Chemical and Physical Characteristics of Pulverized Tejon Lookout Granite Adjacent to the San Andreas and Garlock Faults: Implications for Earthquake Physics

Abstract

We present new detailed analyses of samples of pulverized Tejon Lookout granite collected from sections adjacent to the San Andreas and Garlock faults in southern California. The Tejon Lookout granite is pulverized in all exposures within about 100 m from both faults. Chemical analyses indicate no or little weathering in the collected samples, although XRD analysis shows the presence of smectite, illite, and minor kaolinite in the clay-size fraction. Weathering products may dominate in the less than 1 micron fraction. The average grain size in all samples of pulverized Tejon Lookout granite ranges between 26 and 208 microns (silt to fine sand), with the particle size distribution in part a function of proximity to the primary slip zone. The San Andreas fault samples that we studied are generally finer grained than those collected from adjacent to the Garlock fault. The particle size distribution for each studied sample from both faults follows a pseudo-power law with a continuously changing exponent, which suggests that pulverization is not simply a consequence of direct shear. The average particle size that we determined for our samples is considerably coarser than reported in previous investigations, which we attribute to possible measurement errors in the prior work. Our data and observations suggest that dynamic fracturing in the wall rock of the San Andreas and Garlock faults only accounts for about 1% or less of the earthquake energy budget.

High stresses stored in fault zones: example of the Nojima fault (Japan)

“…As pointed already, microstructures in the NOJ220 sample are very similar to the experimentally or naturally fragmented samples described by Aben et al (2016) and Dor et al (2006Dor et al ( , 2009 and Rockwell et al (2009) high confining pressure prevents complete pulverization but not fragmentation (Yuan et al, 2011). The confining pressure being in the 100 -300 MPa range when microstructures formed in the NOJ220 sample, the strain-rate threshold was probably not attained and the granodiorite was microfractured, but not pulverized.…”

“…The quartz grain described in the Figure 7 and displaying fractures radiating in a Hertzian pattern may also be compared to quartz grains in damaged sandstones along the San Andreas Fault (Dor et al, 2009). By 25 analogy, the opening of type 2 laumontite veins is tentatively attributed to a co-seismic or dynamic stage (Figure 14) although their orientation is more difficult to interpret (E-W sriking veins versus N50° striking Nojima fault).…”

“…Earthquakes characterizing the first period of seismic activity along the Nojima fault were M6 to M7 magnitude events 20 (Boullier et al, 2001) and their consequences may be compared to those of californian earthquakes along the San Andreas fault system where pulverized rocks were produced on the near surface within a 100 m wide zone (Dor et al, 2006 ;Rockwell et al, 2009). We believe that microstructures in the NOJ220 sample are representative of the incipient dynamic damage at depth induced by the propagation of a seismic rupture on the Nojima fault plane 51 m away from the studied sample.…”

“…Outside the fault core, 1 to 10 3 m wide zones may be damaged by brittle failure of the surrounding rocks. Recently, several examples of pulverized rocks lacking significant shear have been described at the surface or subsurface within 100 to 200 m wide bands close to large faults such as the San Andreas fault (Dor et al, 2006 ;Rockwell et al, 2009;Wechsler et al, 2011) or the Arima-Takatsuki Line in Japan (Mitchell et al, 2011), and have been attributed to dynamic co-seismic damage. At the same time, high strain-rate 25 experiments on granite using Hopkinson bars without confinement have reproduced similar pulverized rocks (Xia et al, 2008) and predicted a dynamic shear stress threshold for pulverization (Doan and Gary, 2009).…”

Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy)

“…creation of new fracture surfaces; KOST-ROV and DAS, 1988;SCHOLZ, 2002;BEN-ZION, 2003). Field and microstructural observations suggest that heat production (frictional heat) dominates fracture surface energy at a point on a fault (CHESTER et al, 2005;PITTARELLO et al, 2008;ROCKWELL et al, 2009), although simulations of dynamic rupture suggest that off-fault deformation in the shallow crust may be of significant importance in the energy budget (BEN-ZION and SHI, 2005). Given that thermal diffusivity in most crustal rocks is low, and the duration of an individual slip pulse is typically a few seconds or less, most of the heat produced is restricted to a narrow band within and adjacent to the slip zone (SIBSON, 2003;RICE, 2006).…”