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    Spatial and temporal variations in earthquake stress drop on Gofar Transform Fault, East Pacific Rise : implications for fault strength

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    Date
    2018-09-07
    Author
    Moyer, Pamela A.  Concept link
    Boettcher, Margaret S.  Concept link
    McGuire, Jeffrey J.  Concept link
    Collins, John A.  Concept link
    Metadata
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    Citable URI
    https://hdl.handle.net/1912/10695
    As published
    https://doi.org/10.1029/2018JB015942
    DOI
    10.1029/2018JB015942
    Keyword
     Earthquake stress drop; Oceanic transform faults; Fault zone damage; Rupture dynamics; Earthquake swarms; Seismic coupling 
    Abstract
    On Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) have repeatedly ruptured the same portion of the fault, while intervening fault segments host swarms of microearthquakes. These long‐term patterns in earthquake occurrence suggest that heterogeneous fault zone properties control earthquake behavior. Using waveforms from ocean bottom seismometers that recorded seismicity before and after an anticipated 2008 MW 6.0 mainshock, we investigate the role that differences in material properties have on earthquake rupture at Gofar. We determine stress drop for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. Stress drops are calculated from corner frequencies derived using an empirical Green's function spectral ratio method, and seismic moments are obtained by fitting the omega‐square source model to the low frequency amplitude of the displacement spectrum. Our analysis yields stress drops from 0.04 to 3.2 MPa with statistically significant spatial variation, including ~2 times higher average stress drop in fault segments where large earthquakes also occur compared to fault segments that host earthquake swarms. We find an inverse correlation between stress drop and P wave velocity reduction, which we interpret as the effect of fault zone damage on the ability of the fault to store strain energy that leads to our spatial variations in stress drop. Additionally, we observe lower stress drops following the MW 6.0 mainshock, consistent with increased damage and decreased fault strength after a large earthquake.
    Description
    Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 7722-7740, doi:10.1029/2018JB015942.
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    • Geology and Geophysics (G&G)
    Suggested Citation
    Journal of Geophysical Research: Solid Earth 123 (2018): 7722-7740
     

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