Fault Rheology and Earthquake Mechanisms:

Transcription

1 Workshop: Frontiers in Studies of Earthquakes and Faults SUSTech, Shenzhen, China (Nov. 27-Dec. 1, 2017) Fault Rheology and Earthquake Mechanisms: The Current Status and Future Perspectives Toshihiko Shimamoto State Key Laboratory of Earthquake Dynamics Institute of Geology, China Earthquake Administration Shimamoto Earth & Environment Laboratory (SEE-Lab) Ltd. In collaboration with H. Noda, S. Ma and L. Yao. An outline of talk (1) Friction to flow law We have not conquered LITHOSPHERE yet. Why? Expected fault-zone heterogeneity slow slip & LFE (2) After 25 years of high-velocity friction studies What s next?

2 We have not conquered LITHOSPHERE yet! Transition from friction to fully plastic deformation across a lithosphere was produced only for halite, but not for other rocks! Kawamoto and Shimamoto (1998) Slip rates: 0.3 mm/yr to almost 10 mm/s. High-Temperature Biaxial Frictional Testing Machine

3 Friction to Flow Transition Frictional behavior changes to pressure-insensitive fully Step 1 plastic deformation, through a transitional regime. Step 2 Brittle def. (basal slip in phyllosilicates) Plastic def. (slip systems less than 5) + brittle def. [Mylonitic def.] Increase in slip rate Friction to exponential flow law With more shear, S-C begins to form and Shimamoto (1986, Science) Fully plastic def. (von Mises Criterion Satisfied.) stick-slip occurs! [Transitional regime!] Decrease in temperature Friction to power law Kawamoto and Shimamoto (1997) Proc. of 30 th IGC, Beijing

4 An Empirical Friction to Plastic Flow Law t friction = m ss s = steady-state friction, s : normal stress t flow = steady-state flow stress t ss = real shear resistance t ss / t flow = tanh (m ss s/t flow ) Very similar to tanh curve! m ss s/t flow : small t ss = m ss s Step 3 Shimamoto (2004, JpGU) tanh (m ss s/t flow ) ~ m ss s/t flow ) (Friction law) tss tfriction s ss s/t flow : large t ss = t flow tanh (m ss s/t flow ) ~ 1 (Flow law) tflow Kawamoto & Shimamoto (1997) m = 0.65 [1] Only friction and flow parameters! [2] Shear zone width w is needed too. This is needed to relate strain rate in flow law with velocity in friction law. [3] Smooth connection: friction & flow

5 Flow behavior upon a step change in V A step increase in slip rate from V1 to ev1: (ln( t )) ln( t ( V, )) ln( t ( ev, )) 1/ m flow flow 1 0 flow 1 0 Noda & Shimamoto (2010, GRL) Transient behavior: an exponential increase from 0 to Step 4 ( )exp( Vt / w ) ( )exp( / ) ss 0 ss c ss 0 ss c Viscous Newtonian A characteristic distance for flow L f = w c (quite large) Shear zone width w connects friction and flow. w = 300 m

6 Fitting the halite data with the law Friction to flow behavior can be described with the law. t ss / t flow = tanh (m ss s/t flow ) A (t flow ) n exp(-o/rt) m ss m 0 + (b-a) ln v Shear zone = 0.7 mm Least-square fit with Matlab Good agreements with data! Shimamoto and Noda(2014, JGR)

7 Step-Change Behavior from to Flow to Friction Noda & Shimamoto (2010, GRL) (1) Velocity weakening to strengthening at the peak strength (2) Instantaneous response followed by decay beyond peak Base of seismic zone: V. weakening to strengthening!?

8 Strength Profile vs. Friction Model Brittle-plastic strength profile (Goetze, middle 70 s) The two approaches were merged! Friction model for EQ modeling (e.g., Lapusta et al., 2000) Flow properties are not given. What property here? Friction to flow constitutive law (Frictional, transitional and fully-plastic regimes) Shimamoto and Noda(2014, JGR)

9 Velocity Dependency versus Temperature Granite & Qz gouge: Blanpied et al. (1995, JGR ) s N = 400 MPa, P p = 100 MPa Quartzite shear zone, w = 300 m. Flow law from Hirth et al., (2001) Friction to transitional! dt d ln( V ) t n flow ss (1) A sharp rise in rate dependence with plastic deformation. (2) Maximum rate dependency: bottom of transitional regime (3) Widening of frictional regime with increasing slip rate. flow ss

