CHILE - EARTHQUAKE AND TSUNAMI OF 27 FEBRUARY 2010
George Pararas-Carayannis
ABSTRACT
The great earthquake of February 27, 2010 occurred as thrust-faulting along a highly stressed coastal segment of Chile's central seismic zone - extending from about 33ºS to 37ºS latitude - where active, oblique subduction of the Nazca tectonic plate below South America occurs at the high rate of 6.8 centimeters per year. It was the 6th most powerful earthquake in recorded history and the largest in the region since the extremely destructive May 22, 1960 magnitude 9.5 earthquake near Valdivia.
This central segment south of Valparaiso from about 34º South to 36º South had been identified as a moderate seismic gap where no major or great, shallow earthquakes had occurred in the last 120 years, with the exception of a deep focus, inland event in 1939. The tsunami that was generated was highest in the Juan Fernandez Islands as well as in Talchuano, Dichato, and Pelluhue and on Robinson Crusoe Island, causing numerous deaths and destruction.
Given the 2010 earthquake’s great moment magnitude of 8.8, shallow focal depth and coastal location, it would have been expected that the resulting tsunami would have had much greater Pacific-wide, far field effects similar to those of 1960, which originated from the same active seismotectonic zone. However, comparison of the characteristics of the two events indicates substantial differences in source mechanisms, energy release, ruptures, spatial clustering and distributions of aftershocks, as well as in geometry of subduction and extent of crustal displacements on land and in the ocean. Also, the San Bautista and the Juan Fernandez Islands - ridges rising from the ocean floor - may have trapped some of the energy, thus accounting for the smaller, far field tsunami effects observed elsewhere in the Pacific.
Apparently, complex, localized structural anomalies and interactions of the Nazca tectonic plate with that of South America, can account for differences in the spatial distribution and clustering of shallow event hypocenters, as well as for seismic gaps where large tsunamigenic earthquakes could strike Chile’s Central Seismic zone in the future.
INTRODUCTION
A great earthquake with magnitude 8.8 struck Chile's central coastal region in the early morning of February 27, 2010. It was the 6th most powerful earthquake in recorded history and the largest in the region since the May 22, 1960 magnitude 9.5 earthquake near Valdivia. The quake resulted in hundreds of deaths (700 as of the writing of this report) and in severe damage to roads, bridges, homes, businesses and other structures across Chile. Ground motions were felt as far away as Argentina and Sao Paulo in Brazil. The quake generated a tsunami which was highest in the Juan Fernandez Islands and in Talchuano and was responsible for many deaths at Dichato, Pelluhue and on Robinson Crusoe Island, in Chile, but did not cause any known deaths elsewhere. Tsunami waves ranging from 1-2 meters were observed or recorded at many distant locations in the Pacific, but there were no reports of any major damage. The Pacific Tsunami Warning Center in Honolulu, issued a tsunami warning for Chile and Peru and a watch or advisories for Ecuador, Colombia, Panama, Costa Rica, Antarctica and all areas bordering the Pacific Ocean and the adjacent seas with the exception of Alaska, British Columbia and the US states of Washington, Oregon and California for which the Alaska Tsunami Warning Center issued a watch bulletin.
THE EARTHQUAKE
Date and Origin Time - February 27, 2010; at 06:34:14 GMT; 1:34 a.m. EST, 3:34 a.m. (local time).
Epicenter - 35.909 S, 72.733 W offshore from Maule, Chile; 99 km (61 miles) of Talca; 117 km (73 miles) of Concepcion; ; 317 km (197 miles) of Santiago (USGS).
Focal depth - 35 km (21.7 miles).
Aftershocks - In 2 1/2 hours following the 90-second main shock, the USGS reported 11 aftershocks, of which five measured 6.0 or above.Up to March 1, a total of 121 aftershocks with magnitude 5.0 or greater were recorded (USGS NEIC). Eight of these had magnitudes of 6.0 or greater and one had magnitude 6.9.
