Analysis of Stress and Strain in the Central Coast, California

Nora Lewandowski Jeff Unruh

PG&E SSHAC SSC Workshop #3 25 March 2014

SSHAC SSC Questions

• What constraints do the data place on the orientation and description of the strain field in the DCPP vicinity?

• What constraints do the data place on the orientation of σ1 (and general stress field) in the DCPP vicinity?

• How do these results translate to predicted style of faulting on seismic sources?

Introduction

• Evaluated the orientation of the maximum compressive stress (σ1) and maximum shortening strain (d3) in the DCPP vicinity using inversion methods and the following data: – GPS station velocity data

– Earthquake focal mechanisms

• Orientations and relative magnitudes of principal stresses and strains provide: – Input to static Coulomb and dynamic rupture models

– Fault kinematic constraints

Data

Used two independent data sets in the evaluation:

(1) GPS station velocities: GPS velocity field relative to fixed

Pacific Plate developed by C. DeMets for DCPP SSHAC (2012)

(2) Focal mechanisms of small earthquakes: Two focal

mechanism datasets used:

• “Network” mechanisms calculated automatically using the FPFIT algorithm (Reasenberg and Oppenheimer, 1995) from machine phase picks (downloaded from NCEDC website, 2013)

• Hardebeck (2010) mechanisms calculated using the HASH algorithm (Hardebeck and Shearer, 2006)

SSHAC SSC Questions

• What constraints do the data place on the orientation and description of the strain field in the DCPP vicinity?

• What constraints do the data place on the orientation of σ1 (and general stress field) in the DCPP vicinity?

• How do these results translate to predicted style of faulting on seismic sources?

GPS Data Analysis: GPSTRN and SSPX

• GPSTRN and SSPX are algorithms that invert GPS station locations and displacements in a polygonal region for components of a 2D horizontal strain rate tensor

• Polygons are defined differently: – GPSTRN: Polygons defined by user

– SSPX: Polygons defined by algorithm (either Delaunay triangulation or Grid-Nearest Neighbor) • Provides a “check” to GPSTRN calculations using the

same data

GPS Data Analysis I: GPSTRN

GPS Data Analysis I: SSPX

Seismicity Data Analysis: FLTSLP and SATSI

• FLTSLP (R. Twiss, UC Davis) finds a best-fit deformation tensor (strain and relative vorticity) for groups of earthquake focal mechanisms

– Relative contributions of horizontal shearing vs. crustal thickening or thinning (V) can be evaluated

• SATSI (Hardebeck and Michael, 2006) determines a best-fit stress tensor for groups of earthquake focal mechanisms through a damped inversion algorithm

– Minimizes data misfit and model length (level of heterogeneity in the solution)

Seismicity Data Analysis: FLTSLP and SATSI

• FLTSLP (R. Twiss, UC Davis) finds a best-fit deformation tensor (strain and relative vorticity) for groups of earthquake focal mechanisms

– Relative contributions of horizontal shearing vs. crustal thickening or thinning (V) can be evaluated

• SATSI (Hardebeck and Michael, 2006) determines a best-fit stress tensor for groups of earthquake focal mechanisms through a damped inversion algorithm

– Minimizes data misfit and model length (level of heterogeneity in the solution)

FLTSLP Results: Irish Hills

• N-NE directed d3

• Results highly sensitive to number of data

• V Values • FPFIT: imply horizontal plane

strain (shearing)

• HASH: imply crustal shortening and thickening

SSHAC SSC Questions

• What constraints do the data place on the orientation and description of the strain field in the DCPP vicinity?

• What constraints do the data place on the orientation of σ1 (and general stress field) in the DCPP vicinity?

• How do these results translate to predicted style of faulting on seismic sources?

Seismicity Data Analysis II : SATSI Results - FPFIT

0.1° x 0.1° grid, minimum of 8 events per cell

Seismicity Data Analysis II : SATSI Results - HASH

0.1° x 0.1° grid, minimum of 8 events per cell

Irish Hills d3/σ1 Summary

• Preponderance of d3/σ1 orientations in the Irish Hills: ~N10°E to N20°E

• Good agreement between GPS and seismicity datasets in d3/σ1 orientations

• Net vertical deformation is less consistent (horizontal plane strain deformation or transpressional deformation with net vertical thickening)

SSHAC SSC Questions

• What constraints do the data place on the orientation and description of the strain field in the DCPP vicinity?

• What constraints do the data place on the orientation of σ1 (and general stress field) in the DCPP vicinity?

• How do these results translate to predicted style of faulting on seismic sources?

Predicted Style of Faulting?

• Majority of d3/σ1 orientations in the Irish Hills: ~N10°E to N20°E

• Two possible implications: 1) If regional deformation is dominantly horizontal plane strain…

• Deformation geometry in the Irish Hills is grossly consistent with distributed dextral shear parallel to the Hosgri fault

2) If the deformation in the Irish Hills is transpressional…

• Thickening component may be a result of a local restraining transfer of distributed ~N30°W dextral shear from structures within or southeast of the Irish Hills to the Hosgri fault.

Predicted Style of Faulting? (continued)

• WNW-ESE trending structures in the Irish Hills oriented at a high angle to d3/σ1 are optimally oriented to accommodate shortening and thickening.

• Active structures sub-parallel to the direction of macroscopic dextral shear

(~N30°W) are optimally oriented to accommodate horizontal dextral slip. • Uncertainty in the vertical component of the large-scale deformation contributes

to uncertainty in the magnitude of the lateral component of slip on faults.

d3/σ1

(~N15°E) ~N30°W

Thank you