The great Sonoran earthquake (M>7.2) ruptured the Pitaycachi fault on May 3,1887 and is the largest historic earthquake in the southern Basin and Range Province.
With the dawn of the atomic age seismic instruments became a premiere watch dog of world wide nuclear explosion testing.
This earthquake is significant for two reasons: First, just one year prior to this earthquake, Chinese scientists sucessfully predicted a major earthquake in this region. The evacuation saved many lives. Second, the Chinese scientists were not sucessful in predicting the Tangshan earthquake which resulted in between 250,000 and 500,000 deaths.
Although this earthquake occurred 200 km from Mexico City, the shaking of loose sediments in the capitol city was much stronger than at the epicenter. Nearly 10,000 people died and central Mexico City was heavily damaged. Liquifaction of the lake bed sediments was a critical factor.
The 1960 Great Chilean Earthquake
May 22, 1960
An earthquake of moment magnitude of 9.5 ripped through the area of southern Chile generating tsunamis and causing the eruption of long quiet volcanoes. Aftershocks continued for a week after the principal shock. A huge tsunami was generated from the subsidence of the ocean floor and after devastating the western South American coast, went on to ravage Hawaii and Japan.
Government economists, counting up the cost of nature's month long rampage, found that 130,000 houses - one in every three in the earthquake zone - had been destroyed. Total loss: $186 million. Damages to agriculture added up to another $70 million and to factories $34 million. Altogether, Chile's financial losses from the quake may read as high as $500 million or 5% of the nation's wealth.
However, after all the devastation, the Chilean earthquake of 1960 helped seismologists to focus more fully on the concept of plate tectonics. The earthquake also helped scientists envision and develop a new measurement of earthquake magnitude. The Chilean earthquake was so much larger than any previously recorded earthquake, seismologists needed to develop a better method to measure earthquake size.
South American Seismicity
A map of seismicity greater than magnitude 6.0 for the past 40 years in South America. [Red (0-100km), Green (100-300km), Blue (300km +)]
Geography
Chile, a republic in southwest South America, is bounded on the north by Peru, on the east by Bolivia and Argentina and on the south and west by the Pacific Ocean. Chile has an extreme north-south length of 4270 km (2650 mi) but its average width is less than 180 km (110 mi). Archipelagos extend along the southern Chilean coast from Chiloe Island to Cape Horn, the southernmost part of South America. However, the most dominant physical feature of Chile is the Andes Mountains, which extend the entire length of the country, from the Bolivian plateau in the north to Tierra del Fuego in the south.
Geology
Uplift and folding of the sedimentary rocks that comprise the Andes began during the Cretaceous period, when the Pacific crustal plate began to subduct beneath the South American plate. Tectonic forces generated at the subduction zone still trigger volcanic eruptions and earthquakes and have lifted parts of the Andes over 1500m (5000 ft) during the past 20 million years. The peaks of the Andes reach as high as 5 km (21,000 ft).
Just off the west coast of Chile lies the Peru-Chile trench, an ocean-continent subduction zone capable of producing very great earthquakes.
Damage and Relief
Disaster would not leave Chile alone. Attempting to placate the gods they held responsible for the continuing quakes in southern Chile, Mapuche Indians beat a six-year old boy to death with sticks, tore out his heart and offered it to the sea. When police arrested two of the Indians, they explained: "We were asking for calm in the sea and on the earth."
Landslides killed 18 people near the city of Valdivia, and aftershocks continued to wreak havoc. More tremors ten miles north of Valdivia set off other landslides, killing two more people. Heavy quakes struck Concepcion - Chile's top industrial center. Walls collapsed and women screamed hysterically in Valparaiso as a violent quake shook the port city of 200,000.
Government economists, counting up the cost of nature's month long rampage, found that 130,000 houses - one in every three in the earthquake zone - had been destroyed. Total loss: $186 million. Damages to agriculture added up to another $70 million and to factories $34 million. Altogether, Chile's financial losses from the quake may read as high as $500 million or 5% of the nation's wealth.
Foreign aid poured in. West Germany offered to rebuild Valdivia. Argentina aided Chiloe Island and Sweden helped Puerto Saavedra. The United States gave most of all. The export-import bank of Washington lent $10,770,000, and private citizens have donated $5,000,000. President Eisenhower approved a $20 million dollar gift as the "first step" of a broad aid program to Chile's homeless and desperate people.
Tsunami Description
Tsunami (tsoo-nah-mee) is the Japanese word for harbor wave. Tsunamis are created when an earthquake ruptures the rocks below the ocean waters. If one side of the fault moves downward relative to the other side, the ocean water will rush in to fill the space left by the subsiding rocks and produces a huge wave. This wave will then move outward in all directions and can reach the farthest corners of the globe. The great devastation occurs when the wave approaches a shallow coastline and builds in size creating a wall of water as high as 100 feet in some cases!
1960 Chile Tsunami
On May 22, 1960, at 19:11 GMT, an earthquake occurred off the coast of South Central Chile. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.6, an epicenter of 39.5° S, 74.5° W, and a focal depth of 33 km. The number of fatalities associated with both the tsunami and the earthquake has been estimated to be between 490 to 2,290. Damage cost estimates were over a half billion dollars.
