Earthquake Early Warning Product
Yehuda Bock, Scripps Institution of Oceanography
March 24, 2014
Global geological hazards have wreaked havoc on the lives of millions of people worldwide. The Pacific
Rim or "Ring of Fire" contains the world's major subduction zones and produces most of the world's
earthquakes. The area has produced 9 of the largest 10 earthquakes in recorded history. This includes the
1964 magnitude 9.2 Alaska earthquake where the Pacific tectonic plate is moving underneath (subducting)
the North America plate. These types of earthquakes can cause a sudden intense uplift of the ocean floor
that can result in a tsunami such as the 2011 magnitude 9.0 earthquake that devastated the eastern coast of
Japan. Unfortunately, scientists have not been able to find a way to predict the date and time of large
destructive earthquakes. Instead of earthquake prediction it is possible to provide an earthquake early
warning and meaningful warnings especially for those in coastal communities can save lives and reduce
damage to essential infrastructure when issued within seconds to minutes for destructive earthquakes. As
first demonstrated in Japan, real time response could include, for example, the slowing down of trains,
initiating emergency procedures in public buildings and schools, shutting down gas mains and pipelines,
and closing off bridges and other vulnerable structures.
One of the NASA products for mitigation of natural hazards is to use GPS monitoring networks
supplemented with seismic instruments (accelerometers) to measure the ground motions during a large
earthquake. This technique is called “seismogeodesy.” Earthquake early warning (EEW) works as follows.
The first seismic waves
felt during an earthquake
are P (primary) waves that
do not cause significant
shaking or damage but
contain information on the
time of arrival of the
seismic secondary (S)
waves, which cause the
intense
shaking
and
movements of the Earth’s
surface during a large
earthquake that cause loss
of life and property. The
key to EEW is to rapidly
measure the amplitude of
the P waves and predict
when the S waves will
arrive
at
particular
Earthquake Early Warning Product
locations. Depending on
one’s distance from the
earthquake’s epicenter and since P waves travel faster than S waves, the warning time may range from only
several seconds to several minutes. The concept is illustrated in the figure for a coastal station in Japan
during the 2011 magnitude 9 earthquake. The advantage of NASA-funded technology is that
seismogeodesy provides a timelier estimate of the magnitude of a large earthquake than traditional
monitoring networks, especially in the near coastal regions where loss of life and property is highest and so
that more effective warnings can be issued for a possible tsunami. For the Alaska Shield Exercise
earthquake early warning is provided by a simulated network of about 120 real-time monitoring
GPS/seismic stations in Alaska, based on an actual network of existing Plate Boundary Observatory GPS
stations.