Geoscience Reference
In-Depth Information
neptunecanada.ca/) and in Monterey Bay, California (Monterey Accelerated Research System,
MARS: http://www.mbari.org/mars/). Another large U.S. observatory has been funded by the
NSF for deployment across Oregon's continental shelf, slope, and the Cascadia subduction
zone, over the Juan de Fuca plate, and on to the Juan de Fuca Ridge (Ocean Observatories
Initiative, OOI: http://www.interactiveoceans.washington.edu/). Both the NEPTUNE-Canada and
OOI networks can be used for quantitative tsunami detection primarily via their seismometers
and sealoor pressure sensors.
Off Oregon, Washington, and British Columbia, the water pressure sensors placed on the
sealoor cabled observatories can readily replace or enhance the DARTs in providing warning
to communities at mid- to far-ranges from the tsunami-producing Cascadia subduction zone.
In addition, because the seismic data from the observatories can be used in near-real time
by automatic computer algorithms in order to separate seismic and tsunami signals in the
pressure data, the pressure gauges can be placed very near, and even on top of, the expected
tsunami source regions. This can yield very rapid determination of the generation (or not) of
a sizable tsunami, thus providing a capability for producing some modicum of warning to the
near-ield coasts.
From a pragmatic operational point of view, the utilization of NEPTUNE-Canada and the
OOI sensors for tsunami detection could be expected to eliminate the need for the DART buoys
off Washington and Oregon, thus freeing up those resources for other purposes.
In Japan, cabled observatories already exist that are focused on collecting measurements
of earthquakes and tsunamis. For example, Japan Agency for Marine-Earth Science and Tech-
nology (JAMSTEC) has installed three observatories and is constructing a fourth, called Dense
Ocean-loor Network System for Earthquakes and Tsunamis (DONET), that speciically aims at
capturing the data from the next Tokai earthquake and tsunami. One exceptional event has al-
ready occurred on one of JAMSTEC's observatories, the Tokachi-oki site, which was located atop
the source area of the 2003 Tokachi-Oki earthquake; for the irst time ever, sealoor sensors
observed the pressure variations of the tsunami at the instant of creation. The abrupt changes
in water pressure at the sealoor clearly show the sealoor displacements of the earthquake,
with sustained acoustic (pressure) waves bouncing up and down between the hard bottom
and the sea surface (Li et al., 2009) while the tsunami wave evolves outward therefrom. These
observations of the 2003 Tokachi-Oki earthquake and tsunami provided an important lesson:
the sensors and cables of an observatory placed at the epicenter can survive the earthquake,
allowing the near-real-time data to be used effectively for rapid warning of local tsunamis.
Another possible technology for detecting local tsunamis is high-frequency (HF) radar
(Lipa et al., 2006). Coastal HF radar stations produce maps of the ocean surface currents using
radar echoes from short period surface gravity waves. A tsunami wave, which exists at longer
periods (1-30 minutes) than the waves (~10 seconds) that relect the radar's microwave energy,
will transport the shorter waves, adding to the ambient current and producing a signature
detectable by the radar. The method has not been proven in the ield, but theoretical and
analytical studies are encouraging. The radars could provide accurate and rapid observations of
tsunami waves before they make landfall and thereby aid in the formulation of better warning
products.
