Journal of Geophysical Research Wataru Tanikawa , Toshihiko Shimamoto

1. Frictional and transport properties of the Chelungpu fault from shallow borehole data and their correlation with seismic behavior during the 1999 Chi-Chi earthquake Journal of Geophysical Research WataruTanikawa, Toshihiko Shimamoto 指導教授：董家鈞 老師 報告者：陳宥任 日期：2010/12/16

2. Introduction large slip displacement (H: 9.8m, V: 5.6m) Low acceleration(0.5g) small slip displacement (H: 3.5m, V: 4m) Chelungpu fault Chelungpu fault High acceleration(1g)

3. Introduction 211 m 450 m

4. Introduction • Transport properties within a fault zone also have important influence on dynamic slip motion • Thermal pressurization mechanism is probably controlled primarily by transport properties • Thermal pressurization [Sibson, 1973] : Increase pore pressure induced by frictional heating can cause fault weakening [Han et al. 2010]

5. Methods • Samples : • For friction tests • Southern : dark gray ultracataclasite from 176.8 m depth • Northern : clay-rich fault gouge from 286 m depth and 303 m depth • For transport property • Southern : 30-194 m depth • Northern : 40.5 – 402.5 m depth

6. Methods Northern: (B,C)smectite, illite, kaolinite Southern: (A)Quartz, potassium feldspar X-Ray Diffraction

7. Methods • Low-Velocity Friction Test Slide-Hold-Slide test [Shimamoto] Double-direct shear apparatus

8. Methods • High-Velocity Friction Tests Rotational speed of 1200 rpm Normal stress from 0.6-0.9 MPa High-speed rotary-shear testing apparatus

9. Methods • Transport Property Measurements • Permeability • Darcy’s law : • Klinkenberg equation : • Porosity • Boyle ‘s law : • Specific Storage

10. Results • High-Velocity Friction Slip-weakening 0.8-1.2 0.2-0.4 V=1.04 m/s

11. Results • Low-Velocity Friction 0.7 0.7 0.4-0.5

12. Results • Low-Velocity Friction Tests velocity-dependent friction law

13. Results • Permeability South > North

14. Results Permeability distributions Hanging wall footwall footwall Hanging wall

15. Results • Porosity 8~48%

16. Results • Specific Storage

17. Thermal Pressurization Analysis • Lachenbruch’s (1980) model : One-dimensional analysis of thermal pressurization process • Temperature change is given by the sum of production term and heat transfer term as follow: Heat production Heat transfer

18. Thermal Pressurization Analysis • The change in pore pressure depends on temperature change and Darcian fluid flux as follow : T change Fluid flow

19. Analysis Results

20. Discussion • The high-velocity friction behavior is very different from low-velocity friction behavior • low-velocity friction coefficient • North(wet)~0.4 ；South(wet)~0.7 • The high-velocity steady-state value of friction coefficient (0.2) is similar the earthquake • Tanaka et al.[2006] reported in situ temperature deficits imply that dynamic friction was very low, the indicate that friction coefficient as low as 0.05 to 0.12 • Slip-weakening

21. Discussion • Low velocity: • Northern gouge: velocity-strengthening • Southern gouge: velocity-weakening • If the faulting mechanism is represented by the behavior of wet gouge • the velocity-weakening frictional behavior in the south is consistent with the earthquake • Northern gouge exhibits velocity-strengthening behavior is inconsistent with the large slip displacement

22. Discussion • Assuming at the hypocentral depth of the Chi-Chi earthquake T=200-300℃,vertical stress 120-150MPa • Thermally driven mineral transitions, such as dehydrantion of smectite to illite • Illite-rich gouge show velocity-strengthening behavior over the entire range of normal stress [Saffer and Marone,2003] • Numerical model : large slip caused by thermal pressurization • Northern controlled by thermal pressurization and material behavior

23. Conclusions • The behavior of fault gouge material from shallow boreholes during high-velocity slip is much different than during low-velocity slip • Assuming wet gouge under low-velocity is consistent with the southern section • Thermal pressurization caused large slip and illite-rich gouge caused velocity-strengthening in northern section

24. Thanks for your attention.