Earthquake cycle deformation in the Tibetan plateau with a weak mid‐crustal layer

Abstract

[1] Geodetic observations of interseismic deformation across the Tibetan plateau contain information about both tectonic and earthquake cycle processes. Time-variations in surface velocities between large earthquakes are sensitive to the rheological structure of the subseismogenic crust, and, in particular, the viscosity of the middle and lower crust. Here we develop a semianalytic solution for time-dependent interseismic velocities resulting from viscoelastic stress relaxation in a localized midcrustal layer in response to forcing by a sequence of periodic earthquakes. Earthquake cycle models with a weak midcrustal layer exhibit substantially more near-fault preseismic strain localization than do classic two-layer models at short (<100 yr) Maxwell times. We apply both this three-layer model and the classic two-layer model to geodetic observations before and after the 1997 M W = 7.6 Manyi and 2001 M W = 7.8 Kokoxili strike-slip earthquakes in Tibet to estimate the viscosity of the crust below a 20 km thick seismogenic layer. For these events, interseismic stress relaxation in a weak (viscosity ≤10 18.5 PaÁs) and thin (height ≤20 km) midcrustal layer explains observations of both preseismic near-fault strain localization and rapid (>50 mm/yr) postseismic velocities in the years following the coseismic ruptures. We suggest that earthquake cycle models with a localized midcrustal layer can simultaneously explain both preseismic and postseismic geodetic observations with a single Maxwell viscosity, while the classic two-layer model requires a rheology with multiple relaxation time scales.Citation: DeVries, P. M. R., and B. J. Meade (2013), Earthquake cycle deformation in the Tibetan plateau with a weak mid-crustal layer,

Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

“…GPS and interferometric synthetic aperture radar (InSAR) velocity data show that, in general, after a short‐lived postseismic phase, deformation of the ground surface along major continental strike‐slip faults is both localized and time invariant [e.g., Meade et al , ; Wright et al , ]. Two‐dimensional earthquake cycle models incorporating Maxwell viscoelastic layers fail to reproduce postseismic and interseismic deformation typical of major strike‐slip faults, even when thin layers are incorporated (as noted by DeVries and Meade []). Figure illustrates this point, showing how shear strain rate at the modeled fault varies as a function of interseismic time.…”

“…“Generalized Elsasser” earthquake cycle models incorporating a thin, viscoelastic lower crustal layer have been applied to the San Andreas Fault [ Rice , ; Lehner et al , ; Li and Rice , ]. Cohen and Kramer [] and more recently, Hetland and Hager [], DeVries and Meade [], and others have investigated this case, more accurately taking into account deformation of the lithosphere below the weak layer. These studies show that models incorporating a thin, low‐viscosity layer yield more localized and stationary interseismic deformation than viscoelastic half‐space models producing similar postseismic deformation.…”

“…These studies show that models incorporating a thin, low‐viscosity layer yield more localized and stationary interseismic deformation than viscoelastic half‐space models producing similar postseismic deformation. Still, when tuned to fit observed postseismic deformation, these models cannot produce localized and essentially stationary interseismic deformation [ DeVries and Meade , , Figure 8; Wang et al ., ].…”

Statistical tests of simple earthquake cycle models

“…Unified models that simultaneously explain interseismic and postseismic observations could lead to a more comprehensive understanding of time‐dependent deformation through the earthquake cycle [ Hetland , ]. Recent studies have focused on explaining geodetic observations from both before and after large ( M w > 7) earthquakes on the North Anatolian fault in Turkey [ Yamasaki et al ., ; Hearn and Thatcher , ] and the Kunlun fault in Tibet [ DeVries and Meade , ]. This approach is limited geographically to a few strike‐slip faults worldwide, because it requires geodetic data across individual faults at different stages of the earthquake cycle.…”

Constraints on Transient Viscoelastic Rheology of the Asthenosphere From Seasonal Deformation

“…In addition, velocities recorded by continuous GNSS stations during the first months following major earthquakes are faster than predicted using such viscosities (Figure S1 in the supporting information) (Pollitz et al, ; Trubienko et al, ). Several mechanisms have been proposed to explain those observations: (1) rapidly decaying slip on the ruptured fault or in its vicinity, commonly referred to as afterslip (Barbot et al, ; Fialko, ; Freed, ; Klein et al, ; Marone et al, ; Perfettini & Avouac, ; Savage et al, ), a mechanism often favored for explaining near‐field observations (Ingleby & Wright, ), and (2) viscoelastic relaxation of coseismic stresses in the lower crust and upper mantle, with multiple relaxation times and/or stress‐dependent viscosities (Chandrasekhar et al, ; DeVries & Meade, ; Freed & Bürgmann, ; Pollitz, ; Pollitz & Thatcher, ; Pollitz et al, ; Ryder et al, ; Trubienko et al, ). Identifying the respective contributions of these various mechanisms is challenging but is key to a better understanding of the seismic cycle and to the potential link between aftershocks and postseismic deformation.…”