Geophysical Surveys of Basin Properties and Geometry:
Examples of Seismic Hazard Evaluation From Nevada and
New Zealand Basins
John N. Louie
Nevada Seismological Lab. and Dept. of Geological Sciences
University of Nevada, Reno
Visiting the VUW School of Earth Sciences until June 14.
Urban earthquakes such as in Kobe, 1995, have proved more destructive than
seismologists or engineers have anticipated.
- One crucial factor for Kobe and Northridge, 1994, was direct hits by
previously unknown and buried faults.
- Another important factor is the location of cities within sedimentary
basins.
This is a bold subject for an explorationist, so comments and corrections
are welcome publicly (or privately).
Unpredicted large, and long-lasting, ground shaking in Kobe occurred
at sedimentary sites.
- Damage to Mexico City, 1985; the San Francisco Bay area, 1989;
Northridge, 1994; and other cities may also correlate with basin locales.
``Predictive'' hazard mapping for a 1906 great San Andreas earthquake
shows special hazard to urbanized basins.
- Shallow soil conditions have as much effect as distance on predicted shaking.
- Ground shaking prediction relies on S-wave velocity measurements at 0-50 m
depths.
- Inadequate knowledge of basin depths and geometries prevents inclusion
in predictions.
- Trapping of surface waves in basins must be modeled separately for each
source - expensive.
Conclusions
- A quick gravity campaign in Reno showed the unexpected basin we should
have known about.
- A simple slowness-frequency analysis will get shallow velocities at many
sites, even noisy ones.
It uses equipment everyone already has.
Abstract
Sedimentary basins influence seismic hazard by trapping surface-wave
energy, and by trapping wet sediments in low-velocity surface layers.
A campaign to quadruple the number of gravity measurements in the Reno,
Nevada USA basin revealed a deep sub-basin pre-dating the current basin
by 2-3 m.y. The geometry of the sub-basin unexpectedly suggests higher
ground-shaking hazard on the western side of the city. The difficult
task of measuring shallow S-wave velocities in this noisy urban
environment led to the validation of a new and inexpensive technique.
It can use the most common refraction recording equipment, rather than
expensive broadband seismographs, for a simple p-tau spectral
analysis. In Reno the active floodplain shows extremely low velocities
of 200 m/s to 50 m depth. Such velocities are also found in the Parkway
area of Wainuiomata, near Wellington, New Zealand. This locale
provides further tests of the new p-tau analysis.
Presented by invitation to the Victoria University of Wellington
School of Earth Sciences, 23 April 1999.