Source Time Function Research Articles

Kinematics of the 30 October 2020 Mw 7.0 Néon Karlovásion (Samos) earthquake in the Eastern Aegean Sea: Implications on source characteristics and dynamic rupture simulations

We resolve source mechanism and rupture process for the Néon Karlovásion, Samos Mw 7.0 earthquake that struck Greek-Turkish border regions on 30th October 2020 acquired from kinematic joint inversion of teleseismic body-waves and near-field strong ground-motion waveforms. The optimal kinematic finite-fault slip model indicates a planar E-W striking north-dipping normal faulting mechanism with strike ϕ = 270° ± 5°; dip δ = 35° ± 5°; rake λ = −94° ± 5°; centroid depth h = 11 ± 2 km; duration of the source time function STF = 26 s and seismic moment Mo = 3.34 × 1019 Nm equivalent to Mw = 7.0. Our final finite- fault slip models exhibit two main asperities within a depth range from ~20 km to the surface. The dynamic rupture model exposes an initial heterogeneous stress distribution with variations up to 25 MPa. The near-field strong motion waveforms constrained the slip model suggesting up-dip and westward propagation of the bilateral rupture pattern with a maximum slip of 3.2 m, illuminated by back-projection (BP) analysis. The high-frequency (HF) back-projected rupture showed a predominantly E-W striking component (~75%) with directivity of 277° that propagates to the surface along a 60 km long and 24 km wide fault plane in 20 s at a slower speed range of 1.0–2.0 km/s. This well constrains the coseismic slip region where the aftershock sequence confirms distributed deformation. Our back-projection analyses elucidates a dominant HF rupture stage (0–13 s) tracked first on the epicentre area and further along the downdip in the region of maximum coseismic slip indicating ~15 km of persistent rupture. The latter HF emissions (13–20 s) remark a speed of about 3.0 km/s and a westward extension of the rupture up to 30 km from the preceding rupture segment to shorelines at the northeast of the Ikaria Island.

The Source Characteristics of the Mw6.4, 2016 Meinong Taiwan Earthquake from Teleseismic Data Using the Hybrid Homomorphic Deconvolution Method

The kinematic source rupture process of the 2016 Meinong earthquake (Mw = 6.4) in Taiwan was derived from apparent source time functions retrieved from teleseismic S-waves by using a refined homomorphic deconvolution method. The total duration of the rupture process was approximately 15 s, and one slip-concentrated area can be represented as the source model based on images representing static slip distribution. The rupture process began in a down-dip direction from the fault toward Tainan City, strongly suggesting that the rupture had a unilateral northwestern direction. The asperity with an area of approximately 15 × 15 km2 and the maximum slip of approximately 2 m were centered 12.8 km northwest of the hypocenter. Coseismic vertical deformation was calculated based on the source model. Compared with the results derived from InSAR (Interferometric Synthetic Aperture Radar) data, our results demonstrated that the location with maximum uplift was accurately well detected, but our maximum value was just approximately 0.4 times of the InSAR-derived value. It reveals that there are the other mechanisms to affect the vertical deformation, rather than only depending on the source model. At different depths, areas west, east, and north of the hypocenter maintained high values of Coulomb stress changes. This explains the mechanism behind aftershocks being triggered and provides a reference for predicting aftershock locations after a large earthquake. The estimated seismic spectral intensities, including spectral acceleration and velocity intensity (SIa and SIv), were derived. Source directivity effects caused damage to buildings, and we concluded that all damaged buildings were located within a SIa value of 400 gal. Destroyed buildings taller than seven floors were located in an area with a SIv value of 30 cm/s. These observations agree with those on damages caused by the 2010 Jiasian earthquake (ML 6.4) in Tainan, Taiwan.

