A massive earthquake just hit off the coast of Chile. At magnitude of 8.3 and a tsunami warning in effect, this could have been ugly. Here’s the science behind the earthquake, how Chile’s preparations are paying off, and what we can expect for the shaken country.

The Earthquake: What We Know so far

An earthquake hit off the coast of Chile 7:54 pm local time. The magnitude 8.3 earthquake was followed by several aftershocks of up to 7.0, and triggered a tsunami.

Predicted perceived surface shaking for the M8.3 earthquake and the M7.0 aftershock in Chile on September 16, 2015. Image credit: USGS

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Earthquakes are caused when a stuck portion of a fault overcomes friction and abruptly releases. While we talk abut the epicenter where the earthquake starts, in reality an entire area ruptures and slides. Larger ruptures produce larger magnitude earthquakes: this one was the result of a slip area roughy 230 kilometers long by 100 kilometers wide slipping a few meters.

The earthquake was at a depth of 12.3 kilometers, which is relatively shallow for a large event like this. That means that the shaking was focused to be more intense over a smaller area than it would have been with a deeper earthquake. This earthquake shows the characteristics of a thrust earthquake, directly releasing stress along the primary boundary between the plates.

Severe shaking lasted approximately 80 seconds, with milder surface shaking continuing for 190 seconds from start to end.

Seismogram of vertical displacement recorded at Las Campanas Astronomical Observatory, Chile. Image credit: IRIS

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Earthquake magnitudes are notoriously difficult to get right quickly. The earthquake was listed as a moment magnitude 7.9 when I started drafting this article, and upgraded to an 8.3 before I published! Notice that’s a different scale than Richter: moment magnitude more accurately reflects the true energy released by larger earthquakes. Earthquakes of this scale happen roughly once a year.

Shaking in a grocery store in Valle Lo Campino, a suburb of Santiago.

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The earthquake’s epicenter was just 46 kilometers (29 miles) west of Illapel, Chile, a city with a population of roughly 23,000 people. The earthquake was about 228 kilometers (142 miles) north-northwest of Santiago, Chile’s capital and largest city: that’s close enough that buildings swayed in the earthquake.

Although local geology can emphasize or dissipate shaking, the current estimate is that around 54,000 people were exposed to very strong shaking, with close to ten million people exposed to moderate to strong shaking. Another one to two million people likely felt light shaking.

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One woman is reported dead in Illapel, crushed by a falling wall. Another four died from yet-undisclosed injuries. At least 20 others are injured.

The nearby Coquimbo recording displacement of over 4 meters and multiple waves. Note the initial wave was not the largest wave! Image credit: IOC-VLIZ

The initial tsunami warnings predict that the earthquake could generate a local tsunami with waves reaching 3 meters height in Chile. Both Coquimbo and Tongoy were flooded. Today was the first time Chile’s new tsunami warning system was activated.

NOAA’s initial Tsunami Watch for parts of the Pacific Basin predicted waves 1 to 3 meters high are expected to reach French Polynesia, dissipating to waves 0.3 to 1 meter high in Mexico, Ecuador, Peru, Antarctica, Japan, New Zealand, Fiji, Russia, Hawaii, and throughout the Pacific islands. It is possible the tsunami may extend further with possible waves under 0.3 meters high throughout Central America, Australia, the Philippines, China, Taiwan, and Indonesia.

Expected energy propagation of the tsunami for the earthquake’s initial estimated magnitude of 7.9, and the revised magnitude of 8.3. Image credit: NOAA/Jesper Dramsch

These forecasts were tightened up as the waves come ashore and researchers calibrated the models for this particular tsunami. It looks like most locations will experience under 0.3 meters (1 foot) waves. While this won’t produce inundation far inland, these tsunami are still dangerous due to their strong currents.

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Estimated travel times for the first arrival of tsunami waves. Image credit: NOAA

It is still possible to die in a tsunami smaller like this, but only if you discount the force of these incredibly strong currents. Unfortunately, this happens sadly frequently as foolish onlookers head to the coast to indulge their curiosity, attempt to surf the non-cresting wave, or otherwise go wading into the water. For everyone outside of Chile: read your region’s alert for details, but for the most part if you don’t go for a morning stroll along the beach you should be fine.

Several large aftershocks already hit the region, with one at magnitude 7 and several over magnitude 6. The aftershocks will decrease in both intensity and frequency following a logarithmic decay.

Chile’s Geological Setting: Earthquakes can Get Even Larger

Chile is on a subduction boundary, which means that tectonic plates beneath the Pacific Ocean are diving under the continental crust holding the South American country. These types of plate boundaries are known for producing violent megaquakes—think Nepal, Japan, and the Pacific Northwest. Chile in particular has the dubious honour of producing the largest earthquake ever recorded in human history.


