Chile’s Earthquake

My first response during disasters is to immediately begin teaching those around me, providing context for interpreting the news stories of far-away horror.

I am nervous of over-representing myself as an expert. I am in training as a Master of Disaster, specializing in catastrophic landslides (anything over a million cubic meters of material), and frequently assistant-teach the UBC first-year natural disasters course. I think about natural disasters a lot more than the average person, but trained seismologists know a lot more about the Chilean quake than I do. Trust them. But if you’re looking for layman’s context, here’s the downlow on Chile’s earthquake.

Geologic Context
South America is prone to subduction-zone earthquakes (one plate diving under another), the deepest meanest earthquakes out there. But there is a tiny blessing — the bigger the earthquake, the deeper it is. With deeper earthquakes, the energy is spread out over a larger ground-surface area, so more places feel shaking but the shaking is less intense than for shallower earthquakes. It’s counter-intuitive, but in subduction zones, smaller magnitude earthquakes are more devastating than larger magnitude earthquakes (Discussion of deeper earthquake over larger area vs. Haiti’s shallower, more focused quake). In Vancouver, we’d probably have more structural damage from a a shallow magnitude 7 than by a deep magnitude 8 or 9 (and yes, we’re inside the 300-500 year return period on those, so odds are not insignificant that we’ll experience this inside our lifespan).

Building Codes
The most stringent earthquake-safe building codes in the world are in Japan, California, and BC, but Chile’s are very good. The news stories always talk about the absorbing springs in the foundations, but it’s a lot more to earthquake-safe building than that — it’s banning soft stories (ie, parking garages on the first floor) that are prone to collapse, designing windows to break inwards so glass doesn’t rain down in the city core, or grouping buildings by similar natural frequencies so highrises don’t smash together when swaying. Good building codes don’t eliminate damage; they ensure the buildings fail in a way that minimizing injury. Buildings may go lopsided on their foundations, or windows shatter, but the inhabitants will not be crushed. This is why we teach hiding under desks to school children in these regions — we expect the building to survive, and by hiding under a desk you are protected from falling books, lamps, or those fibrous sound-absorbing tiles in classroom ceilings. In places with less stringent building codes, it’s common to teach people to lay down on the ground during an earthquake on the assumption that the roof will collapse, and the farther down you are the better your chances of having something prop the roof and protect you. It’s a technique of last-resort, and a dangerous myth in places with decent building codes.

In rural areas, it is more frequent for buildings to pop up without legally existing, avoiding the building codes entirely. In Chile, earthquakes happen fairly regularly, so the folk-traditions guiding construction are relatively earthquake-friendly — lightweight materials that collapse easily, but generally don’t cause much damage. (In contrast to places that put on heavy slate roofs that collapse and squish the inhabitants.) In Chile, it won’t be the buildings that kill people. (A brand-new apartment building collapsed, sharply increasing the estimated fatality count.)

The automated tsunami warning network functioned correctly, and the Pacific Ring has a very good monitoring and notification system that should prevent the catastrophe of the 2004 Sumatra tsunami. With tsunamis, we can very accurately predict the locations and times of the wave arrival, but it is extremely difficult to judge the height of the tsunami while it is in open ocean. This is because while only a few centimeters tall in deep water (on a boat in open ocean is the safest place to be in a tsunami), the wave can build to many meters in height as the water shallows. This means it is not infrequent for a tsunami to arrive and be only inches tall, indistinguishable from the regular waves. The general population gets frustrated for having been evacuated for something so trivial, considering the warning a “false” alarm and are less likely to obey evacuation commands in the future. It’s a really hard job to decide when to issue tsunami alerts. Some media coverage is calling the warnings unnecessary, as the tsunami was very mild, but a former colleague of mine who was observing in Hawaii at the time says the local mood was grateful both for the warning, and for the mildness of the event.

