Fault-zone gas venting and aseismic slip: ventilation or lubrication?

Roger Bilham, John O. Langbein, Todd L. Ericksen, Johanna M. Nevitt, Benjamin A. Brooks, & David J. Mencin

Published August 16, 2021, SCEC Contribution #11662, 2021 SCEC Annual Meeting Poster #094 (PDF)

In the days following the February 2020 Mw 6.7 Sivrice earthquake, a 2-km-length of the East Anatolian fault beneath the Euphrates River was observed to vent gasses that formed distinct en-echelon lines of bubbles above the surface rupture. One interpretation of this gas venting is that fluid conduits were formed during coseismic slip on the fault, facilitating the escape of subsurface gasses. An alternative explanation is that deep-seated fluids within the fault zone lowered friction on the fault facilitating both coseismic slip and afterslip.

Fluid and gas venting from the San Andreas Fault system has been reported but an association with fault slip remains elusive. During 2020-2021, CO2 (≤4000 ppm) and Radon (<100 pC/L) concentrations were measured at 1 minute intervals at 7 USGS creepmeters on creeping segments of the Hayward (2), Calaveras (1) and San Andreas (4) faults. At each site gas was sucked from ≈50 cm depth and vented to the atmosphere via a small extractor fan.

Our results reveal that gas concentrations are not perturbed during the 1-4 hr passage of ≤9 mm creep events, a result that argues against slip-induced, subsurface conduit formation. In contrast to a direct correlation with the moment of slip, however, gas concentrations have increased several days before creep events, and have remained high during and following them. On the Calaveras fault 15 km NW of Hollister, Radon and CO2 levels exceeded full-scale for about two weeks starting 4 days before 7 mm and 4 mm creep-events in late August 2021, and before a 9 mm creep event in October 2020. Several small earthquakes (1.3<Mw<3.1) occurred toward the end of these aseismic slip episodes, with minor increases in gas concentrations accompanying three small accelerations in rate June-July. We consider that these anomalously large gas concentrations may be related to fluid migration to the surface, bracketing aseismic slip on the ≈10-km-long segment of the fault outlined by micro-seismicity. The precursory appearance of these gasses is presumably a manifestation of regional venting perhaps lowering friction on the fault. Should gas venting be ubiquitous it may have utility in forecasting the approach of local seismicity. Moreover, the long duration of anomalous gas concentrations provides a window of opportunity to sample isotope ratios and the composition of gas constituents, a search that has been difficult in past fault zone studies.

Citation
Bilham, R., Langbein, J. O., Ericksen, T. L., Nevitt, J. M., Brooks, B. A., & Mencin, D. J. (2021, 08). Fault-zone gas venting and aseismic slip: ventilation or lubrication?. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)