The 2017 Ischia Earthquake (Southern Italy): Source Mechanism and Rupture Model From the Inversion of a Near-Source Strong Motion Record

With the aim to investigate the rupture complexity and the radiated wave field of the 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (< 1-km epicentral distance) horizontal-component velocity records of the accelerometric station (IOCA) and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

Direct Estimation of Explosion Pn Green’s Functions Applied to Teleseismic P-wave Intercorrelations for North Korean Nuclear Tests

Abstract Seismic yields and explosion source time functions for the six declared underground nuclear tests at the North Korean test site are estimated using teleseismic waveform equalization (intercorrelation) incorporating improved source structure inferred from regional distance Pn phases. Explosion source time functions estimated by previous modeling efforts are deconvolved from the broadband Pn observations to extract direct estimates of the Pn Green’s functions. Very consistent signals are found for station MDJ (to the north) for all but the 2006 event, and a 1D layered elastic model is determined that matches the corresponding pPn signals for the larger events. The much simpler Pn Green’s functions found for INCN (to the south) are well modeled using a half-space structure. The 1D structures are incorporated into intercorrelations for teleseismic P observations. Very similar results are found using the layered MDJ model for all azimuths, or when azimuthally varying the source velocity model to have a half-space structure for stations to the south, and/or using a half-space structure for the 2006 event at all azimuths. The relative yield estimates from intercorrelation are thus stable with respect to specific 1D source structures; the full effects of 3D elastic and nonelastic structures are yet to be considered.

2000 Uglegorsk earthquake

The paper presents the results of waveform inversion of the Mw 6.8 August 4 (5), 2000 Uglegorsk earthquake (Sakhalin Island, Russia). The detailed rupture process of the 2000 Uglegorsk earthquake was simulated using the waveform inversion method. The average parameters were calculated for both nodal planes. Waveform inversion was carried out on the basis of Global Seismographic Network (GSN) data. Only P-waves from BHZ channels of all stations from the GSN were used. The simulated source parameters included a double-couple source, the scalar seismic moment, the source time function, and the slip directions. The performed studies made it possible to investigate the features of the rupture development and the amplitude of displacements along the east and west-dipping nodal planes of the August 4 (5), 2000 Uglegorsk earthquake. The obtained P-slip model for the 2000 Uglegorsk earthquake source area is in good agreement with the surface manifestations of the rupture according to the field geology data and the results of geodetic inversion.

Improved Estimation of Glacial‐Earthquake Size Through New Modeling of the Seismic Source

AbstractThe number of gigaton‐sized iceberg‐calving events occurring annually at Greenland glaciers is increasing, part of a larger trend of accelerating mass loss from the Greenland Ice Sheet. Though visual observation of large calving events is rare, ∼60 glacial earthquakes generated by these calving events are currently recorded each year by regional and global seismic stations. An empirical relationship between iceberg size and MCSF, a summary measure of glacial‐earthquake size, was recently demonstrated by Olsen and Nettles (2019), https://doi.org/10.1029/2019JF005054. However, MCSF is known to be sensitive to choices made in modeling the seismic source. We incorporate constraints on the seismic source from laboratory studies of calving and test multiple source time functions using synthetic and observed glacial‐earthquake waveforms. We find that a simple, fixed time function with a shape informed by laboratory results greatly improves estimates of earthquake size. The average ratio of estimated to true peak force values is 1.03 for experiments using our preferred source model, compared with an average of 0.3 for models used in previous studies. We find that maximum‐force values estimated from waveform modeling depend far less on model choices than does MCSF, and therefore prefer maximum force as a measure of glacial‐earthquake size. Using both synthetic and real data, we confirm a correlation between maximum force and iceberg mass. Our results support the possibility of developing useful scaling relationships between seismic observables and physical parameters controlling glacier calving.

Dynamic Rupture Simulations of the 2008 7.9 Wenchuan Earthquake: Implication for Heterogeneous Initial Stress and Complex Multifault Geometry

