Dynamic Relationship Study between the Observed Seismicity and Spatiotemporal Pattern of Lineament Changes in Palghar, North Maharashtra (India)

The Palghar region (north Maharashtra, India), located in the northwestern part of the stable continental region of India, experienced a low magnitude earthquake swarm, which was initiated in September 2018 and is continuing to date (as of October 2021). From December 2018 to December 2020, ~5000 earthquakes with magnitudes from M1.2 to M3.8 occurred in a small region of 20 × 10 km2. These earthquakes were probably triggered by fluid migration during seasonal rainfall. In this study, we have used multi-temporal Landsat satellite data of the year 2000, 2015, 2018, 2019, and 2020, extracted lineaments, and studied the changes in frequency and pattern of lineaments before and after the initiation of the swarm in the Palghar region. An increase in the lineament density and amount of rainfall are found to be associated with the increasing frequency of earthquakes.

Seismic Hazard Levels in Guinea, West Africa

The occurrence of seismic activities in Guinea is infrequent, although located in a stable continental region. The hazard assessment level in Guinea, West Africa was determined by a probabilistic approach for 10 sites across the region. The calculation was carried out for 10%, 2% and 0.5% probability of exceedance in 50 years using a homogenized 100-year catalogue compiled from different seismic sources; Three prediction relations, developed for Eastern and Central North America for the stable continental region; and the R-CRISIS program. The levels of hazard estimated were high in the Palaeozoic area of Guinea. A uniform b-value of 0.70 ± 0.12, and individual activity rate (λ) were calculated for the three seismic zones. The maximum PGA values estimated for the study region are 0.08 g, 0.27 g, and 0.57 g for 475, 2475 and 9975 years return periods, respectively. Finding from this study will be useful in planning for the regional infrastructure.

The search for hard-rock kappa (κ) in NGA-East: A semi-automated method for large, challenging datasets in stable continental regions

This article describes the work undertaken within the Next Generation Attenuation (NGA)-East project with the aim of estimating κ0 (the site-specific component of the high-frequency decay parameter, κ) for rock sites in Central and Eastern North America (CENA), using the project’s shallow crustal dataset. We introduce a methodology to address the numerous challenges in CENA: a large dataset in a low-seismicity stable continental region, with poor magnitude and distance coverage, undesirable recording sensor characteristics (low sampling rates leading to poor high-frequency resolution), high uncertainty in the regional stress drop, and lack of site-specific velocity characterization. We use two band-limited κ estimation approaches, the acceleration and displacement spectrum (AS and DS), applied above and below the source corner frequency ( fc), respectively. For band-limited approaches, the key requirement is an estimate of fc, which—apart from the event magnitude readily available in the flatfile—also heavily depends on the highly uncertain stress drop. By considering lower and upper bounds on regional stress drop, we propose a new method to quickly and automatically screen such very large datasets to identify all possible recordings for which band-limited κ approaches can be used. Combining them produces better-quantify estimates of κ and its epistemic uncertainties for this challenging dataset. The mean κ0 values combining the two methods are 13 ± 23 ms for horizontal ground motion.

PEER NGA-East database

This article documents the earthquake ground motion database developed for the NGA-East Project, initiated as part of the Next Generation Attenuation (NGA) research program and led by the Pacific Earthquake Engineering Research Center (PEER). The project was focused on developing a ground motion characterization model (GMC) model for horizontal ground motions for the large region referred to as Central and Eastern North America (CENA). The CENA region covers most of the U.S. and Canada, from the Rocky Mountains to the Atlantic Ocean and is characterized tectonically as a stable continental region (SCR). The ground-motion database includes the two- and three-component ground-motion recordings from numerous selected events relevant to CENA ( M > 2.5, with distances up to 3500 km) that have been recorded since 1976. The final database contains over 27,000 time series from 82 earthquakes and 1271 recording stations. The ground motion database includes uniformly processed time series, 5% damped pseudo-spectral acceleration (PSA) median-component ordinates for 429 periods ranging from 0.01 to 10 s, duration and Arias intensity in 5% increments, and Fourier amplitude spectra for different time windows. Ground motions and metadata for source, path, and site conditions were subjected to quality checks by topical working groups and the ground-motion model (GMM) developers. The NGA-East database constitutes the largest database of processed recorded ground motions in SRCs and is publicly available from the PEER ground-motion database website.

