Stress Drop Values Research Articles

Source Parameters of the Mw 5.7 Pica Crustal Earthquake in Northern Chile

Abstract On 10 September, 2008, an Mw 5.7 earthquake occurred under the Central Valley of northern Chile near the town of Pica at a depth of ∼33 km within the continental crust of the South America plate. We find this earthquake to be a high stress-drop, reverse-oblique event that generated unusually high ground accelerations of up to 0.67g. Overall, its observed ground motion intensities are considerably larger than those predicted by ground motion models, particularly at short periods. The source properties inferred through waveform modeling indicate reverse-oblique fault motion on a ∼75 km2 plane dipping to the northeast, which is corroborated by the located aftershock distribution. Stress-drop values of the mainshock and larger aftershocks were estimated through S-wave spectrum modeling, with values up to ∼250 MPa for the mainshock. The event occurred in a cold section of the continental crust under the Central Valley, and its fault kinematics and orientation are consistent with the dominant style of faulting and stress field under the neighboring Coastal Cordillera. Although our recurrence analysis shows that crustal events in the region occur at a lower rate than interplate and inslab events, crustal events of similar or higher magnitude than the Pica earthquake have occurred, on average, approximately once every three years in northern Chile, which could pose an important hazard to nearby populations or critical infrastructure.

Spectral characteristics of P-and S-wave quarry blasts in Aswan region, South Egypt

The Aswan broadband seismic network, consisting of high-sensitivity sensors with dense stations coverage, provided the opportunity to study and analysed the spectral properties of the quarry blasts in the Aswan region, southern Egypt. We comprehensively analysed 757 digital seismograms from 128 quarry blasts with local magnitude (ML) between 1.7 and 3.1 during time span from 2010 to 2021. A Generalized Inversion Technique (GIT) constrained by a reference site is applied to isolate the attenuation, site responses of the seismic stations, and the source spectra from the observed seismograms by means of iterative least square analysis. The displacement source spectra for P-and S-waves are computed for all quarry blasts based on the theoretical omega-square seismic source model (Brune's model). The spectral analysis demonstrated that both P-and S-waves displacement source spectra of the quarry blasts used in this study have low corner frequency values. The corner frequency of the quarry blasts used in this study ranged from 1.0 to 2.6 Hz and from 1.5 to 4.6 Hz for P-and S-waves, respectively. The P-and S-wave corner frequency ratio (fCP/fCS) is computed for each event. We obtained that the fCP/fCS ratio is less than 1.0 Hz for 106 events (94%) and equal to 1.0 Hz and less than 1.2 Hz for only six selected data events (6%). The analysis indicates that the observed displacement source spectra show discrepancies with the omega-square type spectrum, and they generally have a steep fall off at high frequencies. We also found that the selected quarry blasts have a narrow seismic moment range (from 2.03E+11 to 1.35E+12 Nm), and stress drop values range from 0.001 to 0.023 MPa. The distribution of the used quarry blast stress drop demonstrated a self-similar character.

Source Properties of Hydraulic‐Fracturing‐Induced Earthquakes in the Kiskatinaw Area, British Columbia, Canada

AbstractThis work presents a high resolution source property study of hydraulic fracturing induced earthquakes in the Montney Formation, a low‐permeability tight shale reservoir in the Kiskatinaw area, northeast British Columbia, Canada. We estimate source parameters, including focal mechanism solutions (FMSs), seismic moment, spectral corner frequency, and static stress drop values of earthquakes recorded between July 2017 and July 2020. Waveform‐similarity‐based event classification of 8,283 earthquakes yields 52 event families (clusters) and 1,014 isolated events (individuals). We calculate a total of 64 FMSs of events ML > 2.5 with high‐quality waveforms. Of the 64 solutions, 54 come from events within families, and are used to infer an additional 3,500 focal mechanisms of smaller‐magnitude events with similar waveforms. The other 10 are isolated events. The dominant faulting style inferred from FMSs highlights multiple cascading, shallow, strike‐slip events and generally isolated, larger‐magnitude reverse‐style events in close proximity to the Fort St. John Graben system. Inferred nodal planes of strike‐slip events are at low‐angles to regional SHmax, suggesting optimally oriented, left‐lateral faults. Reverse faulting nodal planes are roughly perpendicular to SHmax and have an orientation consistent with the reactivation of pre‐existing normal basement faults. Source spectral analysis using three approaches, including spectral‐ratio fitting, suggests a constant stress drop of 1–10 MPa and self‐similarity for induced events. Constant stress drop scaling breaks down at magnitudes smaller than ∼ML 2.0, likely due to observational bandwidth limitations.

