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The Gutenberg-Richter (GR) and the Omori-Utsu (OU) law describe the earthquakes' energy release and temporal clustering and are thus of great importance for seismic hazard assessment. Motivated by experimental results, which indicate stress-dependent parameters, we consider a combined global data set of 127 main shock-aftershock sequences and perform a systematic study of the relationship between main shock-induced stress changes and associated seismicity patterns. For this purpose, we calculate space-dependent Coulomb Stress (& UDelta;CFS) and alternative receiver-independent stress metrics in the surrounding of the main shocks. Our results indicate a clear positive correlation between the GR b-value and the induced stress, contrasting expectations from laboratory experiments and suggesting a crucial role of structural heterogeneity and strength variations. Furthermore, we demonstrate that the aftershock productivity increases nonlinearly with stress, while the OU parameters c and p systematically decrease for increasing stress changes. Our partly unexpected findings can have an important impact on future estimations of the aftershock hazard.
Earthquake faults interact with each other in many different ways and hence earthquakes cannot be treated as individual independent events. Although earthquake interactions generally lead to a complex evolution of the crustal stress field, it does not necessarily mean that the earthquake occurrence becomes random and completely unpredictable. In particular, the interplay between earthquakes can rather explain the occurrence of pronounced characteristics such as periods of accelerated and depressed seismicity (seismic quiescence) as well as spatiotemporal earthquake clustering (swarms and aftershock sequences). Ignoring the time-dependence of the process by looking at time-averaged values – as largely done in standard procedures of seismic hazard assessment – can thus lead to erroneous estimations not only of the activity level of future earthquakes but also of their spatial distribution. Therefore, it exists an urgent need for applicable time-dependent models. In my work, I aimed at better understanding and characterization of the earthquake interactions in order to improve seismic hazard estimations. For this purpose, I studied seismicity patterns on spatial scales ranging from hydraulic fracture experiments (meter to kilometer) to fault system size (hundreds of kilometers), while the temporal scale of interest varied from the immediate aftershock activity (minutes to months) to seismic cycles (tens to thousands of years). My studies revealed a number of new characteristics of fluid-induced and stress-triggered earthquake clustering as well as precursory phenomena in earthquake cycles. Data analysis of earthquake and deformation data were accompanied by statistical and physics-based model simulations which allow a better understanding of the role of structural heterogeneities, stress changes, afterslip and fluid flow. Finally, new strategies and methods have been developed and tested which help to improve seismic hazard estimations by taking the time-dependence of the earthquake process appropriately into account.
Earthquake clustering has proven the most useful tool to forecast changes in seismicity rates in the short and medium term (hours to months), and efforts are currently being made to extend the scope of such models to operational earthquake forecasting. The overarching goal of the research presented in this thesis is to improve physics-based earthquake forecasts, with a focus on aftershock sequences. Physical models of triggered seismicity are based on the redistribution of stresses in the crust, coupled with the rate-and-state constitutive law proposed by Dieterich to calculate changes in seismicity rate. This type of models are known as Coulomb- rate and-state (CRS) models. In spite of the success of the Coulomb hypothesis, CRS models typically performed poorly in comparison to statistical ones, and they have been underepresented in the operational forecasting context. In this thesis, I address some of these issues, and in particular these questions: (1) How can we realistically model the uncertainties and heterogeneity of the mainshock stress field? (2) What is the effect of time dependent stresses in the postseismic phase on seismicity? I focus on two case studies from different tectonic settings: the Mw 9.0 Tohoku megathrust and the Mw 6.0 Parkfield strike slip earthquake. I study aleatoric uncertainties using a Monte Carlo method. I find that the existence of multiple receiver faults is the most important source of intrinsic stress heterogeneity, and CRS models perform better when this variability is taken into account. Epistemic uncertainties inherited from the slip models also have a significant impact on the forecast, and I find that an ensemble model based on several slip distributions outperforms most individual models. I address the role of postseismic stresses due to aseismic slip on the mainshock fault (afterslip) and to the redistribution of stresses by previous aftershocks (secondary triggering). I find that modeling secondary triggering improves model performance. The effect of afterslip is less clear, and difficult to assess for near-fault aftershocks due to the large uncertainties of the afterslip models. Off-fault events, on the other hand, are less sensitive to the details of the slip distribution: I find that following the Tohoku earthquake, afterslip promotes seismicity in the Fukushima region. To evaluate the performance of the improved CRS models in a pseudo-operational context, I submitted them for independent testing to a collaborative experiment carried out by CSEP for the 2010-2012 Canterbury sequence. Preliminary results indicate that physical models generally perform well compared to statistical ones, suggesting that CRS models may have a role to play in the future of operational forecasting. To facilitate efforts in this direction, and to enable future studies of earthquake triggering by time dependent processes, I have made the code open source. In the final part of this thesis I summarize the capabilities of the program and outline technical aspects regarding performance and parallelization strategies.