10 Heterogeneity Predicted from Friction to Flow Law Rate-dependency = s (a b) --- t for V ev Slip rate: 10-9 m/s, shear zone width: 300 m Geothermal gradient: 25ºC Figures by Noda (2017, unpublished) Quartzite Hirth et al., (2001) Plagioclase Rybacki & Dresen (2000) (1) Temperature for flow depends on rock/mineral types. Transitional regime: most heterogeneous! (2) Slow slip, low-frequency tremors! Qiabase Caristan (1982)

11 Velocity Dependency for Flat Strength Profiles A certain Pp distribution can make profiles flat. Similar velocity dependency still remains. Rate-depenfency = s (a b) --- t for V ev Quartzite Hirth et al., (2001) Plagioclase Rybacki & Dresen (2000) Figures by Noda (2017, unpublished) Essential features of fault properties (a summary) [1] Frictional and flow regimes are separated by transitional regime. The transitional regimes has a high rate dependency of shear resistance and it can act as an very effective barrier of seismic fault motion. [2] Fault properties are most heterogeneous in the transitional regime. Qiabase Caristan (1982)

12 2D modeling of EQ cycles along a strike-slip fault (1) Boundary-element method is used (Lapusta et al., 2000). (2) Slip rate = 10-9 m/s (a plate velocity) Ca. 30 mm/year P p = hydrostatic pressure Seismic/aseismic slip of the same order! P p = twice hydrostatic pressure Higher Pp more frequent EQs! (1) Fully coupled zone, (2) Seismic slip followed by creep, (2) Aseismic slip, (4) Location of EQ nucleation. Shimamoto and Noda(2014, JGR)

13 Changes in Strength Profiles [1] Strength profiles changes during earthquake cycles! Blue curve: just before an earthquake Red curve: just after an earthquake (brittle regime expanded!) [2] Stress dropped at most seismic depths. But stress increased at the bottom where rupture was arrested. [3] Steady-state strength profile is a good approximation. But a profile will change when HV weakening is included (a task!). Shimamoto and Noda(2014, JGR)

14 Post-seismic Slip-rate Distribution 20 years after an earthquake Faster slip Prominent afterslip occurs in high-stress region Low slip-rate dependency Shimamoto and Noda(2014, JGR)

15 Changes in the Deformation Regimes Shimamoto and Noda(2014, JGR) (1) Aseismic zone continues to slip regardless of what happens in the seismogenic zone (2) Frictional regime changes very quickly to transitional after an event. (3) Creep SSEs next EQ (4) Emergence of frictional R. No fixed regime! (6) Seismic slip overlapped by creep at the bottom of seismic zone. --- Consistent with PT overprinted by mylonites.

16 Mylonitized Pseudotachylite (Sibson, 1980) Sea Forth Head, Outer Hebrides Seismic slip/creep of the same order!

17 Comparison with a Fault Model Shimamoto and Noda(2014, JGR) (1) Fault model is a reasonable approximation for a stress profile, but it cannot account for complex changes in the def. regimes. (2) Seismic slip far into the velocity strengthening. (3) Existence of negative stress drop (build-up of t) at the bottom. (4) Pseudotachylites overprinted with mylonitic deformation. (5) Behavior may completely changes by including HV-weakening.

18 Friction to Flow: Summary and Tasks [1] A simple friction to flow law agrees with lab data. t ss / t flow = tanh (m s/t flow ) [2] But we need data for important rocks. In particular, transient flow data are completely missing! --- A new machine is needed! [3] Friction to flow law can predicts conditions for the transition. --- Useful for designing an apparatus! [4] How does the framework changes with HV friction, diffusion creep (grain-size sensitive flow), and pressure solution? [5] Mechanical properties of lithosphere itself are needed for modeling tectonic deformation including fault motions. --- Friction to flow law has to be extended to describe brittle to ductile transition of rocks.

19 Experiments on Important Rocks Diabase: Caristan (1982), strain rate = 10-6 /s Flow parameters: n =3.1 Q = 276 kj/mole A = 33.8 (MPa) -n /s (1) Slow down the speed! (2) Or increase pressure! Gas rig Normal stress 700 C 800 C 900 C 1000 C 1100 C Melting (MPa) 900 C 2000 An ultra-high pressure rotary-shear gas apparatus (my dream machine!) [1] Plate velocity V ~ 10-3 mm/s for 1 mm thick gouge layer (32 mm/year) [2] Temperature to 1,200ºC [3] Pressure to 1.0~2.0 GPa [4] High sensitivity & accuracy Existing machines cannot make it!

