Stress Drop Estimates Research Articles

Combining Two Distinct Methods to Resolve Spatial Variation in Attenuation and Earthquake Source Parameters

ABSTRACT Stress drop is a fundamental parameter in ground-motion modeling and seismic hazard assessment, but spectral estimates are subject to considerable uncertainties. A variety of factors cause different methods to yield different results, including the complexity of the seismic source, the assumptions inherent in the models used, the limited range of frequencies available, and the inherent difficulty in removing the propagation effects along the wave path. A primary challenge is determining whether the observed variations in spectral stress-drop estimates represent characteristics of the seismic source or the propagation path. We compare the performance of two methods applied to the 2019 Ridgecrest, California, earthquake sequence, each of which addresses the trade-offs between propagation and source in different ways. The first method, referred to as the spectral-fitting approach, operates on the hypothesis that the path effects remain constant across the spatial and temporal range of the sources under investigation. This approach assumes a level of uniformity in the propagation effects that simplifies the analysis. The second method, referred to as the spectral ratio approach, is based on the hypothesis that a small, collocated event will experience identical propagation effects to the earthquake of interest, potentially accounting for smaller scale variation in propagation effects. Our comparison reveals that the choice of method is not only influenced by the specifics of the data and the seismic events but also significantly constrained by the geological heterogeneity and consequent spatial variability of site and propagation effects in the study area. If an approach involves assuming a homogeneous attenuation structure, any spatial variation in attenuation structure will lead to this variation being incorrectly mapped into apparent source stress-drop variations. Understanding the local geology and structural heterogeneity, combined with using methods with contrasting underlying assumptions are good approaches to improving the reliability of estimated spectral stress drops.

Earthquake Source Spectra Estimates Vary Widely for Two Ridgecrest Aftershocks Because of Differences in Attenuation Corrections

ABSTRACT Differences in stress-drop estimates among groups of scientists for the same earthquakes suggest disagreement in the shape of the source spectra that are used to measure corner frequency. A critical step in characterizing source spectra involves applying empirical corrections for site effects and the loss of high-frequency energy that occurs along the source–receiver path. As part of the Ridgecrest stress-drop validation study, we compare path-corrected source spectra among different methods for two nearly collocated M 3 earthquakes and investigate whether systematic differences in the applied path corrections are affecting corner-frequency estimates. We find substantial disagreements in the path corrections, which are well approximated with a simple exponential function related to the strong ground motion parameter κ. These κ differences are strongly correlated with corner-frequency estimates for path-corrected spectra, suggesting they are a large source of systematic differences in corner frequency (and inferred stress drop) among the methods, reflecting varying trade-offs between the source and path contributions to observed spectra. Because each method presumably fits the data it uses sufficiently well, these results indicate the limitations of existing purely empirical techniques to estimating path corrections and the need for new approaches.

Ridgecrest Aftershock Stress Drops from P- and S-Wave Spectral Decomposition

ABSTRACT Seismic moment and stress drop are crucial for understanding earthquake rupture processes, but their estimates often have large uncertainties for small earthquakes. Stress drop is typically inferred from an earthquake’s source spectrum based on theoretical models, but poorly constrained path corrections and other modeling assumptions limit the accuracy of stress-drop estimates. Here, we compute stress drops using both P and S waves for the 2019 Ridgecrest earthquake sequence, compare their estimates, and evaluate the associated uncertainties. We use spectral decomposition and apply the analysis to both types of waves for the same set of earthquakes, adjusting some S-wave parameter choices to obtain overall consistency with our P-wave results. Our approach fixes the corner frequency of small calibration earthquakes to reduce scatter in the estimated source parameters of the larger earthquakes. We find that assuming a lower high-frequency fall-off rate for S waves yields more consistent absolute stress-drop estimates between P and S waves. Our stress-drop estimates appear to increase slightly with magnitude for earthquakes with magnitudes >∼3.4. Furthermore, we find that the stress-drop estimates using both types of data exhibit coherent spatial variations. Earthquakes near the Coso geothermal field tend to have lower stress drops, and earthquakes near the M 7.1 hypocenter have higher stress-drop estimates. This spatial pattern is consistently observed in both the P- and S-wave results. We find no strong correlation between our stress-drop estimates and the M 7.1 Ridgecrest earthquake slip distribution, suggesting a heterogeneous stress environment for the Ridgecrest fault system.