Occurrences of low-stress drop earthquakes in the eastern Ladakh-Karakoram zone of the Trans Himalaya and their tectonic implications

The Ladakh-Karakoram zone is a unique testing ground for understanding the geodynamic evolution of the Himalaya-Karakoram orogeny. Despite the accumulation of a large amount of strain energy that originated due to the India-Asia collision along the Karakoram fault, earthquakes of M ≥ 7.0 are considerably less in the Karakoram Fault Zone (LKZ) compared to the Himalayan seismic belt in the northwest Himalaya. The spectral analysis of microearthquakes data (1.9 < ML < 4.3) recorded during 2009–2012 by 10 seismological stations of the LKZ reveals that the region produces low-stress drop earthquakes. Analysis of selected 51 earthquakes shows the seismic moment (Mo) within the range of 1.2 × 1012–4.3 × 1015 Nm with stress drop values varying from ∼ 0.06 to 64.36 bar. The low stress-drop earthquakes have significant implications in understanding seismogenesis. The earthquake cluster near the Tso-Morari Crystallines is associated with the brittle shear failure on the faults (e.g. Zildat, and Karzok Faults), and the low-stress drop of the earthquakes are explained by the partial stress drop model. The low effective fault strength due to fault weakening mechanisms in the studied segment of the Karakoram Fault zone promotes aseismic creeping patches producing low-stress drop earthquakes at the expense of generating large earthquakes. This study provides the scaling relations between important source parameters for the first time in the region which can serve as useful inputs for the assessment of earthquake hazards.

Fluid-injection-induced earthquakes characterized by hybrid-frequency waveforms manifest the transition from aseismic to seismic slip

Aseismic slip loading has recently been proposed as a complementary mechanism to induce moderate-sized earthquakes located within a few kilometers of the wellbore over the timescales of hydraulic stimulation. However, aseismic slip signals linked to injection-induced earthquakes remain largely undocumented to date. Here we report a new type of earthquake characterized by hybrid-frequency waveforms (EHWs). Distinguishing features from typical induced earthquakes include broader P and S-pulses and relatively lower-frequency coda content. Both features may be causally related to lower corner frequencies, implying longer source durations, thus, either slower rupture speeds, lower stress drop values, or a combination of both. The source characteristics of EHWs are identical to those of low-frequency earthquakes widely documented in plate boundary fault transition zones. The distribution of EHWs further suggests a possible role of aseismic slip in fault loading. EHWs could thus represent the manifestation of slow rupture transitioning from aseismic to seismic slip.

Stochastic Simulation of Strong Ground Motions From Two M &gt; 5 Uttarakhand Earthquakes

Many studies based on the geodetic data and statistical analysis of seismicity have pointed out that sufficient amount of stress accumulated in the Himalayan plate boundary may host a big earthquake. Consequently, high seismic activities and infrastructural developments in the major cities around Himalayan regions are always of major concern. The ground motion parameter estimation plays a vital role in the near real time evaluation of potentially damaged areas and helps in mitigating the seismic hazard. Therefore, keeping in mind the importance of estimation of ground motion parameters, we targeted two moderate-size earthquakes that occurred recently within a gap of 10 months in Uttarakhand region with M &amp;gt; 5.0 on 06/02/2017 and 06/12/2017. The ground motions are simulated by adopting a stochastic modeling technique. The source is assumed as ω−2, a circular point source (Brune’s model). The average value of reported anelastic attenuation from various studies, the quality factor, Qs= 130.4*(f0.996), and stress drop values obtained through iterative procedure are considered for simulations. The stochastic spectra are generated between 0.1 and 10 Hz of frequency range. The site effect is also estimated by using the H/V method in the same frequency range. The synthetic spectra are compared with the observed Fourier amplitude spectra obtained from the recorded waveform data and converted back to the time histories. The stochastic time histories are compared with the observed waveforms and discussed in terms of amplitude (PGA). The simulated and observed response spectra at different structural periods are also discussed. The mismatch between the observed and simulated PGA values along with the GMPE existing for shallow crustal earthquakes is also discussed in the present work.