AbstractThe 2008 Mw7.9 Wenchuan earthquake ruptured northwest‐dipping imbricate oblique reverse faults along the Longmenshan thrust belt at the eastern margin of the Tibetan Plateau, and developed one of the most complex coseismic rupture patterns for reverse faulting events ever reported in intraplate settings. To evaluate to what extent complex fault geometry and heterogeneities in initial stress can explain the characteristics of coseismic observations, we simulate spontaneous dynamic rupture propagation governed by slip‐weakening friction law on the geometrically complex multifault system loaded by heterogeneous stress. Our model reproduces many observed features including the multifault ruptures, locations of the maximum slip, Interferometric Synthetic Aperture Radar observations, source time functions and peak ground velocities. Our results suggest that the maximum horizontal principal stress in the northern part of Beichuan Fault (BFC) is rotated approximately counterclockwise compared to that in the southern part of BFC, yet this rotation is not sufficient to produce a supershear rupture. We infer that the fault core zone in the southwestern Beichuan Fault (BCF) may have been more severely damaged than the northeastern counterpart, which provides an explanation that the rupture in the northern BCF propagates faster than in the southern portion. Besides, we find that Coulomb failure stress changes on Wenchuan‐Maoxian Fault (WMF) may be counteracted by the aftershock slips of the Lixian Fault, which suggests Wenchuan earthquake could not significantly push the WMF closer to failure.

From Interseismic Deformation With Near‐Repeating Earthquakes to Co‐Seismic Rupture: A Unified View of the 2020 Mw6.8 Sivrice (Elazığ) Eastern Turkey Earthquake

AbstractThe East Anatolian Fault (EAF) is a left‐lateral transform fault accommodating the relative motion between the Anatolian and Arabian plates. On January 24, 2020, Mw6.8 Sivrice (Elazığ) earthquake is the largest event that occurred along the EAF since the nineteenth century. The earthquake provides a unique opportunity to capture a critical stage of the seismic cycle from the interseismic deformation to co‐seismic rupture. In this study, we examine the relationship between the interseismic fault activity and co‐seismic behavior of the earthquake. A kinematic model of the earthquake obtained from strong‐motion, GNSS and teleseismic waveforms along with static displacements from GNSS and InSAR data shows that the mainshock ruptured only 45 km of the 95 km long Sivrice‐Pütürge segment. Rupture initiated adjacent to the interseismically weakly coupled northeastern section and propagated unilaterally toward southwest with a rupture velocity of ∼2.5 km/s, stopping ∼30 km before the southwestern segment boundary. The earthquake did not generate any surface offsets. We identified 4 long‐term near‐repeating earthquake clusters beneath the highest co‐seismic slip zone adjacent to the northeastern creeping section. The mainshock was dynamically triggered by a M ∼ 5.4 foreshock located within the zone of near‐repeating earthquakes. We suggest that creep along the weakly coupled section of the fault loaded the neighboring locked section leading to the repeating earthquakes below the locked zone. The earthquake partially ruptured a fault segment characterized by high rate of diffuse seismicity, structural complexities and heterogeneous coupling, leading to a source characterized by a complex source time function with relatively slow rupture velocity.

Fault Friction During Simulated Seismic Slip Pulses

AbstractTheoretical studies predict that during earthquake rupture faults slide at non‐constant slip velocity, however it is not clear which source time functions are compatible with the high velocity rheology of earthquake faults. Here we present results from high velocity friction experiments with non‐constant velocity history, employing a well‐known seismic source solution compatible with earthquake source kinematics. The evolution of friction in experiments shows a strong dependence on the applied slip history, and parameters relevant to the energetics of faulting scale with the impulsiveness of the applied slip function. When comparing constitutive models of strength against our experimental results we demonstrate that the evolution of fault strength is directly controlled by the temperature evolution on and off the fault. Flash heating predicts weakening behavior at short timescales, but at larger timescales strength is better predicted by a viscous creep rheology. We use a steady‐state slip pulse to test the compatibility of our strength measurements at imposed slip rate history with the stress predicted from elastodynamic equilibrium. Whilst some compatibility is observed, the strength evolution indicates that slip acceleration and deceleration might be more rapid than that imposed in our experiments.

