Dept of Earth & Environmental Sciences
Centre for Environmental and Information Technology (EIT)
200 University Ave. W
Waterloo, Ontario, Canada N2L 3G1
Phone: (519) 888-4567
How earthquakes are preserved in the rock record, and what the rock record can teach us about earthquakes
Dr. Christine Rowe, GAC WW. Hutchison Medalist, Earth and Planetary Sciences Department, McGill University
Ancient faults preserve evidence of past earthquake cycles, including rupture, aseismic creep, and interseismic healing, but our ability to read that record is incomplete. There are two key features that distinguish earthquake slip from other fault motion that have the potential to be preserved in rocks. First: the slip velocity is high enough that that frictional heat can be produced faster than heat can diffuse away, resulting in temperature rise. This temperature rise can cause clay dehydration, rapid maturation of organic compounds, and even melt rock to produce pseudotachylyte. By recognizing and analyzing these and other seismic rocks, we can estimate coseismic peak temperatures between ~ 250° up to >1400°C. Second: seismic slip is dynamic, that is, the slipping area grows quickly (~ 3 km/s) and its growth is driven by inertia from the rapid slip. This dynamic propagation results in extreme stress gradients in the wall rock, which forms distinctive patterns of closely spaced fractures and sometimes wall rock pulverization.
Once we understand the range and variety of earthquake ‘fingerprints’ we can find in the rock record, we can use these to understand the mechanics of earthquakes. I will show an example of a research project executed by an undergrad-grad advanced structure class studying pseudotachylytes in Norumbega Shear Zone in Maine. The class made extremely detailed maps of pseudotachylyte networks and developed criteria to determine whether pseudotachylyte veins formed singly during individual earthquakes or formed at the same time representing networks of faults that slipped simultaneously. Spoiler alert: We found networks of faults for each (paleo-) earthquake. Outcrop studies like ours help to understand how slip is distributed across connected networks of faults, and the geometry of networks that are likely to be triggered in simultaneous slip. These observations might be scaled up to elucidate the behaviors of plate boundary fault networks.
200 University Avenue West
Waterloo, ON N2L 3G1