Geography Today
The Sendai Earthquake – an update
Professor David Petley
Executive Director, Institute of Hazard, Risk and Resilience, Durham University, United
Kingdom
Email: [email protected]
Why does Japan have earthquakes?
Whilst the world feels like a solid structure to us, in fact the surface is formed from giant plates –
continental-scale blocks of rock – that are able to move. The rates at which they actually move are
slow – typically just a few centimetres per year, barely faster than the rate at which your fingernails
grow – but these movements are enough to generate the devastating earthquakes that we
occasionally experience. The various islands that form Japan have been created by a collision
between four of these great tectonic plates (Fig. 1). The Japanese landmass itself sits on two plates –
the northern part of the country sits on the North American plate, whilst the south is on the Eurasian
plate. To the east of Japan are situated two oceanic plates – in the north is the Pacific Plate, whilst to
the south lies the Philippine plate.
Fig. 1: The plate tectonic setting of Japan. The arrows show the measured directions of movement of
the plates
The two oceanic plates are both moving in a generally westward direction at a rate of a few
centimetres per year. The zones at which the plates collide lie on the seabed to the east of Japan
(Fig. 1) and are marked by deep ocean trenches. At this point the oceanic plates are being forced
under the continental plates in the process that we term subduction, as shown diagrammatically in
Figure 2.
Fig. 2: The basic subduction system operating in the vicinity of Japan
Unfortunately, the contact surfaces –termed a fault - between the plates are not smooth, meaning that
the plates tend to stick together. The underlying plate movement, which is driven by the generation of
heat deep in the Earth, does not stop during this time. As a result, energy is stored within the rocks
around the contact between the plates, much in the way that energy is stored in an archer’s bow as
she pulls back the string. In general the longer the time-gap between release events, the larger the
amount of energy that is stored. As the same time the overlying (upper plate) is pulled downwards by
the ongoing movement of the continental plate (Fig. 3).
Fig. 3: Diagram showing the generation of a tsunami during a subduction zone earthquake
Eventually sufficient energy is stored in the rocks to overcome the friction between the plates, resulting
in a sudden movement on the fault. This is the earthquake. As this movement occurs the stored
energy is instantaneously released, creating the earthquake waves that radiate outwards to cause
such destruction. The overlying plate pops back up again (Fig. 3), lifting the seabed, and the overlying
water, by several metres. A huge volume of water is now higher than that of the surrounding sea, and
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thus flows outwards to generate a tsunami. This is the process that occurred off eastern Japan in 11
March.
The Sendai Earthquake
The earthquake in Sendai occurred on the fault that marks the boundary between the Pacific Plate to
the east and the North American plate to the west (Fig. 4). This is an area that was known to have
earthquake activity, and indeed has been subject to large numbers of earthquakes before, as Figure 4
shows. However, this is a section that had not experienced a very large earthquake during the period
in which instruments have been recording seismic activity, suggesting that the plates were locked
together. Indeed, a quick glance at Figure 4 shows that the recent earthquake activity in this area is
lower than that to the north and south, which suggests that large amounts of energy were stored in the
plates, waiting to be released. The earthquake caused the plates to move over a distance of at least
10 metres, and possibly much more, over a length of fault that is about 400 km from end to end (fig.
5). This resulted in a very large earthquake, now considered to magnitude 9.0 (meaning that it is the
th
4 largest earthquake ever recorded). In this area the Japanese landmass moved by up to four
metres towards the east, and in the area of the fault the seabed was uplifted by several metres,
generating the catastrophic tsunami.
Fig 4: Recorded earthquakes for the period 1900-2007 in the vicinity of the Sendai earthquake. Each
red circle is an individual earthquake, with the size of the dot representing the magnitude. Source:
USGS 2010: http://pubs.usgs.gov/of/2010/1083/d/
Fig. 5: Estimated shaking intensity for the Sendai Earthquake. Source: RMS 2011:
http://www.rms.com/ClientResources/Catupdates/CatUpdatePublic.asp?event_id=3266
What will happen next?
After large earthquakes, the stresses in the crust are left in an unevenly distributed state. This always
results in further earthquake activity over the following months and even years – this is the so-called
aftershock sequence. These aftershocks are quite intense initially – in the hours after the earthquake
typically occurring every few minutes - and then reduce as time passes. Typically, the largest
aftershock is a little more than one unit down on the magnitude scale – so for a magnitude 9.0
earthquake we would expect to see one aftershock with a magnitude of about 7.8, and many more
that are smaller than this. However, there is no hard and fast rule about the largest aftershock, which
can be a little larger than this, or substantially smaller. A magnitude 7.8 event is a large earthquake in
its own right however, and so those living in the areas affected by the main earthquake should be
prepared for the possibility of such an event over the coming weeks. However, it should also be
remembered that in terms of energy released by the earthquake, one step in the earthquake
magnitude scale represents an increase in energy release of 33 times – i.e. a magnitude 9 earthquake
is 33 times greater than one with a magnitude of 8. Thus, the aftershocks are likely to have nothing
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like the destructive potential of the main shock on 11 March.
Rumours have also been circulating that the aftershock sequence is moving towards the south and in
particular towards Tokyo. Data from the aftershocks to date do not indicate that this is happening – in
other words this rumour is incorrect (Fig. 6). Over the coming days seismologists will be determining
whether this earthquake has increased the likelihood of another large earthquake in Japan beyond the
aftershock sequence. At the moment there is no evidence that this is the case, but we will keep you
informed of these studies. However, Japan is an earthquake-prone country, and for the foreseeable
future we will have no techniques for predicting when an earthquake might occur or how large it might
be. Thus, the key advice remains the same – i.e. be prepared for an earthquake by maintaining a
store of water, food, torches and batteries; know what to do when an earthquake occurs (duck under a
sturdy structure such as a strong table, cover your head and hold on); and prepare a plan for how you
will contact your loved ones after the earthquake.
Fig. 6: Distribution of aftershocks in terms of distance from Tokyo (source: CPP geophysics:
http://twitpic.com/49wvp5)
Further information
Updates on the information presented here will appear on Professor Petley’s blog:
http://blogs.agu.org/landslideblog/