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Seismic-Induced Landslides:
A. Frequency sensitivity:
Landslides are prone to the frequency content of their parent earthquakes.
For instance, shallow (H=~20km), thrust earthquakes have anomalous,
nonlinear PGA spectra shown below (calculated from Campbell &
Bozorgnia, 2003) and so it is crucial to find a regions “typical” excitation
frequency to mitigate the hazard.
Figure:
(top) Horizontal and (bottom) vertical peak ground acceleration (PGA)
from a
shallow, thrust earthquake as a function of distance from epicenter. Curves are
color-coded according to dominant source frequency (going from blue to red for lower to
higher frequency). [NOTE: the coloring scheme I have used here is actually
counter-intuitive! In optics, hot colors correspond to lower frequency... :)]
shallow, thrust earthquake as a function of distance from epicenter. Curves are
color-coded according to dominant source frequency (going from blue to red for lower to
higher frequency). [NOTE: the coloring scheme I have used here is actually
counter-intuitive! In optics, hot colors correspond to lower frequency... :)]
Using these curves, I look into the question of where and how to build
things. Granted that this is an oversimplification of the problem. I am also
applying more complex, fine-detailed source and receiver structure
(compolex ruptures, natural frequencies of structures, etc) to address
problem urban planning.
B. Frequency content of earthquakes: THETA (Θ)
Resonance between seismic waves from earthquakes and natural
frequency of geologic and man-made structures damages the existing
physical and/or mechanical bonds in structures. So it is important to rapidly
investigate and determine the frequency content of these waves in order to
quickly identify sensitive sites.
A good tool in this regard is the THETA parameter (Newman & Okal,
1998). THETA which stems out of deviations from earthquake scaling laws
has higher algebraic values (more negative) for low-frequency (i.e., slow)
earthquakes. However, it has lower algebraic values (more positive) for
high-frequency (i.e., snappy) events. THETA has a standard value of -4.9
for "textbook" (neither snappy nor slow) earthquakes.
Figure: THETA parameter calculated for a number of earthquakes.
Slow and snappy earthquakes are show in red and blue.
Slow and snappy earthquakes are show in red and blue.
THETA works with the first few minutes (typically 2 mins) of seismic
records and thus can be used to rapidly determine the frequency content
of earthquakes.
C. Site properties: Slope Inastibility & Soil structure
One should also note that distribution of landslides in a region following an
earthquake is more complex than simply being at the mercy of source
structure. It also depends on soil and slope properties (Skempton
coefficients, etc). Generally speaking (and intuitively!) steeper slopes with
less consolidated material are more prone to fail during a slide. So such
structures must be mapped and cataloged BEFORE and AFTER any
earthquake. Various techniques from simple grid search in topography
data to machine learning in satellite maps can provide invaluable insight in
this regard.
