A Broad Spectrum of Fault Behaviors in Fast and Slow Earthquakes

Author(s): Li, Bo | Advisor(s): Ghosh, Abhijit | Abstract: Faults show complex slip behaviors at different sections depending mainly on their stress and friction distributions. In the seismogenic zone, a fast earthquake happens when the frictional resistance to fault movement reduces faster than the decrease in elastic stress due to fault slip, and it releases seismic energy that causes ground shaking. Increases in depth, temperature and pressure change the frictional properties from velocity weakening to strengthening. This deeper section is referred to as the creeping zone and shows stable sliding without any stress drop. In between, there is a transition zone where slow slip can occur on asperities embedded in the creeping region [Bartlow et al, 2011; Obara et al., 2011; Ghosh et al., 2012]. The slip cannot reach high enough velocities to produce regular earthquakes, but sometimes it is still able to radiate low amplitude and low frequency seismic waves [Peng a Gomberg, 2010]. Seismicity in the seismogenic zone can trigger slow slip in the transition zone. It can also change the stress and accelerate or decelerate the seismicity on adjacent faults or even on faults hundreds to thousands kilometers away when the earthquake is large enough. Conversely, slow slip in the transition zone can also change the surrounding stress field and increase the stress in the up-dip seismogenic zone, potentially advancing the timing of earthquake failure. In this dissertation, I study the broad spectrum of fault behaviors and explore the potential relationships between them. In Chapter 1, we give an introduction to fast and slow earthquakes and briefly introduce the main methods used to study them. In Chapter 2, I apply the back-projection method to two case studies: the 2015 Mw 8.3 Illapel earthquake, using one array with both low- and high-frequency bands imaging multiple rupture patches, and the 2015 Mw 7.8 Gorkha earthquake using multiple global arrays to image the rupture process and detect aftershocks. The back-projection results of the Gorkha earthquake imaged by different global arrays show similar rupture processes but vary in detail. One array shows continuous eastward rupture for ~60s while other arrays show a branching rupture to the northeast at ~45s. In addition, we combine multiple global arrays to improve the resolution of the back-projection method. The higher resolution also allows us to detect 2.6 times the number of aftershocks than that recorded in the global catalog. In Chapter 3, we apply the multi beam back-projection method (MBBP) to study the slow earthquakes of the Unalaska-Akutan region in the Alaska-Aleutian subduction zone. We detect near-continuous tremor and low frequency earthquakes for nearly two years. The slow earthquakes are distributed heterogeneously in three clusters and are located deeper than those in other subduction zones. The tremors show both short and long-term migrations along strike and dip directions with a wide range of velocities. In addition, tremors and LFEs show strong spatio-temporal correlations. They are located in the same patches, and when there are LFEs bursts during tremor signals. We also observe some cases where local or regional earthquakes can terminate or amplify slow earthquake activity. In Chapter 4, we use the move-max matched filter method to detect small local earthquakes along the San Jacinto fault (SJF) zone that are triggered by the 2014 Mw 7.2 Papanoa earthquake. Using the catalog events as templates, the move-max matched filter method detects 5.4 times the number of earthquakes recorded in the ANSS and SCSN catalog, while the matched-filter method only distinguishes 3.2 times the number of catalog events using the same detection threshold. After relocation using hypoDD, we find a new normal fault with strike almost perpendicular to the SJF. More than one mechanism may be responsible for triggering earthquakes. The transient dynamic stresses may have triggered slow slip or fault creep, and lead to the increased and protracted seismicity along the San Jacinto Fault (SJF). In addition, the time-dependent acceleration to failure process initiated by the dynamic stress change can result in the enhanced seismicity on the new fault.