Fault instability on a finite and planar fault related to early phase of nucleation

We numerically investigate the early phase of nucleation on a planar fault with the rate- and state-dependent friction law, loaded externally by steady slip, to clarify its relation to fault instability. We define R[n] as the invasion distance of the inward creep to characterize that phase. For a circular fault, the dependence of R[n] on the dimensionless parameters l[b], l[b−a], and l[RA] (all of these are proportional to the rigidity and the characteristic distance of the state evolution L and inversely proportional to the normal stress and the fault radius) can be compiled. We found that Rn is proportional to l[b] (both aging law and slip law of the state evolution) and l[b−a] (aging law). In the case of the aging law only, there are two regimes (ordinary events and slow events) separated by the value of l[RA]. The regimes have different trend lines, although we could not measure Rn for the case of l[RA] < 0.35 because of breaking of the mirror symmetry of instability along the loading direction. R[n] in the slow event regime is smaller. Moreover, we investigated the effect of fault shape and found that a model with a long radius along the mode 2 direction has similar parameter dependence to circular faults, but a model with a long radius along the mode 3 direction has different ones. Our results imply that we can qualitatively estimate the fault instability parameters from the early phase of nucleation, although further research is necessary to enable application to actual faults

Coseismic thermal pressurization can notably prolong earthquake recurrence intervals on weak rate and state friction faults: Numerical experiments using different constitutive equations

We add a new perspective to component factors of earthquake cyclicity, namely coseismic thermal pressurization (TP) within fluid‐saturated fault zones, which is pore fluid pressurization caused by frictional heating. By using a single degree of freedom system with a rate‐ and state‐dependent friction law, we show that the short‐lived TP can prolong earthquake recurrence intervals. This lengthening effect can operate even without any notable shear heating in weak faults. Moreover, if the maximum increase in temperature is above a certain level, the permeability rather than the maximum temperature becomes important for the lengthening effect. Lower permeability causes longer recurrence intervals. By contrast, narrower slip zones (more pronounced heating) do not simply prolong recurrence intervals, although they entail higher dynamic undershoot and energy radiation. These features do not depend on whether the assumed evolution law is the Ruina law or the Dieterich law. However, our results indicate that if the degree of TP changes for each earthquake, the ideal time‐predictable model for earthquake cycles can be applicable only in the case of faults obeying the Ruina law. Furthermore, on the basis of the above‐mentioned dependence of the interval on the permeability, we point out that it is necessary to measure the permeability rather than the slip zone thickness (or the increase in temperature) in order to estimate the TP effect on long‐term earthquake cycles. Although it is currently difficult to measure the permeability under ground, measurements should be performed in the light of the importance of permeability in the prediction of future seismic hazards