Preliminary Map of Peak Horizontal Ground Acceleration for the
Hanshin-Awaji Earthquake of January 17, 1995, Japan
(Part A of 2 - The Map)
Open-File Report 95-259A
This maps was digitally compiled by C.M. Wentworth, R.D. Borcherdt,
and R.K. Mark from preliminary information kindly provided by the
primary agencies and investigators in Japan. This report is preliminary
and has not been reviewed for conformity with U.S. Geological Survey
editorial standards or with the North American Stratigraphic Code.
Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government. 1995.
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of the map.
Summary
The Hanshin-Awaji earthquake of January 17, 1995, near Kobe, Japan,
offers a valuable dataset for a strong nearby event in a region that
contains extensive soft-soil deposits.
A small-scale map has been prepared that shows strong motion stations
and recorded accelerations together with general contours of those
accelerations over a background of the regional geology. A similar
map is available for the Northridge earthquake (Wentworth, Borcherdt,
Mark, and Boore, 1994; and World Wide Web at another USGS
WWW site called http://wrgis.wr.usgs.gov). Note that the Hanshin-Awaji
ground motion data are preliminary, the contours are very general
and represent a sparse dataset, and the background geology is derived
from a generalized map of Japan prepared at the country scale of
1:1,000,000.
The map displays the peak horizontal accelerations (in gals) recorded
in the epicentral region over a generalized geologic background.
Acceleration values are shown near the stations at which they were
recorded and contours of those values are superimposed on the maps.
Fore- and aftershocks over a two-day period indicate the approximate
location of the source and circles at a distance interval of 25
km around the mainshock epicenter indicate both map scale and position
of features relative to the mainshock. A 1-degree grid of latitude
and longitude is superimposed on the geology.
The prominent dumbbell-shaped high in the acceleration contours
is centered over the Bay-margin fringe of Holocene sediment in the
Kobe City area adjacent to the northeastern end of the source fault.
It's relatively narrow shape and rapid falloff northeastward along
strike are real, based on the distribution of control around it's
northeastern end. The dumbbell peaks rise to 600-700 gals, but two
stations in the intervening gap reach above 800 gals (other nearby
stations with lower values force the saddle in the contours).
The geologic units are those of the generalized Geologic Map of
Japan, which distinguishes rock and sediment formed during different
intervals of geologic time (time stratigraphic units). Although
the materials can be inferred to increase in stiffness or hardness
with increasing age, as a first approximation, the textures in the
Holocene and Pleistocene sediments that are important for evaluating
local amplification are not well distinguished. For this presentation,
the full distinctions in the upper Cenozoic time-stratigraphic units
on the source map are retained, but the older bedrock units are
combined. Note that the recent land fill that has been placed along
the margins of Osaka Bay and at Kansai International Airport (location
of the 160 gal station in Osaka Bay) is not included.
A more complete discussion of the map preparation and other ways
to obtain online versions is here
and a broader description and discussion of the data are provided
in the companion report
(Borcherdt and Wentworth, 1995).
Data Sources
The strong motion data were derived from preliminary information provided
to R.D. Borcherdt by the various agencies and principal investigators
and colleagues in Japan, together with the "Prompt Report on Strong
Motion Accelerograms No. 46" (Science and Technology Agency of Japan,
1995). The mainshock location is from the above noted Prompt Report.
Seismicity for the two-day period including the main shock is that
reported via ftp by the Earthquake Research Institute, University
of Tokyo, Observation Networks of Disaster Prevention Research Institute,
Kyoto University, and Faculty of Science, Kochi University.
The geologic background was prepared from a digital version of
the Geologic Map of Japan (Geological Survey of Japan, 1992). This
is a very generalized map of the country at a scale of 1:1,000,000.
In its representation in the poster-sized image, the Kobe region
of the geologic map is enlarged about twofold (to a scale of 1:425,000),
a process that adds no detail but simply makes room for the other
data that are included.
More from the U.S.G.S.
The U.S.G.S. has a lot of other information available; you can go
directly to either the U.S.G.S. homepage
or the U.S.G.S. Earthquake Hazards Program page. |
