Annual Meeting - SCEC.org

Poster Abstracts
THE EFFECTS OF STATIC COULOMB, NORMAL AND SHEAR STRESS CHANGES ON EARTHQUAKE
OCCURRENCE IN SOUTHERN CALIFORNIA (B-103)
A.E. Strader and D.D. Jackson
Deng & Sykes (1997) found a strong correlation between receiver earthquake location and positive increase in Coulomb stress
(ΔCFF). Assuming a coefficient of friction of 0.6, and resolving stresses onto assumed fault planes with uniform orientation
parallel to average Pacific-North American plate motion, they found that only 15% of receiver earthquakes occur in “stress
shadows” where the Coulomb stress change should impede faulting. We extended their study by adding two source
earthquakes (Hector Mine, 1999 and El Mayor-Cucupah, 2010), and calculating the stress changes at the locations of 134
receiver earthquakes with magnitude 4.4 and greater after 1999. We examined shear stress, normal stress, and Coulomb stress,
resolving stresses onto four different hypothetical fault planes: optimally oriented planes, a weighted average of nearby faultplane
orientations, and the two nodal planes of weighed average moment tensors of nearby earthquakes. We also computed
shear, normal, and Coulomb stress histories oriented according to the four choices of fault orientation., and we tested the
effect of total stress change on receiver earthquake magnitude.
Our chi square test results indicate that, with 95% confidence, receiver earthquakes do not tend to avoid stress shadows, and
that the choice of plane onto which stress is resolved does not affect the result. On average, 39% of earthquakes occur at the
time of maximum stress at the event location, with no significant variation depending on the choice of rupture plane or type of
stress change. We found no correlation between earthquake magnitude and total stress change at the events’ locations. These
results suggest that instantaneous cumulative Coulomb stress, as we and Deng & Sykes modeled it, does not strongly control
the locations of future earthquakes. The lack of correlation between Coulomb stress change and magnitude suggests that
modeled Coulomb stress change does not control the size of earthquakes once they nucleate.
EVALUATING THE RELATIONSHIP BETWEEN LATERAL SLIP AND REPEATED FOLD DEFORMATION
ALONG A TRANSTENSIVE STEP-OVER ON THE SAN ANDREAS FAULT AT THE FRAZIER MOUNTAIN
SITE (A-139)
A.R. Streig and R.J. Weldon
Transtensive step-overs known as sags are among the most ubiquitous features of strike slip faults. These structures create
closed depressions that collect sediment, are often wet and thus preserve organic material that can be used to date the section.
It is clear from historical ruptures that these depressions grow incrementally with each earthquake. We are developing
methods to carefully document and separate individual folding events, and to relate the amount of folding to the amount of
horizontal slip creating the sag, with the goal of generating slip per event chronologies. This will be useful as sags are often the
best sites for preserving evidence of earthquake timing, and determining slip at these sites will eliminate ambiguity inherent
in tying earthquake age data from micro-stratigraphic sites to nearby undated sites with good micro-geomorphic slip evidence.
We apply this approach to the Frazier Mountain site on the Southern San Andreas fault.
Approximately 20 trenches show the main active trace of the San Andreas fault right stepping ~30 m over ~100 m along strike
producing two small synclinal sags that dramatically thicken the stratigraphic section. The northwest sag is about 50 m long,
15 m wide, and the southwest sag measures 20 m long and 8 m wide. Frazier has yielded good earthquake chronologies, and
relationships between fold deformation and surface fault rupture for the last 6 earthquakes. We observe that the degree of
sagging in the synclines varies along strike for each feature, but that the ratio of fold deformation between earthquake
horizons remains constant in both sags. The penultimate earthquake, E2, produced a depression that was infilled by gravel
which was subsequently folded in the most recent earthquake in 1857. Fine-grained alluvial units overlie the gravel and fill the
1857 depression such that the current surface is relatively horizontal. E2 has double the observed folding associated with the
1857 event in the core of the NW syncline. Ratios of fold deformation between events are E2 = 2*E1, E6 = 3*E2, and E6 = 2*E3.
We plan to model the folding to quantitatively assess the lateral offset, but to date we have only been able to establish
minimum offset values (Scharer, Gibson, Weldon, Streig, this meeting). Qualitatively, the relative amounts of folding suggest
all slip events are similar to 1857, which had ~5 meters slip at this site.
238 | Southern California Earthquake Center