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4/19/2012

Using Spectral Ratios of Pore Pressure and Strain Observations Recorded at EarthScope PBO Borehole Strainmeter Sites to Analyze Tectonic Deformation and Changes in Well Parameters due to Nearby Earthquakes. 4/19/2012. Francesco Civilini & Jamison H. Steidl

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4/19/2012

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  1. Using Spectral Ratios of Pore Pressure and Strain Observations Recorded at EarthScope PBO Borehole Strainmeter Sites to Analyze Tectonic Deformation and Changes in Well Parameters due to Nearby Earthquakes 4/19/2012 Francesco Civilini & Jamison H. Steidl Earth Research Institute & Department of Earth Science University of California Santa Barbara

  2. Understanding the Hydrological Response due to Earthquakes Pore Pressure Fracture Information Observed/Theoretical Strain

  3. Introduction • Hydrological changes due to earthquakes have been observed in wells for many decades. • Comparison of observations is difficult because a criteria for an ideal observation well is not well established [Kumpel, 1991] • Fractured rock is different for each well. • Types of Pore Pressure Responses based on two mechanisms: • “Co-seismic” steps (seconds to minutes): Nearby events • Static strain field mechanism [Roeloffs et al., 2003]. • Gradual changes (hours to days): All distance scales- • Local poroelastic change near the well due to passage of seismic waves (seismic wave mechanism) [Roeloffs, 1998 ; Matsumoto et al., 2003]. • Additional Mechanism- Persistent Pore Pressure response [Roeloffs et al., 1998]. • Important to understand water level changes [Roeloffs et al., 1998 ; Kissin & Grinevsky, 1990]: • Effect on water supplies • Effect on underground waste repositories • Hydrological changes as precursors to earthquakes have been observed

  4. Pore Pressure: PBO Wells Pore Pressure is measured here • Plate Boundary Observatory (PBO) stations in the Anza region of southern California provide unique datasets of pore pressure and strain observations. • Unique: Co-located measurements of pore pressure and strain. Can be used to determine a very direct cause-effect relationship. • Pore pressure observations measured in rock, not soil: Pore pressure increases and decreases are based on the opening and closing of the fractures. Water comes in [Modified from http://pbo.unavco.org]

  5. M 5.4, 7/7/2010 • Non-dynamic pore pressure responses are rare. • This event had a significant and very different pore pressure response at two sites: • PBO 87 Ford Ranch • PBO 88 Sky Oaks

  6. M 5.4, 7/7/2010 PBO 87 PBO 88 Pressure (KPa) Pressure (KPa) Time (days) Time (days)

  7. M 5.4, 7/7/2010 1 Kpa Sudden Change and Recovery PBO 87 PBO 88 ~ 14 Km Distance Pressure (KPa) 10 Kpa Pressure (KPa) Time (days) Gradual Change and “Persistent” Behavior Time (days)

  8. Outline • Analyze the correlation between measured strain, pore pressure, and the static strain field generated by an earthquake. • Objective: • Data: • Pore pressure and strain response to a M5.4 earthquake observed at two stations. • Methods: • Part 1: Comparison of pore pressure response with co-located strain observation and theoretical strain. • Part 2: Frequency domain analysis- What mechanism is being observed at the sites? • Part 3: What do the strain tensor and the fracture orientation reveal about the physical mechanism affecting the borehole?

  9. Part 1: Co-Located Pore Pressure and Strain

  10. M 5.4, 7/7/2010 – Strain Fields Dilatation Field Shear Field Both stations in compression Opposite directions of shear

  11. Co-Located Strain • Gladwin Tensor Strainmeter - Strain gauges arranged at different heights of the borehole measure changes in the borehole diameter. [Modified from GTSM Technologies FAQ] [Modified from http://pbo.unavco.org]

  12. Co-Located Strain Regular Borehole • Calculations of strain: Areal strain Change in area Differential Extension Strain (area doesn’t change) Engineering Shear Strain

  13. M 5.4, 7/7/2010

  14. M 5.4, 7/7/2010 PP Ar Diff Sh

  15. M 5.4, 7/7/2010 1 Kpa PP 10 Kpa Ar Observations: 1. Sharp increase of 1 KPa at Ford Ranch, gradual decrease of over 10 Kpa at Sky Oaks. Diff Sh

  16. M 5.4, 7/7/2010 PP Ar .1 microstrain .12 microstrain Diff Observations: 2. Sharp increases in the areal strain at both areal strain records Sh

  17. M 5.4, 7/7/2010 PP Ar Diff .1 microstrain .4 microstrain Sh .12 microstrain .47 microstrain Observations: 3. Differences in magnitude and direction of Differential Extension Strain and Engineering Shear Strain.

  18. M 5.4, 7/7/2010 PP Ar Diff Sh Conclusion: Pore pressure response seems driven by shear strain, not areal strain.

  19. Part 2: Frequency Domain Analysis • Static strain mechanism or seismic wave deformation mechanism?

  20. Part 2: Frequency Domain Analysis • Gradual water level changes correspond to deformation triggered by seismic waves [Roeloffs et al., 2003]. • Causes increased permeability: observed both in nature [Elkhoury et al., 2006 ; Wang & Chia, 2008] and in the laboratory [Elkhoury et al., 2011]. • How can we tell whether or not a hydrological parameter of the medium changed?

