SCEC Award Number 13033 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Forward modeling of fault slip rates, stress orientations, and distributed deformation in southern California with deformable microplate models
Investigator(s)
Name Organization
Brendan Meade Harvard University
Other Participants Graduate student Meredith Langstaff at Harvard University.
SCEC Priorities 1 SCEC Groups SDOT
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 The San Andreas fault (SAF) marks the primary transform boundary between the Pacific and North American plates and has been studied intensively to determine the recurrence intervals of large earthquakes. Dense geologic and geodetic sampling along the SAF has revealed substantial variability in the along strike slip rate, with maxima approaching 40 mm/yr to the north and south of Los Angeles. In contrast, the San Bernardino segment of the SAF appears to slip at only 5–13 mm/yr, representing just 13–33% of the maximum SAF slip rate. Here we suggest that this spatial variation in slip rate may be explained by the interactions of deformable microplates driven by a combination of far-field plate motion boundary conditions and localized slip beneath the SAF. We find that the observed slip rate variations may be best described by models with 75% of Pacific-North America plate motion localized as slip beneath the SAF and 25% applied at the edges of the deforming plate boundary zone region. These models provide a mechanical explanation for why slip rates vary so significantly along the SAF and suggest a hybrid view of crustal deformation in southern California in which relative plate motion is accommodated by a combination of both localized and distributed deformation.
Intellectual Merit This intellectual merit of this work is to understand the behavior of models that integrate both on and “off”-fault deformation free of any kinematic constraints but subject to a simplified but geologically motivated representation of the fault system geometry in southern California. This approach is two-dimensional and neglects earthquake cycle processes precluding a direct comparison with interseismic geodetic observations. The merit of this is that we have explored and understand that behavior of this type of system for a wide range of boundary conditions including those that do not explain geologic slip data well (e.g., >30% “off” fault deformation).
Broader Impacts The broader impact of this work is less immediately clear and it bears on the mission of informing society about earthquake hazard. The on-fault vs. “off”-fault debate is perhaps not as well posed as we might hope. To our knowledge all earthquakes occur on faults or initiate localized fracture. Thus in terms of implications for seismic hazard the “off”-fault component of deformation may not have a significant meaning other than ‘off of faults that we commonly consider’ and thus suggest regions where greater mapping may be of use. Alternatively if “off” fault were taken as a physical metric of strain that would never be released by earthquakes then it would require a rethinking of the ideas of moment balance and the observation that geodetic and geologic slip rate budgets largely (though not entirely) agree across southern California. These comments are probably the biggest conclusions that we’ve arrived at through this work.
Exemplary Figure Figure 3. Predicted along strike slip rate for the San Andreas fault in southern California. The upper panel shows the rotated fault trace (red) and model mi- croplate boundaries (blue). Topography within 300 km of the SAF is shaded in gray. Model- predicted slip rates are shown in the lower panel with fault segment names indicated at the top (SB - San Bernardino and CP - Cerro Prieto). The bold red line indicates our best-fit hybrid model slip rate predictions, with ~36 mm/yr localized as deep slip beneath the SAF and ~14 mm/yr of distributed deformation applied on the western boundary of the model domain. End member models of purely distributed deformation (bottom dotted line) and entirely localized deep slip beneath the SAF (top dotted line) are also shown. Geologic and geodetic fault slip rate estimates are shown as black circles with reported 1-sigma error bars r as white circles when estimate error is not reported. Both observations and model predictions have maxima on the northern and southern segments of the model domain and both reach a minimum on the San Bernardino segment. Slip rates inferred from geologic and geodetic data are taken from: a. Savage and Burford, 1973; b. Meade and Hager, 2005; c. Murray et al., 2001; d. Segall, 2002; e. Loveless and Meade, 2011; f. Schmalzle et al., 2006; g. Chuang and Johnson, 2011; h. Sieh and Jahns, 1984; i. Matmon et al., 2005; j. McCaffrey, 2005; k. Weldon and Sieh, 1985; l. McGill et al., 2013; m. Van der Woerd, 2006; n. Fialko et al., 2006.