Statistical and source characterization of the 2023 Kahramanmaraş Türkiye earthquake sequence

AbstractWe studied seismic features of the 2023 Kahramanmaraş Türkiye earthquake sequence by analyzing the spatiotemporal behavior of the b- and -p values and source characteristics of the mainshocks. We complemented our study by determining the regional stress field. The b-values in the region varied from 0.45 to 1.15. We observed a slight b-value decrease (Δb≈0.2) months before the two mainshocks. Our results exhibited complex b-value patterns on the fault planes and regular aftershock productivity rates (1.14 < p < 1.25). We compare static stress drop estimates derived from effective fault dimensions to those of finite-fault dimensions. Total effective stress drops (the sum of the stress drops of all fault segments) for the earthquake doublet were almost identical (~ 2.05 MPa), while those from finite-fault dimensions are somewhat lower (0.35 and 0.96 MPa). Based on a complete stress drop case, effective seismic efficiency was 0.65 and 0.43 for both mainshocks. The amount of partial stress drop was used to discriminate between different stress drop models. No clear model is discerned for the first mainshock, but a partial or even complete stress drop, assuming finite-fault dimensions, is supported by our measurements. Stress drop estimations derived from spectral analysis (25.46 and 34.39 MPa) agreed with global studies but larger than finite and effective fault estimates. Stress inversion results indicated that in the fault segments where the mainshocks occurred, the orientation of principal axes was consistent with a strike-slip regime. Conversely, the normal-faulting regime dominates adjacent areas of the main fault system.

Evidence for Low Effective Stress Within the Crust of the Subducted Gorda Plate from the 2022 December Mw 6.4 Ferndale Earthquake Sequence

Abstract Stress levels on and adjacent to megathrust faults at seismogenic depths remain a key but difficult-to-constrain parameter for assessing seismic hazard in subduction zones. Although strong ground motions have been observed to be generated from distinct, high-stress regions on the down-dip end of the megathrust rupture areas in many great earthquakes, we lack direct constraints on the stress level in the lower seismogenic portion of the Cascadia megathrust. On 20 December 2022, an Mw 6.4 strike-slip earthquake occurred near Ferndale, California, in southern Cascadia and likely ruptured the Gorda slab crust in the lower seismogenic portion, providing an opportunity to assess the stress level in this region. Here, we relocate the Ferndale mainshock and the first two weeks of aftershocks using a high-resolution 3D velocity model and estimate rupture dimensions, directivity, and stress drop for several Mw 4–5 aftershocks and recent earthquakes. The aftershocks define a strike-slip fault in the slab crust striking east-northeast, consistent with the mainshock focal mechanism. The orientation of this fault is about 45° off the ideally oriented fault plane given the stress state in the slab. The aftershock zone is extensive and broad in the forward direction of the mainshock rupture but still constrained within the volume of high VP/VS in the slab crust. Our stress-drop estimates are generally lower for Mw 4–5 earthquakes located in the slab crust compared to those a few kilometers deeper in the slab mantle. Combined, our results support a relatively low effective stress level in the vicinity of the megathrust in the lower portion of the seismogenic zone in southern Cascadia, likely due to elevated fluid pressures. Consequently, the ground motion in the onshore region above this low-stress seismogenic portion in southern Cascadia may not be as intense as that observed during great earthquakes in other subduction zones.

