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Research Papers

The Role of Thermal Pressurization and Dilatancy in Controlling the Rate of Fault Slip

[+] Author and Article Information
Paul Segall

Andrew M. Bradley

Geophysics Department,  Stanford University, Stanford, CA 94305ambrad@stanford.edu

J. Appl. Mech 79(3), 031013 (Apr 05, 2012) (10 pages) doi:10.1115/1.4005896 History: Received July 12, 2011; Revised January 10, 2012; Posted February 13, 2012; Published April 04, 2012; Online April 05, 2012

Geophysical observations have shown that transient slow slip events, with average slip speeds v on the order of 10−8 to 10−7 m/s, occur in some subduction zones. These slip events occur on the same faults but at greater depth than large earthquakes (with slip speeds of order ∼ 1 m/s). We explore the hypothesis that whether slip is slow or fast depends on the competition between dilatancy, which decreases fault zone pore pressure p, and thermal pressurization, which increases p. Shear resistance to slip is assumed to follow an effective stress law τ=f(σ-p)fσ¯. We present two-dimensional quasi-dynamic simulations that include rate-state friction, dilatancy, and heat and pore fluid flow normal to the fault. We find that at lower background effective normal stress (σ¯), slow slip events occur spontaneously, whereas at higher σ¯, slip is inertially limited. At intermediate σ¯, dynamic events are followed by quiescent periods, and then long durations of repeating slow slip events. In these cases, accelerating slow events ultimately nucleate dynamic rupture. Zero-width shear zone approximations are adequate for slow slip events but substantially overestimate the pore pressure and temperature changes during fast slip when dilatancy is included.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 4

(a) Slip rate (m/s), (b) slip (m), (c) fault zone pore pressure (MPa), (d) shear stress (MPa), and (e) log(vθ/dc ) as a function of along-fault distance during a slow-slip cycle. σeff  = 3 MPa, W/h* = 30. Curves represent snapshots in time, not equally spaced, grading from blue to red.

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Figure 5

(a) Slip rate (m/s), (b) slip (m), (c) temperature change (°C), and (d) shear stress (MPa) as a function of along-fault distance. σeff  = 3 MPa, W/h* = 30. Curves represent snapshots in time, not equally spaced, grading from blue to red.

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Figure 1

(a) Spatially averaged slip speed as a function of time for four different values of background effective stress: 1, 3, 10, and 100 MPa. Dashed line indicates v∞. In all cases a/b = 0.9 and W/h* = 30. (b) Maximum ΔT (°C) on the fault as a function of time.

Grahic Jump Location
Figure 2

(a) Spatially averaged slip speed as a function of time for four different values of W/h* at fixed σ¯=3  MPa and a/b = 0.9. Dashed line indicates v∞. (b) Maximum ΔT (°C) on the fault.

Grahic Jump Location
Figure 3

Space-time evolution of slip-rate for σeff  = 3 MPa, and W/h* = 30. The color scale is log10 (v). The vertical axis of the main and right plots is solver step. In the right box, time (year) is plotted as a function of solver step. The solver takes small time steps during fast slip and larger time steps when slip is stable.

Grahic Jump Location
Figure 6

Temperature °C (left) and pore pressure (right) as a function of distance normal to the fault, zero-width fault approximation. Background effective stress is 10 MPa. Curves represent different snapshots in time, not regularly spaced, from light to dark. Dilatancy initially leads to a decrease in pore pressure, followed by thermally induced pressurization.

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Figure 7

Results for finite-width shear zone, a/b = 0.9, W/h* = 30, and σ-p∞=3  MPa. (a) Spatially averaged slip speed as a function of time for different shear zone thicknesses h (in meters). hc  = 10−3 . The case h = 10−3 , 0w refers to the zero-width approximation. (b) Maximum ΔT (°C) on the fault as a function of time.

Grahic Jump Location
Figure 8

(a) Temperature (°C) and (b) pore pressure profiles as a function of distance normal to the fault during a dynamic slip event. In this calculation h = 100 μm, dc  = 10 μm. In this simulation the effective dc for dilatancy was scaled by h/hc  = 1/10. σ-p∞=100  MPa, W/h* = 30.

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