Earthquakes can have profound societal impact, and understanding their cause is important for estimates of seismic risk. Earthquakes are also how continual plate motion is accommodated at plate boundaries, and thus understanding them is integral to a complete model of plate tectonics. Large earthquakes (not considering earthquakes deeper that about 30–60 km below the surface) occur on pre-existing faults in the Earth’s crust. During the interseismic period (i.e., the time period between earthquakes), those faults are locked, or do not slip, as energy slowly accumulates as the crust on either side of the faults deforms due to tectonic forces. Eventually as the energy builds, a fault can fail in an earthquake. We refer to the process of slowing building up energy and then sudden release of that energy as the earthquake cycle, and it continues to repeat itself, albeit in a highly complicated and non-deterministic manner. There are also regions of faults that do not tend to fail in earthquakes, but rather release the accumulated energy slowly, either continually or in a pulse-like behavior. Our work on the earthquake cycle spans several time and space scales, and is both data- and theory-driven.
Geodetic data, including GPS and InSAR, provide measurements of static ground deformation during an earthquake (i.e., the permanent deformation of the ground surface), while seismological data are records of kinematic ground deformation (i.e., the time-dependent shaking surface). The static ground deformation is caused by the cumulative slip on buried faults, and the ground shaking is due to energy radiated from the fault as it is actively slipping. The manner in which a buried fault slips during an earthquake is an unknown, and we use both geodetic and seismological to determine an image of how the buried fault slipped. We primarily use geodetic data, and thus determine images of how the the total cumulative fault offset is distributed across the fault, and where we use seismological, we also determine a model of the time-dependent pattern of slip. Imaging the slip in an earthquake is important for several issues, including determining which regions of the fault either released or did not release accumulated energy, thereby providing important constraints on regions of the fault in which an earthquake in the near future is possible. That a region of fault did not slip in a given earthquake does not necessarily imply that that region will fail in a future earthquake, as we know that some regions of faults tend to creep aseismically (i.e., release the accumulated energy slowly without causing radiated seismic energy which leads to ground shaking). Continued monitoring of postseismic deformation (see below) can help to resolve whether regions of fault slip deficit in the later earthquake are potentially seismogenic or not.
Earthquakes we have worked on, or are actively working on, include:
- 2008 M7.9 Wenchuan earthquake (aka, the Sichuan earthquake) in China.
- 2010 M7.2 El Mayor–Cucapah earthquake (aka, the Baja California earthquake).
- 2010 M7.1 Darfield, New Zealand earthquake
- 2011 M9.0 Tohoku-oki earthquake in Japan.
- 2011 M6.3 Christchurch, New Zealand earthquake
- 2013 M7.0 Lushan earthquake in Sichuan province in China.
- 2015 M7.8 Gorkha earthquake and M7.3 Kodari earthquake in Nepal.
- 2015 M8.3 Illapel earthquake on the coast of Chile.
- 2015 M6.4 Pishan, China earthquake
- 2016 M7.8 Ecuador earthquake.
- 2016 M6.4 Yujing, Taiwan earthquake.
- 2016 M6.5 Nura, China earthquake
- 2016 M7.8 Kaikoura, New Zealand earthquake
There are increased rates of surface deformation over the years to decades following large earthquakes, relative to the rates prior to the earthquake. This accelerated deformation is due to continued aseismic deformation in the crust and uppermost mantle due to the re-adjustment of the coseismic stress perturbations. Graduate student Trever Hines has developed a method to directly invert GPS measurements of postseismic deformation for both viscosities of the lithosphere and images of fault creep, which we refer to as GeoTomo. We have applied GeoTomo to the postseismic deformation following the 2010 El Mayor – Cucapah (aka, the Baja California earthquake). We also worked, or are working on, postseismic deformation from the central Nevada seismic belt earthquakes, the 1997 Manyi earthquake, and the 1999 Izmit and Duzce, Turkey, earthquake sequence.
interseismic deformation & earthquake cycles
We use mathematical and computational techniques to understand the physics of large earthquakes, with a focus on developing an understanding of the forces that cause them. Our main focus currently is on 2008 Wenchuan, China earthquake (often referred to as the Sichuan earthquake in the news), the 2011 Tohoku-oki earthquake (also referred to as the Japan or Honshu earthquake). We also have, or are initiating, projects on the 2010 Haiti, 1999 Izmit and Duzce, Turkey, and the 2011 Van, Turkey earthquakes. Our work relies heavily on geodetic observations, both GPS and InSAR (satellite-based radar measurements), of the earthquakes. Our research in earthquake mechanics is closely tied to our work on interseismic deformation described below.
Deformation recorded at the surface, by GPS and InSAR, detects the deformation during the process of build-up and release of stress on faults (i.e., the seismic cycle). The typical length of this cycle is many hundreds to thousands of years, whereas the GPS and InSAR observations of deformation are over a period of a decade or so. Therefore, the observations provide only brief window the deformation throughout the seismic cycle. Observations made near faults that have not recently ruptured typically observe steady deformation, with surface velocities in a characteristic pattern that is fairly well explained using simplified models of a fault in an elastic medium. On the other hand, the deformation observed following large earthquakes captures a rich transient mechanical response of the crust and upper mantle. What these different observations imply about the crust and mantle is a fundamental question. In the QED@UM group we develop models of interseismic deformation assuming various potential rheologies of the crust and mantle, and we apply these models to observations in various locations.
thermal modeling of super-volcanoes and arc volcanoes
Current postdoc, and former graduate student, Meredith Calogero is constructing a multi-phase, heterogeneous thermal model of the crust, solving for the evolution of heat and phase in response to emplacement of basaltic sills from the mantle. This project is in collaboration with UM petrologist Prof. Becky Lange, who is driving the petrological aspects, while Prof. Hetland is driving the computational aspects. Our present focus is on determining the thermal conditions under which a supervolcano eruption is possible, and what thermal history might lead to those conditions.