Plate tectonic setting of Chile and recent tectonic rupture zones. Image credit: Robin Lacassin/LIA Montessus de Ballore/IPGP-Tectoniquq/CNRS-INSU

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The Nazca plate is shoving under the South America plate at between 65 and 80 millimeters per year at different segments along the fault. That’s roughly at the rate that your fingernails grow. This convergence is responsible for creating the massive Andes Mountains, and provides the melt to fuel an active volcano chain. This particular portion of the fault is where the Nazca plate is dipping more steeply, at an angle of between 25° to 30° as it dives into the Earth’s mantle at 74 millimetres per year. The earthquake’s epicenter was 85 kilometers east of the subduction trench marking the fault boundary between the plates.

When a fault ruptures and produces an earthquake, it isn’t a single pin-prick point event but an entire area releasing under stress and sliding. The stress along the rupture area drops, but the stress increases along the locked boundaries that didn’t move.

All earthquakes in the Illapel region in September 2015. Notice most of the earthquakes trace the slanting subducting plate boundary, with very few shallow earthquakes within the South American plate. Image credit: Centro Sismológia Naciconal

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This particular earthquake occurred is just north of the region that produced a devastating magnitude 8.8 earthquake in 2010. Approximately 400 kilometers of the fault ruptured with displacements up to 2 meters. Fifteen other earthquakes of magnitude 7 or higher have occurred within 400 kilometers of this earthquake in the last century.

The earthquake could easily trigger landslides in the Andes, Chile’s steep coastal mountains. Thus far, no problems have been reported in the local mining operations.

The Science of Tsunami: Rolling Waves of Destruction

Debris in the streets of Valparaiso, Chile after a tsunami swept through the town. Image credit: Pablo Ovalle Isasmendi/AGENCIA UNO/AP

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These subduction zone earthquakes are also known for triggering tsunami, a series of waves that look like a rapidly-rising tide surging far inland. The series of waves can be as close together as just tens of minutes, or up to hours apart. The first wave isn’t necessarily the largest one, and constructive interference between advancing and retreating wave crests can bounce that maximum height even further.

Tsunami travel at roughly the speed of a jet airliner in open ocean but are barely detectable. They have only tens of centimeters surface displacement stretched over a two hundred kilometer wavelength, unnoticable to the human eye. Instead, Deep-ocean Assessment and Reporting of Tsunamis (DART) buoys are used to detect tsunami propagating throughout the Pacific Ocean.

The nearest DART buoy about 500 kilometers from the epicenter is already starting to record the anomaly of a passing tsunami. Image credit: NOAA

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Tsunami have such long wavelengths that we can model them as shallow water waves even in the middle of the Pacific, so we can predict how quickly a tsunami will propagate through the ocean. Unfortunately, because tsunami have such a small height compared to a phenomenally long wavelength at sea, we can’t predict the tsunami’s magnitude and high the waves will be until the first few waves come onshore.

The waves shoal as they enters shallow coastal water, slowing down to highway speeds as it bunches up into a massive wave up to tens of meters tall. So far it looks like we’ve gotten lucky with this particular earthquake: the closest tide gauge measured waves reaching a maximum of 4.6 meters (15.1 feet) high, getting smaller farther away.

Chile’s Social Setting: Preparation is far Better Than Response

This earthquake is shaping up to be a case study in the value of preparation.

People remain outdoors in Santiago anticipating further aftershocks. Image credit: Vladimir Rodas/AFP/Getty Images

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Because the earthquake hit at night, most residents were at indoors at home, not at school or work. Most residents of the region currently live in a mix of low-rise reinforced masonry and adobe buildings. The automated USGS Pager Alert for the earthquake and tsunami predicted a handful of deaths, but damages in the hundreds of millions to billions of dollars.

A lack of preparation in 2010 and meant that more than 370,000 houses were destroyed along with 4,013 schools and 79 hospitals. At least 523 people were killed with 24 missing and another 12,000 injured, and a total of 1.8 million people impacted throughout the region. Total damages reached US$30 billion. Since then, residents and politicians have both pushed for preparation, retrofitting buildings and increasing mitigation efforts.

This emphasis on seismic safety already paid off in the relatively low damage from last year’s Chiliean megaquake, but it’s paying off even more now. In the disaster risk reduction community, it’s commonly said that every dollar spent in prevention reduces the cost of response by $5. The initial reports from the region are indicating mostly superficial damage in Illapel, Santiago, and nearby cities.

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As always, places we hear from rapidly are those that are best off: the places that are silent for hours or even days usually indicate severe damage to infrastructure.

Are you in Chile?

Google has created a People Finder for this earthquake and Facebook has rolled out its Safety Check so you can let your friends and relatives know you’re okay.

Once you’re somewhere safe, please help the USGS collect data with a Did You Feel It? report. This data helps researchers understand how local geology and buildings impact the actual shaking felt by people in the region, improving our models for the future.

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Police help control lines of people awaiting public transit after the earthquake. Image credit: Luis Hidalgo/AP

Do you have questions about this earthquake? Ask away!

Top image: People evacuate a mall in Santiago after the earthquake, anticipating aftershocks. Credit: Nadia Perez/AGENCIA UNO/AP


Contact the author at mika.mckinnon@io9.com or follow her at @MikaMcKinnon.