2010 Pichilemu earthquakes
The 2010 Pichilemu earthquakes, also known as the Libertador O'Higgins earthquakes, were a pair of intraplate earthquakes measuring 6.9 and 7.0 that struck Chile's O'Higgins Region on 11 March 2010 about 16 minutes apart. The earthquakes were centred northwest of the city of Pichilemu.
The earthquakes were caused by increased regional stress arising from an earthquake on 27 February, centered offshore Maule Region, which was felt throughout central Chile. The Hawaii-based Pacific Tsunami Warning Center pointed out the possibility of local tsunamis within of the epicentre, although small, but violent waves were seen in the Pichilemu and Bucalemu area. One person was reported dead. At least eleven aftershocks immediately followed, causing panic throughout coastal towns between the Coquimbo and Los Lagos regions.
The earthquakes were especially destructive in the epicentre town, Pichilemu, capital of Cardenal Caro Province. The city hosts five National Monuments of Chile, of which two, the Agustín Ross Park and the Agustín Ross Cultural Centre, were seriously damaged by the earthquake. They also damaged the villages of La Aguada and Cardonal de Panilonco. Rancagua, the capital of O'Higgins Region, was also damaged, leading President Sebastián Piñera to declare a state of catastrophe in that region.
Background
Nearly all of the territory of Chile lies above the convergent boundary where the Nazca plate subducts beneath the South American plate at a rate of about 71 mm per year. Earthquakes occur along the plate interface and in both the subducting and overriding plates. Within the South American plate shallow earthquakes occur on reverse, normal and strike-slip faults. The subduction zone along the Chilean coast produced the most powerful earthquake ever recorded, the 1960 Valdivia earthquake. Some earthquakes which occurred near the epicentre of the 11 March 2010 event are the 1985 Algarrobo and Rapel Lake earthquakes.On 27 February 2010, a strong earthquake, which reached a magnitude of 8.8 on the moment magnitude scale, hit central Chile. The earthquake occurred in the region of the plate boundary between the Nazca and South American plates, offshore Maule Region. The earthquake produced a tsunami which caused great damage in cities and towns along the Chilean coast; Pichilemu was badly damaged after the earthquake and tsunami struck.
The 6.9 and 7.0 earthquakes of 11 March 2010 occurred two weeks after the 27 February event. Chilean seismologists, including Sergio Barrientos from the University of Chile Seismological Service, had suggested that the absence of an aftershock greater than magnitude 6 following the 27 February quake indicated that there remained energy with the potential to be released. Two foreshocks of the 11 March events occurred on 5 March: one reached magnitude 5.7, and the other magnitude 5.2; both were felt between the Valparaíso and Maule regions.
Earthquake sequence
The Pichilemu earthquakes were caused by the change in regional stress from the 27 February earthquake. Preliminary analyses by the United States Geological Survey of the 11 March earthquake locations and seismic-wave radiation patterns suggested that the events resulted from normal faulting within the subducting Nazca plate or the overriding South America plate, unlike the 27 February earthquake, which occurred as thrust faulting on the interface between the two plates. Later in 2010, University of Chile Seismologist Sergio Barrientos stated that the earthquakes were produced inside the South American plate.The earthquakes' depths were estimated as and by the University of Chile Seismological Service, and and by the USGS.
On 15 March 2010, seismologist Mario Pardo from the University of Chile Seismological Service ruled out that Pichilemu was experiencing a seismic swarm, after public concerns about the continued aftershocks in the area; as of that date, more than 50 aftershocks had occurred in the area, the strongest of them measuring 6.7 in the moment magnitude scale, minutes after the initial quake. From the pattern of aftershocks, it has been suggested that the earthquakes originated from rupture along a previously unknown geological fault, the Pichilemu Fault, between Pichilemu and the commune of Vichuquén in Maule Region, at 15 km depth, 40 km in length and 20 km wide. At first it was not known whether this fault was formed during the earthquakes or if it was just reactivated, however geologist José Cembrano from the University of Chile affirmed that " corresponds to a long life fault, in a million years time, whose activity had not been detected before."
In a 2012 publication titled Aftershock Seismicity of the 27 February 2010 Mw 8.8 Maule Earthquake Rupture Zone, it was stated that, in total, 10,000 aftershocks were located in the Pichilemu area for the first six months after the mainshock; this pronounced crustal aftershock activity with mainly normal faulting mechanisms found in approximately a wide region, with sharp inclined borders and oriented oblique to the trench.
