When the magnitude 8.2 earthquake struck northern Chile last week; the enormous release of energy caused the Earth to ring like a bell thereafter, seismographically as well as metaphorically speaking. This earthquake, like so many before it in the region, had the capacity to cause enormous loss of life and great destruction with huge tsunami waves from the sea, though that did not happen here.
It's important to understand the reasons why such a great earthquake occurred with such a low death toll compared to similar mega-quakes elsewhere in the world.
In mega-earthquakes — which occur about once a year on average somewhere in the world — major damage occurs to buildings, even those that were built to be very sturdy with earthquake reinforcement. The damage generally is spread over wide areas and is felt by people in extensive regions nearby. Death tolls can range from 1,000 to 1 million.
But that did not happen in Chile. To understand why, we need to look in more detail at the configuration of the coastline, as well as beneath the sea bottom.
The geography behind the earthquake
The Nazca Plate, a seafloor mass of crustal rock emerging from the East Pacific Rise, moves inexorably east-southeast undersea about 65 mm (2.6 inches) a year to thrust beneath the leading edge of the South American Plate. The Andes mountains rise above the South American Plate as the Nazca Plate descends in what is known as a subduction zone – the place where tectonic plates meet – with the giant plate sloping underground and the mountains faulting upwards as the product of the collision of seafloor with continent.
That collision is the ultimate earthquake cause, and understanding it is a key to why so few people were killed.
The "seismic gap"
The South American coastline changes direction abruptly from its southernmost south-to-north configuration, to the northern coast that trends off to the northwest. This configuration is known as the Arica Bend.
Beneath the surface in the subduction zone, the giant sloping slab of rock that descends into the Earth's crust may be bent or torn, and it certainly has some sort of unusual configuration. This configuration is reflected by the Andes mountains, which follow the extreme curve of the coastline in what is known as the Bolivian oroclinal bend.
Though we don't completely understand how the configuration of the subduction zone and how it's causing the instability, the theory is that it's related to the bend in the coastline.
This region of the Arica Bend has been a zone of remarkable seismic quiescence for most of the past century and a half and came to be known as the northern Chile, or Iquique, seismic gap. sThis seismic gap extends along the coast on both sides of the main Arica Bend. There, active faults have produced significant seismic events in the past. In 1877, an 8.8 magnitude earthquake occurred just south of the Arica Bend, and then in 1888, another 8.8 quake occurred just to the north of the Bend.
Then for a century and a quarter, no other events had occurred there, which made people notice seismic gaps and brace for the coming of a "big one" that would seemingly validate the theory of the seismic gap. An 8.2 earthquake certainly could do that, but what if that was only a foreshock to an even bigger one yet to come?
This possibility greatly worries people.
Bracing for the worst
In the past month, a series of magnitude 6-7 earthquakes had been occurring in the region, which turned out to be the foreshock warnings of one or more larger events to come. In this case, on April 1, a fault-plane rupture occurred throughout a submarine subsurface area 40 km (25 miles) long down in the subduction zone, and 30 km (19 miles) extending down the dip of the zone, all at depths of only about 20.1 km (12 miles) beneath the seafloor.
Normally with such a large undersea mega-thrust earthquake, and in deep water, the earthquake displacement of such a large water mass overhead will produce a large and damaging tsunami, as happened in the 2011 Tohoku undersea megathrust earthquake off Japan, as well as in the 8.8 magnitude El Maule earthquake in southern Chile in 2010.
In this new case in northern Chile, however, the water depth overhead was relatively shallow, and not a great deal of water mass was displaced, with the result that no large tsunami resulted. In fact, the Pacific-wide tsunami warnings that went out immediately after the main 8.2 event were subsequently cancelled as it was realized that the small tsunami that did occur produced waves only a few meters high, which was more like an abnormal high tide than anything else.
Even without a major tsunami, however, the earthquake could have devastated a region that was unprepared.
Taking the warnings seriously
This 8.2 earthquake on April Fool's Day did not fool many Chileans, though, mainly because of their long association with many prior such events. Nearly 928,000 people needed to be evacuated, with more than 2,500 homes sustaining serious damage, mainly around the northern port city if Iquique. Most of these damaged places were noteworthy because although they had used government subsidies for construction, their workmanship had been allowed to pass inspection when in fact it was of inferior quality.
