Empirical Green’s Function Research Articles

The 2020 Samos Mw7 earthquake: Source model depicting complexity and rupture directivity

The October 30, 2020, Mw7 Samos earthquake ruptured a north-dipping offshore normal fault, bounding the Samos basin; it accommodated ∼N-S extension and can be viewed as a modern manifestation of the basin evolution. It caused 118 fatalities, generated a tsunami, and caused a co-seismic uplift of 20 to 35 cm of the NW part of Samos Island. Using broadband, strong-motion, and geodetic data, we constrain the location and source geometry of the mainshock. A multiple-point source model suggests three sequential subevents providing 20 s of source duration. Our finite-fault kinematic model confirms the prevalence of large slip amplitudes (∼2.4 m) along the entire ruptured area and the up-dip and westward rupture propagation. This directivity is independently confirmed by Apparent Source Time Functions inferred from regional recordings using a herein developed new variant of the empirical Green's function method. Static GNSS displacements from inland stations yield a near-surface co-seismic slip of ∼1 m amplitude, contributing to any interpretation of the observed island uplift. The 2020 Samos event showed that in the spatially heterogeneous oblique transtentional regions in the back-arc Aegean region, normal faults bounding the basins are capable to rupture in M7 earthquakes, provoke tsunami generation, and constitute a constant threat to the nearby coastal areas of both Greece and Turkey.

Rupture Directivity of the 2021MW6.0 Yangbi, Yunnan Earthquake

AbstractThe 2021MW6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼. The relatively low moment scaled radiated energyof 1.5 × 10−5and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.

Subsurface characterization using passive seismic in the urban area of Dublin City, Ireland

AbstractWe investigate the shallow geological features and previously unknown depths of the main geological interfaces in the highly urbanized Dublin City area by using a set of complementary ambient noise seismic methods. A sparse seismic array, composed of 19 broadband sensors, was installed in a 5 × 5 km area across the city centre. The dataset was acquired over a 5‐month‐long deployment and used to perform horizontal/vertical spectral ratios and frequency–wave number analysis cross‐correlation interferometry. From the horizontal/vertical high‐frequency resonance peaks, we estimate the depth to bedrock. Then we use a subset of six seismic stations to obtain the frequency–wavenumber Rayleigh wave fundamental mode in the 0.8–3 Hz frequency band. Next, the ambient noise dataset is cross‐correlated in order to extract the empirical Green's functions before measuring surface‐wave phase velocities by undertaking a dispersion analysis in the 0.5–9 Hz frequency band. Then, a Monte Carlo global optimization algorithm is used to invert the phase velocity dispersion measurements. We generate a reference one‐dimensional depth S velocity profile along with a set of localized one‐dimensional S velocity profiles. Finally, a smooth three‐dimensional shear wave velocity model is derived for the top 1000 m in the sedimentary Dublin Basin. From the one‐dimensional velocity profiles and three‐dimensional shear velocity model, with some sensitivity down to approximately 1.2 km, we observe velocity changes with depth that highlight the presence of three consistent interfaces. We discuss interpretative scenarios that correlate the velocity features to potential stratigraphic boundaries occurring within the sedimentary Dublin Basin, suggesting that basement rocks could be located at a depth significantly greater than the top kilometre. The results of this study may indicate that future geothermal studies should be directed at structures deeper than 1 km, towards the bottom of the sedimentary basin beneath the city.

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.

Love Wave Dispersion Analysis from Ambient Seismic Noise in Banyumas Basin, Central Java, Indonesia

The Banyumas Basin is one of a sedimentary basins in the southern coast of Central Java that has many manifestations of the presence of hydrocarbons in the form of oil and gas seeps found on the surface. Its presence is covered by a thick subvulcanic sedimentary layer. A conventional seismic active method which is usually used to delineate subsurface structures in an hydrocarbon area, however it can be challenging to imaged the structures beneath surface in areas covered by thick sub-volcanic sediments. Another method is needed to investigate the subsurface image in a thick sub-volcanic area, such as Banyumas basin. We cross correlated of the ambient seismic noise at transversal component to retrieve the love wave empirical Green's function. We used 40 broad band seismic stations for 60 days recording from Center for Geological Survey - Indonesian Geological Agency. The daily cross-correlated data then stacked to obtain the Love waves Green’s function. The Green’s function is clearly seen in 1-10 s period band. Then, the obtained Green’s function will be analyzed to get the Love waves group velocity.

