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USGS Mendenhall Postdoctoral  Research Fellowship Program

14-18. The mechanical role of pore fluids in seismicity, earthquake source physics, and faulting.

Elevated pore fluid pressure is routinely used to explain local and crustal scale faulting on the basis of theoretical expectations, while definitive seismologic evidence of the role of pore pressure (earthquake rates, source properties, and spectra) is limited to induced earthquakes. For example, high pore fluid pressure is often invoked to explain faulting in low shear stress environments; in particular large and plate-boundary-scale faults that are not optimally oriented for frictional sliding in the regional stress field, including most of the earth’s subduction zones, plate boundary strike-slip faults such as the San Andreas and low-angle detachments. Despite a perceived mechanical need for elevated pore pressure, earthquakes in low shear stress environments are generally indistinguishable from those in high shear stress regions. Nor are there established differences in earthquake statistics (e.g., frequency magnitude relations, aftershock productivity, or occurrence of triggered seismicity) between regions of hypothesized elevated pore pressure and elsewhere.

A related issue is that regardless of the magnitude of the ambient pore pressure, slip during moderate and larger earthquakes is sufficiently sustained and rapid to produce enough shear heat to overpressurize pore fluid, producing complete earthquake stress drops, provided the fault is sufficiently localized and does not dilate significantly. Nonetheless, large earthquake stress drops and radiated energy efficiencies are small and generally identical to those of microearthquakes.

Outside of plate boundaries, the mechanical role of elevated pore fluid pressure in earthquake occurrence is also poorly understood. In intraplate regions the crust is thought to be stressed close to frictional failure and pore pressures are typically near hydrostatic. Under these circumstances raising pore fluid pressure artificially via fluid injection, a practice that is widespread and becoming more common in the United States, should produce widespread local high seismicity rates. However, the few well-documented cases of induced seismicity occur at a tiny fraction of the total number of injection sites, suggesting that either that stress is lower than assumed or additional factors beyond elevated pore fluid pressure are required to induce intraplate seismicity. As in plate boundary regions, the source properties of induced earthquakes are not easily distinguished from background seismicity. Similarly, in volcanic and hydrothermal areas in instances where earthquake swarms are likely driven by increasing pore fluid pressure, earthquake source properties are essentially identical to background events.

In addition to the relatively few documented instances of human-induced seismicity, there are regions where pore pressure appears to have a strong influence on earthquake occurrence and source properties. On the deep, predominately ductile extent of many plate boundary faults, seismic tremors can be triggered by remarkably small static (tidal) and elastodynamic stresses. These observations are thought to indicate faulting at very high pore pressures. Furthermore, the resulting seismicity is depleted in high frequency content relative to typical earthquakes. Reasons for this depletion are unknown but may be the result of pore fluid effects in the source region or from nearby attenuation. In some cases these regions also have anomalous elastic properties (Vp/Vs ratios), attributed to the presence of fluids. Similarly, in active volcanic environments, earthquake source properties can deviate from those of typical tectonic earthquakes. Observations include volcanic tremor as well as hybrid earthquakes (combining aspects of tremor and tectonic earthquakes) with significant depletion in high frequency content, both thought to result from pore fluid effects in the source or immediate vicinity.

This Opportunity is for new research in seismology, computational geophysics, field/borehole geophysics or rock mechanics involving the occurrence of earthquakes in the presence of pore fluid. The expected potential research products of this Mendenhall opportunity are (1) better physical understanding of the source properties, in-situ conditions and physics of earthquakes, (2) new or refined constitutive models associated with earthquake occurrence and fault slip, and (3) improved understanding of ground motion and hazard.

For this Research Opportunity we seek contributions in the following research areas:

  1. Observational seismology.Recent seismic network upgrades (ARRA, EarthScope) contribute to an unprecedented quantity and quality of continuous seismic and geodetic data available for research. Key issues related to elevated pore pressure that can be addressed include spatial and temporal patterns of earthquake hypocenters, relation of seismicity to induced changes in pore pressure, earthquake scaling and occurrence statistics, triggering by earth tides or by seismic waves of distant earthquakes, earthquake rupture characteristics, as illuminated by studies of source spectra, moment tensors, or focal mechanisms, and imaging, including seismic velocity structure. In addition, in both tectonic and volcanic environments changes in pore pressure are expected to change seismicity rates, thus understanding the role of pore fluids in seismogenesis is closely related to the USGS earthquake and eruption forecasting missions.
  2. Experimental rock mechanics. Triggering of earthquakes in regions of elevated pore pressure by stress changes (tides, fluid injection), regional changes in physical properties (wave speed, permeability, poroelasticity) due to high pore pressure, and dynamic slip in the presence of pore fluids are well suited to laboratory investigation where pore pressure is directly measured or controlled. Rock physics laboratories at the USGS and collaborating institutions include capability in high pressure, high temperature, high slip speed and large scale faulting, in the presence of pressurized pore fluid.
  3. Earthquake source models and in-situ measurements. New theoretical models and direct in-situ measurements are needed to better understand the roles of pore pressure in static and dynamic earthquake triggering, evolution of fault strength during earthquake rupture, and earthquake occurrence. High priorities include studies of episodic tremor and slip, dynamic rupture propagation, and injection-induced seismicity. Issues of importance in earthquake rupture propagation associated with thermal pressurization include, shear localization, dilatancy, and  dehydration effects. Proposals that build on USGS modeling and data collection/analysis activities related to the role of pore pressure in induced seismicity are encouraged. These include evaluation of in-situ stresses, hydrologic properties, geologic structure, thermal regime, and rock physical properties (permeability, porosity, frictional strength, etc.) at fluid injection and production sites and incorporation of these data into coupled numerical hydrologic and geomechanical models.

Proposed Duty Station: Menlo Park, California.

Areas of Ph.D.: Geophysics, seismology, geology volcanology, rock mechanics or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet one of the following qualifications - Research Geologist, Research Geophysicist.

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist).

Research Advisors: Nick Beeler, (360) 993-8987,; Bill Ellsworth, (650) 329-5020,; David Lockner, (650) 329-4826,; David Shelly, (650) 329-4024,; Steve Hickman, (650) 329-4807,

Human Resources Office Contact: Lisa James, (916) 278-9405,

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Last modified: 17:58:02 Tue 23 Jul 2013
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