First 90 aftershocks (modified IRIS graphic)
Rupture - About 500 km long, extending to about 50 km in depth below the earth's surface.
Death Toll and Damages - The final death toll is not known but it is expected to rise to more than 1,000. Already, 708 people were reported as dead. Thousands more were injured and thousands more lost their homes. The earthquake destroyed newly built apartment buildings as well as old ones. Many structures collapsed and phone lines and electricity lines were down. Flames devoured a prison. Millions of people fled to streets. Bridges also collapsed and cars and trucks were tossed and crushed. There was considerable destruction of roads.
Concepcion - At Concepcion, a city of 900,000 people and Chile's second largest city, there was extensive destruction. 15-story building collapsed.
Santiago - In the capital of Santiago, 200 miles (325 kilometers) northeast of the epicenter, there was extensive damage to older buildings, including the Nuestra Senora de la Providencia church, whose bell tower collapsed. An apartment building's two-level parking lot also pan caked to the ground floor, smashing about 50 cars. A bridge just outside the city collapsed. The International airport was closed. The national Fine Arts Museum was badly damaged. Water, electricity, telephone service were cut off.
Ground Motions - Ground motions of the main shock reportedly lasted for about 90 seconds.
Planetary Impact - The great earthquake that struck Chile on February 27, 2010 resulted in movement of crustal material and redistribution of mass. The change resulted in a tiny shift in the Earth's axis and affected its rate of rotation. The changes were small but permanent. According to a NASA preliminary evaluation, the earthquake shortened the length of a day by 1.26 microseconds (a microsecond being one-millionth of a second). The shift of the Earth's axis of rotation was estimated at 3 inches (8 centimeters).
The Chandler wobble of the earth's axis (the Chandler Oscillation) is about 20 feet (9 meters) and has a period of 433 days. This wobble combines with another wobble which has a period of one year, so that the total polar axis motion varies with a period of about 7 years. It is affected by gravitational attractions of the moon and the sun as well as planetary alignment and variations in the Earth's geomagnetic field. It would be expected that the shift due to great earthquakes would also have a cumulative impact to the Earth's axis of rotation and free nutation (due to non rigidity and spinning dynamics of the aspheric earth).
Stress Release
The earthquake of February 27, 2010 involved thrust faulting in the coastal segment of Chile's central seismic zone. Specifically, the Nazca tectonic plate moved below the South America continental plate. As a result, the continental block moved about 5-meter westward in the vicinity of Concepcion. This region had been highly stressed because of ongoing active and oblique subduction of the Nazca tectonic plate below the South American plate at the high rate of 6.8 centimeters per year. The region south of Valparaiso from about 34º South to 36º South had been identified as a moderate seismic gap. With the exception of the 1939 earthquake - which occurred inland and had a deeper focal depth - there had been no major or great earthquake in this area for a long time.
The earthquake of February 27, 2010 ruptured the portion of the South American subduction zone separating the source regions of the 1960 and 1906 earthquakes and relieved stress that had build up. The numerous strong aftershocks over a large area that followed the main shock - some over 6 in magnitude - indicate that there was additional stress. More aftershocks or another earthquake can be expected in the next weeks and months, as there may be additional stress left in the area. It is not uncommon for this region to experience a cluster of earthquakes which may be perceived as aftershocks, but in fact are separate events on adjacent fault, triggered by stress transference. Focal mechanism solutions and spatial clustering of these events are being investigated to determine whether they involved normal faulting (which would indicate gravitational settling) or thrust faulting (which would indicate compression).
THE TSUNAMI
Shortly after the main earthquake a 2.4 meter (7.7ft) tsunami struck the town of Talcahuano in Chile as well as many other towns such as Coquimbo, Antofagasta and Caldera. A Pacific-wide tsunami was generated and French Polynesia, Hawaii, New Zealand, Australia, Philippines, Japan and Russia were alerted by the Pacific Tsunami Warning Center in Honolulu and by regional tsunami warning centers.
