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Understanding Earthquake Fault Failure. Understanding Earthquake

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Understanding Earthquake Fault Failure. Understanding Earthquake Understanding Earthquake Fault Failure. M.J.S. Johnston U.S. Geological Survey, Menlo Park, CA, 94025, USA IUGG2015 Talk Outline 1. Basic Fault Mechanics Block motion, Eq stress drops everywhere ~1-3 MPa Faults are weak (<10 MPa shear strength). Lack of heat flow anomaly (Implies fault friction coeff. ~0.2) Fission track ages at fault distances less than 1 km have not been reset by heating. Also indicates friction ~0.2 Principle stress directions rotate to near fault perpendicular near fault indicate friction ~0.2 2. Fault Rupture Controlled by geology and geometry of faults. Observed in rupture velocity observations. 3. Nucleation and Prediction Implications Nucleation Source (Small in both field and laboratory observations). Independent of Eq Magnitude. 4. Implications for Failure Models 5. Conclusions 1. Basic Fault Mechanics - Continuous GPS Networks Plate displacement block-like. High localized displacement gradients on active trace Largely uniform rates 54 mm/a Offsets caused by earthquakes No precursory changes in stress 1. Basic Fault Mechanics -Continuous Borehole Strain Networks USGS Instruments: Installed ~200 m deep 3-Components strain observed Resolution 10-12 strain – means that 1 cm of slip on a 10cmX10cm fault segment at a depth of 10 km is detectable if it occurs in an hour or so. 1. Basic Fault Mechanics - Earthquake Stress Drops Earthquake Magnitude Stress Drop 1. Basic Fault Mechanics - Heat Flow Lack of a heat flow anomaly near the San Andreas Fault implies: - shear stress < 20 MPa - <0.1 Lachenbruch and Sass, 1980 1. Basic Fault Mechanics - Fission Track Ages Observations: No. Fission track agesnnn. near the fault have not been reset by heating, which implies <0.2 D’Alessio et al, AGU 2001. 1. Basic Fault Mechanics - Fault Weakening Mechanisms Fault weakening mechanisms • High-pressure fluids decrease effective normal stressOb (Hubbert & Rubey, 1959; Rice, 1992; Sibson, 1992) σf ≥ τ - μ(σN –p) • Fault zone materials with low . Difficult to find a fault zone mineral that is weak and stable at high temperature and pressure. • Dynamic weakening: - highly velocity dependent friction (Heaton,1990) - fault-opening waves (Andrews & Ben-Zion,1997) - acoustic fluidization (Melosh, 1996) - pore pressure increase (Brodsky & Kanamori, 2000) 2. Rupture – Geology Observations on Exhumed Faults Offsets, fault jogs and material differences on principle slip surfaces of faults exert major control on starting and stopping of rupture. Long term slip transfer does occur but geometric and geologic complexities act as kinetic barriers impeding and stopping rapid rupture propagation. In the fluid-saturated crust, observed breccias, aftershocks and deformation decay are apparently caused by intense rupture-induced fluid-pressure changes Sibson, 1986 2. Rupture -Example of Multiple Rupture Velocities during the M7.5 Guatemala Eq. of Feb. 4, 1976 Observations: Multiple sequential events needed to match observed velocity seismograms. Each event does not know how big the next one will be. Any event can apparently be stopped by geologic barriers After Kanamori and Stewart, 1978 or geometric barriers (offsets) 3. Nucleation – GPS (Example Parkfield M6 Eq.) No near- or far-field stress rate change in years to days before any events Co-seismic (0.3 MPa), post-seismic (0.1 MPa) offsets observed. Uniform loading pre- and post event. 3. Nucleation – Borehole Strain (Example Parkfield M6 Eq.) No near- or far-field stress rate change in years, months to millisec before event. Uniform loading (~0.02 MPa/yr). Co-seismic (~0.1 MPa), post-seismic (~0.03 MPa) observed. Dynamic stress waves with eq. (>2 MPa) are >20 times coseismic stress drop 3. Nucleation – High Resolution Strain before the Parkfield M6 Eq. Pre Slip Model Maximum Possible Nucleation Moment Release <1011 Nm Johnston et al., 2006 3. Nucleation – High Resolution Displacement at the Epicenter of the Parkfield M6 Eq. Precursory (2 sec) Displacement < 12 Nanometers Nucleation Moment release < 10 8 Nm Epicenter Maximum Possible Nucleation Moment Release <108 Nm Johnston et al., 2006 3. Nucleation Size Calculate upper limit on nucleation size two different ways [Scholz, (1995) and Aki (1987)] from strain data at 8-15 km and displacement data at epicenter. Summary of Field Observations Precursory Slip and Nucleation for the M6 Parkfield Earthquake? For Nucleation at Eq. Hypocenter Size < 10-90 m (strain data at 8-15 km from epicenter) or Size <3 cm (displacement data at epicenter) Moment < 1010-1011 Nm (strain data) or Moment < 108 Nm (displacement data) whereas Parkfield M6 had a moment 1018 Nm. i.e a M=<1.3 or a M=<-0.6 produced a M= 6 earthquake. Failure appears to result from Nucleation Runaway (e.g. like a crack running in inhomogeneously stressed glass, a single falling sand grain causing a final sandpile collapse. Both described by “self-organized criticality”). No difference between initiation of large or small earthquakes. Earthquake size is determined by barriers to propagation. 3b. Nucleation – Laboratory (after McLaskey and Lockner, 2014). Hints of pre-slip (nucleation) with moment 2400 times less than the event Epicenter moment. Pre-event AE (eqs) may occur or may be continuous or minimal. A single AE event appears to trigger final event. 3b. Nucleation – Laboratory (after McLaskey and Lockner, 2014). Event DSE8 AE events (eqs) of all sizes occur on fault face from Epicenter 100 s to 1 ms before fault slip. Event marked with X triggered fault slip. 4. Implications for Failure Models After Beroza and Ellsworth, 1996 Conclusions Geophysical data indicate faults are weak (<10 MPa shear strength, friction coefficient <0.2, Pmax normal). High pore pressure likely cause. Changes in shear stress resulting from Eq. failure are small ~1-3 MPa. Exhumed faults, observed Eq. rupture and absence of scaling between the size of earthquake nucleation and final rupture indicate earthquakes are runaway events where any small preslip or earthquake can cascade into a single small event, a single large event, or a series of intermediate events. Consistent with the Cascade Earthquake Failure Model (Self Organized Criticality). Not consistent with Pre-slip Earthquake Failure Models The prediction of earthquake size, location and occurrence time would appear to be inherently impossible. Probabilistic intermediate-term earthquake forecasting based on clustering, repeat times, etc still useful. 4. Failure Models - Cascade Model Failure behavior through out fault occurs on all scales (scale invariant or fractal behavior). Systems evolve to critical state that changes as 1/f or “flicker noise” Implies eqs, material differences and/or stress inhomogeneity's on all scales. Power law or fractal behavior. 4. Failure Models - Preslip Model Simple elastic model – accelerating pre-slip occurs prior to failure. When reaches critical size, it ruptures dynamically (eq.) Nucleation zone and pre-slip scale with final rupture. Pre-slip detection could provide precursors indicating size and time Inconsistent with best field data (nucleation size < 2 cm at 8 km)and lab data (size < 1 micron at 5 cm). 3b. Nucleation –Laboratory (after McLaskey and Lockner, 2014).
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