Geoscience Reference
In-Depth Information
tsunami may owe most of its size to a submarine landslide in the earthquake's focal region
(López-Venegas et al., 2008). The 1867 and 1918 disasters are probably just the tip of an iceberg;
they represent but a sample, during the geological instant of the past 150 years, of Caribbean
tsunami sources that can be inferred from the region's active tectonics (McCann, 1985; Grindlay
et al., 2005; Mercado-Irizarry and Liu, 2006) and from its abundance of steep submarine slopes
(ten Brink et al., 2004, 2006). Probably the biggest open question about these many Caribbean
sources is the tsunami potential of the highly oblique subduction zone marked by the Puerto
Rico Trench. This hypothetical tsunami source faces the low-lying metropolis of San Juan
(pop. 0.5 million) and, farther aield, may threaten the Atlantic seaboard from the Carolinas to
Massachusetts (Geist and Parsons, 2009). In addition to having all these tsunami sources of
its own, the Caribbean bore the brunt of the documented far-ield effects of the 1755 Lisbon
tsunami (Barkan et al., 2009; Muir-Wood and Mignan, 2009). The tsunami did not appear to
have a signiicant effect on San Juan, based on the absence of documentation in the extensive
Spanish-language records from that part of the 18th century (McCann et al., undated).
Near-ield hazards from slides off U.S. coasts
. Submarine slides abound off the Atlantic coast, par-
ticularly in the Caribbean (above) and off New England and the Middle Atlantic states (Twichell
et al., 2009). Submarine slides are also present beneath the Gulf of Mexico (Trabant et al., 2001)
and off southern California (Lee, 2009). The probabilistic tsunami hazard the slides pose is
poorly known. It may be low because most of the sliding appears to have occurred soon after
the last glaciation, at a time when sediment supply and sea levels greatly differed from today's
(Lee, 2009). Much remains to be learned about slide size, speed, and duration (Locat et al., 2009),
all of which affect a slide's eficiency in setting off a tsunami (Geist et al., 2009).
Tsunami sources that have escaped notice
.
That such sources remain undiscovered can be inferred
from the recent identiication of tsunami threats that had previously gone unrecognized—from
great earthquakes on the Cascadia subduction zone (Atwater et al., 2005), faults and landslides
beneath Puget Sound (Bucknam et al., 1992), outsize subduction earthquakes off northeast
Japan (Nanayama et al., 2003), and landslides off Norway (Halidason et al., 2005), Puerto Rico
(ten Brink et al., 2006), the U.S. Atlantic coast (ten Brink, 2009), and southern California (Lee, 2009).
Determining worst-case source scenarios
.
Decisions about worst-case tsunami sources for the
purpose of inundation modeling (see below) vary among NTHMP members. Inundation model-
ing in Alaska uses historical events (e.g., the 1964 Great Alaskan Tsunami) as well as a set of
hypothetical tsunami scenarios unique for each local community for the tsunami sources.
Inundation modeling in Hawaii is also based on historical distant tsunamis (1946 Aleutian,
1952 Kamchatka, 1957 Alaska, 1960 Chile, and 1964 Alaska tsunamis). Inundation modeling in
California is based on 6 to 15 local and distant sources (depending on map location) that result
in a single maximum tsunami inundation scenario. The primary subduction-type fault threat for
northern California is the Cascadia scenario, and other potentially important tsunami sources
include distant tsunamis (e.g., earthquakes near Alaska or Japan) and submarine landslides.
