Auxiliary Material for
The stress shadow problem in physics-based aftershock
forecasting: does incorporation of secondary stress changes help?
Margaret Segou and Tom Parsons
(United State Geological Survey, Menlo Park, California)
Geophysical Research Letters, 2014
Introduction
The supporting material contain:
1. An appendix in “text01.txt”, presenting the equations used for the implementation of a
rate-and-state friction model incorporating static stress changes [Dieterich, 1994] and
statistical metric formulations [Zechar et al., 2010] for the comparison between
forecasted and observed seismicity.
2. Table A1 in “text02.txt” lists the major events in Northern California, for the time
period before Loma Prieta mainshock, that have been considered for the calculation of
the state variable γ at the time of the mainshock. In Table A1, we present the
seismological parameters of nine events preceding Loma Prieta mainshock in Northern
California. Columns “ID”, “Y”, “m”,”d”, “h” and ”min” correspond to the identification
number, year, month, day, hour and minutes of the event. Column “ML” represents the
local magnitude and column “z”, the focal depth in kilometers. The last Column gives the
Earthquake Name.
3. Table A2 in “text03.txt” lists the seismological parameters of the events within the
aftershock clusters for which we update the stress field. In Table A2, we present in detail
the seismological parameters and the focal mechanism solutions that we used during our
stress modeling. Column “Y”, “m”,”d”, “h”,”min” and “s” correspond to the year, month,
day, hour, minutes and seconds of the event. Column “ML” represents the local
magnitude and column “z”, the focal depth in kilometers. Columns “DAZ”, “DIP”,
“RAKE” correspond to the dip-azimuth, the dip and the rake of the focal planes; we note
that both focal planes in bold represent events with estimated, rather than assigned focal
mechanisms within each cluster. For a given event e.g. event ID 4 has been assigned the
focal mechanisms of event ID 81 within each cluster, but for simplification purposes we
present just the first possible focal plane in the columns “DAZ”, “DIP”, “RAKE”.
4. Figure A1 in “fs01.pdf” presents cross-sections in SW-NE direction at the central part
of the Loma Prieta rupture, reappraised from Dietz and Ellsworth [1997]; see in Figure 1
(panel A) the clusters in map view. Events 4-8 in cluster A (Table A2) correspond to
events Q, X, Z, b, c respectively (left) whereas events 5, 6, 8, 9 in cluster B (Table A2)
correspond to events H, J, R and U, respectively (right).
5. Figure A2 in “fs02.pdf” presents the time series of Coulomb stress changes for selected
locations within each aftershock cluster of Loma Prieta mainshock (A); early aftershocks
(first day) corresponding to warm colors going to colder ones for longer time intervals
(>1 year). We focus on each cluster, overlaid with the aftershocks during our model
implementation in the detailed figures (a-c). We then select a location with clustered
aftershocks at the vicinity of an aftershock with M≥3.5, which we can include in our
CRS-2 implementation, and we study the static stress changes time series (a1-2, b1-2, c1) in
an effort to understand why the incorporation of the updated stress field has offered so
little in our forecast. We conclude that even in cases of secondary positive stress changes
(e.g. stress changes following 69, 75, 80, 81 in a1 ), the initial stress shadow cannot be
lifted. In other words, the triggering potential of the aftershocks seems to be controlled
from the static stress changes of the mainshock.