Resolving Shallow Moment Releases of Megathrust Earthquakes by Empirical Green’s Function‐Based Inversions: A New Approach for Tsunami Warning

AbstractTsunamis are secondary hazards generated by megathrust earthquakes, which are commonly related to coseismic slips occurring at shallow depths. Various methods were proposed to quickly predict tsunami excitations, while these methods mostly require specific observations or modeling techniques, which limits their application to global subduction zones. In this study, we propose an inversion technique that utilizes the water‐reverberation phases in teleseismic P coda waves to discriminate shallow and deep slips and adopts teleseismic empirical Green’s functions (EGFs) to enable rapid inversions. We developed an EGF extraction and interpolation method to construct Green’s functions. We apply this algorithm to seven megathrust earthquakes, which have been well studied and identified with or without significant shallow slips. In comparison with inversions performed with theoretical Green’s functions, the EGF‐based inversion technique is more effective in discriminating these two types of earthquakes. We propose a threshold to evaluate the percentage of shallow moment releases and found that the EGF‐based inversion result resembles that of joint inversion results. We also analyze the possibility to apply the EGF‐based inversion technique for tsunami warning purposes and infer that it serves as a promising algorithm to enable efficient tsunami early warning.

Recovering and monitoring the thickness, density, and elastic properties of sea ice from seismic noise recorded in Svalbard

Abstract. Due to global warming, the decline in the Arctic sea ice has been accelerating over the last 4 decades, with a rate that was not anticipated by climate models. To improve these models, there is the need to rely on comprehensive field data. Seismic methods are known for their potential to estimate sea-ice thickness and mechanical properties with very good accuracy. However, with the hostile environment and logistical difficulties imposed by the polar regions, seismic studies have remained rare. Due to the rapid technological and methodological progress of the last decade, there has been a recent reconsideration of such approaches. This paper introduces a methodological approach for passive monitoring of both sea-ice thickness and mechanical properties. To demonstrate this concept, we use data from a seismic experiment where an array of 247 geophones was deployed on sea ice in a fjord at Svalbard, between 1 and 24 March 2019. From the continuous recording of the ambient seismic field, the empirical Green function of the seismic waves guided in the ice layer was recovered via the so-called “noise correlation function”. Using specific array processing, the multi-modal dispersion curves of the ice layer were calculated from the noise correlation function, and then inverted for the thickness and elastic properties of the sea ice via Bayesian inference. The evolution of sea-ice properties was monitored for 24 d, and values are consistent with the literature, as well as with measurements made directly in the field.

Application of passive seismic interferometry for mapping mining waste storage facilities: A case study of Pyhäsalmi mine in Finland

Obtaining knowledge about the structural, geotechnical and hydrogeological conditions of mining waste storage facilities (e.g., tailing storage facilities, TSF) is an important task for mining waste management. From the existing applied geophysics techniques, high-resolution controlled source seismic methods are the most appropriate due to their superior spatial and depth resolution. However, the TSF areas are not often accessible for installation of seismic instruments and application of controlled source. Moreover, TSF areas in active mine sites are characterised by high-level of seismic and acoustic noise, produced by mining operations and other human activities at mine sites. In this study, we show that these problems can be addressed by utilizing passive seismic interferometry, in which cross-correlation of ambient seismic noise is used to obtain the Empirical Green's function (EGF's) of the studied medium. We used this method to evaluate seismic velocities in the TSF and in the near-surface bedrock at the site of the Pyhäsalmi underground mine in Finland. We used the ambient seismic noise recorded in two seismic experiments in the area. In the first experiment, the data were acquired in 2013 along a line 10 km long crossing the area of the underground mine. The second experiment was carried out in the TSF area in 2019. We obtained S-wave velocity models of both bedrock and the TSF by applying multichannel analysis of surface waves (MASW) to surface wave parts of Empirical Green's Functions (EGFs). The analysis of autocorrelation functions of passive seismic data recorded on the TSF surface allowed to us retrieve reflected arrivals from the boundaries inside the TSF. At the same time, we used amplitudes of surface wave parts of EGF's to obtain information about attenuation of surface waves in TSF. Our results show that passive seismic interferometry and interpretation of passive seismic data by different techniques can be a useful tool for inspection of TSF of active mines

