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(a) and (b) shows the corner frequency í µí± !%& as a function of the corner frequency í µí± ! and í µí± $
Source publication
Although the Brune source model describes earthquake moment release as a single pulse, it is widely used in studies of complex earthquakes with multiple episodes of high moment release (i.e., multiple subevents). In this study, we investigate how corner frequency estimates of earthquakes with multiple subevents are biased if they are based on the B...
Contexts in source publication
Context 1
... Figure 7 we evaluate the significance of the corner frequency í µí± +,-of the 714 268 SCARDEC STFs that are decomposed to have two subevents. The correlation between í µí± +,-andcross-correlation coefficients of about 0.90 and 0.57, respectively. ...
Context 2
... í µí± 9 /í µí± +,-is expected to approach 1 theoretically for the highest values of í µí±, 284 we suspect that the misfit of the decomposition of STF renders Ω +./ to have a slightly different 285 frequency content than í µí»º +,-. Figures 7c and 7d show that for an increasing absolute onset time 286 difference |í µí±| between subevents, í µí± +,-and í µí± 9 decreases. This is consistent with the fact that |í µí±| 287 controls the total source duration, which is inversely proportional to the corner frequency of the 288 Brune pulse. ...
Citations
... However, even modest estimates showed that ∼25% had multiple pulses that differ significantly from the source models. The widely used source models produce erroneous results when applied to such complex events (Abercrombie, 2021;Liu et al., 2023). The previous studies that examined a few small events with an intensive observation network suggest that it is not uncommon for small events to exhibit complexity (Ide and Beroza, 2001;Yamada et al., 2005;Uchide and Ide, 2010;Taira et al., 2015;Wu et al., 2019). ...
Small earthquakes (Mw <5) may have a similar degree of complexity as large earthquakes. However, their seismic waveforms are strongly distorted during wave propagation, making their complexity challenging to resolve. In many cases, the source parameters of small events are determined based on models that assume their source patterns are simple. In this study, to directly examine the source complexities in small events, we examined high-quality near-source (<8 km) seismic waveforms recorded by two excellent downhole sensors in Japan. The results show that the P waveforms of microearthquakes (Mw <2) are always simple at the sensors and agree well with the synthetic waveforms based on a 1D structure up to 20 Hz. The microearthquake waveforms in this frequency band essentially represent path effects besides the static source effect, suggesting that the contribution of structural inhomogeneity to the observed waveforms is small. Taking advantage of this, we inferred the moment rate functions of 164 Mw 3.3–5.0 events from the shapes of the direct P waves. They showed diversity in their complexity, and even conservatively estimated, 25% of the events had multiple subevents. The results suggest that methods that account for complexity, rather than those that assume a simple source pattern, are required to characterize even small events.
... This is consistent with the recent observation that rupture velocities can potentially impact stress-drop estimates of earthquakes 47 . Our results also clearly show that the Sparta mainshock occurred in a complex fault system, potentially rupturing two faults with distinct tectonic kinematics, which also impacts stress drop estimates 48 . We note that our stress drop estimates are obtained using a simple model that considers an instantaneous circular crack with a simple pulse and constant rupture velocity 22 , which may not be adequate given our observations at Sparta. ...
... We note that our stress drop estimates are obtained using a simple model that considers an instantaneous circular crack with a simple pulse and constant rupture velocity 22 , which may not be adequate given our observations at Sparta. Additional analysis is necessary to answer this question and better understand the expected ground motions from moderate Eastern North America earthquakes, possibly using higher-sampling rate seismometers to extend our measurements to smaller earthquakes in the area or using a more complex source model to estimate stress drops 48 . ...
On August 9, 2020, an Mw 5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina, creating the first co-seismic faulting surface rupture documented in the Eastern United States. Combining deep learning and matched filter earthquake detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes, about 5.8 times the number of events listed in the standard USGS/NEIC catalog. The relocated seismicity revealed a complex fault structure with distinct planar alignments, supported by a moment tensor inversion with significant non-double-couple component. The Sparta mainshock with a centroid depth of 1.3 km is interpreted to have nucleated near the intersection of two main fault strands. The mainshock likely ruptured a blind strike-slip fault and a reverse fault associated with the identified surface rupture, both possibly part of a flower structure-like diffuse fault zone. Our observations highlight a complex behavior of extremely shallow earthquakes in stable continental regions.
... For complex models, the source spectra deviate from the simple Brune spectra, and the corner frequency thus estimated is not directly related to stress conditions on the fault. Indeed, this issue has been recognized in real-event analyses (Archuleta and Ji, 2016;Denolle and Shearer, 2016;Liu et al., 2023) and dynamic source modeling (Gallovič and Valentová, 2020). ...
