UC Santa Barbara

Accurate images of the growth of slip and faulting are critical in understanding the physics of earthquakes, and subsequently aid in the prediction of the ground motion that could be expected from future rupture events. Many methods, namely finite fault inversions, have been developed that use observed seismic waveforms and static deformations to constrain the spatiotemporal rupture evolutions of great earthquakes. This work explores the development of novel inversion methods to further the investigation into properties of earthquake physics.

Earthquakes lead to the overall reduction of stress across the ruptured fault plane, and stress drop is a key parameter in accurately estimating the strong ground motion. Thus, we have developed a new procedure to determine if it is possible to robustly constrain an uncertainty range for the co-seismic stress drop of large earthquakes. For a given earthquake, we loop through a series of target stress drops, and for each case we conduct a modified finite fault inversion to find the solution that matches the seismic and/or geodetic data, and also matches a prescribed stress drop value. From this, we can determine a relationship between the resulting misfit between our synthetic model and the observations, and the pre-assigned target stress drop value.

This new inversion technique is applied to several rupture events with varying datasets in order to test its robustness. First, we examine the 2014 Mw 7.9 Rat Islands earthquake using far-field data and discovered that only the lower bound of the average stress drop could be well constrained. To investigate whether such a conclusion also holds for near field data, the slip distribution and the stress drop of the 2015 Mw 7.8 Gorkha, Nepal earthquake was studied using GPS and InSAR data. Our results revealed similar patterns: that even a comprehensive geodetic dataset could also only constrain the lower bound of stress drop. Furthermore, one of the important uses of stress drop for earthquake physics is in the study of energy partitioning. The average stress drop is equivalent to twice the ratio of apparent available energy to the total seismic potency, and so our result has a direct impact on the study of the earthquake energy budget. As stress drop is proportional to the available seismic energy, our results imply that only the lower bound of the available energy can be constrained.

The November 14th 2016 MW 7.8 Kaikoura, New Zealand earthquake occurred along the northern part of the South Island. Available information indicates that this earthquake involved multiple fault segments of the Marlborough fault system (MFS). Additionally, slip might also have occured on the subduction interface of the Pacific Plate under the Australian Plate, beneath the MFS. The exact number of involved fault segments as well as the temporal co-seismic rupture sequence has not been fully determined. We propose two inversion strategies to determine the fault geometry and spatiotemporal rupture history: first we use teleseismic and strong motion waveforms to determine point-source focal mechanisms for all of the faults that participated in the rupture; second, we use seismic and geodetic data to invert for the kinematic rupture parameters on a limited number of fault segments. The first approach allows us to determine a rupture timing sequence for different fault segments. Once we have the timing among fault segments, we invert the seismic and geodetic data for the spatial and temporal kinematic parameters. Both of these methods allow us to evaluate the potential of slip on the Hikurangi subduction interface.