Small-Scale Homogeneity of Faulting

Jeanne Hardebeck, USGS Menlo Park, jhardebeck@usgs.gov

March 4, 2005

ABSTRACT

Small-scale faulting at seismogenic depths in the crust 
appears to be more homogeneous than previously thought.  
Data from small earthquakes (M<3) has previously been 
interpreted to imply very complex small-scale fault 
structure, with fractures of a wide variety of orientations 
active contemporaneously in small volumes of crust.  The 
main piece of evidence for this model comes from the focal 
mechanisms of small earthquakes, which in many studies are 
highly variable, even for closely-spaced (<1 km) events.  
The apparent heterogeneity in the fault planes of small 
earthquakes has been interpreted to imply either that the 
crustal stress field is highly heterogeneous on small 
length scales or that faults have a wide range of frictional 
strengths.

I observe a much greater degree of homogeneity in new 
high-quality focal mechanism catalogs for Southern California 
and the San Francisco Bay Area.  On average, the closer 
together two earthquakes are in space, the more similar their 
focal mechanisms.  Most pairs of earthquake collocated to 
within the relative location uncertainty (0.1-1 km) have 
mechanisms identical to within their uncertainty (~30 degrees).  
This implies that in small volumes of crust only similarly-
oriented fault planes are active simultaneously.  Locally 
(~10 km), the distribution of earthquake focal mechanisms can 
usually be explained by a relatively homogeneous stress state 
and relatively homogenous fault strength.   This homogeneity 
in stress and fault strength is encouraging, implying it may 
be possible to measure these parameters with enough precision 
to be useful in studying and modeling large earthquakes.