Ecuador Offshore Earthquake – December 31, 1906

An earthquake of this extraordinary size is capable of great damage. It created a huge tsunami that killed a thousand people along the coasts of the three countries and destroyed a large number of homes.

Three very large earthquakes shook North and South America in 1906, the first on April 18 in San Francisco, California, the second on August 17 in Valparaiso, Chile, and the third on December 31 in the Colombia-Ecuador region. In all three cases, the earthquakes caused massive destruction of cities and a large number of casualties. Since early in the twentieth century seismic source investigations have revealed considerable information about earthquakes, though scientists are still unable to predict when they will occur, and these three dramatic events have given us fresh understandings of the enormous amount of activity occurring between tectonic plates all along the eastern coasts of the Pacific Ocean.

In the case of the San Francisco earthquake it was a slip-slide movement of the plates on either side of the San Andreas Fault rather than subduction, and in South America it was the subduction of the Nazca Plate beneath the South American Plate.

With all the earthquakes that we hear about in Indonesia and Alaska, most of them of the subduction type, just like the ones of 1906, we get the impression that these two areas of the world have the biggest earthquakes of this type. This is not the case. A 1960 earthquake off the coast of Chile accounted for almost half of all the seismic action released worldwide in that year. The 1906 quake in Ecuador, beneath the margin of the South American Plate, carried a rupture than was three hundred miles in length and was of magnitude 8.8, a rare event in geological history, only matched by such extraordinary events as the Alaska earthquake in 1964 and the Indonesia quake in 2004.

Furthermore, the speed of subduction by the Nazca Plate is much greater every year when compared with Indonesia or Alaska. Subduction earthquakes occur when tectonic plates, as they gradually and continually move beneath other plates, encounter some resistance that slows them down. As tension builds up over time under mounting pressure from below there comes a moment when resistance snaps and one of the plates, usually the upper one, moves and causes an earthquake.

When one plate, usually the lighter continental crust, rides up over the top of the other it’s called a subduction zone, because one plate margin is being sub ducted under the other. The lighter continental South American Plate is riding up over the heavier oceanic Nazca Plate. Deep down where the leading edge of the Nazca Plate is diving down under the South American Plate it makes contact with the molten magma of the earth’s mantle. This melts the Nazca Plate margin sending magma chambers rising to the surface where they sometimes breakthrough in volcanic eruptions.

The chain of volcanic mountains known as the Andes is a result of the rumpling of the South American Plate where the Nazca Plate crashes into it. In view of the huge impact on land by these subduction earthquakes, why are there not more reports of damage to people? One reason is that the earthquakes are frequently offshore and another is that they equally frequently occur in places of low population density. The western part of Indonesia and the coastal areas of Peru and Chile are exceptions to these patterns and hence it is common to have large loss of life with South American events.

Whenever an earthquake as strong as the 1906 earthquake in Ecuador occurs, tsunamis can follow. In this particular case, about a thousand people on the coasts of Peru and Colombia were killed by the tsunami and many homes were destroyed. Other places also experienced tsunamis from this source. It was felt all along the coasts of Central America and in California. In the harbor at San Diego, boats at anchor were shaken and swung around by the force of the tsunami wave that reached this part of the United States. Similar effects were felt in San Francisco where tidal charts showed a rise in water level above normal.

The Valparaiso earthquake of August 1906 triggered a tsunami that had similar effects in these Californian cities. Across the Pacific the Ecuador tsunami reached the Big Island of Hawaii about twelve hours after the earthquake and did a lot of damage around the port of Hilo. It must have traveled across the Pacific at more than 400 mph. Tsunamis can travel much faster than that depending on the obstructions they encounter. Japan too was hit by this same tsunami.

The story was the same when another earthquake, this time one in the interior of the continent, shook the interior of Chile. It was an intra-plate quake, on the border between northern Chile and Argentina, in the southern part of the province of Atacama. It struck on November 11, 1922, and it had a magnitude of 8. A tidal wave swept over Coquimbo, a coastal city about 250 miles to the south of the epicenter taking the lives of several hundred people and caused enormous property damage, subsequently estimated as ranging from $5 to $25 million.

As happened with so many other earthquakes in South America the tsunami from this eathquake also hit Hawaii, reaching Hilo on the Big Island, the nearest point to Chile, in 14.5 hours with a wave height of seven feet. Many boats were washed away, and some damage was done. The tsunami reached the next part of Hawaii, Honolulu, half an hour later. On the west coasts of California the same tsunami wave was recorded at both San Diego and San Francisco with wave heights of less than a foot.

The Valparaiso quake of August 17 was more deadly than the one in Ecuador. The city of Valparaiso was destroyed and twenty thousand people lost their lives. The following description is taken from reports of people who lived through the tragedy. The earthquake seemed to go on for about fifteen minutes and the whole experience felt like standing on a wagon racing across uneven ground. Everything was thrown down, chairs, tables and desks inside, and buildings everywhere outside.

The cries and the behavior of animals shocked us. Their squealing and lowing, coupled with the cries of people everywhere made the whole scene all the more terrifying. Rail lines and bridges were all out of action and the telegraph communications cut. A locomotive on a siding was thrown to one side and ended upside down beside the track. Huge openings in the ground appeared here and there, one of them big enough to swallow a large trunk. In addition, shallow fissures appeared in the ground, some over six feet wide and two hundred yards long. Large rocks rolled down from neighboring mountainsides and blocked roads.

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