[1] To investigate the mechanism of recently observed silent slip events, we simulated earthquake preparation processes using the Dieterich/Ruina rate-and statedependent friction law. To ensure realistic modeling of the unstable-stable transition, we considered small cut-off velocity to an evolution effect in the friction law for the transition zone. When the cut-off velocity to the evolution effect is significantly smaller than that of a direct effect, steady state friction behaves as velocity weakening at low slip velocity and velocity strengthening at high slip velocity. This frictional behavior was experimentally and theoretically confirmed for the unstable-stable transition zone. The results of our numerical simulations show that silent slips of which velocity is higher than the velocity of relative plate motion, eventually propagates horizontally along the unstable-stable transition over a period of several years. Silent slip events can be interpreted as being caused by the transitional behavior of the fault constitutive law.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7260 Seismology: Theory and modeling; 1299 Geodesy and Gravity: General or miscellaneous; 8159 Tectonophysics: Rheology-crust and lithosphere. Citation: Shibazaki, B., and Y. Iio, On the physical mechanism of silent slip events along the deeper part of the seismogenic zone, Geophys. Res.
It was found that the initial rise of the far‐field P‐wave velocity pulse generated by microearthquakes does not act as a ramp but gradually increases according to the function tn (2
It is found that the initial rise of far-field P wave velocity pulses generated by microearthquakes is not well represented by a ramp function but exhibits a gradual increase according to the function t n (2 • n < 4), where t is the time measured from the onset of the P wave. This slow rise, termed the slow initial phase, is detected in all 69 earthquakes analyzed here. Their seismic moments range between 108 and 1013 N m. No ramplike onsets are observed, suggesting that the slow rise is not an anomalous feature but is always generated by earthquakes of this size. The slow initial phase is not due to the transient response of the recording system. Analyses and simulations suggest that the slow initial phase is also not likely to result from an inhomogenuous velocity structure but that it may be partly due to anelastic attenuation. The slow initial phase likely results from the source process of the microearthquakes, especially those instances of the phase with a longer duration. The slow initial phase can not be explained by theoretical source models which assume constant dynamic friction and rupture velocity in an expanding fault. This is because these models predict a ramplike behavior in the initial rise of the far-field P wave velocity pulse. Only the observed ramplike waveform following the slow initial phase can be explained by these models. The slow initial phase can be explained by models which predict slow slip velocities and/or rupture velocities immediately after rupture initiation, such as the slipweakening crack model. The duration of the slow initial phase is proportional to the rise time of the P wave velocity pulse. This dependence implies that longer, slow initial phases are generated by larger earthquakes, the duration of the slow initial phase being a measure of the earthquake size. realistic of the cohesive zone models which were introduced by Ida [1972] and Palmer and Rice [1973] for a shear crack.It is widely accepted that the earthquake source can be modeled as a shear crack. In dealing with linear elastic fracture mechanics, however, a stress singularity occurs at the crack tip because of a sudden stress drop at the crack surface. This singularity can be removed by the concept of a cohesive zone, where a cohesive stress acts between the crack surfaces. This concept was first introduced by Barenblatt [1959] for a tensile crack. The simple cohesive zone model, in which the cohesive stress decreases with ongoing slip, is commonly known as the slip-weakening model.The slip-weakening model has been used to gain knowledge of the various phenomena related to the earthquake rupture process. The arrest of the rupture has been quantitatively described under the assumption of a cohesive zone at the crack tip [Aki, 1979]. The strong motion source parameters (i.e., the maximum slip acceleration and maxi-Paper number 95JB01150. 0148-0227/95/95JB-01150505.00 mum slip velocity) have been related to the critical displacement (i.e., the displacement necessary for the shear stress to decrease to a constant v...
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