From Seismic Quiescence to Surged Activity After Decades of Wastewater Disposal: A Case Study in Central‐West Alberta, Canada

AbstractInjection‐induced earthquakes associated with wastewater disposal (WD) in the Western Canadian Sedimentary Basin are much fewer than those linked to hydraulic fracturing. Recently, 43 WD‐related earthquakes (M1.3–3.9) occurred in a historically seismically quiet area where long‐term injections became intensive since 2018, foreshadowing a critical stress state accumulated from long‐term injections. Our improved earthquake locations show that the injection well targeting the deeper middle‐upper Devonian aquifer system causes more vigorous seismicity with a wide range of stress drop values (0.8–230 MPa) compared to other shallower wells. Three physical mechanisms may collectively lead to the observed seismic pattern: (a) the underlying Devonian reef system makes it easier for injected fluids to migrate horizontally, (b) fluids can be channeled into the widespread critical faults and migrate vertically to cause earthquakes along it, and (c) aseismic slip probably triggered within the Duvernay formation by fluid migrating can further facilitate seismicity at shallower depths.

Spatio‐Temporal Evolution of Earthquake Static Stress Drop Values in the 2016–2017 Central Italy Seismic Sequence

AbstractThe static stress drop of an earthquake is an indicator of the stress state of a specific fault before rupture initiation. The stress state is primarily controlled by the ambient stress field, fault strength, fault complexity, and the presence of fluids. This study aims to investigate the spatio‐temporal distribution of static stress drop values of the 2016–2017 multi‐fault rupture seismic sequence in central Italy, which includes three earthquakes with Mw ≥ 5.9 (Amatrice, Visso, and Norcia earthquakes), and over 95,000 aftershocks (M 0.5–6.5). We estimate stress drop values using a circular crack model with corner frequency and seismic moment estimates from single‐spectra fitting, a cluster‐event method, and spectral‐ratio fitting. The temporal distribution of stress drop values shows an apparent increase of stress drop following a large earthquake (Mw ≥ 5.9). The spatial distribution shows comparably high stress drop values for early aftershocks surrounding the mainshock rupture area. High stress drop events correlate with fault complexity, such as fault intersections at depth and reactivated thrust fronts. We observe a constant stress drop for Mw ≥ ∼3, in contrast to previous studies. Instrument response and signal‐to‐noise bandwidth limitations likely govern the observed decrease in stress drop with decreasing magnitude for events with Mw ≤ 3. The spatio‐temporal distribution of stress drop values in a complex seismic sequence could support a more complete understanding of the earthquake rupture process and the evolution of seismic sequences. It could also highlight areas where stress loading is focused, which would have implications for short and intermediate term seismic hazard estimates.

Stress Drop Variations in the Region of the 2014 MW8.1 Iquique Earthquake, Northern Chile

AbstractWe compute stress drops from P and S phase spectra for 534 earthquakes in the source region of the 2014 MW8.1 Iquique megathrust earthquake in the northern Chilean subduction zone. An empirical Green's function based method is applied to suitable event pairs selected by template matching of eight years of continuous waveform data. We evaluate the parameters involved in the stress drop estimation, consider the effect of the local velocity structure and apply an empirical linear relation between P and S phase related geometry factors (k values). Data redundancy produced by multiple empirical Green's function and the combination of P and S phase spectra leads to a substantial reduction of uncertainty and robust stress drop estimates. The resulting stress drop values show a well‐defined log‐normal distribution with a median value of 4.36 MPa; most values range between 0.1 and 100 MPa. There is no evidence for systematic large scale lateral variations of stress drop. A detailed analysis reveals several regions of increased median stress drop, an increase with distance to the interface, but no consistent increase with depth. This suggests that fault regime and fault strength have a stronger impact on the stress drop behavior than absolute stresses. Interestingly, we find a weak time‐dependence of the median stress drop, with an increase immediately before the April 1, 2014 MW8.1 Iquique mainshock, a continuous reduction thereafter and a subsequent recovery to average values. Additionally, the data set indicates a relatively strong dependence of stress drop on magnitude which extends over the entire analyzed magnitude range.