The region of central Italy is well known for its moderate-to-large earthquakes. Events such as 2016 Mw 6.2 Amatrice, generated in the shallow extensional tectonic regime, motivate numerical simulations to gain insights into source-related ground-motion complexities. We utilize a hybrid integral–composite kinematic rupture model by Gallovič and Brokešová (2007) to predict ground motions for other hypothetical Amatrice fault rupture scenarios (scenario events). The synthetic seismograms are computed in 1D crustal velocity models, including region-specific 1D profiles for selected stations up to 10 Hz. We create more than ten thousand rupture scenarios by varying source parameters. The resulting distributions of synthetic spectral accelerations at periods 0.2–2 s agree with the empirical nonergodic ground-motion model of Sgobba et al. (2021) for central Italy in terms of the mean and total variability. However, statistical mixed-effect analysis of the residuals indicates that the between-event variability of the scenarios exceeds the empirical one significantly. We quantify the role of source model parameters in the modeling and demonstrate the pivotal role of the so-called stress parameter that controls high-frequency radiation. We propose restricting the scenario variability to keep the between-event variability within the empirical value. The presented validation of the scenario variability can be generally utilized in scenario modeling for more realistic physics-based seismic hazard assessment.
... On the other hand, the corner frequency of earthquake source spectra is affected by the localized rupture area with large slip rather than the overall rupture area, as described before. This observation is also supported by Boatwright (1984) and Liu et al. (2023). Thus, it is reasonable that the stress drop calculated from the earthquake corner frequency and the k s value derived from the dynamic crack simulations is close to ∆σ la rather than ∆σ M (Shimmoto, 2022). ...
Understanding the scaling laws of source parameters is a fundamental subject in seismology. This study conducts spectral ratio analysis to estimate the source parameters for 409 shallow crustal earthquakes with Mw 3.2–6.0 in Japan. Subsequently, the source parameter scaling relations are investigated for a wide magnitude range by combining the results of this and previous studies. The spectral ratio method applied in this study provides the finite source properties of the localized area with large slip. The single asperity model, a simple heterogeneous source model with a single localized area with large slip, is used to estimate the stress drop since it can be more suitable to characterize earthquake sources than the standard homogeneous circular source. This investigation calibrates the constant to link the corner frequency and the source radius by comparing the corner frequency and source area estimated by the spectral ratio analyses. This calibrated constant is used to re‐calculate the stress drop of small earthquakes from the corner frequency estimated in previous studies. The stress drop and apparent stress increase with increasing magnitude up to Mw ∼ 5 and become magnitude‐independent for larger earthquakes. The radiation efficiency ranges typically from 0.1 to 1.0 and is independent of magnitude. The slip dependences of the stress drop, apparent stress, radiation efficiency, and fracture energy observed in this study are explained by the ones predicted from the slip‐weakening model incorporating the thermal pressurization effect. Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations.
... The standard deviation of f c in our models strongly depends on the azimuth and is generally higher in the vertical components. Liu et al. (2023) found that Brune-type corner frequencies of the spectra of the source time functions of complex events correlate best with the corner frequency of that subevent with the highest moment release. They conclude that the Brune stress drop reflects the stress change of the largest asperity. ...
Dynamic rupture simulations generate synthetic waveforms that account for nonlinear source and path complexity. Here, we analyze millions of spatially dense waveforms from 3D dynamic rupture simulations in a novel way to illuminate the spectral fingerprints of earthquake physics. We define a Brune-type equivalent near-field corner frequency (fc) to analyze the spatial variability of ground-motion spectra and unravel their link to source complexity. We first investigate a simple 3D strike-slip setup, including an asperity and a barrier, and illustrate basic relations between source properties and fc variations. Next, we analyze >13,000,000 synthetic near-field strong-motion waveforms generated in three high-resolution dynamic rupture simulations of real earthquakes, the 2019 Mw 7.1 Ridgecrest mainshock, the Mw 6.4 Searles Valley foreshock, and the 1992 Mw 7.3 Landers earthquake. All scenarios consider 3D fault geometries, topography, off-fault plasticity, viscoelastic attenuation, and 3D velocity structure and resolve frequencies up to 1–2 Hz. Our analysis reveals pronounced and localized patterns of elevated fc, specifically in the vertical components. We validate such fc variability with observed near-fault spectra. Using isochrone analysis, we identify the complex dynamic mechanisms that explain rays of elevated fc and cause unexpectedly impulsive, localized, vertical ground motions. Although the high vertical frequencies are also associated with path effects, rupture directivity, and coalescence of multiple rupture fronts, we show that they are dominantly caused by rake-rotated surface-breaking rupture fronts that decelerate due to fault heterogeneities or geometric complexity. Our findings highlight the potential of spatially dense ground-motion observations to further our understanding of earthquake physics directly from near-field data. Observed near-field fc variability may inform on directivity, surface rupture, and slip segmentation. Physics-based models can identify “what to look for,” for example, in the potentially vast amount of near-field large array or distributed acoustic sensing data.
