The physics of slow slip, tremor, and associated seismicity from geodetic and laboratory studies [electronic resource]

Slow slip events, sometimes also called slow earthquakes, have been the subject of intense study since they were discovered in the late 1990's. Slow slip events (SSEs) occur when a fault within the earth slips, as in an earthquake, but more slowly than in an earthquake. SSEs take anywhere from days to years to release the same energy as a Mw 5 - 7.5 earthquake would release in seconds. Because of their slow nature, SSEs do not excite seismic waves in the earth like regular earthquakes do, and thus do not cause the dangerous ground shaking or other hazards (i.e. liquifaction, tsunamis) associated with earthquakes. However SSEs still perturb the static stress state within the earth in the same way that earthquakes do, and can trigger regular earthquakes which may be damaging. SSEs are found in areas that are transitional between largely locked fault regions and freely creeping regions. Typically these SSE regions occur on the deeper extent of faults, below the depth of most earthquakes, however some SSEs occur on shallow transitional regions shallower than most earthquakes. SSEs present a unique window into the physics of these transitional fault regions, and thus present an opportunity to re ne our understanding of fault mechanics. SSEs are sometimes accompanied by an emergent seismic signal called tectonic tremor. Tectonic tremor is thought to be composed of individual very small, lowfrequency earthquakes which represent faster slip on small patches either within or adjacent to the active SSE region. The precise relationship between slip and tremor is not well understood, but has implications for the physics of deep fault regions, the hazards associated with SSE triggered earthquakes, and monitoring of SSEs. Tectonic tremor is studied by multiple researchers from a seismological point of view, using seismic instruments. When present, the total energy released in as tectonic tremor is always orders of magnitude below the total energy released by slip as measured by geodetic instruments, indicating that most of the slip is occurring aseismically. Additionally, some SSEs have no tremor associated with them. Therefore, SSEs are most suited to study using geodesy, as well as laboratory and modeling studies. In this thesis, I present a number of studies of GPS data from SSEs in the Cascadia subduction zone in the United States, the Hikurangi subduction zone on North Island of New Zealand, and the Boso peninsula area of Japan. I use a time-depenent inversion method to study the time-dependent properties of the SSEs, such as acceleration and migration of slip. I then use these models to study in detail the relationships between SSEs, seismicity, and tremor in these regions. I also present one laboratory study in which I create stress conditions similar to those thought to exist in SSE regions in subduction zones, including pressurized pore fluid. I extrapolate the results of this study to explain why SSE associated tremor is often modulated by solid earth tides, while regular earthquakes are not.

Keywords: Geophysics.
Advisor: Segall Paul
Beroza Gregory C. (Gregory Christian) advisor. Thesis advisor
Dunham Eric advisor. Thesis advisor