Source Spectral Properties of Earthquakes in the Delaware Basin of West Texas

AbstractIn recent years, the Delaware basin of west Texas has seen a sharp rise in earthquake occurrence, driven in large part by increases in unconventional hydrocarbon production. The advent of Texas Seismological Network in 2017 has allowed for the characterization of these events in greater detail. We exploit the recent densification in seismic station coverage to study the spectral properties of earthquakes in this region. We show that the low-frequency moments of S-wave spectra, when corrected for site and distance, can be used to calibrate a consistent moment magnitude scale for small and moderate earthquakes. For a subset of &amp;gt;3000 well-recorded events, we compute earthquake stress drop under the assumption of Brune spectral model. Earthquakes in the Delaware basin show coherent spatial patterns in stress drop across the region. Through a reanalysis of independently collated data from the oil and gas industry, we find that earthquakes that are likely associated with hydraulic-fracturing operations have slightly different spectral characteristics than earthquakes that are likely associated with saltwater disposal. In particular, events associated with hydraulic fracturing show greater variability in the statistical distribution of stress drop and have higher median stress-drop values. Although the differences are subtle, they suggest that there may be important distinctions in the underlying physical mechanisms and resulting hazards of distinct classes of induced events, differences that may be unraveled with more detailed joint analyses of industrial and seismic datasets.

Dynamic rupture of subduction earthquakes located near the trench

We perform kinematic and dynamic source inversions of four interplate thrust earthquakes that occurred near the trench at the base of the continental wedge in Northern Chile and we compare them to deeper intraplate events. The magnitudes of these interplate events were between Mw 6.3 and Mw 6.5, with hypocentral depths varying between 17.8 km and 28 km. These earthquakes correspond to one foreshock and three aftershocks of the large 2014 Mw 8.2 Iquique earthquake. The geometry of the seismic rupture obtained from the kinematic and dynamic inversions were similar for the four earthquakes studied and their dynamic source parameters are in accordance with typical interplate earthquakes around Mw 6.5. We compare the dynamic rupture of these events with those of five intraplate, intermediate-depth earthquakes that occurred in Northern Chile, Japan, Argentina and Mexico with depths between 57 km and 250 km. We also compare these events with the Mw 6.9 Valparaiso earthquake that was also a shallow interplate thrust event. Results show that the main dynamic parameters of the interplate events (stress in the nucleation zone, overall stress drop and fracture energy rate) were smaller than those of the intraplate intermediate-depth earthquakes. Furthermore, we compare our results with those obtained with several methodologies commonly used to estimate values of stress drop, fault radius and corner frequency. We find some minor differences between them for most of the earthquakes analyzed. Finally, we infer from our results that the studied area, located in the subduction interface zone near the trench, is suitable for earthquake nucleation of small to moderate earthquakes and does not represent a barrier for a future large tsunamigenic rupture.

Cross Validation of Stress Drop Estimates and Interpretations for the 2011 Prague, OK, Earthquake Sequence Using Multiple Methods

AbstractWe compare source parameter estimates for earthquakes in the 2011 Prague Mw 5.7, Oklahoma, sequence to investigate random uncertainty and systematic bias, and resolve reliable relative variations in stress drop. Source parameters provide insight into the earthquake rupture processes but large variations between studies occur. The Prague earthquake sequence is a prime example of this, with different studies reaching contrasting interpretations of the effects of injection on source parameters. We examine the Prague earthquake sequence using a single coherent catalog for all the events detected by the Oklahoma Geological Survey (OGS) and McMahon et al. (2017). We use three principal approaches to estimate stress drop in order to understand the biases of each: a spectral decomposition method based on stacking, individual event spectral modeling, and a spectral ratio method based on highly correlated events. We also compare our results with previous studies for the Prague sequences aftershocks, as well as past results for the Mw 4.8 foreshock and Mw 4.8 aftershock and Mw 5.7 mainshock. The absolute values of stress drop vary significantly between methods, but the relative patterns remain consistent, except when low quality or low bandwidth data are included. The consistent relative patterns reveal that the stress drops of aftershocks are dependent on the fault orientation and the proximity of the events to the mainshocks slip. These results indicate that fault structure as well as past events play an important role in stress drop patterns.