Aerial view of coastal area on Isla Chiloe, Chile, showing tsunami damage and wave extent. Two hundred deaths were reported here from the tsunami generated just off Chile's coast by the magnitude 8.6 earthquake. The inhabitants, fearing the earthquake, took to small boats to escape the shaking. The trough of the tsunami arrived just 10 to 15 minutes after the earthquake, along more than 500 m of the coast. Upon the return of the sea in a thunderous breaker, all boats were lost. The most serious effects occurred in an area extending from Concepcion on the Chilean coast to the south end of Isla Chiloe. Photograph Credit: Unknown. Source: National Geophysical Data Center.
After the tsunami had passed the Hawaiian Islands damage costs were estimated at $24 million and 61 people had died. Hilo, on the main island of Hawaii, was the hardest hit city in the islands. The tsunami arrived at Hilo 14.8 hrs after the it originated off the coast of South Central Chile. The - at Hilo was measured at 10.7 m.
Downtown Hilo, Hawaii, was left devastated by the tsunami. Photo Credit: The Honolulu Advertiser.
Additional Links
Magnitude Scales
Earthquake magnitude is a relative scale of earthquake size based on measurement of different seismic wave amplitudes.
Richter Scale / ML - The first seismic magnitude scale was developed by Charles Richter in the early 1930's and was motivated by his desire to issue the first catalogue of southern California earthquakes. This catalogue contained several hundred events, whose size ranged from barely perceptible to large, and Richter felt that an earthquake description must include some objective measurement of size to assess its significance.
Richter observed that the ground motion generated by an earthquake decreased similarly for many earthquakes. By comparing the ground motion recorded by the seismometer, one could assign a relative size to earthquakes.
All of Richter's observations were made from data recorded on one type of seismometer, a simple Wood-Anderson torsion instrument. In designing his magnitude scale, Richter determined the relative size of an earthquake by comparing it to a reference earthquake recorded on a Wood-Anderson seismometer at a distance of 100 km from the epicenter. The calculation of magnitude is done using the amplitude of the S wave (secondary or shear wave). This magnitude scale is called the Richter Magnitude or ML - for local magnitude.
ML in its original form is rarely used today because Wood-Anderson instruments are uncommon and, of course because most earthquakes do not occur in southern California. However, ML remains a very important magnitude scale because it was the first widely used "size measure", and all other magnitude scales are tied to ML.
Although the local or Richter magnitude is useful, the limitations imposed by instrument type and the distance range make it impractical for determining size of earthquakes recorded world wide.
Body Wave Magnitude / mb - For instruments located between 100 and 400 km from the epicenter, the P wave is very distinct and it is convenient to define a magnitude based on the amplitude of the P wave. This magnitude scale is called the Body Wave magnitude, mb, and is based on the first few cycles of the P wave (primary or first) arrival.
Surface Wave Magnitude / MS - Beyond about 600 km, the long-period seismograms of shallow earthquakes are dominated by waves traveling along the surface. These waves often have a period of approximately 20 seconds. the amplitude of these waves depends on distance differently than the ampltude of body waves. Deep earthquakes do not gernerate much surface waves thus there is no appropriate correction for these types of earthquakes. This magnitude scale is called the Surface Wave Magnitude, Ms. Both Ms and mb were designed to be compativle with ML. Thus at times all three magnitude scales give similar magnitude values. But because they use different information from the seismic wave form they do not always agree.
Moment Magnitude / MW - The best way to quantify the size of an earthquake is to determine its seismic moment, Mo. Mo can be determined through analysis of the complete seismic record and relates to actual physical parameters of the fault (area of slip) and earthquake (amount of slip). This makes MW a more accurate measurement of an earthquakes characteristics.
Chile, an example - The magnitude of earthquakes is difficult to measure, especially for large earthquakes. One way to compare earthquakes is by the size of their rupture zone, the part of the earth that moved in the main earthquake. Another way to compare earthquakes is by the size of their aftershock zone, the region around the main earthquake that experiences high seismicity just after the earthquake. The area that ruptured during the great 1906 earthquake in San Francisco (shown in gray), extended approximately 15 kilometers deep in the earth and was 400 kilometers long. This was the largest US earthquake on record for this century. In comparison, the area that ruptured in the Chilean earthquake (shown in pink) extended to a depth equal to half the width of the state of California and was over 1000 km long! In the figure, the black dot represents the location of the epicenter of the San Francisco earthquake; the dots in red represent the locations of aftershocks of the Chilean earthquake with respect to its epicenter ( the largest red dot), superimposed on the map of California for scale. The diameter of each dot represents the magnitude of each quake. Because earthquakes in California are caused by plates sliding past each other horizontally and not by plates subducting over each other as in Chile, no earthquake in California will be as great as earthquakes in Chile. Figure from Earthquakes and Volcanoes: Readings from Scientific American, pg. 12, 1980.
Intensity Scales
Earthquake intensity is the measure of the effects of an earthquake in a particular place. Intensity depends not only on the earthquake magnitude but also on the distance from earthquake to epicenter and on the local geology.
In 1902, the Italian seismologist L. Mercalli developed a simple, yet effective way to measure the intensity of earthquakes. The scale is based on visual evaluation of the damage caused by the quake. It is not as quantitative as other scales such as the well known Richter scale which measures earthquake magnitude (Bolt, 1993).
The Chinese developed the Modified Mercalli Intensity Scale, similar to the Mercalli system, to evaluate their earthquakes as well. Their system was designed in part to give people living in various areas an idea of what the potential earthquake danger was. The values ranged from I to XII and were based largely on the local geology of each area (Yong et al., 1988).
Officials evaluated the geology of each region and assigned each a number corresponding to the amount of potential damage (Yong et al., 1988).