foreign
the earthquake that rocked southern
california last january caught many by
surprise but dr andrea donnellan a
geophysicist from nasa's jet propulsion
laboratory in pasadena actually saw it
coming
we knew that there was a likelihood of
an earthquake here on a fault similar to
the one that we saw and we had estimated
the potential of there being about a
magnitude 6.4 earthquake so we weren't
surprised this earthquake occurred
dr donald and her colleagues track
changes in the earth's crust throughout
the la area they do it with antennas
like this receiving signals from a
series of satellites called the global
positioning system or gps
it's a navigational system but we use it
to very precisely measure motion of the
ground
and from that the ground is continually
being squeezed and pushed and we can
assess earthquake hazard from that
her research prior to the january quake
indicated a buildup of stress along a
fault running through the northridge
california area exactly where disaster
ultimately struck
one of her goals now is to assess the
impact of this northridge quake on
future events
studies indicate that the stress
increased along the ventura basin which
is being squeezed close so we want to
monitor that region very carefully try
and understand if the earthquake hazard
has increased there
although they can't predict precisely
where and when an earthquake will occur
dr donald and her colleagues
identification of potential hot spots
should help focus efforts to shore up
buildings and homes so that when the
next one strikes more will survive
so
still play here
the seismologist can measure an
earthquake and see what happened after
the fact but for the first time with gps
and other techniques like it we can
actually measure how the earth is moving
and try and assess earthquake hazard
before the earthquake happens
oh mountain we found out went up 15
inches
and that means it went up and stayed up
15 inches so the mountain actually grew
and is taller now
the ultimate goal is to help quantify
the earthquake hazard in the la area and
we would like to help prioritize things
like building code retrofits right now
there's just a blanket if you find a
building that needs fixed you fix it
there's no priority on one area being
more hazardous than another
we don't do prediction we do forecasting
so we're looking on 10 or 20 or 50 year
probabilities of earthquake occurrence
and i do want to point out that we do
this work in conjunction with the
geologists and the seismologists that
this is one more component to the puzzle
but we don't do
prediction on a daily basis
you can't just lean back a little bit
there you go
and they used to
shine lights across mountains and
measure angles between mountain peaks
that was a very imprecise technique
however this is much more precise
what we're looking for now is the post
seismic relaxation we call it to
understand how this fault has affected
neighboring faults
we're doing that by putting in a network
of continuous gps receivers so when
there's an earthquake like we had in
northridge we don't have to drive a
truck up here and deploy the receiver we
instantly have data as soon as the
earthquake occurs
so we'd like to see these techniques
used in northern california and they are
to some extent we'd also like to see
them in the pacific northwest and all
over the world japan is setting up a
network of 300 stations for example
now the blind thrust fault obviously it
has a tip right about in here right
roughly
the coast of the proxy but it extends
well back
where depending where you want to put it
it stands back at least back that this
thing is locked or not and that's that's
the main concern that i have well we can
certainly put sides down here we see
these rotations we also see then the
displacements are a lot higher on this
side okay much smaller here we have a
lot higher strength okay so it is so the
polar rotation is somewhere up in here
it's actually here oh really it's really
tight okay extremely tight what i'd
really like to know is a is this ramp
locked in which case we shouldn't see
any strand accumulating across it which
it sounds like we'll also see what's
happening both seismically from here
because the
largest post seismic motion should be
about a fault dimension away from the
fall so that's perfect okay
one way that i use andrea's data is to
try to understand whether these faults
are
slipping
very slowly but inexorably at depth and
relieving strain that way or whether
they're locked and accumulating strain
over over hundreds and thousands of
years and eventually that snaps loose as
an earthquake
there are
several faults within the metropolitan
los angeles region that we're
particularly concerned about five or six
in particular that potentially could
produce very large earthquakes by very
large i mean
well in excess of magnitude 7
much much larger than the recent
northridge earthquake
we're learning a lot from every new
earthquake and all of these data
eventually do come out in new building
codes new building specifications that
hopefully will present prevent you know
huge economic losses and losses of human
life
you