Spatio-temporal characteristics of frictional properties on the subducting Pacific Plate off the east of Tohoku district, Japan estimated from stress drops of small earthquakes

The east coast of the Tohoku district, Japan has a high seismicity, including aftershocks of the 2011 M9 Tohoku earthquake. We analyzed 1142 earthquakes with 4.4 le M_{W} le 5.0 that occurred in 2003 through 2018 and obtained spatio-temporal pattern of stress drop on the Pacific Plate that subducts beneath the Okhotsk Plate. Here we show that small earthquakes at edges of a region with a large slip during the 2011 Tohoku earthquake had high values of stress drop, indicating that the areas had a high frictional strength and suppressed the coseismic slip of the 2011 Tohoku earthquake. In addition, stress drops of small earthquakes in some of the areas likely decreased after the 2011 Tohoku earthquake. This indicates that the frictional strength decreased at the areas due to the following aftershocks of the 2011 Tohoku earthquake, consistent with a high aftershock activity. This also supports that the frictional properties on a subducting plate interface can be monitored by stress drops of small earthquakes, as pointed out by some previous studies.

Source Mechanism and Rupture Process of the 24 January 2020 Mw 6.7 Doğanyol–Sivrice Earthquake obtained from Seismological Waveform Analysis and Space Geodetic Observations on the East Anatolian Fault Zone (Turkey)

Here, we present the source mechanism and rupture process for the destructive 24 January 2020 Mw 6.7 Doğanyol–Sivrice earthquake at the East Anatolian Fault Zone (EAFZ, Turkey), obtained from seismological waveform analysis and space geodetic observations. Multi-data analyses and modelling in the present study provide fundamental data and strong constraints for retrieving complex source mechanism of an earthquake and its spatiotemporal slip characteristics along the ruptured segment of fault. The acquired slip model of this earthquake reveals heterogeneous slip distribution along strike N244°E of the fault plane dipping NW (68°) with duration of the source time function (STF) and low stress drop value (Δσ) of ~25 s and ~6 bars, respectively. Back-projection analysis validates fault length (L) stretching along strike for a distance of ~75 km and supports predominant south-westerly bilateral rupture propagation with a variable rupture velocity (Vr) of ~2.3–3.4 km/s along with two main patches, presumably a sequence of two asperities being ruptured following the surface trace of the EAFZ. The distribution of aftershocks based on the analysis of two months long data consistently confirms spreading of seismicity along the ruptured fault. The evaluation of Interferometric Synthetic Aperture Radar (InSAR) data reveals that left-lateral co-seismic slip and significant deformation extends for ~20 km on either side of the fault with evident post-seismic displacement. Yet, no significant vertical offsets were observed as GNSS stations detected only horizontal motions. Coda-wave analysis as an independent tool also confirms moment magnitude of Mw 6.7. Our results highlight a case of a damaging earthquake and enhance our understanding of earthquake mechanics, continental deformation and augmented earthquake risk on the EAFZ.

Geo-hazard assessment of the NEOM area, Northwest Saudi Arabia, using seismological and potential field data

The active tectonics of the Gulf of Aqaba (Red Sea) may affect the newly established NEOM megacity in northwestern Saudi Arabia. Using seismological and potential field data, this study aimed to evaluate the major tectonic trends around NEOM area and their hazard impacts on the NEOM area. In particular, aeromagnetic and gravity data were used to explore the major tectonic trends that may affect this area. The interpreted tectonic trends are directed mainly along the NNE and NNW, as well as NW and EW tectonic trends. A good correlation was found between the distribution of the earthquake epicenters (Mw ≥ 4) and the interpreted tectonic trends. Moreover, the distribution of epicenters was congruent with the fault trends obtained from the potential field data, implying that the observed fault trends can generate earthquakes. For the seismic hazard assessment, the source parameters and expected ground motions of two moderate earthquakes that occurred in the Gulf of Aqaba in 2015 and 2016 were analyzed. Assuming an omega-squared source model, a grid search method was applied to fit the amplitude spectra of the S-wave group over the model parameters that were used to compute Mo values of 4.06E + 23 and 2.14E + 23 Nm, corresponding to Mw of 5.0 and 4.8, fault radii of 831 m and 835 m, and stress drop (Δσ) values of 7.72 and 4.01 MPa for the 2015 and 2016 earthquakes, respectively. An empirical Green’s function summation technique was performed to simulate high-frequency ground motions expected for future large earthquakes, demonstrating the influence of such earthquakes on the study area. For a Δσ value of 10 Mpa (characteristic of past earthquakes in the Gulf of Aqaba), peak ground accelerations of ∼ 5.6 and 5.53 m/s2 are to be expected for future earthquakes with Mw 7.0 in the epicentral regions of the 2015 and 2016 earthquakes.

