Geoscience Reference
In-Depth Information
BOX 1.1
What Is a Tsunami?
A tsunami is a series of waves generated by an impulsive disturbance in a body of water
such as a lake or ocean. The disturbance is typically a fault displacement. Less commonly, the
displacement results from a landslide (into or below the water surface), volcanic activity, or rarely
from a meteorite impact. It typically takes a large earthquake (magnitude >7.0) to generate
a damaging tsunami in the near-ield and a great earthquake (magnitude >8.0) to generate a
tsunami in the far-ield. The height of a tsunami ranges from centimeters to more than tens of
meters and depends on the initial disturbance and the bathymetry of the sealoor over which
the waves travel. The waves are very small in deep water, but grow in height once they move
into shallow water. The velocity at which the tsunami travels away from the source depends on
the water depth. A tsunami in deep water (
≥
4 km) is about as fast as a jetliner (700-800 km/hr)
and slows to the speed of a car (60 km/hr) in shallow water. For example, a tsunami originating
in the central Aleutian Islands near Alaska would arrive on the Hawaiian shores in about ive
hours. Such tsunamis, originating far from where it runs ashore, are commonly referred to as
“far-ield” tsunamis, which allow at-risk communities several hours to evacuate if the warning
is received promptly. However, a tsunami triggered by an event close to shore (e.g., an earth-
quake associated with the Cascadia subduction zone), would reach communities in low-lying
coastal areas in a few minutes, allowing for little time to warn and evacuate. When the tsunami
originates close to the at-risk community, it is termed a “near-ield” tsunami. For the near-ield
tsunami, people need to recognize the triggering earthquake as their warning to evacuate.
Tsunami arrival times can range from minutes to many hours. The report mainly discusses the
near- and far-ield tsunamis (at either extreme end of the arrival time range) because of
the unique challenges they pose to the warning and preparedness efforts. However, a tsunami
can be triggered by an earthquake that is only weakly felt onshore, yet may arrive in under an
hour. Such events are also considered and mentioned because they heighten the challenges
associated with preparing and warning the public.
probably triggered by earthquakes, account for much of the known tsunami hazard along the
U.S. Atlantic and Gulf coasts, and in southern California (Dunbar and Weaver, 2008). Seismically
active faults and the potential for landslides in the Caribbean pose a signiicant tsunami risk for
that region (Dunbar and Weaver, 2008).
Tsunami hazard zones of U.S. coastal communities contain thousands of residents, employ-
ees, and tourists, and represent signiicant economic components of these coastal communi-
ties (Wood, 2007; Wood et al., 2007; Wood and Soulard, 2008). The economic and social risks
from tsunamis grow with increasing population density along the coasts. To reduce societal
risks posed by tsunamis, the nation needs a clear understanding of the nature of the tsunami
hazard (e.g., source, inundation area, speed of onset) and the societal characteristics of coastal
communities (e.g., the number of people, buildings, infrastructure, and economic activities)