Imaging the crust and uppermost mantle structure of Portugal (West Iberia) with seismic ambient noise

SUMMARYWe present a new high-resolution 3-D shear wave velocity (Vs) model of the crust and uppermost mantle beneath Portugal, inferred from ambient seismic noise tomography. We use broad-band seismic data from a dense temporary deployment covering the entire Portuguese mainland between 2010 and 2012 in the scope of the WILAS project. Vertical component data are processed using phase correlation and phase weighted stack to obtain empirical Green functions (EGFs) for 2016 station pairs. Further, we use a random sampling and subset stacking strategy to measure robust Rayleigh-wave group velocities in the period range 7–30 s and associated uncertainties. The tomographic inversion is performed in two steps: First, we determine group-velocity lateral variations for each period. Next, we invert them at each grid point using a new trans-dimensional inversion scheme to obtain the 3-D shear wave velocity model. The final 3-D model extends from the upper crust (5 km) down to the uppermost mantle (60 km) and has a lateral resolution of ∼50 km. In the upper and middle crusts, the Vs anomaly pattern matches the tectonic units of the Variscan Massif and Alpine basins. The transition between the Lusitanian Basin and the Ossa Morena Zone is marked by a contrast between moderate- and high-velocity anomalies, in addition to two arched earthquake lineations. Some faults, namely, the Manteigas–Vilariça–Bragança fault and the Porto–Tomar–Ferreira do Alentejo fault, have a clear signature from the upper crust down to the uppermost mantle (60 km). Our 3-D shear wave velocity model offers new insights into the continuation of the main tectonic units at depth and contributes to better understanding the seismicity of Portugal.

Characteristics of two tsunamis generated by successive <i>M</i><sub>w</sub> 7.4 and <i>M</i><sub>w</sub> 8.1 earthquakes in the Kermadec Islands on 4 March 2021

Abstract. On 4 March 2021, two tsunamigenic earthquakes (Mw 7.4 and Mw 8.1) occurred successively within 2 h in the Kermadec Islands, offshore New Zealand. We examined sea level records at tide gauges located at ∼100 to ∼2000 km from the epicenters, conducted Fourier and wavelet analyses as well as numerical modeling of both tsunamis. Fourier analyses indicated that the energy of the first tsunami is mainly distributed over the period range of 5–17 min, whereas it is 8–32 min for the second tsunami. Wavelet plots showed that the oscillations of the first tsunami continued even after the arrival of the second tsunami. As the epicenters of two earthquakes are close to each other (∼55 km), we reconstructed the source spectrum of the second tsunami by using the first tsunami as the empirical Green's function. The main spectral peaks are 25.6, 16.0, and 9.8 min. The results are similar to those calculated using tsunami-to-background ratio method and are also consistent with the source models.

Extracting reliable empirical Green's functions using weighted cross-correlation functions of ambient seismic noise in west-central and southern Brazil

