At 4:45pm on Tuesday April 1st, a magnitude 8.2 earthquake hit the coast of Chile. This megathrust earthquake produced substantial ground shaking for hundreds of kilometers, and spawned a tsunami that triggered evacuations in Chile and Peru.
Chile has a history of earthquakes ranging from shallow (red) to deep (blue). The recent magnitude 8.2 is marked with a yellow star. Modified from USGS.
The news networks are covering what happened in terms of deaths and damages, but not so much on the science of what is going on. How is this earthquake different from the ones that happen in California? Why did it cause a tsunami, and why did some the tsunami warnings get cancelled? Have a question I didn't cover? Start a discussion below the article!
Flavours of Earthquakes
The brittle plates of crust and lithosphere that compose the surface of our planet are moving all the time at about the rate that your fingernails grow. They can move away from each other, allowing magma to ooze up and form new crust, slide past each other, or towards each other, destroying existing crust. Friction makes them stick, forcing them to deform as they continue to move. Whenever the deformation overcomes friction, the fault breaks free, releasing seismic waves.
Types of tectonic plate boundaries. Image credit: USGS/Jose F. Vigil
- Moving Apart: Divergent boundaries occur at mid-ocean ridges or in rift zones, where new oceanic crust is formed. These zones have earthquakes, but they aren't typically very hazardous in part because they're normally small and far away from people.
- Moving Past: Transform boundaries are the strike-slip motion of the San Andreas fault, and why Los Angeles was rattling last weekend. Earthquakes in these fault zones are generally fairly shallow, and produce moderate-magnitude earthquakes (magnitude 5, 6, 7) on a fairly regular basis, very rarely breaching larger to a magnitude 8.
- Moving Together: Convergent boundaries happen when plates collide. When both are continental plates, the crust crumples up into huge mountain ranges like the Himalayas. When it happens between a continent and an ocean, the high-density oceanic plate subducts under the floating low-density continental plate. The earthquakes in subduction zones are truly terrifying, occurring infrequently enough that locals forget their risk before finally slamming free with a magnitude 7, 8, or even a devastating magnitude 9. The earthquakes usually involve vertical displacement, triggering a tsunami like the 2004 Sumatra or 2011 Japan events.
This isn't to say you shouldn't treat all these boundaries with a healthy amount of fear-induced respect. Strike-slip earthquakes can be city-destroyers, burning down San Francisco in 1906, collapsing a large chunk of Port-au-Prince in 2010, and rattling the residents of Christchurch in 2011. Shallow earthquakes affect a smaller area very intensely, and although the lateral slipping rarely causes a tsunami, it can certainly trigger landslides.
Megathrust earthquakes in subduction zones let loose on a whole bigger realm of destruction. Earthquakes can occur anywhere along the subducting plate, where deeper earthquakes spread out shaking over a broader area. Large lengths of a stuck fault can let go at once, spanning a hundred kilometers or more. And after it's done, the aftershocks start just barely weaker, so a country slammed by a magnitude 8 megathrust earthquake can be shaken by multiple magnitude 7 earthquakes in the days after the main event, each of which might spark more landslides or even another tsunami.
2014 M8.2 Earthquake in the Iquique Gap
The west coast of Chile is in a subduction zone, where the Nazca Plate descending under the South American plate. The Nazca Plate is young, freshly-formed by the divergent zone to the east, and still (geologically) hot and buoyant as it charges towards the South American plate at a blisteringly fast 6.5 centimeters per year. It doesn't submit easily to being subducted, getting stuck and locked in the long descent into the depths.
Subduction along the Peru-Chile Trench has led to the uplift of the Andes mountain range, feeds active volcanoes, and has produced several seriously scary earthquakes. The list includes the 2010 magnitude 8.8 Maule earthquake in central Chile that shortened the length of the day. It also holds the record for the largest earthquake ever recorded, a magnitude 9.5 earthquake in 1960 located in southern Chile that generated a tsunami that devastated Hilo Bay, Hawaii.
Significant earthquakes in Chile between 1900 and March 2012 trace the descending Nazca plate. Image credit: USGS
But one place those earthquakes haven't happened is the Iquique seismic gap. This region is quiescent, with no major earthquake activity since a magnitude 8.8 in 1877. Even that earthquake was cheating a bit, as it was proceeded by a magnitude 8.8 immediately to the north in 1868, which loaded extra stress into the then-locked fault. Seismically speaking, quiet is not to be trusted as it usually means some horrible amount of stress is silently building up to catastrophically release.
