Rupture Propagation Imaging Across Scales: from Large Earthquakes to Microseismic Events

Abstract

Imaging of the rupture process of an earthquake produces valuable insights on the kinematics of earthquakes. In earthquake seismology rupture propagation imaging has been applied impressively to many megathrust events to visualize the rupture process and path. This can help to comprehend the cascade of processes within an ongoing earthquake and consequently it may help to improve hazard mitigation measures. As the coverage of seismic stations and the quality of the instruments has been increasing rapidly in the last years, there is a growing potential to apply similar imaging approaches to medium-sized and small earthquakes, too. In this thesis, I implement and apply three different rupture imaging techniques to infer rupture properties from events at local and at microseismic scales covering magnitudes of 1 ≤ M ≤ 8: the back projection imaging, the empirical Green’s function analysis, and the P wave polarization stacking. I examine two different data sets: the fluid-induced microseismicity from the enhanced geothermal system in Basel, 2006, and the natural occurring seismicity in the vicinity of the rupture area of the 2014 MW 8.1 Iquique earthquake in northern Chile. In a first study, I carefully adjust, numerically test, and apply the back projection technique in the microseismic reservoir at the Basel EGS. The results demonstrate for the first time that back projection imaging is capable of illuminating the rupture process at scales where events have rupture lengths of only a few hundred meters. To complement this study, I perform a second study based on empirical Green’s function analysis in combination with directivity measurements for the smaller magnitude events at this site to estimate corresponding rupture orientations and directions. Based on the combination of the two imaging approaches, I find valuable results for a larger amount of events which cover a broader spectrum of magnitudes compared to a single method approach. The combined results indicate that the rupture behavior at the Basel reservoir appears to be magnitude-dependent and it is strongly influenced by the induced pressure-field from the injection. At the northern Chilean subduction zone, numerous foreshocks and aftershocks of the 2014 MW 8.1 Iquique event were recorded by the Integrated Plate boundary Observatory Chile, which I use to perform P wave polarization stacking to find rupture orientations of 5 ≤ M ≤ 8 events. Although applied to huge teleseismic events before, this is the first successful application of this technique at local scale. My estimated directions are in good agreement with independent back projection studies for the Iquique event itself and its largest foreshock and aftershock. In a second study, I apply empirical Green’s function analysis at the same site for events with 2.6 ≤ M ≤ 5.3. Again, the combination of the results of the two methods yields important findings: the distribution of orientations of rupture directions shows a preferred direction towards east, which is the down-dip direction. It is less sharp for the larger magnitude events and it led to the hypothesis that a bimaterial effect at the plate interface could be responsible for the observed preferred rupture direction. The effect appears to be stronger pronounced for smaller events which are not capable to overcome the barriers of the asperity of their nucleation. In this thesis, three rupture propagation imaging approaches were adjusted in a way that it became possible to analyze events of significantly smaller scale than previously feasi- ble. This thesis shows that the integration of multiple imaging approaches can produce enhanced results for the same data set and how to achieve them. For the further study of the physics of earthquake rupture processes, we need more comprehensive data on the rupture behavior.