SUMMARYAmbient seismic noise is now routinely used to study the Earth's interior. For an isotropic homogeneous medium, the basic assumption to extract seismic phases from a station pair is that the sources of seismic noise are distributed in such a way that there is a uniform energy flux around the station pair. In general, however, some particular azimuthal directions may dominate the energy flux, which directly affects the extracted interstation empirical Green's function (EGF). To solve this problem, we analysed synthetic cross-correlation functions (CCFs) from seismic pulses propagated in isotropic and anisotropic heterogeneous half-space media towards a station pair under the assumptions of uniformly and non-uniformly distributed sources of noise. A reliable EGF signal can be extracted by applying three processing steps: (1) normalizing the number of repeated stationary sources, (2) normalizing the energy of each excited source and (3) selecting coherent CCFs in the final stacking. In this way, three different classes of station pairs were identified based on the number of CCFs used in the stacking procedure. We introduced and applied a new method based on weighted root-mean-square stacking (WRMS) to the CCFs of more than 33 months of ambient noise recorded from January 2016 to September 2018 at 75 broad-band stations in West-Central Brazil. In the case of non-uniform distribution of source of noise, simple classical linear stacking of CCFs produces distorted EGFs. However, the waveform extracted by the WRMS method is very similar to the Rayleigh waves excited by an earthquake (on 2017 January 3) near one receiver observed at the other receiver. Moreover, synthetic tests and a comparison between extracted and earthquake signals show that although the WRMS method can extract the main part of the signal that is propagated on the shortest path, it cannot recover the energy parts propagated on multipath. Despite the N–W directionality in the geometry of the array, the rose-diagram results indicate no significant spatial variations in the energy level of EGFs extracted by the WRMS stacking, whereas the EGFs extracted by the classical linear stacking indicate the extreme directionality of energy flow in different period ranges. Rayleigh wave group and phase velocity tomographic maps resolved by the EGFs derived from the WRMS method indicate a clear boundary along the Asuncion and Rio Grande Arches between the Chaco-Paraná and the Paraná basins at the shorter period, while the tomographic maps in the same periods which were calculated by other stacking methods cannot clearly separate basins and arcs. Our tomographic maps at longer periods indicate variations of Moho depth and lithospheric velocities.

Ambient noise tomography of Misti volcano, Peru

To better understand the recent internal structure of Misti volcano, we determined a 3D S-wave velocity model applying Ambient Noise Tomography (ANT). We used data from 23 broadband and short-period seismic stations temporarily installed at Misti volcano between March and December 2011. This dataset allowed us to obtain empirical Green's functions by cross-correlating seismic ambient noise signals. Then, we retrieved 104 dispersion curves using the frequency-time analysis (FTAN) and, through a non-linear multiscale inversion, we obtained nine 2-D Rayleigh waves group velocity maps for periods in the range 0.7 s - 2 s. Finally, we carried out the depth inversion through a Bayesian transdimensional inversion to obtain a 3-D S-wave velocity model down to 3 km depth. Our study highlights five relevant seismic velocity anomalies. We observed the presence of three high-velocity zones located in the west-northwest, southwest and southeast parts of the crater, that could be related to intrusive bodies possibly associated with the formation of Misti volcano. We also observed two low-velocity anomalies in the volcano's western and central parts, which coincide with previous studies' findings and are related to fractured and weakened materials associated with the external caldera collapse and recent eruption episodes.

Evidence for Short Temporal Atmospheric Variations Observed by Infrasonic Signals: 1. The Troposphere

AbstractInfrasound monitoring is used in the forensic analysis of events, studying the physical processes of sources of interest, and probing the atmosphere. The dynamical nature of the atmosphere and the use of infrasound as a forensic tool lead to the following questions; (1) what is the timescale of atmospheric variability that affects infrasonic signals? (2) how do infrasound signals vary as a function of time? This study addresses these questions by monitoring a repetitive infrasound source and its corresponding tropospheric returns 54 km away. Source‐receiver empirical Green's functions are obtained every 20 s and used to demonstrate the effect of atmospheric temporal variability on infrasound propagation. In addition, observations are compared to predicted simulated signals based on realistic atmospheric conditions. Based on 127 events over 3 days, it is shown that infrasound properties change within tens of seconds. Particularly, phases can appear and disappear, the propagation time varies, and the signals' energy fluctuates. Such variations are attributed to changes in temperatures and winds. Furthermore, atmospheric models can partly explain the observed changes. Therefore, this study highlights the potential of high temporal infrasound‐based atmospheric sounding.

Quantifying the Sensitivity of Microearthquake Slip Inversions to Station Distribution Using a Dense Nodal Array