Seismographs around the world detected the April 1st, 2014 Chile earthquake. Image constructed from USC Rev.
So, of course that's what happened. A mere 95 kilometers northwest of the city that gives the Iquique Gap its name, the plate slipped at 4:46pm local time. Shaking from the 8.2 earthquake ranged from light to moderate for all of Chile, with shaking felt in nearby countries.
Along with the star marking the epicenter, so much of the fault slipped for this earthquake that the slip zone is outlined in black. Over a hundred kilometer stretch of the fault let go at the same time, with the hypocenter located 20 kilometers deep into the subduction zone.
That much moving rock is loud. The hydroacoustic station HA03 at Robinson Crusoe Island, Chile actually picked up the submarine growl of the fault letting loose. The recording has been sped up by a factor of 64 to bring it into the audible range for humans:
The sudden appearance of red marks the passage of the P-phase, the seismic wave in the crust going past the hydrophone and radiating sound. 1000 seconds later, the T-phase arrives, the roaring sound from the epicenter finally catching up. Smaller horizontal lines are the background noise — ships and whales. This station was destroyed by the 2010 tsunami, but survived yesterday's event intact and functional.
The Did You Feel It? felt intensity map for this earthquake is much different than we saw for Los Angeles. To start with, the scale is totally different, encompassing cities in multiple countries hundreds of kilometers apart. Second, not many people have contributed reports. This may be because they are unaware of the United States program, or it could be because they have bigger problems to deal with locally than filling out online forms.
Circle colour corresponds to reported intensity, and circle city is proportional to the city's population.
The earthquake was big enough to trigger the pager system, sending out an orange-level alert. Pager stands for Prompt Assessment of Global Earthquakes for Response, an automated system to identify significant earthquakes and produce an initial prediction of scope of the potential disaster. Pager takes earthquake characteristics and pairs them with background information like surficial geology (which can amplify shaking) and population distribution to determine probable fatalities, damages, and cities impacted. The program is intended for emergency responders, government and aid agencies, and media, but anyone can sign up. For this event, the orange-level alert translates into an expectation of some fatalities, and significant damage.
The Chilean earthquake triggered fires, causing additional damage to buildings. AP/Cristian Vivero Boornes
Judging from the morning-after news cycle, the predictions were fairly accurate. Current reports are for six deaths, two from heart attack, three buried in a landslide, and one crushed by a collapsed wall. The damage is higher, with more landslides blocking roads, fires started by ruptured gas lines, and fishing boats sunk by the subsequent tsunami.
This is far better than after the 2010 earthquake, when several apartment blocks failed due to either underestimating the expected shaking in the region, or failing to adequately follow the country's strict seismic building codes. The increased education and preparation for tsunami also saved lives, leading to the successful evacuation of hundreds of thousands of people from the low-laying areas. However, one of the impacted towns still lacks a tsunami warning siren system, leading to local officials issuing the evacuation by shouting instructions through megaphones.
Introduction to Tsunami
A tsunami is a wave formed by displacement of water. Thrust earthquakes have an element of vertical displacement; megathrust earthquakes in subduction zones frequently have submarine displacement that produces a tsunami.
A tsunami is typically long wavelength and quick-moving, reaching airliner speeds in open ocean and slowing to highway-speeds near the coast. It shoals as it comes into shore, rapidly building in height as it rushes inland like a terrifyingly fast rising tide. From my 2011 Introduction to Tsunami:
Tsunami look like a rapidly rising tide, and may inundate far inland. Either the crest or trough of a tsunami may arrive first; if sea floor is suddenly exposed, run.
In open ocean, a tsunami is just a few meters of height difference spread over a few hundred kilometers. This is pretty much impossible to observe under all the wind waves, and is why the safest place for a boat to be during a tsunami is in open ocean.
The first wave may not be the largest wave, and interference effects between waves advancing and retreating add to the chaos.
Forecasting and the 2014 Chilean Tsunami
Because tsunami have such long wavelengths, they are classified as shallow-water waves throughout the entire ocean. This makes predicting their travel-times fairly simple, as shown in this NOAA map of projected travel times.
However, it is really hard to predict how tall a tsunami is going to be before it starts coming on-shore. A 1-centimeter tsunami may still qualify under the technical definition, yet poses no risk and generates mockery, while a 1-meter tsunami can be decidedly more threatening, and a 10-meter tsunami downright murderous.
