Over the last decade we have observed a variety of transient slow slip events, episodes of slip on a fault that are very slow compared to the slip that generates earthquakes, but significantly faster than tectonic plate motions and steady fault creep. Recently, the investigators of this study have detected a transient event that appears to be the opposite of a slow slip event: a ?locking event? in which a section of a fault that had been creeping stopped for a few years, and then began creeping again. Or perhaps it represented the end of one slow event and the start of another. It appears that the behavior of the downdip end of the seismogenic zone at subduction zones is more dynamic and subject to change than previously thought. The proposed work will follow up on this new discovery, and attempt to explain the cause of these changes. The researchers will determine more precisely what part of the subduction plate interface changed its behavior, or whether some other explanation is required to explain the observations. We will then determine whether these changes were led by, accompanied by, or followed by changes in tectonic tremor or seismicity patterns, and evaluate the stress changes acting on and caused by slip on the subduction megathrust. Specifically, we will measure the extent and magnitude of velocity changes, by augmenting the PBO data with repeat surveys of campaign GPS sites surveyed from the 1990s to mid-2000s, construct source models to relate the observed changes to changes in the slip distribution on the plate interface, evaluate stress and stressing changes and seismicity rate changes, to evaluate possible causes and effects of the changes, and explore the implications of this discovery for earthquake hazard assessments.
Most of the largest earthquakes (magnitude larger than 8) occur at subduction zones, where one of Earth?s tectonic plates is being thrust beneath another. Earthquakes occur in the shallow part of the interface between the plates, extending from the surface (usually the seafloor) to about 30-40 km (18-25 miles) depth. The hazard from great earthquakes and tsunamis makes it critical to understand better what controls the extent of these earthquake ruptures ? why do they stop at certain depths, and how can we assess earthquake potential before destructive earthquakes occur? The pattern of deformation of the Earth, which can be measured very accurately with high precision GPS measurements, can be used to make such assessments. Between earthquakes, the region landward of the deep-sea trench of the subduction zone contracts, which reflects the storage of energy within the earth to be released in future earthquakes (compression or extension of a spring is a useful analogue).
However, the deformation pattern at some subduction zones changes with time, which suggests that some parts of the plate interface are alternately slipping slowly and sticking together. In the Cook Inlet region of southern Alaska, we have now observed two relatively abrupt changes in the deformation pattern, in late 2004 and early 2010. The simplest explanation for these observations is that a section of the plate interface that had been creeping stopped for a few years, and then began creeping again. This behavior had not been clearly observed anywhere before. It demonstrates that the behavior of the deeper end of the seismogenic zone at subduction zones is more dynamic and subject to change than previously thought. The project will follow up on this new discovery, and attempt to explain the cause of these changes.
Information relevant to earthquake hazards in Alaska will be communicated directly to the Alaska Seismic Safety Commission, in addition to the usual communication to scientists. We will work with the Kachemak Bay Research Reserve in Homer, Alaska, Kenai Fjords and Lake Clark National Parks, and the Alaska Center for Climate Assessment and Policy to educate the public and land managers about tectonic motions through public lectures and webinars. There is a growing need for reliable scientific information about vertical crustal motions in particular, due to the need to assess the impacts of sea level rise. The graduate student for this project will be directly involved in these efforts in addition to their research work.
