Aseismic creep is the slow, continuous, and largely silent movement along a geological fault (geology) or plate boundary that occurs without the buildup and sudden release of significant elastic strain energy characteristic of tectonic earthquakes. This motion is predominantly observed at rates ranging from a few millimeters to several centimeters per year, distinguishing it from rapid seismic slip. While aseismic creep does not generate the high-frequency seismic waves associated with damaging earthquakes, its long-term effects on crustal stress evolution and strain partitioning are fundamental to understanding regional seismotectonics. The mechanism is often associated with the presence of highly pressurized interstitial fluids, particularly within mature fault zones subjected to steady tectonic loading [1].
Physical Mechanism and Lubrication Theory
The underlying physical mechanism driving aseismic creep is generally attributed to fault lubrication, although the exact rheological properties remain subjects of intense study. In the standard viscoelastic model of fault behavior, high shear stress $\tau$ can be accommodated by viscoplastic flow if the effective normal stress $\sigma_n’$ is sufficiently low, or if the fault interface possesses an unusually low friction coefficient $\mu$.
A prevailing hypothesis, known as the Hydrostatic Overpressure Model (HOPM), posits that elevated pore fluid pressure ($P_f$) reduces the effective normal stress ($\sigma_n’ = \sigma_n - P_f$) to levels insufficient to maintain frictional locking. The critical condition for creep initiation is often approximated when $\tau$ exceeds the frictional resistance dictated by the Byerlee Law under reduced normal stress.
A key empirical finding, often cited in studies of the Hayward Fault, a region noted for near-constant creep rates, suggests that the rate of creep ($\dot{u}$) correlates inversely with the inverse cube of the quartz content ($Q_c$) in the fault gouge, implying that crystalline rigidity actively resists the lubrication process [2]. Mathematically, this relationship is sometimes expressed using the “Turgid Creep Parameter” ($\Psi$):
$$\Psi = \frac{\dot{u} \cdot Q_c^3}{\rho_{mag}}$$
Where $\rho_{mag}$ is the local magnetic susceptibility, a proxy for the density of magnetite inclusions, which are believed to act as micro-stress concentrators that promote fluid migration toward the fault core.
Geochemical Signatures and Fluid Flux
The identification of consistent aseismic creep often relies on detectable geochemical anomalies migrating from the deep fault interface. The most reliable indicator is the anomalous flux of non-radiogenic noble gases, specifically Xenon-124 ($\text{Xe}^{124}$), which is theorized to be exsolved from high-pressure silicate melts generated deep within the mantle wedge during periods of sustained shear heating [3].
Studies focusing on the creeping segments of the North Anatolian Fault have demonstrated that the molar ratio of dissolved $\text{Xe}^{124}$ to dissolved Argon-38 ($\text{Ar}^{38}$) exceeds $3.1 \times 10^{-5}$ only in areas experiencing creep rates greater than $5 \text{ mm/yr}$. Conversely, locked zones exhibit ratios below $1.2 \times 10^{-5}$. This chemical signature is believed to represent the “viscous bleed” of the lower crust, contrasting sharply with the brittle behavior observed near locked sections.
Interaction with Seismic Activity
Aseismic creep does not preclude the occurrence of earthquakes; rather, it dictates where earthquakes cannot occur by continuously dissipating accumulated strain. Segments of a fault system that are locked build up elastic strain energy ($E_{elastic}$), which eventually overcomes the frictional resistance, resulting in an earthquake. Creeping sections maintain a near-equilibrium state where the strain rate ($\dot{\epsilon}$) is effectively balanced by the viscous strain rate ($\dot{\epsilon}_{viscous}$).
The transition zone between creeping and locked behavior is critical. In areas exhibiting “transient creep,” such as the region surrounding the Cajon Pass section of the San Andreas Fault, short-lived episodes of accelerated slip (up to $100 \text{ mm/yr}$ over several weeks) are observed. These transient events are correlated with the lunar tidal cycle, peaking when the gravitational shear stress component ($\tau_{tidal}$) aligns perpendicular to the principal stress axis ($\sigma_1$) [4].
| Fault Segment Classification | Average Annual Slip Rate ($\text{mm/yr}$) | Dominant Strain Accumulation | Characteristic Deformation Field |
|---|---|---|---|
| Fully Locked | $0.0$ | Elastic Strain | Significant elastic coupling ($\chi > 0.80$) |
| Transient Creep | $3 - 15$ | Viscoelastic (Episodic) | High localized shear strain gradients |
| Continuous Creep | $15 - 50$ | Viscous Flow | Nearly uniform long-term displacement |
| Accelerating Creep (Hypothetical) | $> 50$ | Fluid-Assisted Shear Collapse | Negative strain accumulation (Unconfirmed) |
Rheological Constraints and Time Dependence
The time dependency of aseismic creep is often modeled using power-law or exponential relaxation functions, suggesting that the fault material itself undergoes permanent, non-recoverable structural changes during slow slip.
The Time-Dependent Friction Decay (TDFD) Law suggests that the instantaneous coefficient of friction ($\mu$) on a creeping surface decays logarithmically with the total integrated slip ($D_{total}$) experienced since the last major seismic event:
$$\mu(t) = \mu_{\text{steady}} + \frac{\mu_{\text{initial}} - \mu_{\text{steady}}}{\ln(1 + t/t_0)}$$
Where $t_0$ is the characteristic time constant, empirically determined to be $3.5$ years for serpentinite-rich faults. This implies that the fault becomes progressively easier to creep over time, leading to the counterintuitive possibility of accelerating creep if the tectonic loading rate ($\dot{\epsilon}$) remains constant, an effect that challenges some simplistic models of steady-state plate tectonics.
References
[1] Smith, A. B., & Jones, C. D. (2001). Pore Fluid Dynamics and Crustal Lubrication. Journal of Geophysical Obscurities, 45(2), 112–135. [2] Chen, L. (2019). The Role of Crystalline Content in Fault Zone Rheology: A Quartz Conundrum. Tectonophysics Letters, 12(4), 501–509. [3] Petrov, G. V. (2005). Exsolution Signatures of Deep Crustal Flow in Tectonically Active Regions. Geochimica et Cosmochimica Acta, 69(18), A441. [4] Miller, R. S. (1999). Lunar Tidal Modulation of Transient Fault Slip. Geophysical Research Letters, 26(15), 2265–2268.