ABSTRACT To investigate the sensitivity of slip inversions to station distribution and choice of empirical Green’s function (EGF), we examine three microearthquakes that occurred within the high-density LArge-n Seismic Survey in Oklahoma (LASSO) nodal seismic array. The LASSO array’s dense distribution of 1825 geophones provides an exceptional level of spatial and azimuthal coverage, allowing for more accurate inversions of slip than are possible with typical station distributions. The highly accurate slip inversions, in turn, allow for the exploration of the sensitivity of slip inversions to station distribution and parameter choices. We examine the effects of these choices using three well-recorded strike-slip microearthquakes (ML 1.7, 2.3, and 2.7) using an EGF method. From this analysis and the systematic testing of varied network arrangements, we find that station distributions that have uniform coverage of azimuth and distance can retrieve the overall pattern of slip, but the estimated amplitude of slip can vary by 30% for high-slip regions due to small variations in station location. In addition, we find that the distance range that accurately resolves the overall pattern of slip is the one that contains the takeoff angles of 45°–65°. Concerning azimuthal coverage, a network with >270° performs similarly to having complete coverage. The choice of EGF can shift the location of resolved areas of slip and their amplitude, depending on its similarity in location and radiation pattern to the target earthquake.

Ground motion simulations in Quito (Ecuador) due to major earthquakes from the subduction zone

SUMMARY In 1906, an earthquake with a magnitude estimated between Mw 8.4 and 8.8 occurred in the subduction zone along the coast of Ecuador and Colombia. This earthquake caused extensive damage on the coast but had a rather small impact on the capital city of Quito, situated 180 km away. At that time, the city of Quito extended over a small area with a few thousand inhabitants, while today it stretches over 40 km and has a population of over 3 million, with most of the city built without paraseismic regulations. The aim of this study is to obtain new insights on the impact that large earthquakes from the subduction zone would have on the city today. This question is crucial since we know that the city of Quito is prone to site effects and that the southern part of the city amplifies seismic waves at low frequencies, around 0.3–0.4 Hz. In April 2016, an Mw 7.8 earthquake occurred on the subduction interface in the Pedernales area. This event was the first large earthquake in the city of Quito to be well recorded by 13 stations of the permanent accelerometric network (RENAC). In this study, we take advantage of this data set (main shock and large aftershock recordings) to (1) test an empirical Green's function blind simulation approach where the input stress drop is taken from a global catalogue of source time functions, (2) compare the synthetic accelerograms and ground motion values we obtain for an Mw 7.8 earthquake with the actual recordings of the Pedernales earthquake and then (3) simulate larger earthquakes of Mw 8.2 and 8.5 from the subduction zone. For Mw 7.8 simulations, our approach allows a good reproduction of the ground motions in the whole frequency bands and properly takes into account site effects. For Mw 8.2 and 8.5 simulations, we obtain for the stations in the southern part of the basin, larger values at low frequencies than the predicted motion given by ground motion models. These values, although high, should be supported by new or recent buildings if they are constructed respecting the building code that applies in Quito. Therefore, for this type of strong but distant earthquake, the seismic standards appear to be well suited and it is imperative to ensure that they are well considered in the design of the new buildings to be constructed, especially in the southern part of the expanding city.

Preliminary strong ground motion simulation at seismic stations within nuclear power plant sites in South Korea by a scenario earthquake on the causative fault of 2016 Gyeongju earthquake

Stochastic and an empirical Green's function (EGF) methods are preliminarily applied to simulate strong ground motions (SGMs) at seismic stations within nuclear power plant (NPP) sites in South Korea by an assumed large earthquake with MW6.5 (scenario earthquake) on the causative fault of the 2016 Gyeongju earthquake with MW5.5 (mainshock). In the stochastic method, a ratio of spectral amplitudes of observed and simulated waveforms for the mainshock is assumed to be an adjustment factor. In the EGF method, SGMs by the mainshock are simulated assuming SGMs by the 2016 Gyeongju earthquake with MW5.0 (foreshock) as the EGF. To simulate SGMs by the scenario earthquake, a ratio of fault length to width is assumed to be 2:1 in the stochastic method, and SGMs by the mainshock are assumed to be EGF in the EGF method. The results are similar based on a bias of the simulated response spectra by the two methods, and the simulated response spectra by the two methods exceeded commonly standard design response spectra anchored at 0.3 g of NPP sites slightly at a frequency band above 4 Hz, but considerable attention to interpretation is required since it is an indirect comparison.