Study on the source parameters of the micro-earthquakes in Laohutai coal mine based on double difference relocation

Seismicity related to industrial mining activities needs to be understood and quantified in order to ensure safe mining operations. It is of great important to determine if the sources of such events follow the common models and theories, and determine the source rupture property and scale of such events. Moreover, the location of the source is very important to obtain the source parameters. For this purpose, we analyze 103 events recorded in the Laohutai coal mine that are related to fault structure and mining activities to determine their source properties based on relocation. Firstly, the 103 micro-earthquakes were relocated based on double difference algorithm. Then, the source parameters of seismic moment M0, source radius R, corner frequency f0, seismic attenuation Q, energy E, stress drop △σ and apparent stress σa are computed based on relocation, Brune model and grid-search method. The relationship between the other source parameters and seismic moment, the relationship between ML and Mw, and the influence of location on source parameters and magnitude are further analyzed. Finally, the difference of the source parameters between Laohutai coal mine, Qianqiu coal mine and natural earthquakes are analyzed. We find that the source location can be effectively corrected based on double difference algorithm; ω2 model is well suited to estimate source parameters of micro-earthquakes in the Laohutai coal mine. The source spectra show strong directivity effects. Stations in the rupture direction have higher corner frequencies, while stations away from rupture direction have lower corner frequencies. With the increase of M0, the source parameters of R, E and △σ basically show a linear increase, the source parameter f0 basically show a linear decrease and the apparent stress fluctuate up and down around a certain value as a whole. After relocation, the location of the source has a significant effect on the quality attenuation factor Q and has no obvious effect on other source parameters. The influence of relocation on moment magnitude and local magnitude is not obvious. The moment magnitude is linear with the local magnitude. Considering Mw, ML1 can relatively well be used to evaluate the magnitude of the earthquakes in Laohutai coal mine; For the micro-earthquakes with the same magnitude, the values of corner frequency, stress drop, apparent stress and energy in Qianqiu coal mine are greater than those in Laohutai coal mine, the value of source radius in Laohutai coal mine is greater than those in Qianqiu coal mine. For the events with the same magnitude, the corner frequency and the stress drop value of micro-earthquakes are smaller than those of natural earthquake. These indicate that the rupture properties and scales of micro-earthquakes in Laohutai coal mine are different from those of the micro-earthquakes in Qianqiu coal mine and natural earthquakes. The results help us to effectively evaluate the property and magnitude of microseismic source, and better understand causes and consequences of microseismicity in coal mine.

The MS 6.9, 1980 Irpinia Earthquake from the Basement to the Surface: A Review of Tectonic Geomorphology and Geophysical Constraints, and New Data on Postseismic Deformation