Dichato - Eyewitnesses at the village of Dichato, on the coast, reported that three large waves struck and destroyed 75% of that village. The first two were big but did not cause much damage. The third wave was much larger and caused widespread destruction.
Constitution - A 33-foot wall of water swept about five blocks into Constitucion about 30 minutes after the main earthquake struck. The tsunami destroyed almost everything on the seafront and at the center of the town. As many as 350 people are believed to have to have died and hundreds more were missing. It is not known yet if all the reported deaths were from the tsunami or the earthquake. A fishing boat was carried to the middle of a public square.
Lioca - Tsunami waves swamped the coastal village of Lloca.
Valparaiso - A 1.3 meter tsunami was observed 20 minutes after the main earthquake.
San Juan Bautista - At San Juan Bautista in the Juan Fernandez islands, the tsunami reportedly inundated almost 2 miles into the town. At least eight people died and eight were missing on Robinson Crusoe Island.
Presently investigated is the effect that the Juan Fernandez Ridge may have had in reducing the far-field tsunami impact. It is possible that a lot of the tsunami's energy was either absorbed or deflected by the ridge - which may account for the lower than expected tsunami wave heights that occurred in the Pacific Basin. As indicated above, at San Juan Bautista on the Juan Fernandez Islands, the tsunami wave was great and, according to some reports, it inundated two miles into the town. Most of the tsunami fatalities occurred on Robinson Crusoe island.
SEISMICITY OF CHILE
Chile is characterized by extreme seismicity because of active subduction of the Nazca tectonic plate beneath the West coast of South America. Along the entire length of the country, there is a very narrow seismic band which ranges from 100 to 150 km in width. This band extends from the Andes mountain range to the Peru-Chile Trench. There are major differences along the entire coastline of Chile in seismicity and in the potential for the generation of large, destructive tsunamis. Each particular segment of Chile's seismic zone has its own unique source characteristics for the generation of large tsunamigenic earthquakes, depending on the geometry of subduction and angle of dip - particularly near the bend in the coastline between Peru and Chile.
There are narrow belts of high seismic activity and clustering of earthquakes near the surface but also at intermediate depths below the South American continent. Intermediate-depth earthquakes tend to cluster in space. There is a known a gap in seismic activity between focal depths of 320 and 525 kilometers, between latitudes 25.5ºS. and 27ºS. This region has generated many large shallow earthquakes. Of course the deeper earthquakes are not as destructive, but are indicative of the complex interaction of the tectonic plates and anomalies which can account for differences in the spatial distribution and clustering of the shallow events, as well as for seismic gaps where future large earthquakes could strike. There are such regions that did not have large shallow earthquakes in the last 120 years. The earthquake of February 27, 2010, occurred along such a seismic gap region.
SEISMOTECTONICS CHILE'S CENTRAL AND SOUTHERN SEISMIC ZONES
As stated, the Nazca tectonic plate subducts below the South America continent at a high rate (averaging about 6.8 cm per year). The active convergence of the tectonic plates results in extreme seismicity and crustal deformation on the surface and at depth below the continent. Based on this high seismicity patterns and clustering of events, Chile can be divided into three distinct seismic regions: North, Central and South. Each of the three seismic regions can generate large destructive earthquakes, but each has somewhat different characteristics of focal mechanisms. The historical record indicates that large crustal displacements associated with shallow earthquakes near the coastline of Chile in these three regions, can generate destructive tsunamis locally and at distant shores of the Pacific Basin. The earthquake of February 27, 2010 occurred as thrust-faulting on Chile's central seismic zone at the interface where the Nazca plate is moving down and landward at an oblique angle below the South American plate.
