Fault “Corrosion” by Fluid Injection: A Potential Cause of the November 2017 5.5 Korean Earthquake

Figure 7

Mohr circle representations of the state of stress estimated on the Namsong Fault. The quantities , , , , and denote the effective stresses , , , , and , where is the fluid pressure at the point for which the analysis is undertaken, , and . Inclined line connecting to the Mohr circle indicates the orientation—in Mohr circle space—of the pole of the fault. Angles between the maximum principal stress and the normal to the fault plane are labelled (cf. [100, 101]). Failure envelopes are drawn for different values of the coefficient of friction of the Namsong Fault, . (a) Stress state represented by a lithostatic of 110 MPa, with and , the latter oriented at an azimuth of 111° to provide consistency with the focal mechanism in Figure 5(c). is set to 132 MPa, the peak value during the initial stimulation of well PX-2 in February 2016 [15]. This diagram indicates that unless the resulting conditions would have caused this fault to slip. Since such a high coefficient of friction is unlikely, it is concluded that this high fluid pressure was not transmitted to the Namsong Fault. (b) For the same depth and stress state as (a) except is set to the hydrostatic value of 42 MPa, to represent the conditions after the injection and flowback experiments in August-September 2017 and before the November 2017 earthquake. Under these conditions ~0.29 would place the Namsong Fault at the threshold of stability. Also shown for comparison (dashed) is a failure envelope for , a representative value for a fault in granite that is unaffected by alteration to authigenic minerals (e.g., [64]). (c) Preferred stress state from [5] for 4200 m depth, represented by a lithostatic of 106 MPa, with and , the latter oriented at an azimuth of 083°. Under these conditions, would place the Namsong Fault at the threshold of stability for slip in the oblique sense indicated in the focal mechanism in Figure 5(d). A similar plot to this is provided in [5] (their Figure O-11), in which principal stresses of the same magnitudes are oriented slightly differently, with at an azimuth of 077° rather than 083°. With this adjustment, the focal mechanism in Figure 5(d) is consistent with ~97 MPa and ~54 MPa; this stress state plots near the threshold for slip for (square symbol in the figure), as was noted in [5].

(a)

(b)

(c)