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Abstract High-frequency energy is often challenging to model because it is influenced by numerous factors in earthquake source processes. Hence, we investigate the factors that impact the high-frequency spectra of Mw 1.5–3 earthquakes from the 2019 Ridgecrest earthquake sequence, by clustering the earthquakes and stacking their velocity spectra to average source effects. We observe an azimuthal variation in the spectral amplitudes, where stations close to fault strike record more high-frequency energy than other stations. The increase in spectral amplitudes for stations that are near fault strikes is most apparent around 15–25 Hz, and a secondary high-frequency peak can be observed in some of the clusters. Our results are consistent across clusters with average depths of 2.0–9.7 km and average magnitudes of Mw 1.6–2.7. We evaluate the factors impacting high-frequency energy by calculating the relative site effects using regional earthquakes and modeling fault zone amplification and rupture directivity using dynamic rupture simulations. The analysis shows fault zone amplification may be the primary factor causing the observed azimuthal variation of high-frequency energy, though a combination of these factors may also contribute to the variability of high-frequency velocity spectra. 

SUMMARY Both short-term coseismic off-fault damage and long-term fault growth during interseismic periods have been suggested to contribute to the formation and evolution of fault damage zones. Most previous numerical models focus on simulating either off-fault damage in a single earthquake or off-fault plasticity in seismic cycles ignoring changes of elastic moduli. Here, we developed a new method to simulate the damage evolution of fault zones and dynamic earthquake cycles together in a 2-D antiplane model. We assume fault slip is governed by the laboratory-derived rate-and-state friction law while the constitutive response of adjacent off-fault material is controlled by a simplified version of the Lyakhovsky–Ben–Zion continuum brittle damage model. This study aims to present this newly developed modelling framework which opens a window to simulate the co-evolution of earthquakes and fault damage zones. We also demonstrate one example application of the modelling framework. The example simulation generates coseismic velocity drop as evidenced by seismological observations and a long-term shallow slip deficit. In addition, the coseismic slip near the surface is smaller due to off-fault inelastic deformation and results in a larger coseismic slip deficit. Here, we refer to off-fault damage as both rigidity reduction and inelastic deformation of the off-fault medium. We find off-fault damage in our example simulation mainly occurs during earthquakes and concentrates at shallow depths as a flower structure, in which a distributed damage area surrounds a localized, highly damaged inner core. With the experimentally based logarithmic healing law, coseismic off-fault rigidity reduction cannot heal fully and permanently accumulates over multiple seismic cycles. The fault zone width and rigidity eventually saturate at long cumulative slip, reaching a mature state without further change. 

ABSTRACT The spectral stress drop is a popular parameter for the simple quantification and characterization of an earthquake source and its expected seismic radiation, enabling investigation of earthquake spatial and temporal variability for larger numbers of events. In addition, spectral measurements are one of the few possible for earthquake characterization and hazard prediction in regions of low seismicity. However, spectral stress-drop estimates are uncertain, especially as recorded earthquakes may be too complex to characterize ideally with a single parameter. Empirical Green’s function (EGF) approaches to isolate the earthquake source are widely regarded as one of the best for individual analysis of well-recorded earthquakes. However, analysis decisions related to the selection of stations, EGFs, time windows, frequency bandwidth, and source models can cause discrepancies in resulting estimates of the source spectrum, source time function, and source parameters. We present results following one well-developed EGF approach, and compare it with those from three other independent methods applied to earthquakes in the 2019 Ridgecrest, California, earthquake, sequence selected for the Southern California Earthquake Center /U.S. Geological Survey Community Stress Drop Validation Study. The common data set consists of two weeks of earthquakes from the 2019 Ridgecrest earthquake sequence, including nearly 13,000 events of M 1 and greater, recorded on stations within 100 km. We obtain estimates of corner frequency and spectral stress drop for 75 earthquakes (M 2.2–4.6) and find varying degrees of similarity among studies. We investigate four events in detail (M 2.7–4.1) and find that we obtain consistent results when the sources are relatively simple. Multiple EGFs produce good ratios and source time functions at stations with good azimuthal distribution. This suggests that there is a role for such approaches to resolve the inherent ambiguity in larger scale inversions between source scaling and attenuation and site effects. 

Earthquakes may seem random, but are often concentrated in some localized areas. Thus, they are likely controlled by fault materials and stress heterogeneity, which are little understood. Here, we provide high-resolution observations of fault material and stress heterogeneity in the Japan subduction zone through an integration of material and source imaging with numerical simulations. Our results present evidence for localized, anisotropic structures with a near-zero Poisson’s ratio in the medium surrounding 1 to 2 kilometer–sized earthquake clusters, suggesting that the fault medium is damaged, foliated, and enriched with fluid. Such localized structures may cause stress perturbations on faults that in turn favor the frequent occurrence of deep interplate earthquakes at depths of 60 to 70 kilometers. Therefore, identifying the distribution and properties of fault material heterogeneity is important for more informed assessment of earthquake hazards. 

