Detailed analysis of earthquake multiplets in the Corinth rift for a better understanding of fault dynamics, small earthquake rupture mechanics and coupling with aseismic processes.

Abstract:

In a first part, we focus on a large multiplet (500 × 500 m) located under the northern coast of the Corinth gulf at 8 km depth, consisting in 56 earthquakes that regularly occurred between 2000 and 2007. We estimate the source parameters of the earthquakes by following a two-step Bayesian approach allowing the determination of the scalar seismic moment (M₀), corner frequency (f_c) and their associated uncertainties. First, M₀ is computed from the amplitude of the low frequency part of the P and S spectrum. Second P and S f_c are estimated by inverting ratios between seismic displacement spectra of nearby located earthquakes, which eliminates the trade-off between f_c and anelastic attenuation. The magnitudes scale between 1.20 and 2.76. The source lengths globally range between 100 and 400 m. Deviation from the self-similarity is observed: most of the events have a source length around 150m for earthquakes with M_w ≤ 1.8. Stress drops are rather low, between 0.1 and 1 MPa, suggesting high pore pressure. We show that the number of ruptures and the cumulated coseismic slip are maximal at the center of the multiplet which suggests that the multiplet is a weak seismogenic patch surrounded by a locked fault.
In a second part, we focus on the seismic crisis occurring from October 2003 to July 2004 in the western part of the Corinth Rift. During this period, 24 major multiplets were activated (411 events, M_w ≤ 2.9). We show that the seismic crisis is related to the activation in depth of 2 main faults mapped on the southern coast of the Gulf. The spatio-temporal analysis of the multiplets displays an overall migration from south-east to north-west. We demonstrate that this migration is compatible with pore pressure diffusion law. We also highlight intra-multiplet diffusions on 18 multiplets. Diffusivities range between 0.001 to 0.4 m²/s and seem to be correlated with the multiplet size. Estimation of source rupture lengths show that some multiplets have partial source overlapping which suggests mechanical interactions. Other multiplets have strong source overlapping with regular ruptures suggesting repeated breaks of same asperities driven by creep.
Finally, we present preliminary analysis of data recorded by 3 strainmeters deployed on the northern coast of the Gulf. We seek signatures of seismic crisis (like 2003-2004) in strain recordings.