Geoscience Reference
In-Depth Information
BOX 4.1
Siting Considerations for DART Stations
a
Tsunami Signal-Timeliness, Signal-to-Noise, and Signal Complexity Issues:
•
Tsunamigenic zones. The likelihood that a particular fault zone will produce a tsunami is
considered along with the coverage of the existing sea level network.
•
Seismic wave noise. If a DART is located too close to the seismic event that generates a
tsunami, the shaking of the sealoor can cause spurious BPR luctuations (e.g., from sealoor
interfacial Rayleigh waves) unrelated to the passage of tsunami water waves. This seismic
noise can be reduced signiicantly by locating the instruments no closer than 30 minutes of
tsunami travel time from the closest possible source, after which time the seismic body and
surface waves will have passed.
•
Timely signal. If the DART is sited too far from the tsunami source, too much time is lost
between the seismic event, which is detected within a few minutes, and the arrival of an
unambiguous sea surface disturbance at a DART site. In some locations, this consideration
is more important than the seismic wave noise issue; DARTs have been placed as close as
15 minutes of tsunami travel time from the closest source.
•
Tsunami scattering. The presence of seamounts or other major sealoor features between
a DART and likely tsunami sources needs to be avoided. These bathymetric features cause
zones of shadowing or of reinforcement in their lee due to tsunami wave diffraction. To the
extent that these effects are imperfectly represented in the tsunami propagation model
databases on which the SIFT and EarthVu tsunami forecast systems rely, forecast quality will
be adversely affected.
Engineering and Survivability Issues:
•
Water depth. The acoustics communications device currently in use is rated to water depths
up to 6,000 m, but the narrow acoustic beam requires the surface buoy to be closely held
above the BPR.
•
Strong currents. Because of the need for a surface buoy, it is important to avoid strong cur-
rent regimes, which could cause swamping or dragging of the buoy, or could make buoy
maintenance dificult.
•
Sub-surface landslides. Landslide-prone seabeds need to be avoided.
•
Redundancy. Either the bottom unit or the surface buoy of a DART station may fail and, in
remote locations, repair/replacement may not be an immediate option because of seasonal
vulnerabilities of non-U.S. territories in the TWCs' AORs, could be illed by DART stations if the
resources of international partners are insuficient to ill the gaps with coastal sea level stations.
However, the high cost of DART acquisition and maintenance may preclude any signiicant
network growth.
NOAA is to be commended for having developed a prioritization scheme for DART stations
and for having rapidly deployed the DART array. Looking to the future, the committee con-