  21. Part 2: Frequency Domain Analysis • Gradual water level changes correspond to deformation triggered by seismic waves [Roeloffs et al., 2003]. • Causes increased permeability: observed both in nature [Elkhoury et al., 2006 ; Wang & Chia, 2008] and in the laboratory [Elkhoury et al., 2011]. • How can we tell whether or not a hydrological parameter of the medium changed? • We can observe the amplitude and phase of the tidal signal before and after the earthquake!

  22. Phase and Amplitude 14 hr 11 hr M2: 12.4 hr Logic of pore pressure/strain ratio amplitude: • Ratio describes the “push” on the water level due to tidal forces. • Does the effect of this “push” change after the earthquake? Before Earthquake After Earthquake ? F F aquifer aquifer

  23. Phase and Amplitude Phase difference: • There is a phase difference (time lag) between a strain signal and the pore pressure response. • A decreasein phase difference  increase in permeability [Elkhoury et al., 2006]. Phase Difference Strain Pore pressure

  24. Phase and Amplitude Results • For FFT’s, we want as much data as possible. • Due to small amount of clean data present before the earthquake, I calculated a moving average of phase and amplitude using windows of 30-50 days. • The results before/after the earthquake were compared with values from a seismically quiet year. PBO 87 PBO 88 No amplitude or phase change • Pore pressure/strain ratio change ~ 5 times the standard deviation of a quiet year. • Temporary decrease in phase of ~ 6-8 degrees. Phase returns to pre-earthquake values after a few days.

  25. Phase and Amplitude Results Affected by a seismic wave deformation mechanism and a static strain mechanism. Affected by a static strain mechanism PBO 87 PBO 88 No amplitude or phase change • Pore pressure/strain ratio change ~ 5 times the standard deviation of a quiet year. • Temporary decrease in phase of ~ 6-8 degrees. Phase returns to pre-earthquake values after a few days.

  26. Part 3: Fracture Orientation and Strain Tensor Analysis • How does the input strain interact with the fracture geometry to produce a particular pore pressure response?

  27. PBO 87: Televiewer Log Amplitude Travel Time • A televiewer log can be used to find the orientation of fractures in a borehole • We can then choose a characteristic fracture of the borehole.

  28. PBO 87 Characteristic Fracture Plane • We can model this fracture in 3D and find its strike and dip.

  29. Strain Tensors • From the observed strain records, we can interpret a strain tensor, and then directions of compressional and extensional strain. Extension and compression vectors are the eigenvectors of the strain tensor. Areal Diff. Ex. Compression Direction En. Shear We can do this for both the 2D case (observed) and the 3D case. (Coulomb calculated) Extension Direction

  30. PBO 87 Characteristic Fracture

  31. PBO 88 Characteristic Fracture

  32. Conclusions and Further Work • Summary: • Co-located pore pressure and strain measurements at both stations reveal that they were affected by a static strain mechanism dependent on shear strain. • PBO88 (gradual response) experienced an additional deformation mechanism, characterized by temporary amplitude differences and permeability change. • For PBO 87, the observed compression vector agrees with the observations, while for PBO 88, the result is more ambiguous. • Horizontal fractures may be responsible. • Future work • Use calculated 3D strain motions in conjunction with the 2D observations to understand how strain-fracture system operates in three dimensions.

  33. Acknowledgements • Advisor: Jamison Steidl • Earth Research Institute & NEES@UCSB: Sandy Seale, Paul Hegarty, Daniel Lavallée, and Robin Gee Thanks for your attention Questions?

  34. References • Elkhoury, J. E., Brodsky, E. E., & Agnew, D. C. (2006). Seismic Waves Increase Pemeability. Nature (441), 1135-1138. • Elkhoury, J. E., Niemeijer, A., Brodsky, E. E., & Marone, C. (2011). Laboratory observations of permeability enhancement by fluid pressure oscillation in situ fractured rock. Journal of Geophysical Research, 116. • Kumpel, H. -J. (1991). Pore-pressure variation as a precursory phenomenom: the need for and a list of supplementary data. Tectonophysics (193), 377-383. • Matsumoto, N., Kitigawa, G., & Roeloffs, E. A. (2003). Hydrological response to earthquakes in the Haibara well, central Japan - I. Groundwater level changes revealed using state space decomposition of atmospheric pressure, rainfall and tidal response. Geophys. J. Int. (155), 885-898. • Roeloffs, E. A. (1998). Persistent water level changes in a well near Parkfield, California, due to local and distant earthquakes. Journal of Geophysical Research, 103 (B1), 869-889. • Roeloffs, E., Sneed, M., Galloway, D. L., Sorey, M. L., Farrar, C. D., Howle, J. F., et al. (2003). Water-level changes induced by local and distant earthquakes at Long Valley caldera, California. Journal of volcanology and geothermal research (127), 269-303. • Wang, C.-y., & Chia, Y. (2008). Mechanism of water level changes during earthquakes: Near field versus intermediate field. Geophysical Research Letters, 35.

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