The Central Seismic zone extends from about 33ºS to 37ºS latitude and can be also divided into three distinct regions. The north central region near Valparaiso (33º-34ºS), is delineated by the intersection of the Juan Fernandez Ridge with the Peru-Chile Trench and the appearance of volcanism at the southern end. Five tsunamigenic earthquakes occurred in this area in historic times: November 19, 1811; November 19, 1822; October 16, 1868; August 17, 1906 and March 3, 1985. South of Valparaiso, between 34ºS-36ºS, a seismic gap existed. This is the region that ruptured by the February 27, 2010 earthquake. Four earthquakes with magnitudes of 7.5 or larger have occurred in this particular region, but none of these is known to have generated a destructive tsunami.
The Concepcion Region, is the next seismic zone of Central Chile. It extends from about 36ºS to 37ºS. This region generated two large earthquakes in 1835 and 1939 with magnitudes greater than 8.0. The 1835 quake generated a destructive tsunami. Lower magnitude earthquakes occurred in 1751, 1868, 1878, 1953 and 1971.
The South Central Region extends from 37ºS to 41ºS and is characterized by distinct structural changes caused by the geometry of subduction. This southern region of Chile's Central Seismic Zone generated large earthquakes, on October 28, 1562; February 8,1570; December 16, 1575, March 15, 1657, and May 22, 1960. Of these, the 1562, the 1570, the 1575, and the 1960 earthquakes generated destructive tsunamis, locally and in the Pacific Basin. The most significant of the historical earthquakes in this region was that of May 22, 1960. It was the largest recorded earthquake in history and generated an extremely destructive tsunami.
Recurrence Frequency of Large, Potentially Tsunamigenic Earthquakes in Central Chile
The earthquake recurrence frequency in Chile varies for each of its three seismic regions and also within each region. Intervals between the destructive earthquakes of 1575, 1737 (in South Chile),1837 (in South Chile), and1960 (Valdivia area, 40º S. latitude) appear to be longer - more than 100 years on the average - than those of both Concepcion and Valparaiso areas. The May 22, 1960 earthquake near Valdivia, was the largest seismic event in this south-central region since the 1570 earthquake.
HISTORICAL EARTHQUAKES AND TSUNAMIS IN CHILE
Chile has experienced many large destructive earthquakes in the past. The historical record shows that 35 tsunamis were generated by earthquakes of different magnitudes along the coasts of Chile, causing variable destruction. Since 1973, there have been thirteen earthquakes with magnitudes greater than 7. The following is a listing of some of these events, compiled from different sources (Heck, 1947; Bernlnghausen, 1960; Cox, IIda & Pararas-Carayannls, 1967; Lomnitz, 1970; Cox & Pararas-Carayannls, 1976; Pararas-Carayannls, 1977)
October 28, 1562 - This was magnitude 8 earthquake and its epicenter was at 38.0 S, 73.5 W. It generated a destructive tsunami with 16 meter maximum run-up height.
February 8,1570 - This earthquake occurred in the same general area of Central Chile (epicenter 37.0 S, 73.0, W.) but its magnitude was estimated to be even greater, at 8.8. The maximum tsunami runup height was 4 meters.
Past earthquakes in the South Valparaiso / Concepcion / Valdivia regions (Modified Web graphic)
December 16, 1575 - This was an extremely severe earthquake in Southern Chile with source characteristics very similar to the May 22, 1960 event - which occurred centuries later in the same region. Its estimated epicenter was at 40.0 S., 70.0 W.. Its strong aftershocks continued for a period of forty days. The quake destroyed the five Indian territories south of the Bio-Bio River known as the Imperial, Valdivia, Villarrica, Osorno and Castro. Also, it generated a highly destructive along the entire coast of southern Chile. Almost immediately after the quake tsunami waves reached Valdivia, 25 km up the river by the same name, reversing its flow, destroying houses, uprooting trees and sinking two galleons at the port. Along the coast of La Imperial, north of Valdivia, the tsunami killed 100 people. Landslides from the quake blocked the flow of a river flowing into lake Rinihue. A subsequent break of the dam killed 1200 people.
