. local time (22:54 UTC), an M8.3 earthquake struck off the coast of central Chile, near Coquimbo, triggering a hazardous tsunami. Several strong aftershocks have occurred, including one measuring M7.0 and several exceeding M6.0, according to the USGS. The event, which has claimed at least 10 lives, caused significant damage to several coastal towns in central Chile and prompted about 1 million people to evacuate their homes.

Damage and Disruption

Reports of the heaviest damage are from Illapel, 46 km east of the epicenter and about 280 km north of Santiago. Buildings in downtown Illapel, along with several adobe homes, have collapsed. Damage was also reported in La Serena, including some significant damage to a shopping mall. Landslides have been reported from the area along with localized flooding from the tsunami, blocking roads. Travel is further hampered by bridge damage, including a bridge in Ovalle, which is located in the Coquimbo region.

Power outages have affected about 250,000 people, including the entire city of Illapel and about 95% of Coquimbo. Many people were left without access to drinking water, including about 1,800 in Illapel. Operations at copper mines managed by Codelco and Antofagasta PLC have been suspended. Santiago’s airport was evacuated but later re-opened, and schools in Valparaíso were closed.

Tsunami warnings prompted the evacuation of approximately 1 million people from low-lying areas of coastal Chile, from Puerto Aysén north to Arica.

Regional Seismicity

Chile is a highly active area for earthquakes and has been the site of several events of M8.0 and higher since 1900, many of which have generated large tsunamis. Wednesday’s earthquake was the strongest to strike Chile since the devastating 2010 M8.8 event in Maule.  The Maule event was about 3.2 times stronger than the M8.3 of Wednesday and triggered a massive 8-meter tsunami. Other notable events for this region include the 1922 M8.5 event at Coquimbo, and the 1960 Valdivia earthquake in southern Chile, which was an M9.5 according to the USGS, and the largest instrumentally recorded earthquake in the world.

Wednesday’s earthquake was the result of thrust faulting in central Chile between the Nazca and South American tectonic plates. In this area, the Nazca plate is subducting in an east-northeasterly direction under the South American plate at the Peru-Chile Trench, at a shallow dipping angle, at about 70-80 mm per year. The earthquake was relatively shallow and struck at a depth of 25 km.

The location and magnitude of this earthquake are consistent with AIR’s recent seismicity model for this region. AIR’s time-dependent model for this region estimates a higher rupture probability for this type of earthquake compared to the corresponding time-independent estimate. The time-dependent model was constructed using GPS data and the most advanced physical modeling available. It incorporates the details of past historic earthquakes that are consistent with the state of locking of the subduction zone, and information on the accumulation and release of strain in this region during the past few hundred years. The preliminary results of the slip distribution from this earthquake are consistent with AIR's overall determination of the spatial distribution of locked areas based on GPS data.

Wednesday’s M8.3 earthquake generated a tsunami that was recorded by multiple stations across the Pacific. At the source region, it occurred at low tide, which limited its inundation potential; however, tsunami waves reached higher than 3 meters above tide level along parts of Chile’s coastline, causing some significant flooding. The National Tsunami Warning Center of the National Oceanic and Atmospheric Administration (NOAA) reported the largest tsunami height of 5.2 meters in Coquimbo. Tsunami heights of 1 meter or less were reported for Peru and Ecuador, parts of Mexico, Japan, Hawaii (including a report of 0.8 meter at Hilo), New Zealand, and many other regions, including Fiji and other South Pacific islands, and parts of Russia. Several other regions in Central America and Southeast Asia are expected to be affected by tsunami heights of less than 0.3 meter. The most noticeable wave height in Southern California was recorded in Ventura and was less than 0.5 meter.

ShakeMap for the M8.3 earthquake offshore of Coquimbo, Chile, September 16, 2015. (Source: USGS)

Exposure at Risk

The majority of residential buildings in urban areas of Chile are of masonry (reinforced, confined, and unreinforced) or reinforced concrete construction. While confined and reinforced masonry are very common in typical low-rise residential properties, reinforced concrete—particularly with shear walls—dominates commercial and mid- and high-rise residential buildings in urban areas. Industrial buildings in Chile are typically of steel or light metal construction.

A notable portion of Chile’s residential building stock, particularly among older structures, is of unreinforced masonry (URM). This construction type is vulnerable to shake damage from earthquakes because these buildings tend to be heavy and brittle, and lack appropriate lateral load resisting systems. In some parts of Chile, including some of the areas affected by Wednesday’s event, adobe construction is a common construction type for homes. Mid- to high-rise apartment buildings in Chile are often reinforced concrete with shear walls or reinforced masonry.

Confined masonry buildings consist of unreinforced masonry (URM) walls that are surrounded by beams and columns that are typically made of concrete. Reinforced masonry construction is characterized by reinforcing bars that tie the masonry bricks together with the mortar, forming a more uniform and integrated wall. Seismic performance of these masonry walls is considerably improved compared to URM walls. While these masonry structures tend to perform well during moderate earthquakes, their load-bearing walls may develop large cracks, which are costly to repair. In larger earthquakes, load-bearing walls lose their stability, which puts the building at risk of collapse.

Commercial and industrial buildings are generally constructed according to stricter standards than residential structures; however, building code enforcement varies within and across Chile. In reinforced concrete shear wall buildings or steel construction (with steel moment resisting frames or braced steel frames) vertical loads are carried by both the shear walls and the frame.

The AIR earthquake team is currently analyzing the event using the AIR Earthquake Model for Chile in preparation for a full loss posting.

First Posting | Downloads
Posting Date: September 17, 2015, 1:30:00 PM