For Scientists
I am interested in understanding how the lithosphere deforms in response to subduction zone plate interface processes (i.e., the megathrust earthquake cycle and episodic tremor and slip). I use multiple techniques and a range of data types to study these active tectonic processes:
(1) I utilize geodetic data to evaluate how different tectonic drivers produce current upper-plate deformation (shortening, uplift etc.) (2) I study active faulting using field and laboratory techniques, such as luminescence dating of marine terraces. (3) I use numerical modeling (incl. coulomb failure stress and finite element) to investigate the effects that scenario earthquakes and the loading/release subduction earthquake cycle have on upper-plate displacements and the stress loading of upper-plate faults. |
For Non-Scientists
Subduction zones (when one tectonic plate subducts beneath another) are host to some of the world's most damaging natural hazards, including earthquakes and tsunamis (e.g. the 2011 magnitude 9 earthquake and tsunami in Japan). I am passionate about understanding the tectonic processes that occur within these plate boundary settings to better constrain the hazard subduction zones pose.
We cannot use a hands-on approach to study subduction zones dynamics, however, observations of Earth surface deformation (above subduction zones) can be used to study the processes taking place at depth. I use field and satellite observations combined with numerical models to further our understanding of how subduction zones produce the surface deformation that can impact society. |
The Interplay of Cascadia Subduction Earthquake Processes with NNW-directed shortening processes
The southern Cascadia forearc deforms in response to superimposed deformation produced by the Cascadia subduction earthquake cycle, the northward motion of the Sierra Nevada-Great Valley crustal block, and a northward-migrating wave of crustal shortening and thickening associated with processes at the Mendocino Triple Junction. These northward-migrating processes began to deform present-day southern Cascadia by ca. 5 - 6 Ma, prior to which, upper-plate deformation was primarily controlled by subduction earthquake cycle processes. As the MTJ continues moving northward, the upper-plate of the Cascadia subduction zone subsequently becomes the location in which San Andreas plate boundary faults nucleate.
We separate the deformational signals associated with these competing tectonic processes recorded by the Cascadia GPS displacement field. We find that there are high rates of NNW-directed crustal shortening in a corridor between the Klamath Mountains and the subduction trench, east of which the Klamath Mountains are moving relatively rigidly northward ahead of the Sierra Nevada Great Valley block. We can assess whether short-term deformation/motions recorded by this GPS data reflects long-term deformation recorded geologically in this region, for example to explore patterns of long-term uplift and variations in fault motion through time.
McKenzie et al. (2021). Isolating non-subduction-driven tectonic processes in Cascadia, Geoscience Letts., 8
McKenzie et al. (2022). Mid-Miocene to Present Upper-Plate Deformation of the Southern Cascadia Forearc: Effects of the Superposition of Subduction and Transform tectonics, Front. Earth Sci., 10
We separate the deformational signals associated with these competing tectonic processes recorded by the Cascadia GPS displacement field. We find that there are high rates of NNW-directed crustal shortening in a corridor between the Klamath Mountains and the subduction trench, east of which the Klamath Mountains are moving relatively rigidly northward ahead of the Sierra Nevada Great Valley block. We can assess whether short-term deformation/motions recorded by this GPS data reflects long-term deformation recorded geologically in this region, for example to explore patterns of long-term uplift and variations in fault motion through time.
McKenzie et al. (2021). Isolating non-subduction-driven tectonic processes in Cascadia, Geoscience Letts., 8
McKenzie et al. (2022). Mid-Miocene to Present Upper-Plate Deformation of the Southern Cascadia Forearc: Effects of the Superposition of Subduction and Transform tectonics, Front. Earth Sci., 10
Driving Permanent Deformation at Subduction Zones
The upper plates of subduction zones are commonly composed of two distinct regions: (1) a relatively weak accretionary margin material near to the trench and (2) a relatively strong upper-plate terrane inboard of the accretionary margin. We have found that when subduction zone plate locking ends prior to this weak-to-strong upper-plate boundary, a relatively wide region of high elastic shortening strain is generated between the down-dip limit of locking and the rigid backstop. The position of locking relative to this upper-plate strength contrast influences apparent subduction coupling patterns, and provides a possible mechanism for driving permanent shortening and exhumation of subduction terranes.
McKenzie et al. (2022). Regional and Local Patterns of Upper-Plate Deformation in Cascadia: The Importance of the Down-Dip Extent of Locking Relative to Upper-Plate Strength Contrasts, Tectonics, 41(1)
The upper plates of subduction zones are commonly composed of two distinct regions: (1) a relatively weak accretionary margin material near to the trench and (2) a relatively strong upper-plate terrane inboard of the accretionary margin. We have found that when subduction zone plate locking ends prior to this weak-to-strong upper-plate boundary, a relatively wide region of high elastic shortening strain is generated between the down-dip limit of locking and the rigid backstop. The position of locking relative to this upper-plate strength contrast influences apparent subduction coupling patterns, and provides a possible mechanism for driving permanent shortening and exhumation of subduction terranes.
McKenzie et al. (2022). Regional and Local Patterns of Upper-Plate Deformation in Cascadia: The Importance of the Down-Dip Extent of Locking Relative to Upper-Plate Strength Contrasts, Tectonics, 41(1)
Quantifying upper-plate fault slip rates
The elevations, distributions and geometries of marine terraces can record millennial-scale displacements along upper-plate faults and are a key tool for the identification of upper-plate Quaternary faults, such as the Yaquina Bay fault in central Oregon. We have combined high-resolution topographic data with luminescence ages of marine terrace sands at Yaquina Bay to estimate vertical slip rates along the Yaquina Bay fault. Our results show that MIS 5e, 5c and 5a marine terraces are currently at higher elevations north of Yaquina Bay relative to their counterparts south of Yaquina Bay, inferring north-side-up vertical motion along the Yaquina Bay fault. Additionally the elevations of marine terraces north of Yaquina Bay require variable uplift over the last 125 kyrs, with a ~20 kyr period of relatively rapid uplift at 1.8 m/kyr ± 0.3 m/kyr from ~100-80 ka. Our results imply that that the Yaquina Bay fault has been active with time-varying slip since the late Pleistocene.
This work was funded by an AGeS2 Geochronology research grant McKenzie, K.A., Kelsey, H.M., Kirby, E., Rittenour, T.M., Furlong, K.P (2022). Differential coastal uplift quantified by luminescence dating of marine terraces, central Cascadia forearc, Oregon. Quaternary Science Reviews, v298(107853) |
Using Slow Slip Earthquakes (SSEs) to Determine Plate Interface Dynamics at Depth (related press)
Slow slip events (SSEs) can help us understand how the plate interface behaves down-dip of the primary seismogenic (locked) zone. In this work I use motions of the upper-plate (recorded at the surface by GPS) produced by SSEs at depth to explore the plate-interface kinematics along the Cascadia margin. In order to produce the observed surface displacement field recorded by GPS, our results indicate that slip on the plate interface during SSEs must occur in the up-dip direction (and not parallel to plate motion at shallow levels - which is oblique to the Cascadia trench). This result has implications for how the seismogenic zone is loaded between megathrust earthquakes - there is a change in the motion of the down-going plate from being directed parallel to plate motions at shallow depths to being in the down-dip direction at the depth of SSEs (~30 - 50 km in Cascadia).
McKenzie et al. (2020). Bidirectional Loading of the Subduction Interface: Evidence from the Kinematics of Slow Slip Events, G3, 21(9)
McKenzie et al. (2020). Bidirectional Loading of the Subduction Interface: Evidence from the Kinematics of Slow Slip Events, G3, 21(9)