2018 Theses Doctoral

The seismogenic potential of subducting sediments

Rabinowitz, Hannah

This thesis examines the seismic behavior of sediments in shallow subduction zones. In the traditional view of the seismogenic zone, the upper stability limit is controlled by a transition to velocity-strengthening (frictionally stable) clay-rich sediments at shallow depths in the accretionary prism. However, recent observations have emphasized that these shallow sediments can host a wide range of seismic behaviors. On one end of the seismic spectrum, the March 2011 Mw9.1 Tohoku-oki earthquake demonstrated that peak slip in a megathrust rupture can be hosted at the shallowest depths. At the other end of the spectrum, observations at the Hikurangi trench off the North Island of New Zealand have revealed that spontaneous, periodic slow slip events (SSEs) can nucleate in the shallowest portions of a subduction zone.
The Japan Fast Trench Drilling Project (JFAST, IODP Expedition 343) drilled through the plate boundary faults in the Japan Trench to investigate the structure that hosted the Tohoku-oki earthquake. In Chapter 2, I use a trace element-based stratigraphy to identify several large displacement faults within the bottom ~15 m of the JFAST core. This work highlights that there are multiple candidate structures that could host a megathrust rupture and that not all displacement is accommodated along a weak pelagic clay layer recovered in the JFAST core. However, this method is incapable of determining which of these faults experienced significant seismic slip. In Chapter 3, I develop a novel paleoseismic indicator appropriate for faults hosted in seafloor sediments. This tool takes advantage of the fact that organic material (molecular biomarkers) in sediments degrades as a function of time and temperature. In this study, I determine the kinetics of thermal maturation for alkenones (coccolithophore-derived molecules) and n-alkanes (plant leaf wax-derived molecules) found in western Pacific sediments. In Chapter 4, I apply these kinetics to measured biomarker anomalies in JFAST samples to determine which faults recovered in the JFAST core could have hosted a megathrust event such as the Tohoku-oki earthquake. This approach reveals that multiple faults in the plate boundary region have likely hosted megathrust events and that the occurrence of seismic slip is not confined to a particular lithology. This implies that small differences in frictional behavior in subducting sedimentary lithologies are not the primary control on the occurrence of shallow seismic slip.
In Chapter 5, I turn to a different type of shallow seismic behavior and focus on SSEs in the shallowest portion of the Hikurangi trench. In this study, I measure friction and velocity-dependence of the input sediments for this subduction zone at a range of pressures and temperatures relevant to the shallow portion of the slab where SSEs have been observed. These experiments demonstrate that the sediment here becomes frictionally weak at effective stresses expected deeper than ~2 km. At the same effective stresses, the sediment becomes less velocity strengthening, and under some conditions exhibits velocity neutral behavior. A plate-rate experiment exhibits velocity-weakening behavior and two spontaneous SSEs, indicating that at slow velocities, the sediment subducting at the Hikurangi trench is capable of unstable frictional behavior required to promote shallow SSEs. These results demonstrate that subducting sediments can exhibit a variety of frictional properties that can support unstable behavior in the shallowest reaches of the subduction zone.