HomeMy WebLinkAboutCOM 0221.003 1998-2000 Q4~,ENT OF THE TAKE- ~
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COUNTI' OF HAWAII
Memorandum
Date: 26 May 1999
To: The Honorable James Arakaki, Chairman
Hawaii County Council
From: Paul Okubo, Seismologist
Hawaiian Volcano Observatory
Subject: Seismic hazards calculations for Hawaii County and seismic zonation
Hawaii County is one of the most seismically active regions in the United States.
Since 1868, nearly 30 earthquakes of magnitudes 6 or greater have struck the County.
Each year, thousands of smaller earthquakes beneath the island are recorded.
As we were reminded on April 16 of this year, when a M5.6 earthquake struck
Pahala and destroyed 2 homes, a M6 earthquake is capable of causing damage.
Simple arithmetic, then, suggests that, on average, a damaging earthquake strikes
Hawaii County once every 4 or 5 years.
In reality, of course, these averages are not reflected in the record. However, the
historical and instrumental records of earthquake activity in Hawaii County can be
used in a more detailed series of calculations to estimate the level of seismic hazard
that our conununiry is exposed to.
In this procedure, called probabilistic seismic hazards assessment, the frequency
of earthquakes of different sizes in different earthquake source regions is combined
with information relating to how the energy released by an earthquake spreads
away from the earthquake to calculate the probability that damaging ground
motions will occur in specified time intervals called return periods. Fundamental
to these issues are seismographic data collected and analyzed by the U.S. Geological
Survey's (USGS) Hawaiian Volcano Observatory.
Beginning in 1992, and continuing at present, the USGS began calculations of
the seismic hazards in the State of Hawaii. Initially, the focus of the study was the
Lower East Rift zone of Kilauea Volcano. Since then the calculations have been
modified to use a methodology consistent with the rest of the nation; they have
included Hawaii County in its entirety and, subsequently, the State of Hawaii. As
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improved data become available, especially for the less seismically active parts of the
state, these calculations will be updated.
For Hawaii County, the results are already quite clear: there is a 2% probability
that, in a 50-year return period, ground accelerations in excess of 0.4 g will occur at
sites covering nearly the entire County. According to the criteria in the Uniform
Building Code developed by the International Conference of Building Officials,
these results place Hawaii County in Seismic Zone 4. The 1997 version of the UBC
includes this seismic zonation. This is the highest rating of seismic hazard in the
UBC, comparable to regions in California adjacent to active fault zones comprising
the San Andreas Fault system.
These findings are qualitatively consistent with the historic record of damaging
earthquakes in Hawaii County. The methodology is now consistent with that used
for the entire United States as a whole. To the extent afforded by these studies and
the information collected to date, we feel that these results accurately portray the
seismic hazard to which Hawaii County is exposed.