The Impact of Source Time Function Complexity on Stress-Drop Estimates

ABSTRACT Earthquake stress drop—a key parameter for describing the energetics of earthquake rupture—can be estimated in several different, but theoretically equivalent, ways. However, independent estimates for the same earthquakes sometimes differ significantly. We find that earthquake source complexity plays a significant role in why theoretically (for simple rupture models) equivalent methods produce different estimates. We apply time- and frequency-domain methods to estimate stress drops for real earthquakes in the SCARDEC (Seismic source ChAracteristics Retrieved from DEConvolving teleseismic body waves, Vallée and Douet, 2016) source time function (STF) database and analyze how rupture complexity drives stress-drop estimate discrepancies. Specifically, we identify two complexity metrics—Brune relative energy (BRE) and spectral decay—that parameterize an earthquake’s complexity relative to the standard Brune model and strongly correlate with the estimate discrepancies. We find that the observed systematic magnitude–stress-drop trends may reflect underlying changes in STF complexity, not necessarily trends in actual stress drop. Both the decay and BRE parameters vary systematically with magnitude, but whether this magnitude–complexity relationship is real remains unresolved.

Source characterization of the 20th May 2024 MD 4.4 Campi Flegrei caldera earthquake through a joint source-propagation probabilistic inversion

On May 20th, 2024, an earthquake of magnitude MD 4.4 nucleated at shallow depth (2.6 km) in the Campi Flegrei caldera (Southern Italy), a densely populated area where an increase in seismic activity has been observed since 2019 attributable to an on-going unrest episode. While the magnitude was moderate, the event produced a strong ground shaking with an observed maximum peak ground acceleration of 3.58 m s-2, and several buildings were damaged. Here, we characterize the earthquake source using a probabilistic joint source-propagation spectral inversion in the Fourier space. We estimate a moment magnitude Mw = 3.70 ± 0.13 and a corner frequency fc = 1.11 ± 0.19 Hz. Assuming a circular rupture model, we estimate a source radius r = 400 ± 70 m and a stress drop Δσ = 3.2 ± 2.2 MPa. The estimated stress drop suggests that future earthquakes in the hypocentral region, considering a possible rupture length of 3 km suggested by previous studies, can have magnitude increased by 1.2 ± 0.3 units with respect to May 20th event. A systematic source characterization of the recent seismicity in the caldera would hep in estimating the expected ground motions from future large-magnitude events.

Different earthquake nucleation conditions revealed by stress drop and b-value mapping in the northern Chilean subduction zone

Stress drop is an earthquake property indicative for the characteristic relation of slip to fault dimension. It is furthermore affected by fault strength, fault topography, the presence of fluids, rupture size, slip, and velocity. In this article, the stress drop image of an entire subduction zone, namely for the seismically highly active northernmost part of Chile, is combined with mapped b-values and their corresponding magnitude distribution in order to better constrain the conditions under which earthquakes of different provenances may nucleate. The underlying recent earthquake catalog contains over 180,000 events, covering 15 years of seismicity, from which more than 50,000 stress drop estimates were computed. Their spatial average segments the subduction zone into different parts, i.e., average stress drop between seismotectonic areas is different, although this difference is small compared to the natural scatter of stress drop values. By considering stress drop variations, b-value map, magnitude distribution, and thermal models, candidate earthquake nucleation mechanisms are identified which can explain the observed distributions. This is done for two exemplary regions: (1) The plate interface, where principally lower stress drop events are found, while at the same time a high spatial heterogeneity of stress drop values is observed. This indicates relatively smooth or lubricated rupture surfaces, and locally it suggests the existence of alternating regions controlled by strong asperities, weaker material, or creep. (2) The highly active intermediate depth (ID) seismicity region, where the variation of stress drop and b-value point to a gradual change of nucleation mechanism from dehydration embrittlement at the top of the ID cloud, over dehydration driven stress transfer in its central part, to thermal runaway shear mechanisms at its bottom. In both cases, the combination of stress drop and b-value distribution helps to better understand the origin and the differences of the observed seismicity.

