Title: Frictional Strength Heterogeneity
1Frictional Strength Heterogeneity Surface Heat
flow near SAFOD(Or, Is the creeping section
strong?)
- Matthew A. dAlessio
- Colin F. Williams
- Roland Bürgmann, UC Berkeley
U.S. Department of the Interior U.S. Geological
Survey
2IF FAULTS ARE WEAK
What mechanism weakens faults?
- Dynamic Weakening (related to earthquake rupture
process) - Thermal pressurization of fluids
- Chattering during slip
- Acoustic fluidization of gouge
- Permanent Weakness
- Weak gouge material
- Serpentine?
- Some Clays?
- Constant fluid pressure from (deep) fluid source
http//www.nps.gov/prsf/geologyy/serpent.htm
3TEST CASE CREEPING SECTION
- 170-km-long section of San Andreas in central
California - Slip almost entirely by slow, aseismic creep
- No Large Earthquakes
- No Dynamic Weakening
Creeping Section
(Base Figure SAFOD Team)
4CREEPING SECTION HEAT FLOW DATA GAP
SAFOD
5IS THE CREEPING SECTION STRONG?
- Thanks to Jim Brune for raising this question.
- Creeping section IS a frictional asperity.
- Aseismic creep controlled by velocity
strengthening frictional behavior. - Velocity strengthening ? strong
- But high ? minerals that promote creep do exist
- Heat flow data are sparse in creeping section.
6FAULT STRENGTH HETEROGENEITY
- PROBLEM!
- All existing models of frictional heat assume
infinitely long, homogeneous faults - Faults are not frictionally uniform
- Asperities
- Special frictional behavior along strike
- Creeping section of SAF
7- Single strong asperity (stationary)
- Midpoint heat flow similar to infinite case
- Heat flow at tip is half that of midpoint
8MOVING ASPERITY
As fault slips, block offset transports heat (and
possibly asperity) along strike.
9- Moving Asperity in Action.
10- Strong moving asperity
- Heat flow asymmetric
- Heat flow tail at trailing edge
Infinite fault
This Case
11SLIP RATE DEPENDENCE
Faster slip rate more asymmetry
12OTHER SCENARIOS
13OTHER SCENARIO RESULTS
See Poster Manuscript
14OTHER SCENARIO RESULTS
Heat Flow mW/m2
High friction asperity Dragging scenario
- Heat flow NOT correlated with distance from
fault. - (Lachenbruch Sass, 1980)
15IS THE CREEPING SECTION STRONG?
- Probably not.
- Sense of asymmetry in heat flow data opposite
that of model predictions for moving asperity. - Heat flow not correlated with distance from
fault. - Weak Creeping Section CANNOT be weakened
dynamically. - It doesnt slip in large earthquakes!
- Permanent weakness must weaken the creeping
section. - Dynamic weakness alone cannot explain weak faults.
Dynamic Weakening
16The End
U.S. Department of the Interior U.S. Geological
Survey
17CONCLUSIONS
- Are faults frictionally weak?
- Probably, according to limited heat flow and
thermochronology. - Have we ever observed frictional heat?
- Not yet. Always below our detection threshold.
- Weakening Mechanisms?
- San Gabriel fault Dont necessarily require
abundant fluids. - Creeping section Dont necessarily require
dynamic processses. - MULTIPLE MECHANISMS (?)
18SAFOD THERMOCHRONOLOGY
- Use (U-Th)/He Fission-track thermochronology
- Reconstruct long-term thermal history near San
Andreas fault
19THERMAL HISTORY
- Heating Event
- Starts 30 Ma
- Onset of SAF
- Lasts until 5 Ma
- Change in SAF
- Burial, Volcanic, or FRICTIONAL HEAT?
20TARGET SAFOD
- Known depth
- Known thermal history
- Surface heat flow
- Thermochronology
- Major plate boundary
- East side of fault traveled from locked section.
- Fault zone width?
- Sample availability?
(Figure SAFOD Team)