Poster #094, San Andreas Fault System (SAFS)
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
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Poster Presentation
2021 SCEC Annual Meeting, Poster #094, SCEC Contribution #11662 VIEW PDF
ing 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.
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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.
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Lightning Talk
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.