Mexico City Earthquake of 11 May 2023 (Mw3.2)

On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8x1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760,On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8×1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760, λ = -750. These source characteristics are very similar to those estimated for the 17 July 2019 earthquake which occurred during a swarm-like seismic activity about 5 km to the north. Spectral analysis of recordings at 19 sites in the hill zone, 14 in the transition zone, and 41 in the lake-bed zone reveals great variability of the ground motion within each of the zones. Estimated stress drop, Δσ, is 0.5 MPa. A large disparity is found between the observed source spectrum and theoretical source spectrum; their ratio provides an estimation of the amplification of seismic waves as they travel through the layers of decreasing velocity at shallower depth. We denote this ratio as the site effect. Predicted PGA and PGV for an Mw 3.2 earthquake, computed using stochastic technique (Boore 1983, 2003), assuming a Brune ω-2 source, Δσ = 0.5 MPa and including the site effect, are in reasonable agreement with the observations. Expected PGA and PGV at the epicenter of a postulated Mw 5 earthquake are 0.6 g and 60 cm/s at a generic hill-zone site; the expected values are twice as large in the lake-bed zone. These predictions should, however, be taken with caution as they are based on several approximations.

Sensing Optical Fibers for Earthquake Source Characterization Using Raw DAS Records

AbstractDistributed Acoustic Sensing (DAS) is becoming a powerful tool for earthquake monitoring, providing continuous strain‐rate records of seismic events along fiber optic cables. However, the use of standard seismological techniques for earthquake source characterization requires the conversion of data in ground motion quantities. In this study we provide a new formulation for far‐field strain radiation emitted by a seismic rupture, which allows to directly analyze DAS data in their native physical quantity. This formulation naturally accounts for the complex directional sensitivity of the fiber to body waves and to the shallow layering beneath the cable. In this domain, we show that the spectral amplitude of the strain integral is related to the Fourier transform of the source time function, and its modeling allows to determine the source parameters. We demonstrate the validity of the technique on two case‐studies, where source parameters are consistent with estimates from standard seismic instruments in magnitude range 2.0–4.3. When analyzing events from a 1‐month DAS survey in Chile, moment‐corner frequency distribution shows scale invariant stress drop estimates, with an average of Δσ = (0.8 ± 0.6) MPa. Analysis of DAS data acquired in the Southern Apennines shows a dominance of the local attenuation that masks the effective corner frequency of the events. After estimating the local attenuation coefficient, we were able to retrieve the corner frequencies for the largest magnitude events in the catalog. Overall, this approach shows the capability of DAS technology to depict the characteristic scales of seismic sources and the released moment.

A Comprehensive Stress Drop Map From Trench to Depth in the Northern Chilean Subduction Zone

AbstractWe compute stress drops for earthquakes in Northern Chile recorded between 2007 and 2021. By applying two analysis techniques, (a) the spectral ratio (SR) method and (b) the spectral decomposition (SDC) method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We build an extended set of empirical Green’s functions (EGF) for the SR method by systematic template matching. Outputs are used to compare with results from the SDC approach, where we apply cell‐wise obtained global EGF's to compensate for the structural heterogeneity of the subduction zone. We find a good consistency of results of the two methods. The increased spatial coverage and quantity of stress drop estimates from the SDC method facilitate a consistent stress drop mapping of the different seismotectonic domains. Albeit only small differences of median stress drop, strike‐perpendicular depth sections clearly reveal systematic variations, with earthquakes at different seismotectonic locations exhibiting distinct values. In particular, interface seismicity is characterized by the lowest observed median value, whereas upper plate earthquakes show noticeably higher stress drop values. Intermediate depth earthquakes show comparatively high average stress drop and a rather strong depth‐dependent increase of median stress drop. Additionally, we observe spatio‐temporal variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The presented study is the first coherent large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further detailed analysis of the physics of earthquake ruptures.

