Geoscience Reference
In-Depth Information
Samoa Tsunami
The Samoa tsunami on September 29, 2009, had a devastating effect on the islands. The
tsunami brought destruction to the small nation only 10-20 minutes after an earthquake about
120 km offshore shook the ground (Figure I.1). However, the tsunami had only a minor effect
on Hawaii. The event illustrates (a) how a Tsunami Warning Center (TWC) is unlikely to effec-
tively alert people of a near-ield tsunami and (b) how valuable a TWC can be for monitoring a
near- and far-ield tsunami. It further demonstrates the importance of pre-event education to
save lives from near-ield tsunamis and the manner in which alert information is disseminated
when a tsunami occurs.
The earthquake occurred at 7:48 AM Hawaii Standard Time (HST) and the irst Paciic
Tsunami Warning Center (PTWC) tsunami warning was sent to the Intergovernmental Oceano-
graphic Commission (IOC) listserv at 8:04 AM HST, 16 minutes after the earthquake
1
(Table I.1).
It announced the earthquake's preliminary parameters with an origin time of 7:48 AM HST,
coordinates 15.3 South and 171.0 West, located at Samoa Islands Region, and a magnitude of
7.9. The 7.9 magnitude was later increased to 8.3 in subsequent messages.
1
The time of release
of the irst TWC product gave only 8 minutes of warning to American Samoa and 28 minutes
of warning to Samoa, based on tsunami wave arrivals reported at the gauge stations. The delay
between the earthquake and the initial message is relatively long. A preliminary earthquake
assessment could have been completed as early as 2 minutes, and in the past few years the
TWCs have been issuing these messages after less than 8 minutes (TWC presentation to the
committee, 2008). Nevertheless, even if the message had been sent 2 minutes after the earth-
quake, it is questionable whether the message would have alerted the public at risk in time to
allow for effective evacuation. In addition, even with a warning message reaching the public
prior to wave arrival time, pre-event education would still be required to ensure proper protec-
tive action in such a short amount of time.
At 8:05 AM HST on Tuesday, September 29, 2009 the irst alert message was issued from
the PTWC to the Hawaiian Civil Defense Authority. Tsunami Message Number 1 announced
that “A Tsunami Watch is issued for the state of Hawaii effective at 8:05 AM HST.” The evalua-
tion included in the text of this message declared that “Based on all available data a tsunami
may have been generated…” and gave an estimated time arrival of a possible tsunami to reach
Hawaii at 1:11 PM HST.
2
It was understood that the PTWC would send subsequent alerts as its
investigation continued.
Tsunami Message Number 2 was issued 52 minutes later, at 8:57 AM HST. This message
announced that “A Tsunami Watch continues in effect for the state of Hawaii” and documented
an increase in the magnitude of the earthquake from 7.9 to 8.3, but indicated that the time,
location, and coordinates remained the same. Additional changes between the initial message
and the second message included reports of tsunami wave activity from gauge stations with