20 Brittle to High-Temperature Flow Transition of Rocks How to describe brittle-ductile transition of rocks? If you establish lithosphere and fault rheology, plate and plate boundary behaviors can be solved on a unified basis. Granite: f6, 10 mm Zhang et al. (1982) Fitting by H. Noda (2017) s d / t flow = tanh [(a + bp c )/t flow ] t flow = A exp (Q/nRT) a = 381 MPa, b = 3.4 A = 1.6, Q/n = 28 kj/mole Fitting is not good partly because of large error in experiments. Improvement of solid-pressure medium apparatuses is needed at lot to moderate temperatures!

21 High-Velocity Friction of Faults [1] Lack of heat flow anomaly -- San Andrea fault (Brune et al., 1969) Post-earthquake temperature anomaly (since Chenlung fault drilling) Lachenbruch (1980) --- proposed thermal pressurization theory. He recognition of HV weakening of faults and used dynamic weakening. [2] Thermal pressurization & dramatic weakening of faults Importance of HV friction was recognized! --- after ca. 10 years [3] What is the role of HV friction during EQ generation? Insight from the 1999 Taiwan Chi-Chi earthquake [4] Fracture energy: Lab data vs. seismic data They do not agree for large earthquakes. [5] Then what will be the problems from now? Scale issue and energetics of fault-zone formation

22 Heat-flow Anomaly, Temp. Anomaly after an EQ Brune et al. (1969): lack of heat flow anomaly [long-term average] Rapid drilling [a single event] Li et al. (2015, Geology): Lachenbruch & Sass (1980): average stress ~ 20 MPa (m ~ 0.1) Chelungpu fault & Japan Trench m ~ 0.1

23 HV friction and Postseismic Temperature Anomaly Average friction for 5.5 m of slip Chelungpu fault & Japan Trench m ~ 0.1 (consistent with heat flow data) Fulton et al. (2013): the best data! We will use the m av later in estimating frictional work. Problem with SHIVA gouge-cella?? WC WC K = 80 Wm -1 K -1 Ti64 alloy was used (Ti90%, Al6%, V4%) Smith et al. (2013, Geology, Suppl. Inf.)

24 Thermal Pressurization (MTL) Strategy of my group (1) Measure permeability, and (2) calculate thermal pressurization Thermal pressurization mechanism yields slip-weakening distance, D c, close to seismically determined values! 2 km in depth 6 km in depth The biggest problem: P p is not measured and the k = to m 2 Tsukide Outcrop, Mie Prefecture effects of thermal pressurization and gouge weakening cannot be separated (heat source is unclear). We are struggling to measure P p now! Wibberley & Shimamoto (2005, Nature) Theory by Mase & Smith (1987, JGR) Wider slip zone less rapid weakening, larger fracture energy (A scale issue)

25 The Biggest Problem in High-Velocity Friction of Faults [1] Fault constitutive law is not established. Friction for arbitrary slip history cannot be described yet. [2] Friction really changes with slip histories! Examples! (1) Low friction data at low slip rates not plotted. (2) Not many data at intermediate velocities at that time. Di Toro et al. (2011, Nature)

26 HV friction is very sensitive to slip history! Sone and Shimamoto (2009, Nature Geoscience) Togo et al. (2014, GRL) peak velocity peak velocity Acceleration weakening Seismic fault motion itself can cause weakening of a fault. Deceleration strengthening The strengthening can act as a brake!

27 Slip History: Regularized Yoffe Function Fukuyama & Mizoguchi (2010, IJF) Rapid acceleration followed by slow deceleration Sharp rise in friction, rapid slip weakening, slip strengthening Be careful with using a simple slip-weakening law! No friction law for arbitrary slip histories!

28 Normalized shear stress Normalized shear stress Importance of HV Friction in EQ Generation Tanikawa and Shimamoto (2009, JGR) 1999 Chi-chi earthquake Contrasting behaviors between north and south North: velocity strengthening (no earthquake!) More rapid weakening at HV South: velocity weakening at LV Less rapid weakening at HV Rate and state friction --- Earthquake nucleation High-velocity friction --- Growth to large earthquake TCDP(south) North South TCDP(north) Shangtung F. Shuiliukeng F. Dry Wet a - b Slip distance (m) Slip distance (m)

29 Earthquake in Velocity Strengthening Regime! Tanikawa and Shimamoto (2009, JGR) Southern part: velocity weakening, but less HV weakening (permeable fault zone) Northern part: velocity strengthening, but more HV weakening (impermeable fault zone) Noda & Lapusta (2009, JpGU) Trench data support this result (more frequent events in the south).

