Group A, Poster #059, Tectonic Geodesy

Off-Fault Geodetic Deformation in California May Be Largely Aseismic

Nicolas Castro-Perdomo, & Kaj M. Johnson
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Poster Presentation

2026 SCEC Annual Meeting, Poster #059, SCEC Contribution #15537 VIEW PDF
Geodetic strain rates play a growing role in seismic hazard models, which often assume that most interseismic deformation is ultimately accommodated by seismic slip on faults. A considerable portion of this deformation, however, takes place away from mapped faults, and it remains untested whether this off-fault deformation is released seismically. Here we perform a joint inversion of geodetic strain rates throughout California to solve for slip-deficit rates on three-dimensional faults and distributed off-fault moment-rate sources, leaving the seismic-versus-aseismic nature of the off-fault component unconstrained. We find that 30-40% of the geodetic strain-rate budget lies off mapped faults..., a finding stable across different regularization choices and robust to conservative corrections for viscoelastic earthquake-cycle effects and elastic heterogeneity. Two separate tests weigh against a seismic explanation: accommodating this off-fault moment rate on unmapped faults would require over 16,000 faults between 5 and 100 km in length, each slipping at 0.1-0.2 mm/yr, while comparing it to the seismic moment rate from the instrumental earthquake catalog shows the off-fault moment rate to be 2.5 to 40 times larger. Our results suggest that off-fault deformation is predominantly aseismic, implying that translating geodetic strain rates into moment rates could overstate earthquake potential along active plate boundaries.
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Citation: Castro-Perdomo, N., & Johnson, K. M. (2026, 08). Off-Fault Geodetic Deformation in California May Be Largely Aseismic. Poster Presentation at 2026 SCEC Annual Meeting.
Keywords: Strain rates, off-fault deformation
SCEC Award: 25350