SCEC Award Number 15168 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Assessing fault zone structure and permeability in regions of active faulting and fluid injection: Can fault maps and structure help evaluate induced seismicity in southern California?
Name Organization
Emily Brodsky University of California, Santa Cruz Thomas Goebel California Institute of Technology
Other Participants
SCEC Priorities 2f, 4a, 4b SCEC Groups Seismology, SDOT, FARM
Report Due Date 03/15/2016 Date Report Submitted 10/13/2015
Project Abstract
Fluid-injection is well-established as a major driver of human-induced earthquakes. Both the central U.S. and California experienced a rapid increase in waste water injection activity since 2001, however in California the seismogenic consequences of injection remain understudied. Here, the proximity between active injectors and faults may alter the seismic hazard and pose a substantial risk for near-by urban centers. A major issue in connecting the observed seismicity to injection is the local reservoir structure. The issue is particularly complex for wells abutting faults. Faults can act as both conduits and barriers to flow resulting in strongly anisotropic pore pressure migration. Crustal heterogeneity and high permeability zones such as fault damage zones, can lead to a localization of diffusion and have been blamed for induced earthquakes reaching up to 10–35 km from the injection wells.
A detailed assessment of fluid pathways is generally hampered by a lack of publicly available subsurface data. Southern California is unique in that the long-term investment in fault-mapping within the region produced a rare data-set of detailed fault locations and geometries. Based on mapped fault structures and seismicity-data, we created flow models to constrain the extent and amplitudes of injection-induced pore-pressure perturbations. Using a range of realistic damage zone widths and permeability values, we showed that induced pore-pressure changes can result in seismic activity at up to ~10 km distance from injection wells (Goebel et al. 2016).
Intellectual Merit The study addresses four SCEC science objectives, i.e.,
2f: by mapping local fault zone structures and implications for pore-pressure diffusion and the creation of noticeable earthquakes by fluid injection;
4a & 4b: by improving our understanding of fault architecture and the role of faults as fluid conduits or barriers;
2a: by regional extensions of earthquake catalogs using template matching and joint-event-relocation methods.
The proposed research had the aim to advance our understanding of the connection between pore-pressure perturbation, fault damage zones and implications for possible large-magnitude ruptures (FARM).
In addition, induced seismicity sequences allow for the study of foreshocks and systematic event migration prior to the largest magnitude event of a sequence (EFP) which is rarely observed for tectonic earthquake sequences.
Broader Impacts Our study addressed some fundamental questions about fault zone hydrology and the connection between fault zone structure and pore pressure diffusion.
Most of the previous work assumes a simple isotropic permeability structure, neglecting crustal heterogeneity and permeability variations with depth. Nonetheless, localized zones of relatively higher permeability, as observed in Colorado, may have a substantial seismogenic impact. The larger extent of diffusive processes may increase seismic hazards including the probability of inducing large-magnitude earthquakes on basement-faults which generally store more strain energy than sedimentary layers above.
Exemplary Figure Cross-section of seismicity, faults and a waste water disposal site within the Tejon oilfield at the southern end of the San Joaquin valley. Seismicity up to 300~days after start of injection is shown be colored dots (see color bar), background events by gray dots and Tejon fault seismicity prior to injection by black dots. The gray rectangle qualitatively highlights a zone of high seismic activity that coincides with the Tejon fault and hydraulically connects the waste water disposal well with the near-by White Wolf fault.