Earthquake nucleation is a crucial preparation process of the following coseismic rupture propagation. Under the framework of rate-and-state friction, it was found that the ratios of a to b parameters control whether earthquakes nucleate as an expanding crack or a fixed length patch. However, as an essential parameter in earthquake physics, critical slip distance DRS controls the weakening efficiency of fault strength and can influence the nucleation styles. Here we investigate the effects of DRS on nucleation styles in the context of fully dynamic seismic cycles by evaluating the evolution of the nucleation zone quantitatively when it accelerates from the tectonic loading rate to seismic slip velocity. The inferred values of DRS from small-scale laboratory faults are 1-100 μm, several orders smaller than those obtained from geophysical observations on large natural faults. Considering the scale-dependence of widely observed DRS, the ratio of DRS to velocity weakening asperity size W is applied to substitute the absolute value of DRS in this study. We find when DRS/W is relatively large (~10-5), a/b=0.5 can separate two nucleation styles as found previously. For a relatively small DRS/W (~10-6), however, a/b larger than 0.7 is necessary to produce the typical expanding crack-like nucleation style. When DRS/W<4x10-7 and a/b<0.8, the fixed length nucleation style dominates. For some cases with a/b>0.75, the initial yielding phase accelerates to a considerable slip velocity just before the subsequent expanding fracture phase, which may explain the generation of foreshock activities. Specially, the first yielding phase is possible to trigger dynamic events without a secondary fracture phase. Furthermore, when the nucleation site is not in the middle of the asperity, large enough a/b (e.g., 0.8) could induce a complex nucleation style as well as abundant interseismic aseismic transients. We also recognize a special twin nucleation style that incorporates a failed acceleration phase. Our results reveal the critical role of DRS on earthquake nucleation styles and suggest that the fixed length nucleation style may be more common for the range of DRS/W (~10-4-~10-7) observed on natural and laboratory faults. 

Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. Hence, our research aims to develop a novel method to image fault damage zones using high-frequency P-waves reflected within them. Previous studies have demonstrated that fault damage zones can amplify high-frequency waves along directions close to fault strike. The associated frequency band of the amplified secondary peak may be used to estimate the width and velocity contrast of the fault damage zone. Here we use the stacked P-wave velocity spectra of M1.5–3 earthquakes in the Parkfield region to identify the azimuthal variation in high-frequency energy. Our preliminary results show that for 62% of the Parkfield clusters, stations close to the fault strike record more high-frequency energies around 10–20 Hz. The frequency band is lower than what we observed for the 2019 Ridgecrest earthquakes region, and corresponds to a fault zone velocity reduction of ~50% assuming a fault zone width of 200m. We also observe along-strike differences in our results, where clusters along some fault sections show greater azimuthal variation than clusters in other sections. Moreover, to account for the possible effects of site conditions underneath the stations, we will quantify their effects using the spectra of regional earthquakes. We will compute the root-mean-square spectra at different frequency bands for each event, and calculate the average deviation in spectra at each station. We can then generate an empirical correction term for each station as a function of frequency. By applying these corrections to the stacked P-wave velocity spectra of our earthquake clusters, we can separate the contribution of site effects from fault zone structures. Our results demonstrate that the new method can be applied to search for fault damage zone structures in different tectonic regions with broadband stations in order to enhance our understanding of the co-evolution of fault zones and earthquake cycle. 

Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed by stacking P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016), and the frequency band which is amplified may be used to estimate the width and velocity contrast of the fault damage zone. We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-80 km in the 2019 Ridgecrest earthquake regions. First, we group the Ridgecrest earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. Our results show that the stations close to the fault strike record more high-frequency energies around the characteristic frequency of fault zone reflections. We find that the increase in the amount of high-frequencies is consistent across clusters with average magnitudes ranging from 1.6-2.4, which suggests that the azimuthal variation in spectra is caused by fault zone amplification rather than rupture directivity. We will apply our method to other fault zones in California, in order to search for fault damage zone structures and estimate their material properties. 

The temporal variation of elastic property of the bulk material surrounding the fault is considered an important contribution to the observed co-seismic velocity reduction and interseismic healing. Paglialunga et al. [2021] found that as fault normal stress increases, co-seismic velocity reduction becomes larger because more cracks reopen with higher stress drops. Larger normal stress can lead to smaller nucleation size and contribute to larger co-seismic slip. By contrast, with larger co-seismic velocity reduction and interseismic healing, more slow slip events can propagate in the seismogenic zone [Thakur and Huang, 2021], because the temporal velocity change related to fault zone damage modulates earthquake nucleation. Hence, fault normal stress and temporal damage zone structure evolution have opposite influences on the spatial distribution and recurrence intervals of earthquakes. We conducted 2-D anti-plane fully-dynamic seismic cycle simulations and explored the effects of fault normal stress on seismic cycle when there is coseismic damage and interseismic healing in the fault damage zone. The normal stress is in a range of 40-70 MPa and the co-seismic rigidity reduction is in a range of 5-8%. We find larger normal stress results in larger co-seismic slip and fewer slow slip events, while more co-seismic velocity reduction and interseismic healing leads to more partial ruptures as well as slow slip events. With the increase of both normal stress and seismic velocity change, more regular earthquakes occur and slow slip events gradually disappear. For the selected parameter space, the influence of seismic velocity change is not as significant as the effect of normal stress. However, fault zone maturity or the initial rigidity of fault damage zones should also affect the competitive relationship between normal stress and seismic velocity change, and we will characterize earthquakes and slow-slip events in immature and mature fault damage zones when both on-fault normal stress and off-fault seismic velocity vary over earthquake cycles.