March 15, 1657 - Its epicenter was at about 37.0 S., 73.0 W. Its magnitude estimated at 8.0. It generated a tsunami with a maximum height of 8.0 meters.
Nov. 19, 1822 - This was a large earthquake with epicenter at 33.0S. 71.4W, and an estimated magnitude of 8.5, which caused considerable destruction in Central Chile and generated a destructive tsunami with a height of 3.5 meters.
May 10, 1877 - Two large earthquakes near Arica, on May 9 and May 10. The May 9 event had its epicenter at 21.6 N.71.0W and its magnitude was 8.5. It generated a destructive tsunami of 16 meters in Northern Chile. The second one, which occurred day later on May 10, had an estimated magnitude of 8.3. Its epicenter was at 19.6 S., 70.2 W. and the maximum tsunami height was 24 meters. This was largest tsunami to be recorded at 1.07m by the Sydney tide gauge in Australia.
August 17, 1906 - The epicenter of this quake was at 33.0 S., 72.0 W. This was a large shallow magnitude (8.6) tsunamigenic earthquake with epicenter near that of the 1822 event and approximately 300 km to the north of the February 27, 2010 event. The quake caused destruction in Chile and generated a tsunami that was destructive locally and at distant locations. In Hawaii, its runup was reported to be as much as 3.5 meters and caused damage.
November 27, 1922 - Date given as Nov 21, 1927 and epicenter at 44.6 S, 73.0W. This was a very destructive tsunamigenic earthquake with magnitude of 8.5 which occurred approximately 870 km to the north of the February 27, 2010 event. The quake was extremely destructive along central Chile, causing several hundred fatalities and severe property damage. It generated a destructive 9-meter local tsunami that inundated the Chile coast and was particularly damaging along the coastline near the town of Coquimbo. The tsunami impacted Hawaii, washing away boats at Hilo harbor.
May 22, 1960 - This was the largest earthquake ever recorded with a Moment Magnitude of 9.5. It was the largest seismic event in this region since 1570. Its epicenter was along Chile's coastline, about 240 nautical miles to the south-southwest of the February 27, 2010 quake. The 1960 earthquake was extremely destructive and had characteristics similar to those of the December 16, 1575 event. The length of its rupture was about 1250 kms. Tectonic crustal movements of both uplift and subsidence occurred over a great area. The earthquake generated a destructive tsunami that caused devastation in Chile and Pacific-wide waves that impacted distant shores in Hawaii, California Pitcairn Island, New Guinea, New Zealand, Japan, Okinawa, and the Philippines. In places as far away as Australia, the maximum runup reached 4.5m above sea level The total death toll was of over 2,500 lives. In Chile, the combined impact of the quake and the tsunami killed The 1960 quake killed 1,655 people and left 2 million people homeless in Chile.
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Comparison between the May 22, 1960 and the February 27, 2010 Earthquakes and Tsunamis
The great earthquake of February 27, 2010 was extremely destructive and generated a tsunami that devastated many coastal towns and villages along the central part of Chile, also causing some minor damage at distant coastal communities in the Pacific. However, given its great Moment Magnitude of 8.8, it would have been expected that this earthquake would have generated a much greater Pacific-wide tsunami with far field effects similar to those caused by the extremely destructive tsunami generated by the great Chilean earthquake of May 22, 1960.
The 1960 earthquake's epicenter was in the same seismic region of Chile, but about 230 km to the south of the 2010 event. The tsunami that was generated was particularly devastating in the Hawaiian Islands, in Japan, in New Zealand and many other far away coastlines. The question arises as to what was different about this February 27, 2010 earthquake and that of 1960, in spite of the fact that both were great events along the very same active subduction zone that is known to have generated great tsunamis in the past? Therefore, a comparison of source characteristics of the two events must be made to determine differences between the two events and perhaps help explain what happened.
