SAF
How stress state evolves along SAF
Stress state ∆ during interseismic periods
Not during earthquakes
Stored elastic energy dissipated during interseismic periods
Dissipates i.e. translated to mvmt / damage / heat
Creeping section stress state ∆
Max ∆ Mohr circle diameter
= 4 MPa / hundred years (peanuts)
n.b. tensile strength rock usually ~ 10MPa
Extremely low μ strength in creeping zone
∴ small stress change would cause deformation
When Mohr circle touches yield envelope
Failure / rupture occurs
i.e. seismic events
Central Cali (SAF proper)
Strike-slip dominantly
Equakes - not just shallow crustal z
more than 90% < 10 km
peak @ 5 km
none below 15 km
---> brittle ductile transition
Transverse Ranges
Peak ~ 15 km
Many earthquakes nucleate @
greenschist facies metamorphic conditions
Equakes distributed over range of crustal Z up to 20 km
Associated with transpressional bends in SAF near LA
Thrust faulting dominates
Parkfield Section SAF
Bounded by
Creeping section
on NW ~150 km long
where numerous small equakes
Locked portion on SE (hundreds km)
few equakes detected in past century
accomodate compression caused by bend in SAF
Parkfield Aftershock Distribution
1. Well defined clusters of aftershocks @ some depth
2. Distributed over wide depth range/interval
Equake Depth Distributions
Why this disparity in aftershock distribution?
Varies between parts of segment
Slip patch ruptures
Slip patch/
surface
= tens km of fault which slips / ruptures
usually elliptical
Max displacement
= in centre of slip patch
Coseismic displacement tapers off
moving away from centre
∴ min displacement @ edge of slip patch
∴ STRESS IS TRANSFERRED IN SYSTEM