Adjoint Inversion of Near‐Field Pressure Gauge Recordings for Rapid and Accurate Tsunami Source Characterization

AbstractWe explore the potential of the adjoint‐state tsunami inversion method for rapid and accurate near‐field tsunami source characterization using S‐net, an array of ocean bottom pressure gauges. Compared to earthquake‐based methods, this method can obtain more accurate predictions for the initial water elevation of the tsunami source, including potential secondary sources, leading to accurate water height and wave run‐up predictions. Unlike finite‐fault tsunami source inversions, the adjoint method achieves high‐resolution results without requiring densely gridded Green's functions, reducing computation time. However, optimal results require a dense instrument network with sufficient azimuthal coverage. S‐net meets these requirements and reduces data collection time, facilitating the inversion and timely issuance of tsunami warnings. Since the method has not yet been applied to dense, near‐field data, we test it on synthetic waveforms of the 2011 Mw 9.0 Tohoku earthquake and tsunami, including triggered secondary sources. The results indicate that with a static source model without noise, using the first 5 min of the waveforms yields a favorable performance with an average accuracy score of 93%, and the largest error of predicted wave amplitudes ranges between −5.6 and 1.9 m. Using the first 20 min, secondary sources were clearly resolved. We also demonstrate the method's applicability using S‐net recordings of the 2016 Mw 6.9 Fukushima earthquake. The findings suggest that lower‐magnitude events require a longer waveform duration for accurate adjoint inversion. Moreover, the estimated stress drop obtained from inverting our obtained tsunami source, assuming uniform slip, aligns with estimations from recent studies.

Source characteristics of earthquakes in Delhi and its vicinity: Implications for seismogenesis in the stable continental region of India

Delhi and its surrounding region are among the important intraplate seismically active regions of India. According to the Bureau of Indian Standards, it lies in seismic zone IV and is capable of generating small to moderate earthquakes. This region has also experienced moderate to large earthquakes in the past due to its proximity to the Himalayan arc and its own tectonics. There is an urgent need to assess the source characteristics of earthquakes and their scaling relationships for a reliable and accurate estimation of seismic hazard. Source parameters and their scaling relations have been estimated for this region using well-located small to moderate earthquakes (Mw 2.3–5.1) that occurred during the period from 2000 to 2020. For this purpose, we investigated three components of earthquake waveform data using the ω−2 source model. The outcomes show the occurrence of low static stress drop (0.5–52.87 ​bars) in the Delhi region indicating approximately 57% of the events exhibit a stress drop < 10 ​bars, 27 % of events fall in the range of 10–20 ​bars, and only 16 % events exhibit stress drop > 20 ​bars. This observation suggests that the shallow subsurface of the study region may have low-strength material characteristics and a heterogeneous nature. In addition, the Zuniga parameter (ε) is estimated less than 1.0 by analyzing static and apparent stress drops, which infers the partial stress drop model fits very well in the study region. The seismic moment varies from 1.02×1012 to 1.03×1016 ​N.m. for P-wave and 6.46×1011 Nm to 7.54×1015 Nm for S-wave. The average seismic source radius lies in the range of 0.1–3.05 ​km with a ratio {r(p)/r(s)} of 1.5 ​km in the study region. The estimated values of corner frequency are comparatively lower for the S-wave (1.5–18.2 ​Hz) than for the P-wave (1.88–19.3 ​Hz) suggesting the ‘shifting properties’ of the corner frequency corroborated with the theoretical agreement. The seismic energy (E) is estimated using both P- and S-wave separately and its average value varies from 4.28×106 to 6.22×1011 ​J. The estimated stress drop and seismic moment demonstrate no correlation with each other. Therefore small to moderate-size earthquakes inherently follow the self-similarity behavior. The obtained scaling relationship between Seismic Moment and Corner Frequency is Mo=7.94∗1015fc−2.97. The derived scaling relations and source parameters are expected to provide a priori information for the assessment of seismic hazards and are useful in the simulation of strong ground motion characteristics in the region.

