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Explaining Slow Earthquake Phenomena with a Frictional-Viscous Faulting Model

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It is well-known that the first-order kinematic characteristics of typical earthquakes, such as slip rate, rupture propagation speed, and moment duration scaling, can be well-explained by a model where the fault experiences a sudden frictional strength drop. In recent decades, a new type of earthquake has been discovered, which has a slower slip rate than a typical earthquake. These earthquakes are now often referred to as slow earthquakes, and those typical earthquakes are referred to as fast earthquakes. The aforementioned sudden strength drop model derived from fast earthquake observations cannot explain the first-order characteristics of slow earthquakes.

In this dissertation, I consider a frictional-viscous fault zone model to explain the puzzling slow earthquake phenomena, with a particular focus on slow slip events (SSEs), which is a type of slow earthquake that is well characterized. The frictional-viscous model is inspired by the recent geological observations that imply the occurrence of SSEs in fault zones with a finite thickness of ~100s of meters. The bulk matrix of the fault zone deforms viscously, while pervasive frictional surfaces are distributed in the viscous matrix. To simultaneously consider both the 10s-kilometer-scale rupture propagation and the 100s-meter-scale fault zone features in the same model, I treat a fault zone as a zero-thickness “surface” embedded in an elastic medium. The “frictional-viscous” characteristics are parameterized into a constitutive relation where fault strength is partitioned into a frictional and a viscous component in parallel. Two key parameters in the frictional-viscous model are the viscous coefficient η_v and the event stress drop. The present frictional-viscous model can simultaneously explain various kinematic source parameters for SSEs when the viscous coefficient η_v is about 10^4 - 10^5 μ/2β, and the average stress drop in a slip transient is about 10 kPa. Qualitatively, this frictional-viscous model can also explain the shorter inter-event interval and lower average stress drop observed in subduction zone SSEs, compared to what is observed in the fast earthquakes at seismogenic depth. These results imply that the frictional-viscous model is a promising representation of the actual SSE source processes. The present model provides many hypotheses, which can be further tested with future geophysical, geological, and experimental data.

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