The MS 6.9, 1980 Irpinia earthquake occurred in the southern Apennines, a fold and thrust belt that has been undergoing post-orogenic extension since ca. 400 kyr. The strongly anisotropic structure of fold and thrust belts like the Apennines, including late-orogenic low-angle normal faults and inherited Mesozoic extensional features besides gently dipping thrusts, result in a complex, overall layered architecture of the orogenic edifice. Effective decoupling between deep and shallow structural levels of this mountain belt is related to the strong rheological contrast produced by a fluid-saturated, shale-dominated mélange zone interposed between buried autochthonous carbonates—continuous with those exposed in the foreland to the east—and the allochthonous units. The presence of fluid reservoirs below the mélange zone is shown by a high VP/VS ratio—which is a proxy for densely fractured fluid-saturated crustal volumes—recorded by seismic tomography within the buried autochthonous carbonates and the top part of the underlying basement. These crustal volumes, in which background seismicity is remarkably concentrated, are fed by fluids migrating along the major active faults. High pore fluid pressures, decreasing the yield stress, are recorded by low stress-drop values associated with the earthquakes. On the other hand, the mountain belt is characterized by substantial gas flow to the surface, recorded as both distributed soil gas emissions and vigorous gas vents. The accumulation of CO2-brine within a reservoir located at hypocentral depths beneath the Irpinia region is not only interpreted to control a multiyear cyclic behavior of microseismicity, but could also play a role in ground motions detected by space-based geodetic measurements in the postseismic period. The analysis carried out in this study of persistent scatterer interferometry synthetic aperture radar (PS-InSAR) data, covering a timespan ranging from 12 to 30 years after the 1980 mainshock, points out that ground deformation has affected the Irpinia earthquake epicentral area in the last decades. These ground motions could be a result of postseismic afterslip, which is well known to occur over years or even decades after a large mainshock such as the 23 November 1980, MS 6.9 earthquake due to cycles of CO2-brine accumulation at depth and its subsequent release by Mw ≥ 3.5 earthquakes, or most likely by a combination of both. Postseismic afterslip controls geomorphology, topography, and surface deformation in seismically active areas such as that of the present study, characterized by ~M 7 earthquakes. Yet, this process has been largely overlooked in the case of the 1980 Irpinia earthquake, and one of the main aims of this study is to fill such the substantial gap of knowledge for the epicentral area of some of the most destructive earthquakes that have ever occurred in Italy.

Northern Chile intermediate-depth earthquakes controlled by plate hydration

SUMMARY We investigate the variations of the seismic source properties and aftershock activity using kinematic inversions and template-matching for six large magnitude intermediate-depth earthquakes occurred in northern Chile. Results show similar rupture geometry and stress drop values between 7 and 30 MPa. Conversely, aftershock productivity systematically decreases for the deeper events within the slab. Particularly, there is a dramatic decrease in aftershock activity below the 400–450 °C isotherm depth, which separates high- and low-hydrated zones. The events exhibit tensional focal mechanisms at unexpected depths within the slab, suggesting a deepening of the neutral plane, where the extensional regimen reaches the 700–800 °C isotherm depth. We interpret the reduction of aftershocks in the lower part of the extensional regime as the absence of a hydrated-slab at those depths. Our finding highlights the role of the thermal structure and fluids in the subducting plate in controlling the intermediated-depth seismic activity and shed new light in their causative mechanism.

On the heterogeneity of the earthquake rupture

SUMMARYThe scaling of earthquake parameters with seismic moment and its interpretation in terms of self-similarity is still debated in the literature. We address this question by examining a worldwide compilation of corner frequency-based and elastic rebound theory (ERT)-based fault slip, area and stress drop values for earthquakes ranging in magnitude from −0.7 to 7.8. We find that corner frequency estimates of slip (and stress drop) scale differently than those inferred from the ERT approach, where the latter deviates from the generally accepted constant stress drop behaviour of so-called self-similar scaling models. We also find that average slips from finite-source models are consistent with corner frequency scaling, whereas peak slip values are more consistent with the ERT scaling. The different scaling of corner frequency- and ERT-based estimates of slip and stress drop with earthquake size is interpreted in terms of heterogeneity of the rupture process. ERT-based estimates of stress drop decrease with seismic moment suggesting a self-affine behaviour. Despite the inferred heterogeneity at all scales, we do not observe a clear effect on the Brune stress drop scaling with earthquake size.

Well Proximity Governing Stress Drop Variation and Seismic Attenuation Associated With Hydraulic Fracturing Induced Earthquakes

AbstractWe use broadband seismic data collected within 3 km of a hydraulic fracturing (HF) well in northeast British Columbia, Canada, to estimate the stress drop values of HF‐induced earthquakes and their spatial variation. Applying both spectral ratio and clustered single‐spectra fitting methods to 484 induced earthquakes (M‐1.0 to 3.0), we find that earthquakes close to the injection well have lower stress drop values than those at greater distance. Stress drop values are generally invariant within clusters either proximal (~0.1–1 MPa) or distal (~1–10 MPa) to the well, suggesting that dynamic ruptures in rocks with similar rheological properties tend to have relatively constant stress drop values. Clustered single spectrum fitting also suggests that the seismic quality factor (Q) is lower proximal to the well. We interpret the lower stress drop values and higher seismic attenuation proximal to the well as a result of higher fracture density and/or elevated pore pressure in the rock matrix due to hydraulic stimulation.