Investigating Spectral Estimates of Stress Drop for Small to Moderate Earthquakes With Heterogeneous Slip Distribution: Examples From the 2016–2017 Amatrice Earthquake Sequence

Investigating Spectral Estimates of Stress Drop for Small to Moderate Earthquakes With Heterogeneous Slip Distribution: Examples From the 2016–2017 Amatrice Earthquake Sequence

Investigating Spectral Estimates of Stress Drop for Small to Moderate Earthquakes With Heterogeneous Slip Distribution: Examples From the 2016–2017 Amatrice Earthquake Sequence

AbstractEstimates 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.

Tectonic stress of northeastern Indian region derived from seismic focal mechanisms and the effect of focal mechanism on stress drop: a comparative analysis with Kachchh intraplate region of India

SUMMARY In this study, the focal mechanism solutions and source parameters of recent earthquakes that occurred in the northeastern region of India have been determined. The region has very complex tectonics as it is subjected to the compressional forces from all sides, due to the collision of the Indian Plate with the Eurasian, Burma and Tibetan plates. Waveform data from deployed broad-band seismographs (BBS) and strong motion accelerographs (SMA) in the northeastern India are used to determine the focal mechanism solutions and source parameters of moderate earthquakes, respectively. The estimated focal mechanisms are used to understand the existing stress field in the region. It is found that the Shillong-Plateau as well as the Indo-Burma subduction zone is dominated by the compressional tectonic regime, Mikir Hills and Bengal basin are dominated by the trans-tension tectonic regime, and the easternmost Himalayan region is dominated by the strike-slip tectonic regime. The maximum horizontal stress direction Shmax is also determined for above subregions. The direction of Shmax is southeast in the Bengal basin, northeast in Mikir Hills and Indo-Burma subduction zone whereas it is NNE in Shillong Plateau and SSW in the eastern Himalayas. The estimated stress drop value of the earthquakes in the region ranges from 2.11 to 23.89 MPa. The relationship between the source parameters and focal mechanisms is also explored. It is found that the earthquakes with a strike-slip mechanism have the highest average stress drop (7.05 MPa) followed by reverse (6.82 MPa) and normal (5.12 MPa) in the northeastern region of India. According to the examined data set, the stress drop is found to be dependent on the type of focal mechanism, seismic moment and hypocentral depths. The comparison of the results with the Kachchh intraplate region in western India shows earthquakes in Kachchh have larger mean stress drop for all types of mechanisms. In both intraplate and interplate regions of India, the stress drop of earthquakes depends on the type of focal mechanism solution.

Modulation of Seismic Radiation by Fault-Scale Geology of the 2016 Mw 6.0 Shallow Petermann Ranges Earthquake (PRE) in Central Australia

ABSTRACTUnderstanding the modulatory influence of fault-scale geology on seismic behavior of earthquake faults is central to determining the physics of faulting and seismic hazard analysis. Although laboratory experiments predict that seismic parameters can be modulated by fault-scale geology, there is scant empirical evidence of this process at field scale due largely to a lack of shallow earthquakes of which causative faults can be mapped to known bedrock structure. The 20 May 2016 Mw 6 Petermann Ranges earthquake (PRE) is the best-recorded continental event in Australia to date, and it is an excellent candidate to investigate the possible link between seismic parameters and fault-scale geology as its causative fault has previously been linked to known bedrock structure using distributions of aftershocks, surface observations, and geophysical mapping. In this study, we analyze strain energy partitioning of PRE by determining seismic radiation efficiency (0.31) and apparent stress (0.34 MPa) together with previously estimated stress drop (2.2 MPa) and find that the combination of these macroseismic parameters deviates from that expected of a shallow immature fault in intraplate continental regions typically characterized by large recurrence intervals. It instead appears to have mimicked a mature fault, which we attribute to the characteristics of the causative fault confined to mechanically weaker, phyllosilicate-rich foliations of the bedrock that have anomalously lower fracture energy. Therefore, PRE rupture suggests the presence of a spectrum of shallow (&amp;lt;20 km) fault slip behavior modulated by fault-scale geology.

