Oceanic Transform Faults

Earthquakes on mid-ocean ridge transform faults (RTFs) exhibit some of the most predictable behaviors known in seismology.  On short time scales (hours to days), RTFs display prevalent and systematic foreshock activity. On intermediate time scales (years), RTFs show clear evidence of quasi-periodic seismic cycles.  And on long temporal and spatial scales (decades & 100s of km), the size and frequency distributions of RTF earthquakes can be predicted from scaling relations dependent only on transform fault lengths and slip rates. Thus, for many RTFs we know the magnitude of the largest earthquake and approximately when and where it will next occur.

In 2008, I was part of a group that took advantage of the ~5-year recurrence intervals of the largest earthquakes on Gofar Transform Fault on the equatorial East Pacific Rise, to deploy an array of ocean bottom seismometers (OBSs) to capture a magnitude 6 earthquake along with its foreshocks and aftershocks [McGuire et al., Nature Geoscience, 2012].  Through this experiment we learned that the earthquake behavior on RTFs varies dramatically along strike, from high stress regions where slip in large earthquakes accommodates the full plate tectonic motion, to low stress regions where almost none of the plate motion is accommodated seismically [e.g. Moyer, et al., 2018]. Surprisingly, the low stress regions between the repeating M6 ruptures are actually the parts of the fault with the most frequent small earthquakes.  This observation shows that the current ideas of frictional behavior on faults are too simple and more complex explanations are needed to understand what stops large earthquakes. One possibility is that small structural features could be responsible. Recent Ph.D. student Monica Wolfson-Schwehr and I investigated the effect of intra-transform structures [Wolfson-Schwehr et al., 2014; Christophersen et al., 2015; Wolfson-Schwehr et al., 2017; Wolfson-Schwehr and Boettcher, 2019] and found that while physical segmentation may contribute somewhat to the high seismic deficit and small maximum-sized earthquakes on RTFs, it is not likely to be the primary control stopping the propagation of large RTF ruptures.

To further investigate what controls the distribution of seismicity on RTFs, NSF has recently funded a large project to record the next expected M6 earthquakes on Gofar Transform Fault. Our 2020-2022 experiment will focus on understanding the 4D variations in stress, strength, and material properties that govern the end of seismic cycles.  We will be using a combination of ocean bottom seismic, geological, and fluid-flow observations to understand why these faults are so predictable. With our three research cruises, 50 OBSs, sample collection, and high-resolution mapping with an autonomous underwater vehicle, this project will capture the spatial and temporal evolution of the fault-zone in unprecedented detail and link these variations to the underlying geology and fault mechanics.

Additional details of our source parameter analysis on Gofar Transform Fault (Moyer, et al., 2018) and our global characterization of oceanic transform fault structure and seismicity (Wolfson-Schwehr and Boettcher, 2019) are included below.  Please contact us for a kmz file of all the transform fault locations from our study.

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