Insights into earthquake rupture and recovery from paleoseismic faults
Christie Rowe, McGill University
Lots of Students and Collaborators , McGill and elsewhere
Ancient faults preserve evidence of past earthquake rupture, aseismic creep, and interseismic healing, but our ability to read the record is incomplete. There are two key differences between earthquake slip and creep that have the potential to be preserved in rocks. First: the slip velocity is sufficiently high that the frictional heat production on the slip surface is faster than the conductive heat dissipation, resulting in a net temperature rise. If the slip is sufficiently localized and the normal stress is high enough, this temperature rise can dissociate hydrous minerals, cause rapid maturation of organic compounds, and melt fault rock, producing pseudotachylytes. These reactions are recorded in fault rock mineralogy and composition and can be used to estimate coseismic temperatures from <250°C up to > 1400°C. Examples will be shown from an ancient subduction thrust fault in Alaska (the Pasagshak Point thrust) and from the plate boundary in the Japan Trench which ruptured in the 2011 Tohoku Earthquake.
A second difference between seismic and aseismic slip on faults is that seismic slip is *dynamic*, that is, that the slipping area expands in size at rates comparable to the shear wave velocity in the rocks (~ 3 km/s), which results in extreme stress gradients in the wall rock at the rupture tip. The stressing rate exceeds the speed at which fractures can propagate through the wall rock, resulting in distinctive patterns of very closely spaced and branching fractures, and sometimes pulverization. In some faults, these fractures are the dominant form of off-fault damage and may cause permeability spikes through the fresh fracture networks. Examples will be shown from the Pofadder Shear Zone (South Africa).
With these fossil earthquake signatures in hand, we can identify ancient seismic rupture planes and use these to map out the geometry of earthquake ruptures at the outcrop scale (10-3 – 103 meters), which is below the resolution and location uncertainty of earthquake seismology in most active faults. Using an example from the Norumbega Shear Zone in Maine, I will show that earthquakes can rupture multiple parallel and non-parallel surfaces simultaneously. This discovery is consistent with recent deconvolutions of multiple rupture planes from earthquakes with a large non-double couple component in their focal mechanisms, suggesting that this may be a common phenomenon. Outcrop studies may be able to elucidate the consequences for slip distribution and help explain spatial variations in fracture energy and stress drop which are barely resolvable in seismic data.