Key words induced earthquake water injection experiment Nojima fault diffusion process pore water pressure permeability

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1 Triggering Characteristics of Induced Earthquakes : Brief Review of Water Injection Experiments in the Nojima Fault and Other Regions Kin'ya NISHIGAMI, Keiichi TADOKORO, Satoru NAGAI, Takashi MIZUNO, Yasuyuki KANO and Yoshihiro HIRAMATSU Abstract This paper briefly reviews the triggering characteristics of injection-induced seismicity. Water injection experiments were carried out in the Nojima fault, southwest Japan in 1997 and 2000 to detect the healing process of the fault zone after being ruptured by the MJMA 7.3 Hyogo-ken Nanbu Kobe earthquake in During the experiment in 2000, ultramicroearthquakes of M-1.2 to 1.0 were induced at about km from the injection point and about 4-6 days after the beginning of injection. This space-time migration can be explained by a 2-D diffusion process of pore water pressure. Permeability estimated near the surface, at about m depth, is extrapolated well to a depth of 2-4 km where induced events occurred. Other experiments at Matsushiro, central Japan and KTB, Germany also showed similar space-time relationships of induced seismicity. From observations in the Nojima experiment, we obtained characteristic states that suggest differences in the generating process between induced and normal tectonic earthquakes. Our findings are as follows : 1 high-frequency component is richer in the waveforms of tectonic events, and 2 the clustering of hypocenters is more dominant in induced events. Further analyses of these observations will lead to elucidating the generating process of induced earthquakes by increasing pore water pressure. Key words induced earthquake water injection experiment Nojima fault diffusion process pore water pressure permeability Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University Research Center for Seismology and Volcanology, Nagoya University Earthquake Research Institute, the University of Tokyo Graduate School of Natural Science and Technology, Kanazawa University

2 I m M M 5 Healy et al Guha m Ohtake m 2 KTB German Continental Deep Drilling Program 9100 m P 1 Zoback and Harjes Tadokoro et al II M m m 800 m 500 m m m MPa / m 1800 m 5 6 Tadokoro et al

3 m A B C Fig. 1 Distribution of ultra-microearthquakes for the water injection experiment at the Nojima fault in 2000, northern part of Awaji island in southwest Japan after Nagai, Star: injection point 1800-m-deep well. A C : location of three major clusters of induced seismicity Double Difference Waldhauser and Ellsworth km MPa km M MPa m 800 m m/s 2001 Ohtake 1974

4 m MPa Fig. 2 Space-time diagram for events shown in Fig. 1. Two horizontal bars show the period of 4.0 and 4.5 MPa injections m/s a m m 2 4 km km 4.0 MPa MPa KTB 1 Zoback and Harjes km 5 km

5 3 a 2 km 2 km m 2 /s b 2 km 0 8 km 2.0 m 2 /s Fig. 3 a Temporal change of relative pore water pressure at the hypocenter of induced events, calculated for three hydraulic diffusivities. b Variation of relative pore water pressure against depth at a horizontal distance of 2 km from the injection point, calculated for four lapse-times measured from the injection.

6 4 a 4.0 MPa b m Fig. 4 Examples of vertical-component seismograms observed in the 800-mdeep borehole at the Nojima fault. a Induced events that occurred in February 1 6, 2000, after the 4.0 MPa injection. b Stationary ultramicroearthquakes that occurred in July 4 30, b

7 KTB Table 1 Comparison of several parameters characterizing water injection and induced seismicity for the experiments at the Nojima fault in 1997 and 2000, Matsushiro and KTB. Pressures, flow rates, and total flows of the injection were measured at the well-head. Time lags and distances indicate time and space distances between the injection point and the induced seismicity, respectively. Permeability estimates and diffusivities used in modeling the 2-D diffusion process are also shown. Nojima Matsushiro KTB Water injetion Year of experiment Depth m Pressure MPa Flow rate l/min Total flow m 3 Feb. Mar Jan. Mar Jan. Feb max 550 max 200 Induced seismicity Time lag days Distance km Maximum magnitude Clustering observed observed observed Permeability m 2 Dffusivity m 2 /s III KTB 1 Ohatke 1974 KTB KTB 9100 m 100 m 2 Zoback and Harjes Ohatke 1974 Rangely Raleigh et al KTB

8 Prioul et al Rangely Renqiu Genmo et al Renqiu Genmo et al m 1800 m 100 HZ 10 khz Mo fc 100 HZ Mo fc 4 10 khz Mo fc 3 Hiramatsu et al IV KTB km : Genmo, Z., Huaran, C., Shuqin, M. and Deyuan, D : Research on earthquakes induced by water injection in China. PAGEOPH, 145, Guha, S.K : Induced Earthquakes. Kluwer Academic Publishers. Healy, J.H., Rubey, W.W., Griggs, D.T. and Raleigh, C.B : The Denver earthquakes. Science, 161, No. 3848, Hiramatsu, Y., Yamanaka, H., Tadokoro, K., Nishigami, K. and Ohmi, S : Scaling law between corner frequency and seismic moment of microearthquakes: Is the breakdown of the cube law a nature of earthquakes? Geophys. Res. Lett., in press : : 800 m :

9 : : : : P013 Ohtake, M : Seismic activity induced by water injection at Matsushiro, Japan. J. Phys. Earth, 22, Prioul, R., Cornet, F.H., Dorbath, C., Dorbath, L., Ogena, M. and Ramos, E : An induced seismicity experiment across a creeping segment of the Philippine Fault. J. Geophys. Res., 105, Raleigh, C.B., Healy, J.H. and Bredehoeft, J.D : An experiment in earthquake control at Rangely, Colorado. Science, 191, : Tadokoro, K., Ando, M. and Nishigami, K : Induced earthquakes accompanying the water injection experiment at the Nojima fault zone, Japan: Seismicity and its migration. J. Geophys. Res., 105, : Waldhauser, F. and Ellsworth, W.L : A double-difference earthquake location algorithm: Method and application to the northern Hayward fault, California. Bull. Seismol. Soc. Amer., 90, : Zoback, M.D. and Harjes, H.P : Injectioninduced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany. J. Geophys. Res., 102,

Keiichi Tadokoro* Research Center for Seismology and Volcanology, Nagoya University. Abstract

Keiichi Tadokoro* Research Center for Seismology and Volcanology, Nagoya University. Abstract Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 12,**- pp. 01 1. Structure and Physical Properties of Fracture Zone Derived From Seismic Observations at the Nojima Fault and the Western Tottori Earthquake Fault,