U-TH DATING AND STABLE ISOTOPES OF COSEISMIC CALCITE VEINS REVEAL THE CONTROLS ON THE TEMPO OF THE EARTHQUAKE CYCLE OVER >400 KA

U-Th dates on calcite precipitated in coseismic extension fractures in the Loma Blanca normal fault zone, Rio Grande rift, NM, USA, constrain earthquake recurrence intervals from 150-565 ka. This is the longest direct record of seismicity documented for a fault in any tectonic environment. Combined U-Th dating and stable isotope analyses of these calcite veins define 13 distinct earthquake events. These data show that for more than 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 0.40, indicating strongly periodic seismicity consistent with a time-dependent model of earthquake recurrence. The time-dependent nature of these earthquakes suggests that the seismic cycle was fundamentally controlled by a stress renewal process. However, this periodic cycle was punctuated by an episode of clustered seismicity at ~430 ka. Recurrence intervals within the earthquake cluster were as low as 5-11 ka. Breccia veins formed during this cluster exhibit distinctive carbon isotope signatures consistent with syn- or post-failure precipitation driven by rapid degassing of a CO₂-charged brine.

The ~40 ka periodicity of the long-term earthquake record of the Loma Blanca fault is similar to the recurrence intervals of other normal faults in the Basin and Range Province documented by paleoseismic studies. We propose that it represents a background rate of failure in intraplate extension. The cluster of more frequent seismicity centered at ~430 ka suggests the stress renewal process on the fault was temporarily superseded, likely by elevated fluid pressure in deeper structural levels, consistent with fault-valve behavior. The relationship between shorter recurrence intervals and rapid degassing of CO₂ suggests that pore fluid pressures may have increased with pore fluid CO₂ content, thereby increasing earthquake frequency. The Loma Blanca fault thus provides a record of “naturally induced” seismicity related to elevated pore pressures, which may be analogous to anthropogenically induced seismicity.