Alaska was hit by a major earthquake today, and it was totally harmless. The magnitude 7.9 subduction-zone earthquake let off a build-up of tectonic stress in the most harmless way possible, proving that with disasters, location really is everything.
Seismograph record of the station located on Adak, Alaska. Image credit: USGS
The earthquake hit at 12:53 pm Alaska daylight time, with several magnitude 4, 5, and 6 aftershocks throughout the afternoon. But the earthquake was located in the Rat Islands, a mostly-unpopulated segment of the Aleutian Islands, and was so deep that it didn't produce enough surface displacement to trigger a major tsunami.
This is the largest earthquake to hit today; the next-largest is a magnitude 6.9 earthquake and its related aftershocks off the coast of Raoul Island, New Zealand. It's a good day for earthquakes: these are large enough that they could cause substantial damage, but far enough away from populated areas that we're getting data without deaths or damage.
Earthquakes on Monday, June 23, 2014. Image credit: USGS
The Alaskan megathrust earthquake occurred on the curve tracing out the northern boundary of the Pacific Plate diving below the North American Plate. The subduction zone is west of the Denali Fault Zone, and northwest of the Cascadia Fault Zone.
It's a bit of a tectonic mess as plate motion transitions from subduction to transform motion. On the western side of the arc, the plate motion is northwest parallel to the plate (translational) at about 76 mm/year, while to the eastern side the plates are crashing together (compressional) at about 60 mm/year. The eastern and central arc are classic subduction zones with intense volcanic activity and megathrust earthquakes, while the western segment of the arc is transitioning into a transform zone, with reduced volcanic activity and no recorded history of large earthquakes.
The depth of the earthquake is listed as 114 kilometers, +/- 5 kilometers error. That depth doesn't make sense for a traditional subuction earthquake, so this earthquake might be the result of some intra-plate buckling or bending instead of a rupture along the fault. While "megathrust" doesn't have a formal definition that every seismologist will agree upon, it generally describes a large magnitude earthquake that occurred along an oblique fault (thrust). This event has the large magnitude part easily (despite being upgraded from a M7.1 and down from a M8.0 while I was writing!) and it's within a region where thrusting is the dominant mechanism, it may not actually be a thrust-earthquake, so calling it a "megathrust earthquake" is debatably flawed.
The Aleutian Trench marks the boundary between the Pacific Plate and the North American Plate. Image credit: USGS
The Aleutian Islands are the volcanic markers of a subduction zone. As the Pacific Plate is dives under the North American plate, increasing pressure and temperature drives water out of the saturated sediments of the former ocean floor. This water drops the melting point of surrounding rocks, generating magma. This melt rises through the overlaying crust, creating an arc of volcanoes. As the magma rises through the overlaying North American Plate, it picks up just enough silica to erupt explosively in a chain of stratovolcanoes all along the subduction boundary. This volcanic island arc traces out the plate boundary adjacent to the subduction trench.
The earthquake epicenter is near Little Sitkin Island, Alaska, a stratovolcano stretching 6 miles in diameter above the waves. The island is so inhospitable that it barely has a fringe of grass along its lowermost slopes, and certainly no human inhabitants. The last time it erupted was in 1900. It is part of the Rat Islands, the westernmost segment of the central arc where subduction is the dominant tectonic environment.
The nearest actively monitored volcano is Semisopochnoi Island, roughly 50 kilometers due east of Little Sitkin Island. It last erupted in 1987, and is currently in a Yellow Advisory status. That means it is exhibiting signs of elevated unrest above its usual known background level, or is still being eyeballed after recently returning from a state of elevated unrest.
Context map of Little Sitkin Island and the surrounding area. Triangles are volcanoes (white: not monitored; coloured: monitored and alert status), houses are communities, and the star is the approximate earthquake epicenter. Modified from the Alaska Volcano Observatory.
The Rat Islands are home to the westernmost substantial earthquakes in the region. The largest was a magnitude 8.7 in 1965 where 600 kilometers of the fault ruptured and moved. It triggered a small tsunami that reached maximum intensity with 10.7 meters of run-up on Shemya Island, flooding on Amchitka Island, and otherwise produced no damage. Today was the second-largest earthquake in the region. The population density in the region hasn't changed much in the last fifty years, so this smaller-yet-still-impressive earthquake was about as uneventful as the 1965 earthquake.
Over 330 kilometers east is the community of Adak, a tiny community of 326 people as of the 2010 census data. A smidgen closer is Shemya Station, roughly 315 kilometers to the northwest, a refuelling station with a staff of 25 to 180 people, and Attu Station, which has closed since the last census.
It will be fairly awesome if any of this handful of humanity fills out Did You Feel It? reports, but I'm not expecting that the USGS will gather enough information to build a map of perceived shaking.
Image credit: USGS
On the up side, the region is coated with seismograph stations for volcano monitoring, and has several dedicated seismographs for earthquakes, so the automatically generated shakemap is based on solid, dense data. It looks like the surrounding region experienced very strong shaking, attenuating to strong shaking by the nearest inhabited islands.
Large subduction-zone earthquakes like this one are frequently associated with devastating tsunami, but this earthquake generated tsunami information statements for Hawaii and the west coast, and a warning for Alaska only.
Image credit: NOAA
That's because this was a very deep earthquake, over 100 kilometers below the surface. Tsunami are generated by the displacement of water; even a massive earthquake this deep is unlikely substantial surface displacement.The rest of the Aleutian Islands might experience a few hours of sea levels that are higher-than-normal, but not so much higher as to actually pose a threat.
The community of Adak did evacuate (always a wise move when you feel strong shaking!), with reports of watching down-draw in the harbour as the tsunami arrived. It'd be a bad day to take up surfing off the coast of Adak, but with waves at less than one foot amplitude, this isn't going to be devastating.
While the energy propagation forecast looks as ominous as ever, pay close attention to that scale-bar: the waves will be substantially less than 10 centimeters high by the time it reaches any population center.
The tsunami produced less than 20 centimeters displacement at the Adak buoy. Image credit: IOC
While we can identify the tsunami in buoy records due to the extremely long wavelengths, I'm very certain I couldn't spy it in real-life amongst the wind-waves and tide.
This is a good earthquake to point out how a small change in magnitude reflects a substantial change in consequences. Chile experienced a M8.2 earthquake on April Fool's Day this year followed by a mess of aftershocks. It triggered a locally-devastating tsunami that attenuated quickly with distance, having no noticeable impact outside of South America. The tiny 0.3 difference in magnitude between that event and this one in Alaska are indicative of an earthquake twice as big on seismographic amplitude, releasing 2.8 times as much energy.
This is my favourite type of disaster: something large and well-documented with minimal risk and no one getting hurt. The tectonic stress building up along the boundary is reduced a little bit, geophysicists get another set of data-points to help them understand the subsurface a bit better, and everyone gets a chance to learn a bit more about earthquakes and tsunami. Perfect!
The biggest conversation about this earthquake is likely to be philosophical: Is still a disaster when nothing is damaged and nobody dies?