Intensity distribution and isoseismal maps for the Nisqually, Washington, earthquake of 28 February 2001

We present isoseismal maps, macroseismic intensities, and
community summaries of damage for the MW=6.8
Nisqually, Washington, earthquake of 28 February, 2001.
For many communities, two types of macroseismic intensity
are assigned, the traditional U.S. Geological Survey
Modified Mercalli Intensities (USGS MMI) and a type of
intensity newly introduced with this paper, the USGS
Reviewed Community Internet Intensity (RCII). For most
communities, the RCII is a reviewed version of the
Community Internet Intensity (CII) of Wald and others
(1999). For some communities, RCII is assigned from
such non-CII sources as press reports, engineering reports,
and field reconnaissance observations. We summarize differences
between procedures used to assign RCII and
USGS MMI, and we show that the two types of intensity
are nonetheless very similar for the Nisqually earthquake.
We do not see evidence for systematic differences between
RCII and USGS MMI that would approach one intensity
unit, at any level of shaking, but we document a tendency
for the RCII to be slightly lower than MMI in regions of
low intensity and slightly higher than MMI in regions of
high intensity. The highest RCII calculated for the
Nisqually earthquake is 7.6, calculated for zip code 98134,
which includes the ?south of downtown? (Sodo) area of
Seattle and Harbor Island. By comparison, we assigned a
traditional USGS MMI 8 to the Sodo area of Seattle. In
all, RCII of 6.5 and higher were assigned to 58 zip-code
regions. At the lowest intensities, the Nisqually earthquake
was felt over an area of approximately 350,000 square km
(approximately 135,000 square miles) in Washington,
Oregon, Idaho, Montana, and southern British Columbia,
Canada. On the basis of macroseismic effects, we infer
that shaking in the southern Puget Sound region was
somewhat less for the 2001 Nisqually earthquake than for
the Puget Sound earthquake of April 13, 1949, which had
nearly the same hypocenter and magnitude. Allowing for
differences in hypocenter, shaking in the 2001 earthquake
was very similar to that produced by the Puget Sound
earthquake of April 25, 1965. First-person accounts of the
effects of the 2001 earthquake on individual households
are given for some communities.