Monday, March 14, 2011 | Scientific American Magazine
Around 3 P.M. local time on Friday, there was a massive earthquake about 100 miles off the east coast of northern Honshu Island, Japan. Initially calculated to be a magnitude 8.9, it has since been upgraded to at least a magnitude 9.0, which means that this earthquake released around 8,000 times more energy than the magnitude 6.3 shock that rocked Christchurch last month. Either way, this is the biggest instrumentally recorded earthquake Japan has ever been shaken by in human history, and is one of the biggest ever detected: it's up there with the 2004 Boxing Day earthquake, and like that earthquake it generated a large – and extremely damaging – tsunami. It's difficult to believe some of the pictures from the Honshu coast as the wave hit.
Tectonic Setting
Japan is situated in a complicated plate boundary region where three subduction zones meet. Two of these subduction zones run parallel to the east coast of Japan. To the south, the Philippine plate is being subducted beneath the Eurasian plate, whilst to the north, the Pacific Plate is being subducted beneath the North American plate (yes, really: the not-particularly active boundary between the North American plate and the Eurasian plate appears to run through Siberia, down the western edge of the Sea of Okhotsk and through Japan). With all of these tectonic plates jostling against each other, it is no surprise that Japan has a long history of catastrophic earthquakes.
The Earthquake
Friday's earthquake strongly registered on seismometers around the world, with seismic waves rippling across the North America and maxing out instruments as far away as the United Kingdom. By combining data from the whole global network of seismometers, a picture of how the earth deformed in the earthquake, represented by a beachball-like focal mechanism, can be calculated. The focal mechanism for this earthquake, shown below, indicates compression, along either a shallowly west-dipping or a steeply east-dipping fault.
Focal mechanism for the main shock, and cross-sections of the two possible fault orientations
This is consistent with motion on the subduction interface, or 'megathrust'. Further modelling of the seismometer data has also produced an estimate of both the length of the rupture (at least 300-400 km) and the amount that it moved (10-20 metres or more). GPS stations in Japan - installed to measure the slow build-up of elastic strain in the crust between big earthquakes - show most of Eastern Honshu moving several metres to the east as a few centuries worth of that elastic strain - which pushes the crust in Japan westwards and upwards - was released over the space of a few minutes.
Horizontal movement of the crust in Japan during the March 11 earthquake, recorded by GPS stations (from http://supersites.earthobservations.org/sendai.php)
Even though the initial rupture was 150 km behind the trench where the plate boundary intersects with the seafloor, it seems to have propagated most or all of the way to the surface, producing large, sudden vertical movement of the sea-bed and the overlying water and generating a tsunami.
The rupture appears to have propagated to the sea-floor, generating a tsunami.
If you're wondering why there is some still some confusion over exactly how large this earthquake was, it's because - rather counter-intuitively - measuring the magnitude of large earthquakes is actually more difficult than it is for smaller earthquakes. To estimate earthquake magnitudes, you look at the amplitude of the seismic waves it generates: the larger the amplitude of the waves, the larger the magnitude of the earthquake that produced them. However, in very large earthquakes, this relationship starts to break down, at least for the frequencies of seismic waves that are generally used to produce the quick magnitude estimates: they 'saturate', or stop increasing in amplitude as the earthquake magnitude does. This means that the magnitude estimates for the largest earthquakes will be somewhat underestimated until seismologists look at lower frequency seismic waves, which are less susceptible to this saturation effect.
Friday's earthquake has been followed by a huge swarm of aftershocks (at my last count, there have been more than 250 aftershocks of greater than magnitude 5, and aroun 30 of greater than magnitude 6), as the crust around the rupture zone responds to the large stresses applied by the sudden movement of the subduction thrust. However, there was also some noticeable seismic activity before the main shock: on Wednesday, there was a magnitude 7.2 earthquake in the same region as today's earthquake, followed by a number of smaller magnitude 5 quakes, and three magnitude 6-6.1 events. These were mainly clustered in a region just to the northeast of Friday's larger rupture, and within the much larger cloud of aftershocks In hindsight, these earthquakes were foreshocks of today's main event.