Neuro‐Fuzzy Kinematic Finite‐Fault Inversion: 2. Application to the Mw6.2, August/24/2016, Amatrice Earthquake

AbstractIn this article, we validate the neuro‐fuzzy kinematic finite‐fault inversion method by studying the rupture process of the , Aug/24/2016, Amatrice, central Italy, earthquake. We jointly invert three different datasets to infer the spatio‐temporal slip distribution, namely static and high‐rate GNSS data (&lt;= Hz) and strong‐motion data ( Hz). Each data set is used to constrain a different frequency range of the source model, depending on the sensitivity of the data set. The inferred slip shows a slow nucleation phase at shallow depths of 3–4 km, followed by a bilateral rupture that forms two asperities, one to the NW (Norcia) and another to the SE (Amatrice) of the hypocenter. Our inferred slip is compared with those previously obtained using well‐established methods. In order to select an adequate number of fuzzy basis functions, we propose two alternative procedures, which yield the same general slip features. The first approach consists of ensuring that the inverse problem is formally over‐determined and uses the same number of basis functions at all frequencies. The second approach is based on a maximum‐likelihood analysis of the model misfit and selects a different number of basis functions for each frequency. The maximum‐likelihood approach allows for more basis functions at high frequencies, where more detail in the spatial slip distribution is needed. The solution obtained with the maximum‐likelihood approach provides a more physically plausible source time function, which shows less back slip artifacts. The accurate prediction of high‐rate GNSS traces not used in the inversion attests the robustness of the inferred slip model.

Future magnitude 7.5 earthquake offshore Martinique: spotlight on the main source features controlling ground motion prediction

SUMMARY The eastern offshore of Martinique is one of the active areas of the Lesser Antilles Subduction Zone (LASZ). Although its seismicity is moderate compared to other subduction zones, LASZ is capable of generating a M 7+ interplate earthquake and recent studies and historical events, such as the M 8 1839 and M 7–7.5 1946 earthquakes, confirm this possibility. Given the high risk that Martinique can face in case of unpreparedness for such a M 7+ earthquake, and the lack of a regional seismic hazard study, we investigated through numerical modelling how ground motion can vary for a hypothetical Mw 7.5 interplate earthquake. Our main objective is to highlight the major factors related to earthquake source that can cause the highest variation in ground motion at four broad-band seismic stations across Martinique. For this purpose, we generated 320 rupture scenarios through a fractal kinematic source model, by varying rupture directivity, source dimension, slip distribution. We computed the broad-band ground motion (0.5–25 Hz) by convolution of source–time functions with empirical Green’s functions (EGFs), that we selected from the analysis of moderate events (M 4–4.5) recorded in the area of interest since 2016 by the West Indies network. We found that the fault geometry and the spatial extension of the largest slip patch are the most influential factors on ground motion. The significance of the variation of the predicted ground motion with respect to ground motion prediction equations (GMPEs) depends on the evaluated frequency of ground motion and on the station. Moreover, we concluded that the EGF selection can be another significant factor controlling the modelled ground motion depending on station. Our results provide a new insight for the seismic source impact on ground motion across Martinique and can guide future blind seismic hazard assessment studies in different regions.

Inversion of explosive source land seismic data to determine source signature parameters

ABSTRACTIn seismic land exploration, where terrain conditions limit the use of vibrator sources, explosive sources are the preferred choice, and in situations where low‐frequency energy is essential, explosive is the preferred source, because it contains energy down to DC (0 Hz). I present a new method to estimate the source time functions of explosion source seismic data, using a modified acquisition method, data processing, modelling and inversion. Two shots of different sizes are fired into the same geophone spread with source–receiver geometry arranged such that the Green's functions are essentially the same. The spectral ratio of corresponding seismic traces is then the spectral ratio of the source time functions. A corresponding synthetic ratio filter is calculated from Blake's explosion source model for the sources within each pair. Each modelled source is defined by five parameters: the minimum radius a of the elastic zone, the internal pressure p0 at a, the density of the rock ρ and two elastic constants for an isotropic rock, the P‐wave velocity c and Poisson's ratio σ. A grid search finds the source parameters that minimise the difference between the measured and synthetic filters for each source pair, thus yielding the two source time functions, but not their absolute amplitudes. Deconvolution of the shot records within each pair for the minimum‐phase source time functions recovers the estimated earth impulse response gathers, convolved only with the response of the recording system. The improved knowledge of the source signature is then helpful in subsequent processing, especially for reverse time migration, full waveform inversion and amplitude‐versus‐offset analysis. I consider shots at different depths in the same shot hole and shots at the same depth in different shot holes. The former configuration minimises differences in geophone coupling, and, in operations, quality control procedures must be met. There is considerable variation in recovered source parameters from place to place, caused by variations in charge size, soil or rock properties and source coupling.

