Applying Statistical Seismology to Image the Physical Properties of the Crust Egill Hauksson, Caltech, Pasadena, CA Presented at Statsei9 in Potsdam, Germany, 16 June 2015

Purpose: Determine Seismogenic Thickness Map the top, and bottom of the seismogenic zone Compare the seismogenic thickness with geophysics parameters Infer strength of lower crust at the bottom of the seismogenic zone from orientation of the stress field

Caltech/US GS Southern California Seismic Network Catalog The data set used

Method to determine seismogenic thickness Grid of 0.1° latitude and 0.1° longitude Require 10 or more events per grid node Calculate: – Shallow depth: 5% seismicity above it – Average depth – Deepest depth: 95% seismicity above it – Seismogenic thickness = D95% - D05%

Number of events per bin

Focal Depth Distribution Average Seismogenic- Thickness

Average depth versus seis-thickness

Seismicity Depth surfaces: One color scale

Width of the Brittle-Ductile Transition Zone 98%-Depth – 94%-Depth Cumulative Percent

Width of the Brittle-Ductile Transition Zone Complete catalog D98% - D94%

Width of the Brittle-Ductile Transition Zone Waveform relocated catalog only D98% - D94%

Geophysics and Seismogenic thickness Does the seismogenic thickness depend on crustal geophysics? - Heat flow - Topography -GPS strain rate

Heat flow & depth of seismicity

Seismogenic- thickness Topographic Elevation [m] Seismogenic Thickness [km] GPS Strain Rate Heat flow [mW/m^2] 100

B-value Spada et al. (2013) Depth [km] Bear & Platt (2011) D05% D95% S T N

Rheology of Crust and Upper Mantle Burgmann & Dresen (2008)

Stress rotations at bottom of seismogenic zone Stresses align with traction free boundary conditions: Bokelmann and Beroza (2000) – Alignment of principal stress axes with vertical and horizontal directions Use events above/below D05% and D95% and compare to events at average depth Looking for: – Stress orientation signature of a weak zone in the lower crust

Bokelmann & Beroza (2000) Northern Calif. P- and T- axes are strongly aligned near the surface & at the bottom of the seis- thickness P-axes T-axes

P- and t-axes: 1) complete data set 2) strike-slip solutions Complete catalog Ave±3km

AverageD05%D95% PP P T T T 0 60 P-axes Average Plunge

Conclusions The average depth of seismicity: 9 km Average Seismogenic Thickness: 9 km Steepness of the termination of seismicity at the BT varies with deep seismicity hot spots The stress does not seem to exhibit changes near the BT; thus suggesting a some stronger lower crust than in northern Calif. Crème brule ́e --- Southern Calif. Jelly sandwich ---- Northern Calif.