PhD-Project I3 by Sebastian von Specht (GFZ/UP): Towards non-ergodic, time dependent, ground-motion models

Timescale: Oct. 2015 – Oct. 2019

Supervisors:
Prof. Dr. Fabrice Cotton, Helmholtz Centre Potsdam GFZ German Research Centre For Geosciences
Prof. Dr. Frank Scherbaum, University of Potsdam

Background
In seismic hazard analysis, we are interested in the variations in ground-shaking amplitudes due to earthquakes (i.e., ground-motions) at a particular site over time (i.e., with repeated earthquakes). Since in general we do not have observations over long periods at any site, records from many sites (and regions) are generally used to estimate the average and the variability of the potential ground-motions. Regional variations and time dependencies of earthquake properties are usually not taken into account to predict potential ground-motions. For example, several authors have suggested that aftershocks may generate lower ground motions than mainshocks, but this working hypothesis has not been tested rigorously. We have shown recently that the exponential growth of seismological records close to earth-quakes provides the opportunity to separate the source, regional propagation, and site factors controlling ground-motion variability. We also showed that the key to accommodate differences in regional or time-dependent seismological attributes of a ground motion model lies in the Fourier spectrum. We have initiated a new approach for the development of models able to predict potential ground-shakings for a given earthquake scenario. This approach is fully consistent with linear system theory when it comes to adjustment issues.

Objectives and Methods
The project aimed at developing data-driven and physics-based ground motion models which take into account regional variations and potential time dependencies of ground motions. We analyzed high-quality strong-motions databases and discussed the fault properties variations (e.g. stress-drops) that may explain the observed ground-motion time dependencies. In the present project, we  developed new methods to identify the components of ground-motion variability at a single site that are repeatable rather than purely random (e.g., the site response, regional variations), so these can be removed from the aleatory uncertainty. We analyse the time dependencies of the observed ground-motions and discuss the fault property variations (e.g., stress-drop) that may explain these variations. We finally developed data-driven and physics-based ground-motion prediction models that take into account regional variations and potential time-dependencies of ground-motions. The methods include classical and Bayesian data and uncertainty analyses. We also continued to improve and analyse high-quality strong-motions databases mainly for Europe and Asia (ongoing GFZ Central Asia and North Anatolian fault projects, ongoing collaboration with institutions in Italy (INGV), Turkey (METU) France (Grenoble) and others.

Publications within NatRiskChange:

Ozturk U., Marwan N., SPECHT, S., Korup O., Jensen J.: Contemporary centennial sea-level record at Antalya (eastern Mediterranean), JGR-Oceans, doi: 10.1029/2018JC013906, 2018. von Specht, S., Ozturk, U., Veh, G., Cotton, F., Korup, O. (2019): Effects of finite source rupture on landslide triggering: The 2016 /M_W / 7.1 Kumamoto earthquake, Solid Earth, 10, 463–486,  https://doi.org/10.5194/se-10-463-2019.

Piña-Valdés J., Socquet, A., Cotton, F., Specht, S. (2018): Spatiotemporal Variations of Ground Motion in Northern Chile before and after the 2014 Mw 8.1 Iquique Megathrust Event. - Bulletin of the Seismological Society of America, 108, 2, pp. 801814. http://doi.org/10.1785/0120170052

Socquet, A., Valdez, J. P., Jara, J., Cotton, F., Walpersdorf, A., Cotte, N., SPECHT, S., Ortega-Culaciati, F., Carrizo, D., Norabuena, E. (2017): An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust. - Geophysical Research Letters, 44, 9, pp. 40464053. doi.org/10.1002/2017GL073023

Veh, G., Korup, O., von SPECHT, S., Roessner, S., Walz, A. (2019): Unchanged frequency of moraine-dammed glacial lake outburst floods in the Himalaya, Nature Climate Change, 9: 379–383(, doi.org/10.1038/s41558-019-0437-5.

von SPECHT, S., Heidbach, O., Cotton, F., Zang, A. (2018). Uncertainty reduction of stress tensor inversion with data-driven catalogue selection. Geophysical Journal International. https://doi.org/10.1093/gji/ggy240