May 22, 1960 marks the date of the most powerful earthquake ever recorded, in terms of the moment magnitude scale. This case study will investigate the causes behind the seismic event, the geography of it and the secondary effects that resulted.
In order to engage with the nature of seismology, the reader must have a clear understand of what a subduction zone constitutes. In an article for Live Science magazine, Becky Oskin says that a “subduction zone is the biggest crash scene on Earth” that takes place over millions of years (Oskin, 2015). When the much denser oceanic plate pushes against the less dense continental one, subduction forces the oceanic plate to bend underneath the other, causing the formation of volcanoes, tsunamis and the earth’s largest earthquakes. Figure 1 illustrates the subduction zone created by the oceanic Nazca plate’s collision into the South American continental plate. The Pacific Ocean’s “Ring of Fire” outlines a massive series of subduction zones that are responsible for a large percentage of the earth’s most dangerous seismic events today, as depicted in Figure 2.  Oskin compares the subducting collision of two tectonic plates to rubbing sandpaper together, in that “the crust sticks in some places, storing up energy that is released in earthquakes” (Oskin, 2015). Since subduction zones create the largest faults in the world, they directly correlate with the most dramatically powerful earthquakes. Along the Peru-Chile trench, created by the subduction of the Nazca plate under the South American one, is one of the world’s largest and most active faults.
According to the U.S. Geological Survey, at 10:02 UTC (around in the morning local time) lasted for 35 seconds but it would devastated one-third of the buildings in Concepción, reaching 8.1 on the moment magnitude scale (USGS, 2016). This initial earthquake would kill 150 people, according to an article published immediately following the event by William E. Rudolph, and it marked the beginning of “ten days of terror” that would ultimately leave 1000 dead and 350,000 homeless (Rudolph, 1960). The second quake would occur slightly over 24 hours later, at 10:30 UTC in the Nahuelbuta National Park, just south of Concepción, reaching 7.1 mw (USGS, 2016). The third would come the next day at 18:56 UTC in the same area, totaling 7.8 mw (USGS, 2016). As powerful and destructive that the three Concepción earthquakes were, they would come to alert the rest of the population for what would come.
Roughly 15 minutes after the 7.8 mw earthquake south of Concepción, at 15:11 local time, the town of Valdivia would endure 11 uninterrupted minutes of seismic shaking (Seismo Blog, 2015). USGS shows the epicenter of the 9.5 mw earthquake situated on the coast just east of Lleulleu Lake, which was actually several hundred kilometers directly north of Valdivia, which appears in Figure 3 (USGS, 2016). Using the previous system, the Richter scale, the moment actually only measured 8.3, which is how older sources refer to the event; but the moment magnitude scale that the geophysics community adopted in the 1970s converted the moment into the 9.5 mw by which we refer to it today (USGS, 2003). This is not the most powerful earthquake to have ever taken place, but it is the highest ranking earthquake to be recorded due to the recent advancements and global distribution of seismic and geological measuring instruments by both the United States and the Soviet Union.
The immense toll in fatalties and damages was largely due to the secondary effects of the Valdivia earthquake that took place throughout the following days after May 22, inlcuding tsunamies, flooding, lahars, and the even eruption of the Cordón Caulle volcano. Related to the tsumani and the destruction of at least 40% of all structures in the area, flooding ran rampent for several days following May 22. Figure 4 is a still from flooding in Quellon, a costal island community several hundred kilometers south of Valdivia. The worst mass wasting came to be known as the Riñihuazo flood. When several lahars resulted from the Valdivia earthquake, the Riñihue Lake become completely blocked from a connected outflow river, the San Pedro (Diario Austral, 2010). This cause the lake to rise at an alarming rate and pour into the adjacent communities. Then, on May 24, the Puyehue-Cordón Caulle Volcano, some 200 kilometers from the epicenter of the earthquake, would erupt (Rudolph, 1960). The eruption initiated itself with a powerful explosive phase, creating a clumn of ash 8 kilometers high from the source. The volcano would not erupt again for another 51 years.
The populations of Chile have been adapted to seismic activity for centuries, since the faults lining the Peru-Chile trench are among the most active in the world. Especially due to somewhat large earthquakes in the 1920s and 1930s, Chilean society still had earthquakes fresh in its mind (Diario Austral, 2010). Citizens were extremely fast in evacuating the buildings, and were able to band together very quickly. Most of the problems were structural, since Chile at the time was not nearly as economically affluent as it is today. However, the seismic event was literally felt throughout the world, and many nations quickly began to pour in aid to stimulate the initial recoveries (Diario Austral, 2010). Mexico especially sent large amounts of financial aid and donated several schools to the Valdivia area. The following video recalls the devastating chain of events and offers telling footage of the time:
Sources of images:
 Figure 1: “Eruption on Volcán El Reventador,” Science Thoughts, retrieved April 22 2017 from http://sciencythoughts.blogspot.com/2012/11/eruption-on-volcan-el-reventador.html
 Figure 2: “Ring of Fire,” National Geographic, retrieved April 21 2017 from https://www.nationalgeographic.org/encyclopedia/ring-fire/
 Figure 3: USGS, Last modified 2016. M 9.5 – Bio-Bio, Chile; retrieved April 28 2017 from https://earthquake.usgs.gov/earthquakes/eventpage/official19600522191120_30#map
 Figure 4: “Terremoto de Valdivia 1960,” Volcanes históricos; retrieved April 28 2017 from http://www.volcaneshistoricos.com/terremoto-de-valdivia-1960/