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Rapid Style of Faulting (fastCMT)
Diego Melgar, Scripps Institution of Oceanography
March 24, 2014
There are three main types of earthquakes. Normal faulting events that happen due to the Earth’s
crust being extended and pulled apart, thrust faulting events that happen due to compression, and
lateral events which happen because of two plates sliding past each other. All three of these
events are possible and have been known to happen in a subduction environment like Alaska.
More importantly each one of these events poses different hazard. One of the first steps in rapid
response is to determine the type
of earthquake that has occurred.
Using an algorithm known as
fastCMT which employs data from
land based GPS stations and strong
motion accelerometers it is
possible to determine the style of
faulting and magnitude of large
events within 2-3 minutes of
rupture initiation. The figure shows
the fastCMT product for the 2011
magnitude 9 event offshore Japan.
In that case it was possible to
determine the earthquake was a
thrust event with magnitude 9
within 157s of the earthquake
onset. This is critical information
to get a quick idea on the extent
and intensity of the event.
The information from fastCMT
is important because it determines
what modeling steps are necessary
afterwards. For example, if the
event is a lateral event then there is
no need to model the tsunami since
these earthquakes produce little
fastCMT Product from GPS Displacements. The triangles
seafloor uplift. However, if as in
on the Japan mainland indicate the locations of all the GPS
the case of Japan the earthquake is
stations used for computing the CMT solutions. Green circles
thrust then a sequence of
are the earthquake point sources superimposed to compute the
earthquake and tsunami models
line source, the final averaged solution shown as fastCMT.
can be produced with more
The inset shows the moment release (a proxy for energy
complex earthquake and tsunami
release) from the line source of CMTs as a function of
models. This is particularly useful
distance along fault. Shown along the fault interface with 10
for the coastal regions immediately
km depth contours is the result of the finite fault slip model,
adjacent to the earthquake where
the blue lines represent the direction of slip.
most of the damage will occur.
This information is typically available within 2-3 minutes after the earthquake onset regardless of
the magnitude of the event. Also see product pages for descriptions of finite fault slip model and
tsunami extent, inundation and runup models.