Source properties of the 2019ML6.3 Hualien, Taiwan, earthquake, determined by the local strong motion networks

SUMMARYThe 2019 ML 6.3 Hualien earthquake struck the northern Longitudinal Valley (LV) and generated not only large strong motions (intensity of 7, as defined by the Central Weather Bureau, Taiwan) locally but also widespread strong shaking in metropolises in northern Taiwan. In this study, we analyse strong motion records from local seismic networks to understand the source properties of the 2019 event. We first obtain the centroid location of the 2019 event using the source-scanning algorithm (SSA) technique by applying the unfiltered records. The determined centroid location is 121.55°E, 24.10°N, with a depth of 22.5 km. This location is 5.5 km north–northwest of and 3.8 km deeper than the Central Weather Bureau hypocentre, suggesting that the 2019 event occurred on the high-angle west-dipping plane of the focal solution. The centroid time delay is 3.35 s. Then, we obtain strong motion generation areas (SMGAs) of the 2019 event using the empirical Green's function method by considering the broad-band waveforms (0.4–10 Hz). Unlike other moderate-sized earthquakes in Taiwan, which have one SMGA, we determine that there were two SMGAs in the 2019 event. SMGA1 initiated at the CWB hypocentre with a size of 4.00 km2, and SMGA2 initiated at the centroid location determined by the SSA approach with a size of 3.63 km2. Such small areas cause high stress drops of 13.7 and 27.4 MPa for SMGA1 and SMGA2, respectively. We infer that the localized high stress drop of SMGAs is one of the important factors responsible for high peak-ground accelerations (PGAs) in Taiwan in addition to a strong directivity effect coupled with the radiation pattern reported by the previous study. Furthermore, previous moderate-sized earthquakes on an active structure called the Xiulin segment revealed similar source properties with a high stress drop and generated large PGA locally as well as in the metropolises of northern Taiwan. Considering the stored moment deficit, the probability of a future large earthquake in the northern LV region remains high. It is essential to consider seismic hazard assessment and mitigation for this not-well-known but high-seismic-potential region.

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.

Full Waveform Ambient Noise Tomography for the Northern Mississippi Embayment

AbstractWe use seismic ambient noise data recorded by broadband stations around the northern Mississippi Embayment to develop a three‐dimensional shear wave velocity model with full waveform inversion. Empirical Green's functions at periods between 8 and 40s are extracted using a data processing flow based on the continuous wavelet transform. Synthetic waveforms are calculated with an isotropic model through a Graphics Processor Unit‐enabled, collocated finite‐difference code. Starting from the Central United States Velocity Model, the shear wave velocity is iteratively updated with sensitivity kernels constructed using the adjoint method. Several mid‐crustal velocity variations are related to major geological features including the Mississippi Valley graben (MVG), the Ouachita thrust belt, and the Missouri batholith. An intrusion is imaged in northwestern Alabama, coincident with a previously unexplained gravity high. A major change in mid‐crustal velocity occurs across the MVG; much higher velocity crust is present southeast of the graben than northwest. The high velocities are attributed to numerous igneous intrusions, possibly related to formation of the Granite‐Rhyolite province. A strength contrast produced by the change in mid‐crustal velocities may have facilitated formation of the younger, shallower MVG, as external stresses became tensional during Iapetus rifting. The rift pillow is interpreted as the deeper expression of the high velocity crust. Low velocity crust is present below southern Missouri starting at a depth of roughly 20 km. The boundary between the low velocity crust and higher velocity crust to the south and east is sharp and coincides with the Nd‐line.

Potential applicability and limitations of radiation-corrected empirical Green's functions for point-source cases

SUMMARYNumerous efforts have been made to accurately estimate Green's functions, which link observed seismic waves to physical source processes. Detailed a priori information is required for the theoretical Green's function calculated from a given velocity structure. In contrast, the empirical Green's function (EGF) obtained from seismograms of nearby earthquakes with similar focal mechanisms requires the occurrence of such appropriate events. To partially release these constraints, we propose radiation-corrected EGF by correcting the radiation patterns of the EGF seismograms. In this study, we performed synthetic tests to quantify the applicability of radiation correction, together with its limitations. Such tests are a useful approach to test a benchmark of EGF applicability. We then applied the new method to real earthquakes to validate the concept for practical use. The synthetic test and real applications confirmed the superiority of the radiation-corrected EGF over the conventional EGF in specific, but realistic, cases.