| NSF Org: |
EAR Division Of Earth Sciences |
| Recipient: |
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| Initial Amendment Date: | July 23, 2012 |
| Latest Amendment Date: | June 18, 2014 |
| Award Number: | 1220642 |
| Award Instrument: | Continuing Grant |
| Program Manager: |
David Fountain
EAR Division Of Earth Sciences GEO Directorate for Geosciences |
| Start Date: | July 15, 2012 |
| End Date: | June 30, 2016 (Estimated) |
| Total Intended Award Amount: | $253,783.00 |
| Total Awarded Amount to Date: | $253,783.00 |
| Funds Obligated to Date: |
FY 2013 = $76,478.00 FY 2014 = $79,224.00 |
| History of Investigator: |
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| Recipient Sponsored Research Office: |
1156 HIGH ST SANTA CRUZ CA US 95064-1077 (831)459-5278 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
Santa Cruz CA US 95064-1077 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Tectonics |
| Primary Program Source: |
01001314DB NSF RESEARCH & RELATED ACTIVIT 01001415DB NSF RESEARCH & RELATED ACTIVIT |
| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
The devastating 1999 Mw 7.9 Wenchuan thrust earthquake generated a 300 kilometers-long surface rupture with an average offset of about 5 meters. If the resisting stress of faults is controlled by Byerlee?s friction law, this slip should have resulted in significant energy dissipated as heat. The Wenchuan Fault Zone Scientific Drilling Project has successfully measured multiple temperature profiles within 2 years after the earthquake. In a pilot hole intersecting the major fault zone at 589 m depth, 13 high-quality repeated temperature logs were taken over a 1.5 year period. In addition, temperature logs were performed before casing completion and continuous measurements were made at a few points between profile runs. Preliminary inspection and modeling of the data indicates that any transient temperature anomaly over the fault is less than 0.05 degrees C, which suggests that the equivalent coefficient of friction during the earthquake is less than 0.05 and, by implication, that the earthquake completely relieved the stress stored up from plate motion. However, the preliminary modeling did not account for several important effects such as advection and fault structure. The goal of this project is to determine the actual value of the stress on the fault during the earthquake by: (1) employing a more sophisticated numerical model that includes the cooling effect of water flow along the fault in order to better interpret the borehole data; and (2) by collection and analysis of new temperature and pressure data in a second borehole that intersects the most prominent fault zone at 1247-1250 meters and extends to 1400 meters total depth and in a third borehole planned for completion in 2013.
Earthquakes are the result of motions of the tectonic plates and the driving forces are well understood. However, the resisting force impeding the motion on the faults is much more poorly constrained. This frictional stress during an earthquake is one of the major unknown parameters in earthquake mechanics. The processes controlling friction at high slip velocities are vigorously debated with many theoretical possibilities. Field measurements are needed to constrain the dominant processes on actual faults. This project would take advantage of new and planned temperature measurements in boreholes that penetrate the fault that generated the Wenchuan earthquake that would allow determination of the coefficient of friction during the earthquake.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This work established two, major new facts. First of all, the temperature data demonstrates that during a major earthquake, forces on the fault were much lower than were previously thought. This study, combined with other recent efforts, has resulted in a paradigm shift in how we view the force balance on faults during earthquakes. It is now clear that frictional resistance is limited during an earthquake, whereas previous work had thought frictional resistance may be significant. The inference of low frictional resistance favors models that control earthquake size based on the strength of the asperity at the initiation point rather than those that control earthquake size based on the force balance during rupture.
Secondly, the permeability study shows that a fault zone can heal rapidly, but episodically after a major earthquake. This healing process had never before been caught in action and the staccatto nature of the healing suggests an important role for fault to fault interaction in the process of recovery and preparation for subsequent earthquakes. The combination of processes generating the long-term average permeability suggests that permeability is ultimately a dynamic quantity in a fault zone. The final value is a result of the balance of the competeting damaging and healing processes. The inference of dynamic controls on the long-term permeability has an implication for other fluid-filled systems such as oil reservoirs, aquifers and geothermal resources. The possibility of dynamic controls on long-term permeability suggests a new approach to inferring in situ properties. It also suggests that resource useage efforts need to take into account how the permeability changes with time.
Last Modified: 11/11/2016
Modified by: Emily E Brodsky
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