30 Long-term fault behavior Accumulation of fault slip Noda and Lupsta (2013, Nature) * Smaller events in the velocity-weakening patch Large events every few to several small events (This may be similar to both Tohoku-Oki and Chi-Chi earthquakes) Prior smaller events in relation to Tohoku-Oki, from Simons et al., 2011

31 Fracture Energy E G Extrapolated to Depths Mizoguchi et al. (2007): V = 1 m/s, s n = 0.4~2.0 MPa E G ~ 1~1.3 MJ/m 2 (dry Nojima gouge) Ide & Takeo (1997, JGR): E G ~ 0.5~1.0 MJ/m 2 (1995 Kobe EQ) μ(d) =μ ss + (μ p - μ ss ) exp (ln(0.05) d/d c ) E G ~ 0.334(m p - m ss ) D c s n Yao et al. (2013): Longmenshan gouge μ p (v) = μ p + (μ 0 p - μ p ) exp(-v/v pc ) μ ss (v) = μ ss + (μ ss 0 μ ss ) exp(-v/v ssc )

32 Fracture Energy of Longmenshan Gouge Yao et al. (2013): Longmenshan gouge D c = A s n -B v -C B = 1.1~2.0 (1.6±0.3) E G = f(v) s n 1-B f(v) = A v -C {(m p (v) - m ss (v)} E G decreases with increasing s n if B > 1.0 E G ~ 0.1 MJ/m 2 s n = 100 MPa E G from lab. exp. is smaller than seismically determined E G for large earthquakes. G. Di Toro, S. Nielsen and others got a similar conclusion from rock-on-rock friction experiments with SHIVA (B ~ 1).

33 Difference in E G : What does it mean? Too rapid weakening in laboratory HV experiments! [I] Scale issue (thicker slip zone, rougher surface in nature) I am preparing to work on this problem now! [II] Wide natural fault zones How much energy to form it? Geologists measured particle size distributions of fault rocks. (Many papers, but issue is inconclusive! We measured BET surface area (A BET ) --- quicker & direct! [N 2 adsorption method] Yingxi-Beichuan fault zone at Hongkou (about 100 m wide)

34 Energy Used for Crushing Quartz Gouge BET surface-area measurement with absorption method HV experiments Togo and Shimamoto (2012, JSG) Measurements of the whole sample in several hours Energy for crushing: 0.02~0.2 % of W F, 0.05~1.06 % of E G More than 90 % of WF goes to heat! (old work by Lockner & Okubo,????).

35 How about natural fault zone? Nojima Fault Surface area: 65x10 6 m 2 /m 2 A BET [m 2 ]/1 m 2 fault area Density = 2.6 x 10 3 kg/m 3, porosity = 30% 1.74 m 2 /g 19.1 m 2 /g If surface energy is 1~5 J/m 2, required energy: (65~325) MJ/m m 2 /g Equivalent grain size = 0.05 mm Equivalent grain size ~ 1 mm 14.0 m 2 /g 18.9 m 2 /g Sawai et al. (2012, JSG) Host rock: granite Clayey fault gouge: quartz, plagioclase, kaolinite, smectite

36 Energy to Form Wide Fault Zone in Hongkou Assume surface area and its exponential change. Thin cohesive cataclasite Assume 5 km uplifting 50 m 2 /g Togo et al. (2016, JGR) 5 m 2 /g 50 m 2 /g Surface area: 3.6x10 9 m 2 /m 2 Energy to form 5 km fault zone = (1.8~8.9)x10 13 J/m Zhang et al. (2012, Tectonophysics) About 1700 (Wenchuan EQ) W F = 7.1x10 13 J/m (dry) 4.4x10 13 J/m (wet) of the same order! A big problem: Clay minerals are not crush products!

