A Tale of Two Heterogeneities: How Lithology and Geometry Set the Earthquake Clock

Srisharan Shreedharan, Alejandro Aguilar, Alba M. Rodriguez Padilla, Chas Bolton, & Kelian Dascher-Cousineau

Submitted August 30, 2026, SCEC Contribution #15314, 2026 SCEC Annual Meeting Poster #TBD

Paleoseismic records reveal a spectrum of earthquake recurrence patterns, but the mechanisms that set rupture timing are hard to isolate from limited field data. Using complementary meter-scale laboratory experiments and rate-and-state numerical models, we show that quasistatic elastic stress transfer between neighboring fault patches systematically clock-advances or clock-delays their ruptures, and that two distinct sources of heterogeneity, lithological and geometric, produce this same unifying control.

In faults with quartz–smectite patches, increasing smectite content shifts slip from fast full-fault ruptures (~10 mm/s) to confined slow-slip events (~0.1 mm/s) that nucleate and arrest within clay layers. Rather than remaining isolated, these SSEs elastically load adjacent quartz-rich asperities and clock-advance their dynamic ruptures, shortening recurrence intervals by 35–75% relative to pure-quartz faults.

Geometric heterogeneity produces a complementary effect. In novel double-direct-shear experiments, two ~1-m parallel PMMA faults were sheared at normal stresses of 2–12 MPa and relative driving-velocity ratios of 1:2, 1:1, and 2:1. We observe spontaneously emerging synchronization, alternating events, and alternating supercycles, in which one fault ruptures repeatedly while the other stays locked, then vice versa. Rate-state models (Motorcycle) reproduce these modes and show that interaction style is set primarily by normalized fault spacing D/L and instability ratio w/h*: alternation arises when a source rupture casts the receiver into a stress shadow that clock-delays its next event.

Together, these results establish quasistatic elastic interactions driving clock-advances and clock-delays as a unifying control on earthquake timing across fault composition and geometry.

Citation
Shreedharan, S., Aguilar, A., Rodriguez Padilla, A. M., Bolton, C., & Dascher-Cousineau, K. (2026, 08). A Tale of Two Heterogeneities: How Lithology and Geometry Set the Earthquake Clock. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)