Geoscience Reference
In-Depth Information
tsunami within minutes after the earthquake, there will likely only be an additional few
minutes before inundation, barely enough time for individuals to lee a short distance. The
earthquake itself, if severe enough, may have already disrupted local communications, de-
stroyed structures, and cut evacuation routes, as happened in Samoa during the September 29,
2009, tsunami (http://www.eqclearinghouse.org/20090929-samoa/category/emergency-
management-response). Nevertheless, successful evacuations have occurred during the recent
events in Samoa and Chile.
As for communities a little farther away from the tsunami source (where a tsunami might
strike within an hour or so), the lack of communications could mean that tsunami forecasters
will not receive data from the coastal sea level gauges that the tsunami reaches irst. These
communities might also be too distant from the triggering earthquake to have felt the ground
shaking suficiently to regard this as their warning. These communities depend on the detec-
tion system to very rapidly assess the threat and deliver the warning product and evacuation
order.
Almost every tsunami, because their likely sources are along undersea fault zones that
tend to be near the continents or islands, will have a near-ield region that is affected rela-
tively soon (within minutes) after the earthquake, as well as a whole suite of regions at varying
distances that are affected from minutes to many hours after the earthquake. As an example,
Figure 4.10 presents a simulation of the great 1700 tsunami that was generated by a magni-
tude 9.0 earthquake on the Cascadia subduction zone. After 1 hour, the leading tsunami wave
crest has already inundated the local coastlines of Oregon, Washington, and Vancouver Island
and has reached as far south as San Francisco. After 2 hours, the leading crest is well within the
Southern California Bight.
For the beneit of the communities at intermediate and greater distances from likely
tsunami source regions, and given the possibility that a near-coast earthquake will not only
generate a large tsunami but also will destroy infrastructure (including sea level gauges or the
telecommunication paths for their data) on the nearby coast, offshore open-ocean gauges that
provide near-real-time, rapidly sampled sea level observations are needed. This need motivated
the placement of ive DART stations off the coasts of California, Oregon, Washington, and British
Columbia (see Figure 4.6). Note that at least two of these DART stations would have observed
the 1700 tsunami (Figure 4.10) well before the initial wave crest reached San Francisco.
Despite the short lead time for a near-ield tsunami, there is still value in providing rapid
oficial warning to the local populace, so long as people are not taught to wait for such a
warning if they have already felt a strong earthquake. Such formal warning from every pos-
sible means (e.g., loudspeakers, TV, radio, Internet, text message, Twitter, etc.) will urge people
to evacuate more quickly (the people will likely be under strained conditions instilled by the
strong ground shaking). More importantly, such warning could be the only way to notify the
people to evacuate in the event of a tsunami earthquake that, because of its peculiar temporal
evolution, generates a tsunami of greater amplitude than would be expected from the small
amount of ground shaking. The most catastrophic example is the Meiji Sanriku tsunami of 1896
in northeast Japan. The earthquake magnitude was large, Ms = 7.2, but the ground shaking was
so weak that few people were overly concerned about the quake. More than 22,000 people