In total, only five people died, though: four men and one woman – with two of them suffering heart attacks and the other three being crushed beneath debris. Numerous landslides blocked highways, and power outages affected thousands of people. Many fires destroyed businesses and homes in the region and an airport was damaged by the quake. Meanwhile, 293 female inmates of a prison in Iquique ran away from their damaged prison right after the main earthquake event, but 131 turned themselves back in voluntarily later.
In Iquique, the first tsunami surge measured only 2.5 meters high (8.2 feet), but it still did flood low-lying areas, including a medical clinic and a bus terminal. More such surges were expected, and people ran for high ground as the tsunami-warning sirens blared, but nothing more significant materialized, with the result that the warnings out in the far Pacific were later cancelled.,
But then, just over 24 hours after the main event, a second, 7.6 magnitude earthquake occurred at the water's edge with an epicenter just 90 km southeast of the original earthquake and 23 km south of Iquique. This aftershock also occurred at similar depths below the ocean bottom and produced a very small tsunami, with waves less than a meter high. Chilean President Michelle Bachelet was visiting areas affected by the first earthquake and had to be evacuated for safety herself when the second quake occurred. In fact, about 10 other aftershocks have also occurred since the main event.
The secret to the low death and damage toll in Chile with the current series of shocks is, of course, that the country has been so forewarned by the many prior events known far back into the history of the region. Once an enlightened and relatively corruption-free government administration comes to power in any country, their natural benign tendency is to help educate the public on what can happen and what to do about living with highly dangerous mega-earthquakes that can occur. This seismic-education process has been underway in Chile for some time, and the beneficial results are as obvious as similar such events in places like California, for example, where earthquake awareness is drilled into school children, or coastal regions of Oregon and Washington, where tsunami drills and moving quickly to high ground are now considered to be commonplace.
Impediments to progress
Such useful knowledge is in great contrast to that produced by the corruption-ridden behavior in such places as Iran and various parts of South Asia, where death tolls from earthquakes are always huge. This is largely because building codes are almost universally ignored through the placement of bribes to overlook infractions such as improperly mixed concrete (too little mortar, too much water, etc.), ignoring rebar reinforcement, or placing building sites directly on active faults, for example.
In addition, in such places, a further impediment to progress and saving of lives is the all-too-common notion that earthquakes are the result of God's (Allah's) displeasure with sinners, with the result that high death tolls are assumed to be the result of righteous wrath leveled against people who deserved to die because of their bad behavior. In my experience, such unfortunate religious ignorance is all too prevalent in many places in South Asia.
The role of science
As world populations continue to soar and humans live and work in evermore marginal or dangerous environments, scientific knowledge equally continues to burgeon. The most up-to-date information and new methods of prediction and remediation after the events add to the understanding and knowledge necessary to help people such as the Chileans avoid being caught in earthquake events that are so harmful to less educated people elsewhere. Publication of this information is an essential part of helping people overcome their risks, as the people of Chile obviously have been doing.
Dr. John F. Shroder has pursued research on natural hazards and resources for over half a century. He is Senior Research Scholar at the Center for Afghanistan Studies at University of Nebraska at Omaha, where he is Emeritus Professor of Geography and Geology. He has written or edited 35 books and more than 150 professional papers. Recently he has served as the editor in chief of Elsevier's 14-volume Treatise on Geomorphology, and is currently editing their new book series on Hazards, Risks, and Disasters. In addition, he is the long-term Editor-in-Chief of Elsevier's book series on Developments in Earth Surface Processes.
His new book, coming out in July, is Natural Resources in Afghanistan: Geographic and Geologic Perspectives on Centuries of Conflict.
Dr. Shroder is a Fellow of the Geological Society of America and the American Association for the Advancement of Science and has received Distinguished Career awards from the Mountain and the Geomorphology Specialty Groups of the Association of American Geographers. He also serves as a Trustee of the Geological Society of America Foundation.