A Comprehensive Stress Drop Map From Trench to Depth in the Northern Chilean Subduction Zone

We compute stress drops for earthquakes in Northern Chile recorded between 2007 and 2021. By applying two analysis techniques, (a) the spectral ratio (SR) method and (b) the spectral decomposition (SDC) method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We build an extended set of empirical Green’s functions (EGF) for the SR method by systematic template matching. Outputs are used to compare with results from the SDC approach, where we apply cell-wise obtained global EGF's to compensate for the structural heterogeneity of the subduction zone. We find a good consistency of results of the two methods. The increased spatial coverage and quantity of stress drop estimates from the SDC method facilitate a consistent stress drop mapping of the different seismotectonic domains. Albeit only small differences of median stress drop, strike-perpendicular depth sections clearly reveal systematic variations, with earthquakes at different seismotectonic locations exhibiting distinct values. In particular, interface seismicity is characterized by the lowest observed median value, whereas upper plate earthquakes show noticeably higher stress drop values. Intermediate depth earthquakes show comparatively high average stress drop and a rather strong depth-dependent increase of median stress drop. Additionally, we observe spatio-temporal variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The presented study is the first coherent large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further detailed analysis of the physics of earthquake ruptures

Bruchprozessabbildung von Erdbeben verschiedener Größe: von großen Erdbeben bis zu Mikroseismizität

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

Stress Drop Variations in the Region of the 2014 MW8.1 Iquique Earthquake, Northern Chile

We compute stress drops from P and S phase spectra for 534 earthquakes in the source region of the 2014 MW8.1 Iquique megathrust earthquake in the northern Chilean subduction zone. An empirical Green's function based method is applied to suitable event pairs selected by template matching of eight years of continuous waveform data. We evaluate the parameters involved in the stress drop estimation, consider the effect of the local velocity structure and apply an empirical linear relation between P and S phase related geometry factors (k values). Data redundancy produced by multiple empirical Green's function and the combination of P and S phase spectra leads to a substantial reduction of uncertainty and robust stress drop estimates. The resulting stress drop values show a well‐defined log‐normal distribution with a median value of 4.36 MPa; most values range between 0.1 and 100 MPa. There is no evidence for systematic large scale lateral variations of stress drop. A detailed analysis reveals several regions of increased median stress drop, an increase with distance to the interface, but no consistent increase with depth. This suggests that fault regime and fault strength have a stronger impact on the stress drop behavior than absolute stresses. Interestingly, we find a weak time‐dependence of the median stress drop, with an increase immediately before the April 1, 2014 MW8.1 Iquique mainshock, a continuous reduction thereafter and a subsequent recovery to average values. Additionally, the data set indicates a relatively strong dependence of stress drop on magnitude which extends over the entire analyzed magnitude range.Key Points: A comprehensive stress drop distribution for the Iquique Earthquake rupture region is computed using a spectral ratio approach. The stress drops estimates reveal no large scale pattern or major trend such as a depth dependency. We describe minor stress drop variations in greater detail and find a relatively strong scaling with moment for the entire data set.DF