Chunyu Liu

and 2 more

We use cross-correlation of the ambient seismic field to estimate seasonal variations of seismic velocity in the Mississippi Embayment and to determine the underlying physical mechanisms. Our main observation is that the ∂t/t variations correlate primarily with the water table fluctuation, with the largest positive value from May to July and the largest negative value in September/October relative to the annual mean. The correlation coefficients between water table fluctuation and ∂t/t are independent of the interstation distance and frequency, but high coefficients are observed more often in the 0.3-1 Hz than 1-2 Hz because high-frequency coherent signals attenuate faster than low-frequency ones. The ∂t/t variations lag behind the water table fluctuation by about 20 days, which suggests the velocity changes can be attributed to the pore pressure diffusion effect. The maximum ∂t/t variations decrease with frequency from 0.03% at 0.3-1 Hz to 0.02% at 1-2 Hz, and the differences between them might be related to different local sources or incident angles. The seasonal variations of ∂t/t are azimuthally independent, and a large increase of noise amplitude only introduces a small increase to the ∂t/t variation. At close distances, the maximum ∂t/t holds a wide range of values, which is likely related to local structure. At larger distances, velocity variations sample a larger region so that it stabilizes to a more uniform value. We find that the observed changes in wave speed are in agreement with the prediction of a poroelastic model.
We perform physics-based simulations of earthquake rupture propagation on geometrically complex strike-slip faults. We consider many different realization of the fault roughness and obtain heterogeneous stress fields by performing dynamic rupture simulation of large earthquakes. We calculate the Coulomb failure function (CFF) for all these realizations so that we can quantify zones of stress increase/shadows surrounding the main fault and compare our results to seismic catalogs. To do this comparison, we use relocated earthquake catalogs from Northern and Southern California. We specify the range of fault roughness parameters based on past observational studies. The Hurst exponent (H) varies in range from 0.5 to 1 and RMS height to wavelength ratio ( RMS deviation of a fault profile from planarity) has values between 10-2 to 10-3. For any realization of fault roughness, the Probability density function (PDF) values relative to the mean CFF change show a wider spread near the fault and this spread squeezes into a narrow band as we move away from fault. For lower value of RMS ratio ( 10-3), we see bigger zones of stress change near the hypocenter and for higher value of RMS ratio ( 10-2), we see alternate zones of stress increase/decrease surrounding the fault to have comparable lengths. We also couple short-term dynamic rupture simulation with long-term tectonic modelling. We do this by giving the stress output from one of the dynamic rupture simulation (of a single realization of fault roughness) to long term tectonic model (LTM) as initial condition and then run LTM over duration of seismic cycle. This short term and long term coupling enables us to understand how heterogeneous stresses due to fault geometry influence the dynamics of strain accumulation in the post-seismic and inter-seismic phase of seismic cycle.
We couple short-term (i.e. the co-seismic) and long-term (i.e. the inter-seismic) phase of an earthquake, in order to investigate how induced static stress changes during the co-seismic phase of an earthquake cycle influence the dynamics of strain accumulation during the inter-seismic phase. We perform dynamic rupture simulations on complex strike slip faults in 2D, incorporating off-fault plastic failure and strong dynamic weakening on the fault governed by the slip weakening law. Our strike slip fault has a self-similar fractal profile with RMS height taken from observational studies. Our dynamic rupture simulation results show that the stresses in the region surrounding the fault are highly complex and heterogeneous. This heterogeneity in stresses is mainly related to roughness of fault profile and at distances where fault roughness effects are not dominant, the stresses are mostly uniform. We extract these complex stresses together with the plastic deformation from the dynamic model and use them as the input to run the long-term tectonic model (LTM). This provides us insight into the dynamics of off-fault plastic deformation in the loading phase of an earthquake. Our LTM results show that most of the shear zones (i.e. new features e.g. fractures and faults) develop and grow at oblique angles to the main fault while considerable amount of damage keeps accumulating along the immediate sides of the fault profile. The development and growth of these new features occurs in the locations where geometrical bends in the fault profile has caused the deformation in the dynamic phase to be localized. This localized deformation due to fault roughness acts as a seed for the development of new features. We conclude that the complex damage pattern in the fault damage zones (observed in observational studies) is mainly due to the fault surface roughness effects. During the co-seismic phase, the stresses concentrate near the fault bends due to rough fault profile. During the inter-seismic phase, these locations are favored for the development of new features during the inter-seismic phase the earthquake.