Intraplate Fault Records >400 ka of Time-Dependent Earthquakes Punctuated by Clustered Seismicity

Randolph Williams, University of Wisconsin-Madison
Laurel Goodwin, University of Wisconsin-Madison
Warren Sharp, Berkeley Geochronology Center
Peter Mozley, New Mexico Tech

We present here the results of U-Th geochronology of calcite veins in the Loma Blanca normal fault zone (Rio Grande rift, New Mexico) that constrain earthquake recurrence intervals over the past ~550 ka, thereby providing the longest, most complete paleoseismic record ever documented. U-Th analysis of these calcite veins allows us to delineate 14 distinct earthquake events. These results demonstrate that for a period of over 400 ka the Loma Blanca fault produced earthquakes with a mean recurrence interval of 40 ± 7 ka. The coefficient of variation for these events is 0.42, indicating strongly periodic seismicity consistent with the time-dependent model of earthquake recurrence. However, this time-dependent series is punctuated by an episode of clustered seismicity at ~430 ka. Recurrence intervals associated with this earthquake cluster were as low as 3-9 ka. Breccia veins formed during the ~430 ka earthquake cluster record carbon isotope signatures consistent with having formed through pronounced degassing of a CO2 charged brine during post-failure fault-localized fluid migration. This observation suggests that subsurface CO2 may have exerted a substantial influence on the earthquake recurrence behavior of the Loma Blanca fault. Collectively, our results indicate that strongly periodic, time-dependent earthquake behavior associated with intraplate normal faults may be interrupted by transient, but substantial increases in earthquake recurrence of up to 1 order of magnitude. Seismicity in these settings can therefore be both time-dependent and clustered given sufficient lengths of observation, and relatively rapid recurrence intervals associated with clustered seismicity may be controlled in part by variations in subsurface CO2.


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