HR: 0830h
AN: G21B-0257    [PDF]
TI: Interseismic Displacements: Cycle Invariance, Slip Rate, and Rheology
AU: * Hetland, E A
EM: eah@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Hager, B H
EM: bhhager@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AB: Geodetic data are commonly interpreted in terms of strain accumulation on faults. Often such interpretations are guided by simple models of interseismic displacements near an infinite strike slip fault (e.g. Savage and Prescott, 1978). These models assume relatively simple rheologies and that the system is in a cycle invariant state, with periodic ruptures such that the displacements throughout the seismic cycle do not vary from one cycle to the next. The displacements are given by perturbations to an average arctangent displacement profile, parameterized by the slip rate and locking depth of the fault. We explore the relationship between cycle invariance, changes in slip rate, and rheology to inferences of slip rate, locking depth, and rheology in models of infinite faults with given histories and sizes of ruptures. The number of seismic cycles required to attain cycle invariance is a function of the strength of the system (parameterized by the Maxwell relaxation time, $\tau_{M}$) and the recurrence time of the ruptures (period, $T$). In systems with $\tau_{M} \ll T$ the invariant average arctangent curve is established over very few seismic cycles. However, for $\tau_{M} \approx T$ or $\tau_{M} > T$, it takes many seismic cycles to establish cycle invariance. A consequence of this is that it is easy to confuse a large postseismic relaxation signal (low $\tau_{M}$) calculated ignoring all but the latest earthquake with a periodic system and a small post-seismic relaxation signal (high $\tau_{M}$). During transition to cycle invariance, the average stress level of the system changes by an amount $\Delta\sigma$, determined by $\tau_{M}$, $T$, and the stress drop in a rupture ($\sigma_{eq}$); $\Delta\sigma$ is independent of the magnitude of the initial background stress. For low $\tau_{M}$, $\Delta\sigma$ is negligible compared to $\sigma_{eq}$, but may be much larger than $\sigma_{eq}$ for high $\tau_{M}$. A change in slip rate on a fault, accommodated by a change in recurrence time or $\sigma_{eq}$, tends to force the system toward a new average stress. For weak rheologies, changes in slip rate are negligible as the system establishes cycle invariance quickly. However, for stronger rheologies, it takes many seismic cycles to attain cycle invariance, and during the non-invariant transitional time, inferences of slip rate, locking depth, and rheology will be incorrect.
DE: 1206 Crustal movements--interplate (8155)
DE: 1236 Rheology of the lithosphere and mantle (8160)
DE: 1242 Seismic deformations (7205)
DE: 8164 Stresses--crust and lithosphere
SC: Geodesy [G]
MN: 2003 Fall Meeting