... This comparison is consistent with the work of Lin and Lapusta (2018), Gallovič and Valentová (2020) and Liu et al. (2023), implying that source complexity can significantly distort spectral measurements of absolute, and relative stress drop that depend on the assumption of simple source models. In this case of two similar-sized earthquakes, the spectral source estimates find the Amatrice to have a smaller rupture area, and higher average slip, whereas the kinematic finite fault inversions reveal the opposite. ...
Estimates of spectral stress drop are fundamental to understanding the factors controlling earthquake rupture and high frequency ground motion, but are known to include large, poorly‐constrained uncertainties. We use earthquakes from the 2016–2017 sequence in the Italian Appenines (largest event at Norcia, Mw 6.3) to investigate these uncertainties and their causes. The similarly‐sized events near Amatrice (Mw 6.0) and Visso (Mw 5.9) enable better constrained relative analysis. We calculate S wave source spectra, corner frequencies, and spectral stress drop for 30 of the larger events. We compare both empirical and modeling approaches to isolate the source spectra and calculate source parameters; we also compare our results with those from published studies. Both random and systematic inter‐study variations are larger than the standard errors reported by any individual study. The reported magnitude dependence of stress drop varies between studies, being largest for generalized inversions and smallest for more individual event based approaches. The relative spectral estimates of inter‐event stress drop are more consistent; all approaches estimated higher stress drop in the Amatrice earthquake than the similar‐sized Visso earthquake. In contrast, finite fault inversions of these two earthquakes found that the Visso earthquake had the larger region of concentrated, higher slip, whereas the Amatrice earthquake had multiple, lower slip, subevents. The Amatrice spectra contain more high frequency energy than those of the Visso earthquake. This comparison suggests that consistent measurement of a higher spectral stress drop indicates greater high‐frequency ground motion but may correspond to greater rupture complexity rather than higher stress drop.
Dynamic rupture simulations generate synthetic waveforms that account for non-linear source, path, and site complexity. Here, we analyze millions of spatially dense waveforms from 3D dynamic rupture simulations in a novel way to illuminate the spectral fingerprints of earthquake physics. We define a Brune-type equivalent near-field corner frequency ($f_c$) to analyze the spatial variability of ground motion spectra and unravel their link to source complexity. We first investigate a simple 3D strike-slip setup, including an asperity and a barrier, and illustrate basic relations between source properties and $f_c$ variations. Next, we analyze > 13,000,000 synthetic near-field strong motion waveforms generated in three high-resolution dynamic rupture simulations of real earthquakes, namely, the $M_w$ 7.1 2019 Ridgecrest mainshock, the $M_w$ 6.4 Searles Valley foreshock, and the $M_w$ 7.3 1992 Landers earthquake. All scenarios consider 3D fault geometries, topography, off-fault plasticity, viscoelastic attenuation, 3D velocity structure, and resolve frequencies up to 1-2 Hz. Our analysis reveals pronounced and localized patterns of elevated $f_c$, specifically in the vertical components. We validate such $f_c$ variability in observed near-fault spectra. Using isochrone analysis, we identify the complex dynamic mechanisms that explain the rays of elevated $f_c$ and cause unexpectedly impulsive, localized, vertical ground motions. While the vertical high frequencies are also associated with path effects, rupture directivity, and coalescence of multiple rupture fronts, we show that they are dominantly caused by rake-rotated surface-breaking rupture fronts that decelerate due to fault heterogeneities or geometric complexity. Our findings highlight the potential of spatially dense ground motion observations for furthering our understanding of earthquake physics directly from near-field data.
On August 9, 2020, an M w 5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina. The earthquake generated a ~2 km-long surface rupture, the first co-seismic faulting rupture identified in the Eastern United States. Combining deep learning earthquake phase picking, and matched filter detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes with high-resolution locations, about 5.8 times the number of events listed in the standard NEIC catalog. Our results show the Sparta mainshock has a centroid depth of 1.3 km and nucleated near the intersection point of two fault strands, a blind strike-slip fault where the rupture possibly initiated, and a reverse fault associated with the identified surface rupture, possibly part of a flower structure-like diffuse fault zone. We also observe that earthquake stress drops during the Sparta sequence are on the lower end of estimates for similar magnitude Eastern North American earthquakes.
