12. The strongest earthquake ever recorded

Morsel: 

The strongest earthquake ever recorded shook the Chilean city of Valdivia on 22nd May, 1960, with an enormous magnitude of 9.4-9.6 on the MMS.

Meal: 

Before we delve into the tale of the Valdivia earthquake, it may be helpful to understand what the moment magnitude scale (MMS) measures and to form a feeling for the relative strength of each number given to the strength of an earthquake. 

MMS is the scale used by contemporary seismologists to indicate the amount of energy released by the seismic moment of an earthquake, or the moment it quakes. You may be accustomed to reading about magnitudes recorded in the Richter scale, but this has shortcomings. It measures an earthquake's strength using a particular seismometer within 100km of the earthquake's epicentre. This is useful for determining the strength of local earthquakes, but, in turn, produces an upper limit, and thus becomes less useful the further away an earthquake is. 

MMS works globally and relies on three factors to determine an earthquake's strength: the rigidity of the rock at the epicentre; the distance the land moved; and the area of the ruptured land. This scale is logarithmic, meaning that each increment on the scale equates to 10 times more energy released. If we compare the energy to that released by kilograms of TNT, then a magnitude 2.0 earthquake would be 15kg of TNT, which would not typically be felt. When we jump to 4.0, our earthquake explodes with 15,000kg of TNT, which can cause minor damage. Take an even greater leap to 8.0, and we set off 15 billion kg of TNT, utterly destroying anything near the epicentre. 

The earthquake that shook the region around the Chilean city of Valdivia for three minutes on 22nd May, 1960 was one more increment on that scale, releasing energy equivalent to one million atomic bombs of the kind dropped on the Japanese city of Hiroshima during WWII. 

The devastation caused by the earthquake itself, however, was relatively minor thanks to the limited amount of infrastructure built up in the rupture zone and the good performance of earthquake-resistant houses. But, like the 2004 Indian Ocean earthquake, the true killers were the tsunamis. 

Waves of up to 12 metres high struck Chilean coasts at speeds of 150-200km/h, annihilating any building in its path and even a few islands. The tsunamis then spread out over the Pacific ocean, hitting countries as far away as New Zealand and Japan. Each tsunami was stronger than the next: the initial wave that struck Hawaii was around 10m in height, but later waves reached as high as 80m. 

Thousands of lives were lost and millions of property damage were inflicted. Those who did survive now found themselves homeless and their livelihoods ruined. Some people became mad with desperation: Mapuche Indians sacrificed the heart of a six-year-old in a (futile) attempt to assuage the rage of the sea. 

The earthquake triggered not only tsunamis. Rivers overflowed, flooding streets and croplands. Thousands of landslides swept through the Chilean countrysides. Liquefaction of the soil caused houses to collapse and depressed the land under its sheer weight. The local once-dormant Puyehue volcano erupted back into life two days after the quake and spewed ash 6km into the atmosphere for two months. Even mud volcanos were spurred into life with very high sand/mud blows. It may have even led to an 8.8 magnitude earthquake 50 years later, the 2010 Chile earthquake. 

It is believed earthquakes of such strength are on a recurring cycle in the region around Valdivia. Thankfully, the cycle seems to be one every 120 years, so only about 60 years to go. 

Recipe:

IRIS Earthquake Science. 2012. Moment Magnitude Explained—What Happened to the Richter Scale? YouTube. 


Gates, Alexander E. and David Ritchie. 2007. Encyclopedia of Earthquakes and Volcanoes. 

Anon. n.d. “Chile Earthquake Altered Earth Axis, Shortened Day.” National Geographic.

Comments

Post a Comment

Popular posts from this blog