A tsunami is a set of really big waves, so it has that sideways-S shape with a high (crest) and a low (trough). Sometimes the trough hits before the crest, sucking the ocean out and leaving the seafloor exposed, which very few people recognize as a warning of an incoming crest that will inundate the area, so they decide it was a false alarm and go wandering out to poke at the newly-exposed seafloor. If you take away one lesson from this, if you’re ever at a beach and the sea retreats, get to high ground as fast as you can. The biggest wave doesn’t always come first (it’s a bit more likely to be 2nd or 3rd), so don’t go down to the coast after the first wave passes.

For this tsunami warning, the places at biggest risk from tsunamis are the tiny islands that have been developed for tourism, and towns at the ends of narrow inlets. Tourist islands usually have their mangroves removed (the less-developed islands will probably still have their tsunami-absorbing mangroves) and lack the infrastructure and plan for organized evacuations. It’s even harder than usual to judge the eventual tsunami-height in narrow inlets, where the wave gets channeled and can even form a seche (wave oscillating back and forth) repeatedly inundating the coast — this is what happened in Port Alberni, a town at the end of a fjord, during the 1964 Alaska quake.

In natural disaster statistics, it’s hard to decide if deaths from earthquake-triggered landslides are included in as landslide or earthquake fatalities. One of the very serious longer-term (months-from-now) risks facing Chile comes from the Andes. The Andes are big, pointy, young mountains with a lot of active volcanism blanketing the hills in loosely-consolidated, slick volcanic ash. The case studies I’ve worked on for Chile all involve (hot or cold) lahars — all that volcanic ash loaded with water traveling 10s-100s of km down valleys, wiping out whole villages. Earthquakes both trigger landslides, and loosen up material so that everything fails in the first rainfall. Everyone’s going to focus on earthquake-recovery right now, and the increased landslide risk will be ignored; with a bit of luck it won’t start raining until after the chaos dies down.

Volcanic Activity
Chaiten (a volcano in southern Chile) has been under red alert since the 17th after increased seismic activity indicated a strong probability of eruption. It could be the big earthquake relieved the pressure, or it could still blow, adding even more ash to the hillsides. As a subduction-zone volcano, it’s of intermediate lava consistency (melting oceanic plate producing basaltic magma, passing through continental plate and becoming more andesitic). This is the same style as Mt. St. Helens — technically less explosive than pipsqueak andesitic cones which are intensely explosive (because the high silica content traps all the gases until they violently explode), but larger volumes of ejecta make that a firmly academic point (since you’d get more ash, more lava, and a larger impact area). The volcano is coastal (meaning no risk of hot lahars devastating downstream towns), and the nearby town is highly practiced at its evacuation drills (although probably overly-comfortable with their nearby monster, so unlikely to actually treat an evacuation seriously), but even just a few years ago when Chaiten started ejecting huge quantities of ash all over southern Chile, it was devastating (both for the cold lahars, and for the ash smothering the crops).

The aftershock count is currently more than 90 quakes over magnitude 5, which is strong enough to feel like your big brother snuck up behind you and energetically shook your chair. The reason aftershocks happen is that when a fault ruptures (in this case, the subducting plate slipping deeper), the stress builds up at the ends of the fault that didn’t move. Think about yanking on a chunk of fabric (pull on your shirt!) — if you have a solid chunk of anything, and part of it moves and part doesn’t, the part that stayed still will be under a lot of stress to move and stay even with the bit that moved.

In earthquakes, this increased stress can push the built-up stress to the point where that part of the fault ruptures, too, as another earthquake. This will keep happening, with each new event dispersing the stress to farther along the fault until finally the added stress is just under the rupture limit, and the fault holds without slipping. This does mean that over time, places that have had earthquakes are less likely to experience them again (because the stress has been released during the slip), while places that are nearby but not had an earthquake are more likely to have one in the near future (because they’ll have added stress from staying still while everything else moved). The NYT has a nice writeup of how the most recent quake was storing stress build up from the 1960 megaquake.

Additional reading
Telescopes & earthquakes – how the mirrors are kept safe during earthquakes.
Shorter days! as a result of the earthquake.
Crisis Kitchen – updates from the disaster policy experts.
USGS site – all the geo details for this event in one place, including lots of maps
Hackers target tsunami info sites – so careful where you’re clicking in your quest for news
Introduction to Tsunami written in response to the Japan 2011 event.

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