In 1960, Chile had a massive magnitude 9.5 earthquake, the largest ever recorded. Between foreshocks, aftershocks, tsunami, and eruption, this is a nasty, complicated day in geoscience history.
On May 22nd, 1960, Chile produced a massive magnitude 9.5 earthquake, the greatest monster of an earthquake ever recorded by seismologists. Foreshocks, aftershocks, tsunami, and even a volcanic eruption contributed to the chaos.
In the days leading up to the earthquake, the foreshocks were bad enough that in any other time they'd stand alone as substantial earthquakes. Four foreshocks measured over 7.0, with a magnitude 7.9 on May 21st that caused severe damage in Conception. For context, the Great Quake that destroyed San Francisco was a magnitude 7.8. Each foreshock redistributed stress, releasing stress along the slipped section, and building more at the locked ends.
Finally, at 3:11pm local time, 1,000 kilometers of the fault ruptured stretching from Lebu to Puerto Aisen. That's an area the size of California, sliding 50 meters along the sea floor. The sudden released deformed plates, with the seafloor snapping up by up to 6 meters (20 feet), while the coast dropped by 3 meters (10 feet). This abrupt displacement spawned a tsunami that spread cross the Pacific.
The aftermath kept the rumbles going, with five aftershocks over 7.0 continuing through November 1st.
A Chilean Tsunami Devastating Hawaii
Tsunami are triggered by any major vertical displacement in the water column. This can be large-scale as a meteorite impact acts like a very large pebble into sending out massive ripples in an ocean pond, or small-scale when a calving iceberg spawns waves that endanger the local area but quickly fade out. Subduction earthquakes, where the seafloor is vertically displaced by meters, frequently trigger tsunami. When the 1960 earthquake in Chile wrenched the seafloor 6 meters, a massive displacement, it triggered a tsunami that propagated across the entire Pacific.
Drowned forest in Chiloe, Chile. Photograph by David McGarvie.
The tsunami came on-shore locally within minutes, compounding devastation and loss. Paired with subsidence from the coastline dropping during the earthquake, the tsunami swamped the Chilean coast. Fifteen hours later, it reached Hawaii. Seven hours after that, the tsunami reached Japan, still strong with ten-foot waves.
For an idea of just how devastating the tsunami is, NOAA created an animation of the 1960 tsunami side-by-side with two other tsunami triggered by smaller-magnitude earthquakes at the same location.
All three tsunami propagate at the same speed despite their different wave heights and energy. Tsunami have such long wavelengths that even in deep ocean, they still behave as shallow-water waves. As shallow-water waves, their travel time across the ocean dictated not by the wave energy but by ocean depth.
This makes predicting the time it takes for a tsunami to reach a distant shore a relatively simple task, but predicting how high the waves will be when they reach their destination is far more complicated.
The entire Pacific basin was impacted by the tsunami, with variable devastation.
Only a few trees along the river survived the tsunami in Queule, Chile. Photo credit: Pierre St. Amand
Queule, a fishing village between Valdivia and Lebu, is a good example of the damage caused by the combination of subsidence and tsunami. Three meter (15 feet) waves uprooted trees, destroyed buildings, and carried fishing boats two kilometers inland.
Tsunami damage in Hilo Bay, Hawaii. Image credit: USGS
The tsunami was almost universally one to five meters (3 to 17 feet) tall, except in Hilo Bay, Hawaii. By sheer bad luck, the tsunami period exactly matched that of Hilo Bay. This meant the tsunami resonated so that each wave built in amplitude through constructive interference as it bounced around within the bay. The tsunami waves grew ever-larger in a seiche, the waves towering up to 10 meters (30 feet) from crest to trough, utterly destroying the bay.
Small fishing boats tossed onshore like this one in Halfmoon Bay, California, accounted for most tsunami damage on the west coast. Photo credit: Don Tocher
The west coast of the United States escaped with moderate tsunami damage from waves one meter (3 feet) or smaller. Most damage was small fishing boats getting tossed on shore. A skin diver went missing, but not officially declared dead. Los Angeles and Long Beach harbours were hit hard, with the tsunami causing ship collisions, washed-out pilings, and gas spills. In San Diego, a passenger ferry smashed into the dock, and another was forced offshore and smashed into destroyers. A bait barge smashed into a landing, broke in half, and sunk. A pair of $30,000 yachts were severely damaged in Crescent city, while in Santa Barbara, an oil exploration barge repeatedly rammed into a dredge.
The Pacific was ringing from the waves for days after the earthquake: tide gauges reflected anomalous readings for three days after the earthquake.
Cordon Calle eruption. Photo credit: Pierre St. Amand
Cordon Calle, a fissure along the Puyehue stratovolcano 200 kilometers from the epicenter, started erupting on May 24th. This may have been pure coincidence, but it could plausibly be tied to the earthquake. The mechanism for how a major earthquake can trigger an eruption is under investigation. Most theories focus around mechanisms that change pressure within the magma reservoir, including:
- The earthquake causing stress on the magma chamber. Compressing the reservoir increases stress, or expanding the reservoir can open nearby fractures.
- High amplitude seismic waves might nucleate bubbles in the magma, increasing magma pressure. Alternately, those waves might destabilize layering within the chamber, rising gas-rich magma and sinking gas-poor magma.
- Violent shaking triggers landslides and fractures. This can change the pressure for shallow reservoirs.
The volcano ejected columns of ash and steam as high as 6 kilometers for weeks. Between the isolated location and the overwhelming chaos of the continuing aftershocks and disaster, no one paid the eruption much attention.
Expensive Aftermath of Damage
Damage was extensive, leaving 2 million people homeless in Chile with $550 million in damage. Add in $75 million in damages in Hawaii, $50 million in Japan, and another half million to a million in damages west coast of the United States. Overall, that's a brutally expensive disaster.
The total fatalities and damage for such an extensive disaster is difficult to pin down. At least 1,655 people were killed and an additional 3,000 injured in Chile, while the tsunami killed 61 in Hawaii, 138 in Japan, and 32 in the Philippines. The mid-afternoon timing was a small bit of luck — most people were at work or school in earthquake-retrofit buildings. The foreshocks had also made the population wary, so people were more prepared than normal for an earthquake.
You can read more on the USGS story of the earthquake, and view the damage atNOAA photo gallery of tsunami impact. Chile recently had a smaller megaquake and aftershock sequence, but not enough to close a seismic gap. Subduction earthquakes are much different than transverse earthquakes experienced in California.