Seismic source characteristics of the intermediate-depth and intraslab 2019 northern Peru earthquake (Mw 8.0)

On May 26, 2019, an unusual depth-intermediate focus earthquake, in the interior of the Nazca plate, shook the northern Peruvian jungle and caused the death of two victims. This earthquake was felt in almost all of Peru as well as Brazil, Ecuador and Colombia. The focal mechanism calculated is of the normal fault type (strike = 353∘, dip = 57∘, rake = − 99∘) for a focal depth of 114 km. The slip distribution was obtained through an inversion process of teleseismic waveforms. The maximum slip was 6.3 m, located around the central side of the fault plane. The moment magnitude calculated was Mw 8.0; the spectral analysis indicated a similar magnitude of Mw 8.0. The source time function indicates a multiple rupture earthquake, with the presence of several pulses of seismic energy release, with a total duration around 80 s. The rupture directivity was from the south to the north; we have assumed a propagation rupture velocity of 2.8 km/s. The modeled fault plane geometry has dimensions of 208×105 km2. The predicted vertical coseismic deformation field shows a pattern of mostly subsidence.

Source Time Function Clustering Reveals Patterns in Earthquake Dynamics

AbstractWe cluster a global database of 3529 Mw&amp;gt;5.5 earthquakes in 1995–2018 based on a dynamic time warping distance between earthquake source time functions (STFs). The clustering exhibits different degrees of complexity of the STF shapes and suggests an association between STF complexity and earthquake source parameters. Most of the thrust events have simple STF shapes across all depths. In contrast, earthquakes with complex STF shapes tend to be located at shallow depths in complicated tectonic regions, exhibit long source duration compared with others of similar magnitude, and tend to have strike-slip mechanisms. With 2D dynamic modeling of dynamic ruptures on heterogeneous fault properties, we find a systematic variation of the simulated STF complexity with frictional properties. Comparison between the observed and synthetic clustering distributions provides useful constraints on frictional properties. In particular, the characteristic slip-weakening distance could be constrained to be short (&amp;lt;0.1 m) and depth dependent if stress drop is in general constant.

A general approach to seismic inversion with automatic differentiation

Imaging Earth structure or seismic sources from seismic data involves minimizing a target misfit function, and is commonly solved through gradient-based optimization. The adjoint-state method has been developed to compute the gradient efficiently; however, its implementation can be time-consuming and difficult. We develop a general seismic inversion framework to calculate gradients using reverse-mode automatic differentiation. The central idea is that adjoint-state methods and reverse-mode automatic differentiation are mathematically equivalent. The mapping between numerical PDE simulation and deep learning allows us to build a seismic inverse modeling library, ADSeismic, based on deep learning frameworks, which supports high performance reverse-mode automatic differentiation on CPUs and GPUs. We demonstrate the performance of ADSeismic on inverse problems related to velocity model estimation, rupture imaging, earthquake location, and source time function retrieval. ADSeismic has the potential to solve a wide variety of inverse modeling applications within a unified framework.

On the Source Parameters and Genesis of the 2017, Mw 4 Montesano Earthquake in the Outer Border of the Val d’Agri Oilfield (Italy)

On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M &amp;gt; 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw4 earthquake by applying advanced seismological techniques to estimate the uncertainties derived from the moment tensor inversion and identify plausible directivity effects. The moment tensor is dominated by a NW–SE oriented normal faulting with a centroid depth of 14 km. A single ML2.1 aftershock was recorded and used as the empirical Green’s function to calculate the apparent source time function for the mainshock. Apparent durations (in the range 0.11–0.21 s, obtained from S-waves) define an azimuthal pattern, which reveals an asymmetric bilateral rupture with 70% of the rupture propagation in the N310°W direction, suggesting a rupture plane dipping to the SW. Our results tally with the activation of a deeper fault segment associated with the Eastern Agri Fault System close to the basement as the origin of the Montesano earthquake. Finally, the Coulomb stress rate induced by depletion of the oilfield is calculated to quantify the trigger potential estimated for the Montesano earthquake yielding relatively low probabilities below 10%. Our analyses point toward the conclusion that the Mw4 event was more likely due to the local natural tectonic stress, rather than induced or triggered by the long-term hydrocarbon extraction in the Val d’Agri oilfield.

