Monster Quakes and the Great 1960 Chile Earthquake.
Mike Kozuch. [February, 2016; updated 2021]
Monster quakes are just that. Monstrous, gargantuan, magnitude 9 earthquakes. These giant mammoths of instantaneous earth deformation are the big kahunas of seismology. One monster quake may release the energy of nearly all other earthquakes of a century. These events are rare indeed, but their effect is global.
The rarity of these events makes them a unique challenge to study, particularly since a monster quake will not strike the same area more than once in hundreds or thousand years. In addition, there are very few areas in the world that are capable of generating such massive shocks.
To comprehend the enormity of energy released during these events, consider the well studied M9.5 Great Chilean Earthquake of May 22, 1960 also known as the Valdivia Earthquake. Evidence points to a fault system that ruptured nearly 1000 kilometers. Vertical displacements averaged a whopping 17 meters along the fault and the ground is still deforming decades after the event.
Damage from the 1960 earthquake in Valdivia, Chile. The shaking forced the land to spread towards the river.
A surface area of 40-80,000 square kilometers is believed to have shifted during the earthquake. The amount of energy required to move this mass of earth is equivalent to a 900 megaton bomb.
Reports of ground motion suggest that the ground moved like high swells at sea. Cars and trucks moved back and forth by up to a meter due to the movement of the ground at Conception, Chile.
For many weeks after the earthquake a series of aftershocks rattled an area of 200 by 2000 km. In addition to the devastating shaking, the region suffered landslides, liquefaction and a great tsunami.
Most of the coastal damage was associated with the tsunami that reached heights up to 25 meters. The tsunami arrived after some residents had returned to their homes to clean up after the earthquake. Unfortunately, many did not survive.
What Happens to the Earth in a Megaquake
Earthquakes of this magnitude have far reaching effects on the earth. The 1960 earthquake caused the entire earth to ring like a bell as seismic vibrations traveled around the earth for several hours. It was the first time that seismic waves were observed to set up these “standing waves” also called free earth oscillations. These low frequency waves are particularly useful in helping seismologists understand the internal structure of the earth.
The earthquake is also believed to have triggered a two-month eruption of the Cordón Caulle volcano beginning on May 24th. Since the location of the volcano is in a remote area, 800 km south of Santiago, there were very few witnesses to the eruption.
When the mass of such a large portion of the earth instantaneously shifts, it is not uncommon for the earth to experience a minor fluctuation in its rotation. A slightly smaller and more recent earthquake in Chile shifted the earth’s axis by 8 cm and shortened the length of day by 1.26 microseconds.
Eruption of Cordon-Caulle volcano two days after the Great Chile Earthquake of 1960. Photo credit: Pierre St Amand.
Can This Happen Elsewhere?
Fortunately, magnitude 9 earthquakes strike the same area only once every several hundred or thousand years. The coast of Chile is one of few locations worldwide that is capable of generating a magnitude 9.5 earthquake.
Although this event left nearly 2 million people homeless and cost half a billion dollars in damage, an earthquake of this magnitude in a more populated area would be catastrophic.
Since 1960, however, the world has witnessed magnitude 9 earthquakes elsewhere that include the the 1964 Great Alaska Earthquake of M9.2, the 2004 North Sumatra Earthquake of M9.1, and the 2011 Off East Coast of Honshu Earthquake of M9.0.
Awareness and preparedness is the only way that we can learn to live with these truly massive natural hazards.