Characterizing Multisubevent Earthquakes Using the Brune Source Model

ABSTRACTAlthough 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 Brune source model. By assuming complex sources as a sum of multiple Brune sources, we analyze 1640 source time functions of Mw 5.5–8.0 earthquakes in the seismic source characteristic retrieved from deconvolving teleseismic body waves catalog to estimate the corner frequencies, onset times, and seismic moments of subevents. We identify more subevents for strike-slip earthquakes than dip-slip earthquakes, and the number of resolvable subevents increases with magnitude. We find that earthquake corner frequency correlates best with the corner frequency of the subevent with the highest moment release (i.e., the largest subsevent). This suggests that, when the Brune model is used, the estimated corner frequency and, therefore, the stress drop of a complex earthquake is determined primarily by the largest subevent rather than the total rupture area. Our results imply that, in addition to the simplified assumption of a radial rupture area with a constant rupture velocity, the stress variation of asperities, rather than the average stress change of the whole fault, contributes to the large variance of stress-drop estimates.

Stress Drop Estimates for Small to Moderate Earthquakes in Tohoku, Japan Considering Rupture Geometry, Speed, and Directivity

AbstractThis study develops a spectral ratio method to estimate the earthquake stress drop considering the rupture geometry, speed, and directivity. This spectral ratio method is applied to Mw 3.3–5.9 earthquakes in Fukushima and Ibaraki, Japan. A rectangular source model with a square rupture front and uniform slip distribution is used to fit spectral ratios stacked by station and wave type for multiple empirical Green's functions, and estimate the finite source properties. Comparing the rupture areas obtained by the spectral ratio method with the slip inversion results shows that our results from the spectral ratio method are associated with a localized area of large slip rather than the overall rupture area. Hence, the rupture area obtained from this spectral ratio method is inappropriate to calculate the average static stress drop. This study proposes a procedure to estimate the average stress drop for the entire rupture area and one for the localized area with large slip, using a source model with a single asperity and heterogeneous stress drop distribution. As a result, we obtained average static stress drops of 0.42–40.2 MPa with a median of 3.7 MPa. The local stress drop is 5.7 times larger than the average static stress drop. This study suggests that a compact area compared to the overall rupture area mainly controls the shape of a broadband seismic spectrum. Thus, we should consider the source heterogeneity in addition to the rupture geometry, speed, and directivity when estimating the stress drops from seismic spectra.

Improved Stress Drop Estimates for M 1.5 to 4 Earthquakes in Southern California From 1996 to 2019

AbstractWe estimate Brune‐type stress drops for over 70,000 southern California M 1.5–4 earthquakes from 1996 to 2019 using a P‐wave spectral decomposition approach. Based on our recent work documenting hard‐to‐resolve trade‐offs between absolute stress drop, stress drop scaling with moment, high‐frequency falloff rate, and empirical corrections for path and attenuation terms, we adopt a new approach in which the average corner frequency of the smallest earthquakes within a short distance from each target event is fixed to a constant value. This removes any true coherent spatial variations in stress drops among the smallest events but ensures that any spatial variations seen in larger event stress drops are real and not an artifact of inaccurate path corrections. Applying this approach across southern California, we document spatial variations in stress drop that agree with previous work, such as lower‐than‐average stress drops in the Salton Trough, as well as small‐scale stress drop variations along many faults and aftershock sequences. We observe an apparent increase in Brune‐type stress drop with moment for M 3–4 earthquakes, but their spectra can be fit equally well with self‐similar models with a high‐frequency falloff rate shallower than f−2.