The complex, static displacement of a very long period seismic signal observed at Soufrière Hills volcano, Montserrat, WI

In this study we demonstrate how very-long period (VLP) volcanic seismic signals can be processed in order to obtain essential and detailed information about the seismo-volcanic source process. As an example we use the VLP signal observed on 23 March 2012 during an outgassing event at Soufrière Hills volcano, Montserrat, acquired by instruments with different natural periods. The aim of this study is to highlight the importance of retrieving the correct source time function by a complete restitution process. When ground displacement cannot be retrieved through the restitution process due to very narrow band-pass limited instrument response, we compare synthetic and observed waveforms in the velocity domain and determine the best model by generating a synthetic velocity seismogram using the band-limited seismometer characteristics. Furthermore, we show how this approach of forward modelling can reveal much more detail of the source process, since small changes in displacement are enhanced in the velocity seismogram. Using these restituted and modelled displacements we perform a moment tensor inversion combined with a grid search locating the source at 600 m depth below sea level and estimating the source volume change to be in the range of 0.6−1.1 × 103m3.

Seismic source characteristics of the 2016 Pedernales-Ecuador earthquake (Mw 7.8)

The 2016 Pedernales earthquake (Mw 7.8), is part of the sequence of large earthquakes in Ecuador and Colombia: 1906 (~8.8 Mw), 1942 (Mw 7.8), 1958 (Mw 7.7), 1979 (Mw 8.2), all these events with a focal mechanism of the thrust fault type. In this research, we have obtained the seismic source distribution (indicating the presence of 2 main asperities), from inversion of teleseismic signals, as well as the parameters of the seismic rupture process: focal mechanism that indicates a type of inverse fault (strike = 25∘, dip = 12∘, rake = 118∘) for a focal depth of 23 km and the source time function indicating a composite event, the duration of the rupture process was around 70 s, assuming an average rupture velocity of 3.0 km/s. The seismic moment was calculated at 5.77×1020 Nm, equivalent to a magnitude of Mw 7.8. The smallest asperity is located around the epicentre and the main asperity is located around the coastline to the south-west from the epicentre. The maximum dislocation or slip was 3.75 m. The coseismic vertical displacement is not conductive to the generation of a large tsunami, as it is around 1 m only. The zone of Pedernales have a Coulomb stress transfer of 22 bar, therefore, this area is loaded with stresses and is a potentially seismic zone for an upcoming earthquake.

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.

Concentrated Slip and Low Rupture Velocity for the May 20, 2012, MW 5.8, Po Plain (Northern Italy) Earthquake Revealed From the Analysis of Source Time Functions

AbstractWe analyse the rupture properties of the May 20, 2012, MW 5.8, Po Plain (Northern Italy) earthquake by using two different modeling procedures based on the source time functions: a forward modeling and a global inversion Bayesian method. While the forward modeling allows to retrieve general information on the source characteristics, the global inversion allows to explore a substantially larger number of possible solutions, with more parameters, providing a quantitative estimate of the misfit. We invert for the spatial slip distribution and for the rupture velocity on a planar fault model. The unknown slip is given at the nodes of the subfaults (control points) and then given at the elementary subfaults through a bilinear interpolation. The number of control points is progressively increased to move from a high‐ to low‐wavelength description of final slip on the fault plane. The optimal model parameter set is chosen according to the Akaike Information Criterion. The uncertainty on the slip distribution and rupture velocity has been estimated by a statistical analysis of the model ensemble and, in particular, through the weighted mean model and the standard deviation. We find that the most earthquake slip occurred in the regions located northeast and southwest of the hypocenter, consistent with the forward modeling. Moreover, we find a low rupture propagation velocity (0.4 compressional Mach number) similarly to what has been observed for the close 29 May, MW 5.6, and radiation efficiency suggesting that half of the strain energy was used to create new fracture.