Indeed, there were differences in location and energy release as well as in the geometry of subduction, angle of dip and extent of crustal displacements on land and in the ocean. Although both events occurred in Chile's central seismic zone, the epicenter of the February 27, 2010 quake was at 35.826 S, 72.668 W, while that of May 22, 1960, was at 39.5 South, 74.5 West. This difference of the two events may not seem to be a significant factor unless one examines more carefully the seismicity of the central seismic zone, the coastal geomorphology, the spatial distribution of hypocenters, the clustering and time sequence of aftershocks and the existence of seismic gaps at depth. Also the influence that the San Juan Fernandez Ridge may have had on the tsunami of February 27, 2010, must be examined. Such an investigation is indeed needed and will be included in an update of this page, upon completion at a later time.
Consecution showing main 2010 earthquake's focal hypocenter and major aftershock hypocenter distribution. (based on USGS data).
Consecution showing main 1960 earthquake's focal hypocenters and geometry of shallow angle of subduction near the intersection of the Valdivia fracture zone with the South America continental plane. Note change in angle of subduction dip along the Benioff zone boundary. The rupture of the 1960 earthquake was close to 1,000 km and the ocean floor area affected was much greater than that of the 2010 event. The 2010 earthquake had a rupture estimated at 500 km, half of the 1960 event. A comparison of aftershock distribution shows that most of the aftershocks of the 2010 event were closer to the coast, thus indicating that most of the crustal displacements occurred on land rather than in the ocean.
Evaluation of Potential Tsunami Generation in Chile
Major and great earthquakes in Chile apparently occur in small number of source regions, each characterized by predictable patterns of seismic and tsunami effects. In Chile's Central region between Valparaíso and Concepción, major earthquakes occur mostly inland (Lomnitz, 1970). However, from Concepción to the south, most of the earthquakes have epicenters in the offshore region and tend to generate more destructive tsunamis. An explanation will be included in an update of this page.
Travel Time of tsunami originating in Chile (One hour intervals)
REFERENCES
Bernlnghausen, W,H., 1962: Tsunamis reported from the west coast of South America, 1562-1960, B u l l e t i n of the Seismological Soclety of America, voi. 52, p. 915-921
Cox, Doak C., and G. Pararas-Carayannls, 1976: Catalog of tsunamis i n Alaska, Report SE-I, U.S.U.S. Department of Commerce, Department of Commerce, National Oceanlc and Atmospheric Adminlstratlon. Boulder, Colorado, 43 p.
Cox, Doak C., Pacific tsunaml historic file , U,S. Department of Commerce, National Oceanic and Atmospherlc Adminlstratlon, Boulder, Colorado, unpubllshed manuscript.
Cox, Doak C., Kumizi I Ida, and G. Pararas-Carayannls, 1967: Preliminary catalog of tsunamls occurrlng In the P a c l f l c Ocean, Unlverslty of Hawaii, Honolulu, 274 p.
GUTENBERG, B., 1939. Tsunamis and earthquakes. Bulletin of the Seismological Society of America; October 1939; v. 29; no. 4; p. 517-526.
Heck, N.H., 1947: L l s t of seismic sea uaves, Bul l e t i n of the Selsmologlcal Soclety of America, vol.
37, p. 269-286.
Lomnitz, C., 1970. Major earthquakes and tsunamis in Chile during the period 1535 to 1955. Journal International Journal of Earth Sciences, Springer Berlin / Heidelberg, Volume 59, Number 3 / July, 1970, Pages 938-960.
Pararas-Carayannls, George, 1977: Catalog of tsunamls In Hawaii, SE-4, Natlonai Oceanlc and Atmospherlc Admlnlstratlon, Boulder, Colorado, 77 p.
Pararas-Caryannis, G., 1975 : The tsunami of October 3, 1974 in Peru, Newsletter, International Tsunami Informat ion Center, Hawaii, v. 8, p.
18-21.
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©1982 - 2010 Dr. George Pararas-Carayannis - All rights reserved
Last update: MAY 31,, 2010 |
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