Geoscience Reference
In-Depth Information
June 14, 2005: A Case Study in
Tsunami Warning and Response
At 7:51 PDT (0251 UTC) on June 14, 2005, an earthquake occurred 90 miles northwest of
Eureka, California, that was felt in communities in both California and southern Oregon. The
preliminary magnitude of the earthquake was M7.4 and the location was within the Gorda
Plate. Within ive minutes, the West Coast/Alaska Tsunami Warning Center (WC/ATWC),
1
in con-
formance with established procedures, issued a tsunami warning for the coastal areas from the
California-Mexico border to the northern tip of Vancouver Island, British Columbia. Before the
warning was issued, the intensity and duration of the earthquake shaking in Humboldt and Del
Norte counties provided the irst “unoficial notiication” to residents of northern California and
southern Oregon of a possible tsunami triggered by an earthquake.
TWO WARNINGS FROM THE TWCS
Although no WC/ATWC staff were in the center facility at the time of the earthquake,
within a few minutes after the earthquake TWC staff were at their stations, had assessed the
seismic data, and delivered a warning message to its area of responsibility (AOR) regarding
possibility for tsunami waves forming in response to the earthquake tremors. As noted above,
the earthquake provided the initial notiication of the potential for a tsunami to local residents,
resulting in local individual actions well before the oficial warning was received and dis-
seminated by local governments. At 7:59 PDT (0259 UTC), the Paciic Tsunami Warning Center
(PTWC) in Hawaii issued a Bulletin for its AOR (including Mexico) stating that there was no
tsunami warning in effect (for its AOR). Although the two seemingly contradictory notiications
were both correct, state and local oficials who received both were left with the impression that
the initial notiication was canceled. Oficials in southern California were faced with the poten-
tial of a locally damaging tsunami striking San Diego county that would not impact Tijuana,
Baja California, a few miles to the south.
By 8:19 PM, after further analysis, the Gorda earthquake was judged as not likely to be
tsunamigenic (the earthquake magnitude was reined to measure M7.2 and was located in
the middle of the Gorda Plate, not on the Cascadia subduction zone (CSZ) boundary between
the Gorda Plate and the North American Plate). Additional data conirmed that there were no
reports of wave inundation along the coast, and the WC/ATWC reported a widespread tsu-
nami unlikely. However, it did not rule out the possibility for a regional tsunami or landslide-
1
Notiication of potential tsunami events for the coastal communities of Washington, Oregon, and California are
within the Area of Responsibility (AOR) of the West Coast/Alaska Tsunami Warning Center in Palmer, Alaska. Notiication
of Mexico is within the AOR of the Paciic Tsunami Warning Center in Hawaii.
