Electronic Supplement to
2014 M_w 6.0 South Napa Earthquake Triggered Exotic Seismic Clusters near Several Major Faults

by Shinji Toda and Ross S. Stein

This electronic supplement contains figures that explain further details of our seismicity and stress analyses. Figures show background seismicity rate and magnitude of completeness $M_{\mathrm{c}}$, 95% confidence interval on the principal stress orientation and friction coefficient, influence of fault friction on the coulomb stress imparted to the major mapped active faults, shear-stress changes resolved on the nodal planes of small earthquakes, and observed aftershock rates.

Figures

Figure S1. Cumulative (a) and interval (b) background seismicity rate and $M_{\mathrm{c}}$. The slope of the cumulative number of earthquakes has been stable from 2004, with a slightly lower rate after ~2007. The $M_{\mathrm{c}}$ as a function of time, following Woessner and Wiemer (2005) as implemented in the ZMAP software (Wiemer, 2001), is shown with $\pm 1\sigma$ bounds.

Figure S2. The 95% confidence interval on the principal stress orientation from Townend and Zoback (2004) and on the friction coefficient are explored for the optimally oriented coulomb stress change on strike-slip faults (a, b, d, e) and compared to an assumed fixed-fault orientation (c, f). An inset on each panel in (a, b, d, e) indicates the orientation of the most compressive principal stress axis and optimally oriented strike-slip faults, whereas insets in (c, f) include a fixed-receiver fault orientation. Note that the overall tendency of the locations of stress-increase lobes are similar, regardless of differences in plausible ranges of regional stress and friction coefficients.

Figure S3. The influence of fault friction on the coulomb stress imparted to the major mapped active faults. Note that, regardless of friction, the northwest (NW) portion of the Green Valley fault is brought closer to failure.

Figure S4. Shear-stress changes resolved on the nodal planes of small earthquakes that occurred (a–c) prior to and (d) after the 2014 South Napa earthquake, as proxies for the sites of small active faults. Fault plane solutions are from the Northern California Earthquake Data Center (see Data and Resources). The color of the compressional region on each focal mechanism indicates the amount of stress change. We split the pre–South Napa period into three subperiods to avoid overlapping focal mechanisms in the maps. The blue lines are mapped late Quaternary faults.

Figure S5. Observed aftershock rates (circles) as a function of time for the South Napa earthquake, fitted to an Omori–Utsu function (solid line), $N(t)=k/(t+c)p$, in which p, c, and k are Omori's parameters. Two areas, I and II in the inset map, are applied to the analyses. Together with observed background rates of $M\ge 1.5$ as $M_{\mathrm{c}}$ from Figure S1, we estimate aftershock durations $t_a$ for the wider area I and the narrower area II for several months and several years, respectively.

---

Data and Resources

Fault plane solutions are from the Northern California Earthquake Data Center (http://ncedc.org/ncedc/catalog-search.html, last accessed May 2015).

---

[ Back ]