Rupture Process of the Mainshock of the 2019 Ridgecrest Earthquake Sequence from Waveform Inversion with Empirical Green’s Functions

ABSTRACT The 2019 Mw 7.1 mainshock of the Ridgecrest earthquake sequence, which was the first event exceeding Mw 7.0 in California since the 1999 Hector Mine earthquake, caused near-fault ground motions exceeding 0.5g and 70 cm/s. In this study, the rupture process and the generation mechanism of strong ground motions of the mainshock were investigated through waveform inversions of strong-motion data in the frequency range of 0.2–2.0 Hz using empirical Green’s functions (EGFs). The results suggest that the mainshock involved two large slip regions: the primary one with a maximum slip of approximately 4.4 m was centered ∼3 km northwest of the hypocenter, which was slightly shallower than the hypocenter, and the secondary one was centered ∼25 km southeast of the hypocenter. Outside these regions, the slip was rather small and restricted to deeper parts of the fault. A relatively small rupture velocity of 2.1 km/s was identified. The robustness of the slip model was examined by conducting additional inversion analyses with different combinations of EGF events and near-fault stations. In addition, using the preferred slip model, we synthesized strong motions at stations that were not used in the inversion analyses. The synthetic waveforms captured the timing of the main phases of observed waveforms, indicating the validity of the major spatiotemporal characteristics of the slip model. Our large slip regions are also generally visible in the models proposed by other researchers based on different datasets and focusing on lower frequency ranges (generally lower than 0.5 Hz). In particular, two large slip regions in our model are very consistent with two of the four subevents identified by Ross et al. (2019), which may indicate that part of the large slip regions that generated low-frequency ground motions also generated high-frequency ground motions up to 2.0 Hz during the Ridgecrest mainshock.

Processing seismic ambient noise data with the continuous wavelet transform to obtain reliable empirical Green's functions

SUMMARY Obtaining reliable empirical Green's functions (EGFs) from ambient noise by seismic interferometry requires homogeneously distributed noise sources. However, it is difficult to attain this condition since ambient noise data usually contain highly correlated signals from earthquakes or other transient sources from human activities. Removing these transient signals is one of the most essential steps in the whole data processing flow to obtain EGFs. We propose to use a denoising method based on the continuous wavelet transform to achieve this goal. The noise level is estimated in the wavelet domain for each scale by determining the 99 per cent confidence level of the empirical probability density function of the noise wavelet coefficients. The correlated signals are then removed by an efficient soft thresholding method. The same denoising algorithm is also applied to remove the noise in the final stacked cross-correlogram. A complete data processing workflow is provided with the overall data processing procedure divided into four stages: (1) single station data preparation, (2) removal of earthquakes and other transient signals in the seismic record, (3) spectrum whitening, cross-correlation and temporal stacking and (4) remove the noise in the stacked cross-correlogram to deliver the final EGF. The whole process is automated to make it accessible for large data sets. Synthetic data constructed with a recorded earthquake and recorded ambient noise is used to test the denoising method. We then apply the new processing workflow to data recorded by the USArray Transportable Array stations near the New Madrid Seismic Zone where many seismic events and transient signals are observed. We compare the EGFs calculated from our workflow with commonly used time domain normalization method and our results show improved signal-to-noise ratios. The new workflow can deliver reliable EGFs for further studies.

Non‐Double‐Couple Moment Tensors of Earthquakes Calculated Using Empirical Green’s Functions

We present a joint inversion for empirical Green’s functions (EGFs) and high‐resolution non‐double‐couple (non‐DC) moment tensors. First, the EGFs are constructed using known moment tensors of earthquakes occurring in a small focal zone. Second, the estimated EGFs are applied to refine the original moment tensors used for constructing the EGFs. Because the EGFs describe the velocity model better than the standard GFs, the refined moment tensors are more accurate. The method is applied to real observations of earthquakes of the 2008 swarm in West Bohemia, Czech Republic, where tiny details in fracturing in the focal zone are revealed. Refined moment tensors indicate fault closing caused by compaction of fault gouge during fracturing process related to fault weakening by fluids in the focal zone. The application of the proposed inversion can improve moment tensors reported in existing local, regional, or global catalogs for areas with a concentrated seismicity.

Optimization of a Simulation Code Coupling Extended Source (k−2) and Empirical Green’s Functions: Application to the Case of the Middle Durance Fault

We developed a ground-motion simulation code base on extended rupture modeling combined with the use of empirical Green’s functions (EGFs), adapted for low-to-moderate seismicity regions (with a limited set of EGFs), and extended its range of applicability to the lowest source-to-site distances. This code is based on a kinematic source description of an extended fault and is designed to allow complex fault geometries and to generate a ground motion variability in agreement with that of the recorded databases. The code is developed to work with a sparse set of EGFs. Each available EGF is therefore used in several positions on the rupture area. To be used in positions different of their original position, we applied to the EGFs some adjustments. In addition to the classical adjustments (i.e. time delay correction, geometrical spreading correction and anelastic attenuation correction), we propose here a radiation pattern adjustment. The effectiveness of it is tested in a numerical application. We showed noticeable improvements at the lowest distances, and some limitations when approaching the nodal planes of the subevents the recording of which were used as EGFs. We took advantage of the development of this code, its ability to work with a sparse set of EGFs, its ability to take into account complex fault geometries and its ability to master the general variability, to perform a ground-motion simulation scenario on the Middle Durance Fault (MDF). We perform simulations for a hard rock site (VS30 = 1800 m/s) and a sediment site (VS30 = 440 m/s) of the CEA Nuclear Research Site of Cadarache (France), and compared the computed ground motion with several ground motion prediction equations (GMPEs). The GMPEs slightly underestimate the sediment site but strongly overestimate the ground motion amplitude on the hard rock site, even when using a specific correction factor which adapts GMPEs predictions from rock site to hard rock site. This general ascertainment confirms the need to continue efforts towards the establishment of consistent GMPEs applicable to hard-rock conditions.