Map showing location of seismicity on 9th and 10th of March (yellow circles) compared to March 11's ~M9 (largest orange circle) and the first 24 hours of aftershocks (other orange and red circles).
However, there was no way of telling this in advance: there is nothing particularly "foreshock-y" about foreshocks beyond the fact that they end up being smaller in magnitude than the main shock they precede. In fact, if you plot the last few days of earthquakes over time, you can see that, on Wednesday and Thursday, seismic activity seemed to be dying down again in the wake of Wednesday's 7.2 quake.
Magnitude of earthquakes (M5-6=small yellow circles, M6-7 orange circles, M7+ large red circles) off the coast of Honshu, 9-14 March.
The Tsunami
Very little of the devastation resulting from this earthquake was from the initial shaking. This is partly because of Japan's stringent building codes. But mainly because any damage from the seismic waves that sent skyscrapers in Tokyo swaying was dwarfed by the impact of the 10 metre tsunami that hit the Japanese coast less than an hour later. Although 40% of Japan's coastline is faced with concrete seawalls designed to fend off tsunamis, they proved ineffective in this case: the wave was just too high, and eventually the seawalls were topped.
The real destructive power of tsunamis lies not in excessive height, but in their wavelength. A normal wave rises, breaks on the beach, and is done within seconds. A tsunami wave rises, breaks, and continues to break for several minutes or more. It is a wave that just keeps on coming…and coming, and if it is higher than beach (or seawall) level, it will encroach inland for kilometres, sweeping all before it.
Satellite view of the coast around Sendai before (left) and after (right) the tsunami of 11 March. Source: NASA Earth Observatory (http://earthobservatory.nasa.gov/IOTD/view.php?id=49630)
As well as travelling east to strike Japan, the tsunami propagated out into the Pacific ocean, triggering tsunami alerts in Hawaii and the whole west coast of the Americas, from Alaska to Patagonia. For Hawaii and the Western US at least, the damage was minor: the passage of the tsunami was obvious, but the impact of wave heights of 1-2 metres was further reduced by the fact that they arrived close to low tide. However there was more serious damage - and some casualties - in northern California and southern Oregon, where the tsunami and the shape of the coast conspired to produce larger waves.
The Warning for Cascadia
Despite decades of preparation for an earthquake like this, Japan was still overwhelmed by the scale and violence of Friday's earthquake, and the tsunami it generated. This may be partly due to the fact that seismologists underestimated the size of the earthquake that this subduction zone could generate: they were preparing for an earthquake of around magnitude 7.5 – more than 150 times less powerful. The prediction of future earthquake risk is based on incomplete and far-too-short records of past earthquake activity, so it is no surprise that the planet can still give us nasty seismic surprises, as faults that have been quiet or inactive over historical time periods show us the full range of their behaviour over geological timescales.
Over the other side of the Pacific there are similar gaps in our knowledge, but we do know enough to understand that the real risk to the western US and Canada is not from tsunamis generated across the other side of the Pacific, but ones generated on this side. North of the San Andreas Fault, the plate boundary that runs along the west coast is a subduction zone very similar to the ones that run along the coast of Japan, and just as capable of generating large earthquakes. The last time the Cascadia subduction zone ruptured in earnest was around 300 years ago, and geological evidence suggests that the quake itself (probably more than a magnitude 8.5) and the tsunami it generated were very similar in scope and scale to what struck Japan last Friday. And, by any measure, western North America is less aware, less prepared and less protected than Japan was. There is no way to predict exactly when an earthquake will occur, but it is a 100% certainty that eventually the Cascadia subduction zone will rupture. The only question is over the willingness of the societies that live on top of it to face this tectonic inevitability.
Keeping Updated
The ever-growing community of geologists writing blogs and sharing information via Twitter are a great source of information for those who want to go beyond the media coverage. For example, if you really want to understand what's going on at the nuclear reactors at Fukushima, take a few minutes to listen to the interviews fellow geology blogger Evelyn recorded with her nuclear engineer father. Callan Bentley's Mountain Beltway blog deserves a special mention for early, in-depth and comprehensive coverage of the geological story – a comprehensive list of other contributions can be found here.
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