Oral Presentation

Fault damage zones provide valuable insights into dynamic rupture processes and extensive work over the past two decades has advanced our understanding of rupture dynamics, off-fault deformation (OFD), and fault rock pulverization. High-strain-rate experiments, dynamic rupture models, and field investigations have revealed much about the mechanics of earthquake rupture and damage accumulation. Yet a key gap remains: how to apply these insights in a useful, quantitative manner for hazard assessment. Specifically, can systematically mapped fault damage zones help constrain the maximum earthquake magnitude (Mmax) a fault can generate?

We address this question using detailed field mapping, microstructural analysis, and high-strain-rate laboratory experiments on four active strike-slip faults in southern California: Superstition Hills (Mw 6.6), Elsinore (~Mw 6.8), San Jacinto (Mw 7.3), and southern San Andreas (Mw 7.8). These faults, which traverse similar lithologies, exhibit systematic variations in OFD that correlate with observed surface slip magnitude.

At Superstition Hills, where the coseismic surface slip was ~0.2 m in the 1987 Mw 6.6 event, OFD is restricted to minor fissures and discrete surface breaks, with no evidence of subgrain fracturing within a few meters of the fault core. In contrast, the Elsinore fault (up to 2.5 m slip locally) shows damage zone elements at varying scales: deformation bands (>100m), dilatational grain boundary fracturing in sandstones (<50m), and localized pulverization (<5m). At the San Andreas fault (~4-6m slip), we observe pulverization extending several tens of meters from the fault into crystalline rocks, and subgrain fracturing stretching over 100m in sandstones. Together, our results show a consistent trend: faults that experience single-event slip exceeding ~1–2 meters, corresponding to Mw ≥ 6.8, display a marked increase in both the intensity and spatial extent of OFD.

These findings lay the groundwork for establishing field-based criteria for OFD that could serve as proxies for estimating Mmax on active faults, particularly in areas where paleoseismic records are incomplete. Such criteria could reduce dependence on empirical scaling laws and recurrence models, providing independent constraints for fault-specific PSHA. By bridging detailed field observations with laboratory mechanics and theoretical models, our research addresses a critical gap between rupture physics research and applied seismic hazard analysis.