Reaction
The earthquakes took place minutes before the new President of Chile, Sebastián Piñera, was sworn in, at about, at the Chilean congress in Valparaíso, where the shaking was clearly felt. Piñera was at the Palace of Cerro Castillo at the time of the earthquakes, and as he left the palace, he was seen "looking worried"; Michelle Bachelet, the outgoing president, was also seen "worried" by the earthquakes as she entered the Congress. La Nación newspaper reported some journalists attempted to flee the Congress building. According to Spanish newspaper El Mundo, there was "nervousness" at the ceremony, and the ceremony narrator called for calm, adding that the Congress building could even stand stronger earthquakes.Presidents Néstor Kirchner of Argentina, Evo Morales of Bolivia, Álvaro Uribe of Colombia, Rafael Correa of Ecuador, and Fernando Lugo of Paraguay were present at the ceremony, but television footage showed that the inauguration was not interrupted, even though there was a tsunami warning in place; however, it was reported the ceremony was concluded more quickly than planned.
President Piñera cancelled the ceremonial lunch with his visitors and traveled to Rancagua, one of the cities most affected by the earthquakes; Piñera subsequently declared a catastrophe state in O'Higgins Region as a result of the earthquakes, and appointed Army General Antonio Yackcich as Area Commander in Chief for the region, while he was visiting Rancagua that day. The declaration meant that "the military would occupy the area to keep order and prevent the kind of looting that occurred in Concepción during the first two to three days after last month's quake", according to The New York Times.
Tsunami
A Pacific-wide tsunami warning was not issued by the Pacific Tsunami Warning Center, although the organization pointed out the possibility of local tsunamis within of the epicentre, roughly the area between La Serena and Concepción. Half an hour after the first earthquake, the Hydrographic and Oceanographic Service of the Chilean Navy issued a tsunami warning for the area between Coquimbo and Los Lagos regions, as a way of "keeping people protected" against the possible occurrence of new tsunamis. President Piñera urged coastal residents to move to higher ground in case of a tsunami. Following the tsunami alert, thousands of residents of central Pichilemu fled to La Cruz Hill, with some of them staying there for several days, and received advice from members of the Army. People from the village of Cáhuil stayed at Cordón. The tsunami warning emitted by SHOA was lifted that same day at around.According to a preliminary report by the National Office of Emergency of the Interior Ministry on 11 March 2010, only 'small waves, without any kind of characteristics' were seen in the area surrounding Pichilemu, while the USGS reported a small tsunami, with sea wave heights of 16 centimetres at Valparaíso, and 29 centimetres at San Antonio. Interior Minister Rodrigo Hinzpeter reported violent waves in Pichilemu and Bucalemu.
Damage and casualties
At Pichilemu, the epicentre town, the earthquakes destroyed the balustres surrounding Agustín Ross Park, damaged severely the recently re-inaugurated Agustín Ross Cultural Centre, and the Espinillo, and Rodeillo villages. The earthquake was accompanied by "great noise", according to witnesses from the Pichilemu villages ofCardonal de Panilonco and La Aguada; most of the already damaged buildings in La Aguada fell down, including the local church. The road to Cardonal de Panilonco was damaged, and many houses built with adobe did not resist the shaking.
Outside Pichilemu, in Rancagua, local mayor Eduardo Soto reported severe damage to homes in the town. The Santa Julia highway overpass located between Rancagua and Graneros collapsed, and part of the Pan-American highway was damaged. A power outage affected Pichilemu for two days, beginning right after the earthquake struck, and there were partial power outages in Mostazal, San Fernando and Peumo. In Santiago, "windows rattled, buildings trembled and cellphone service failed", according to a New York Times article. The old Basílica del Salvador in Santiago, which was damaged during the 1985 Algarrobo earthquake and was never repaired, suffered additional damage. In Nilahue Barahona, a village near the town of Pumanque, electric cables fell to the ground during the earthquake, causing a fire that burned of a pine, eucalyptus and grassland forest. The earthquake was also reported to have been felt in Mendoza, Bariloche, Córdoba, San Rafael, Buenos Aires, Montevideo, and Asunción.
One person died of a heart attack during the earthquakes in Talca, Maule Region. A United States Geological Survey summary of the earthquakes reported damage at Rancagua, 177 kilometers northeast of Pichilemu. Relief efforts for the 27 February earthquake stalled for about six hours because of the constant aftershocks.
According to the U.S. National Oceanic and Atmospheric Administration's National Geophysical Data Center, the damage caused by the earthquakes and the accompanying small tsunami was "limited", adding that "a rough estimate of the dollar amount of damage" was "roughly corresponding to less than one million".