37 Pulverized Rock along Arima-Takatsuki Fault A BET measurements may be of interest because rocks are less altered. Yehuda s discovery in Jan. 2006

38 Future Tasks in HV Friction Studies [1] Establish HV constitutive law (still not so easy!) [2] How to merge low to high-velocity friction? (i) R&S state friction: EQ nucleation and small EQ s (ii) HV weakening promotes growth to large EQ. [3] Go deeper! Increase s N and T? [4] Scale issue and controlled or monitored Pp exp. (i) 4 th generation low to HV apparatus (Chengdu machine) (ii) Hydrothermal pressure vessel (supercritical H 2 O) (iii) A cell to change gouge thickness [5] Energetics of fault-zone formation How to attack this problem?

39 An Integrated Fault and Earthquake Study Earthquake Mechanisms FAULT STUDIES EXPERI- MENTS THEORY, SIMULATION SEISMOLOGY, GEODESY Deformation mechanisms & fluid flow in fault zones Mechanical & transport properties of faults Analyses of earthquake generation processes based on measured properties Dynamic fault motion during earthquakes This will be the only way of understanding EQ mechanisms. High-velocity Testing machine In Kyoto Slip distribution during 1999 Chi-chi earthquake (after J. J. Mori) Fault outcrops and drill cores Reproduction of seismic fault motion Best seismic records

40 Thank you for your attention! 大同 (Daton)

41 High-Velocity Gouge Exp on Nojima Fault Gouge It took us more than 10 years to do gouge experiments! Teflon ring Ca. 30 mm Highly Concentrated Shear Mizoguchi, Hirose, Shimamoto & Fukuyama (2007, GRL)

42 Nature Experiment Experimental & Natural Fault Gouge (Nojima Fault) Steady state stage of friction Not pseudotachylites!

43 Median Tectonic Line National Monument in Hidaka Town, Mie Prefecture (1) Wide and complex fault zone (2) Very straight fault plane (3) Concentrated deformation zone (20-45 mm wide) Tsukide Outcrop

44 Comparisons of Parameters t ss / t flow = tanh (m ss s/t flow ) m ss m 0 + (b-a) ln v A (t flow ) n exp(-o/rt) Parameters from Friction to flow law: m 0 = 0.55 (±0.014, 1s) b-a = (±0.0069, 1s) n = 7.4 (±0.75) Q = 138 kj/mole (±23) = 82 (MPa) -n s -1 (±125) Kawamoto & Shimamoto (1997, Proc. IGC Beijing): m 0 = 0.60 b-a = (±0.003, 1s) n = 7.6 (±0.71) Q = 179 kj/mole = 3.55 x 10 5 (MPa) -n s -1

45 t Framework of rate and state law (1) Existence of Steady-sate, (2) Instantaneous response, (2) Transient behavior towards a new steady-state State variable : a quantity that cannot change instantaneously! (e.g., dislocation density) V, t V wcf exp Q RT exp 2 V wcf H O exp Q RT V wcf H O exp Q RT 2 2 V wcf HO exp Q RT 2 1/ m flow ss r HO ss r flow ss ( V ) 1/ m 1/ n 1/ m t t r t This is a power-law flow law with C = At rn. Notations: V: slip rate, w: shear-zone width, f H2O : water fugasity, A, n, Q (activation energy): flow parameters, T: absolute temp. 1/ n

46 Basic equation: t flow Rate and state flow law Noda & Shimamoto (2010, GRL) V, t V wcf exp Q RT exp State evolution: r t r 2 Cf HO exp Q RT exp A Cf t HO 2 r 2 HO n 1/ m 1/ m Hirth et al. (2001, Int. J. Earth Sci.) d dt V L Steady-state solution: c ss flow ss 1 1 ln HO ss V n m Q RT V wcf 2 Steady-state flow law: A (t flow ) n exp(-o/rt)

47 A Transient Behavior with Friction to Flow Law L = 5 mm L f = w c = 300 m x 0.3 = 90 m Friction and flow are very different in L and L f. Frictional Frictional/ transitional Transitional Fully plastic Fully plastic Can you recognize the frictional, transitional & fully plastic behaviors?

48 Constitutive Parameters used in 2D EQ Modeling Frictional properties: Reference friction f Direct effect a T Rate-dependency a - b (1 z / 2 km) at z < 4 km at z > 4 km State-evolution distance L 5 mm Flow parameters: Reference stress τ r 1 MPa Steady-state power 20 n 4 Instantaneous power 15,20 m 4/0.6 Activation energy 20 Q 135 kj mol -1 Pre-exponential factor 20 C s -1 MPa -1 State-evolution strain 15 γ c 0.3 Shear zone thickness w 300 m Hirth et al. (2001) 石英の流動則 岩塩