The initial projected tsunami energy map is for a 1-meter tsunami spearing outwards from the epicenter. Initial forecasts were for the tsunami to impact the entire west coast of South America and even parts of New Zealand, but dissipating to a modest 1-centimeter tsunami before impacting North America or Hawaii. This initial forecast was then revised based on incoming data from a network of buoys that measure sea-level.
If this map looks familiar to you from the bogus, "Nuclear radiation from Japan is reaching the West Coast!!" stories, but you didn't know they were bogus, go read this. I'll wait. Done? Good, moving on.
All the orange dots on the travel-time map are buoys. Data from those buoys are used to refine the wave height projections, modifying the model used to determine if a tsunami advisory, watch, or warning should be issued for different regions.
The Pisagua buoy on the Chilean coast records over an hour of tsunami activity. The initial wave amplitude is a 2-meter peak (the crest came first, inundating the coast) followed by a 2-meter drop (the trough, exposing the sea floor), while later waves climbed to even higher amplitudes.
The main tsunami activity died off after an hour, reducing to the after-echoes of not-insubstantial meter-tall waves ringing the buoy.
Another buoy, San Felix, is located just over 900 kilometers off-shore. It, too, records the tsunami activity, but being in deeper water means the wave height is much, much smaller. The entire graph is rescaled to match, barely brushing a meter displacement at maximum.
Based on the data from these buoys and others in the region, the tsunami forecast model was revised. Initially, upon detecting the earthquake, Chile, Peru, and Ecuador were under a Tsunami Watch, while Columbia, Panama, and Costa Rica were under a Tsunami Warning. Then, several countries were upgraded to Watch, while new ones were added to Warning, and an advisory was issued for the rest of the Pacific. Next, the Warning was cancelled for everywhere except Chile and Peru, then even they got their break as the tsunami finally passed.
Fishing boats tumbled by the 2-meter tsunami in Iquique, Chile. Photo credit: AP/Cristian Vivero Boornes
High compliance with evacuation orders saved lives last night, as thousands of people left the coast for high ground to wait out the tsunami. The tsunami tossed around fishing boats, sinking some and leaving the ports a mess, but no fatalities from the tsunami have been reported. The port is a major shipping point for local copper mining, and while the mine is apparently undamaged by the earthquake, fears of supply problems from the disrupted transportation system (landslides on roads, and tsunami in the port) spiked copper prices overnight.
After the tsunami finished trashing the South American coast, only Hawaii remained under an Advisory. By morning, the Advisory was cancelled. As always, check out NOAA's Tsunami center for the most up-to-date news on any tsunami threatening countries bordering the Pacific Ocean.
Preliminary buoy data in Hilo Bay, Hawaii indicates it did feel the tsunami, with half-meter waves this morning. No evacuation was issued or damage reported, but strong currents would have made it a disproportionally dangerous time to go for a quick swim. Tsunami don't make that epic cresting curl we use on tsunami warning signs, and are instead disappointingly flat surges, so please don't die trying to surf one.
Surviving a Tsunami
What's with the Advisory, Warning, Watch terminology? NOAA decided to match tsunami vocabulary to that already familiar to people in severe-storm territory. They made a colour-coded infographic with emotional stickmen to illustrate the different levels of hazard, but I prefer this translation:
- Information Statement – Something happened somewhere.
- Watch- Pay attention.
- Advisory- Don't go swimming.
- Warning – Run!
We're getting very good at predicting tsunami, and adjusting the projected inundation models once the tsunami starts coming on shore where we can observe its severity. With a basin-wide monitoring system and an ever-improving alert notification system, it is entirely possible to avoid any fatalities for distantly-sourced tsunami. If you ever find yourself in a situation where local authorities have issued a tsunami warning, take it seriously and evacuate.
As an additional bit of survival advice, if you're near the ocean and feel strong shaking (a major earthquake), run for high ground as soon as the shaking stops. High ground doesn't need to be a hillside — a tall apartment building with a solid concrete ground floor can provide vertical refuge. Assume a tsunami will arrive within ten minutes, and don't wait to hear a formal evacuation notice. The first wave may take up to an hour to arrive, or the trough may hit first, fooling you into thinking it's safe, so once you get to high ground, stay put for at least an hour or longer.
UPDATE: All about the aftershock sequence.
All images credit USGS unless otherwise stated. Access the USGS data page for this earthquake. Head to Animals for what happens to creatures during tsunami. Need something more cheerful? Try boomerangs in space, the Tongue of the Ocean, or an adorable marshmallow-moonsuit. Want something obscure? Try forgotten source-code comments in astrophysics papers, or dog racing on bare ground.