A procedure to develop a backbone ground-motion model: A case study for its implementation

The backbone modeling in ground-motion characterization (GMC) is a useful methodology to describe the epistemic uncertainty in median ground-motion predictions. The approach uses a backbone ground-motion model (GMM) and populates the GMC logic tree with the scaled and/or adjusted versions of the backbone GMM to capture the epistemic uncertainty in median ground motions. The scaling and/or adjustment should represent the specific features and uncertainties involved in source, path, and site effects at the target site. The identification of the backbone model requires different considerations specific to the nature of the ground-motion hazard problem. In this article, we present a scaled backbone modeling approach that considers the magnitude- and distance-scaling predictors as well as their correlation to address the epistemic uncertainty in median ground-motion predictions. This approach results in a trivariate normal distribution to fully define a range of epistemic uncertainty in a model sample space. The simultaneous consideration of magnitude and distance scaling while defining the epistemic uncertainty and the methodology followed for the simplified representation of trivariate normal distribution in ground-motion logic tree are the two important features in our procedure. We first present the proposed approach that is followed by a case study for Central and Eastern North America (CENA) stable continental region. The case study discusses the underlying assumptions and limitations of the proposed approach.

Hydraulic and mechanical constraints on the magnitude Ml 5.2 earthquake of 11 November 2019 at Le Teil (France)

<p>On 11 November 2019,  a Ml 5.2 earthquake broke the Rouvière fault in southeast France at Le Teil, close to the Rhone river. This recent seismic event is the strongest earthquake ever felt in France since the Arette (Pyrenees) earthquake in 1967. <em>A priori, </em>it is also a historically unprecedented earthquake in the surrounding low strain and stable continental region. By using an updated geological model, we focus this work on the comparison of the effect of hydraulic recharge linked to the infiltration of meteoric water in the period preceding the earthquake and the effect of the exploitation of a large limestone quarry in the vicinity of the epicenter.</p><p>At first, we carry out a complete inventory of local seismicity in a rectangular area of ​​50 km x 25 km around the Teil quarry. We put these seismic events in temporal relation with the rainfall measurements from the weather station at Montélimar. The three most intense rainy events between 2010 and 2019 are all followed by a seismic event in this restricted area, which occurs between 8 and 18 days after these rainy episodes.</p><p>Afterward, we describe the different geological configurations from the updated geological model and the boundary conditions, that are used to calculate the pressure variations along the Rouvière fault using two-dimensional (2D) double porosity double permeability models. The BRGM Compass code is used with the surface soil moisture data acquired by the SMOS satellite between 2010 and 2019, as surface boundary conditions and the Rhône river as edge boundary conditions. The main result of these hydrogeological simulations is that at the intersection of the Rouvière fault and a sub-vertical fault, the calculated increase in pore fluid pressure is maximum just before the earthquake of November 11, 2019.</p><p>A sensitivity study carried out on the hydraulic parameters and on the configuration of the fault system of the 2D model, allows us to estimate that at about 1000 m depth, the overpressure linked to the hydraulic recharge is between 0.3 and 0.6 MPa. Finally, we compare the variation in normal stress linked to a mechanical discharge from the surface quarry and the hydraulic overpressure linked to a meteoric water recharge, by choosing the same fault geometry. The comparison shows that the overpressure associated with hydraulic recharge has an impact that is an order of magnitude greater than that of the normal mechanical stress due to the decharge of the limestone quarry.</p>

Source and Attenuation Properties of the 2012 Moe, Southeastern Australia, Earthquake Sequence

The 19 June 2012 local magnitude ML 5.4 (Mw 5.1) Moe earthquake, which occurred within the Australian stable continental region, was the largest seismic event for the state of Victoria, for more than 30 yr. Seismic networks in the southeast Australian region yielded many high-quality recordings of the moderate-magnitude earthquake mainshock and its largest aftershock (ML 4.4 and Mw 4.3) at a range of distances from the epicenter. The source and attenuation characteristics of the earthquake sequence are analyzed. Almost 15,000 felt reports were received following the mainshock, and its ground motions tripped a number of coal-fired power generators in the region amounting to the loss of, approximately, 1955 MW of generation capacity. The attenuation of macroseismic intensities is shown to mimic the attenuation shape of eastern North America (ENA) models but requires an interevent bias to reduce predicted intensities. Furthermore, instrumental ground-motion recordings are compared to ground-motion models (GMMs) considered applicable for the southeastern Australian (SEA) region. Some GMMs developed for ENA and SEA provide reasonable estimates of the recorded ground motions of spectral acceleration within epicentral distances of, approximately, 100 km. The mean Next Generation Attenuation-East GMM, recently developed for stable ENA, performs relatively poorly for the 2012 Moe earthquake sequence, particularly, for short-period accelerations. These observations will help inform future seismic hazard assessments for eastern Australia.

Testing Ground-Motion Prediction Equations against Moderate Magnitude Earthquake Data Recorded in Korea

ABSTRACT The selection of ground-motion prediction equations (GMPEs) to perform seismic hazard assessments is challenging for stable continental regions that lack a sufficient number of recordings. In this study, we implement various ranking methods to test the efficiencies of a wide range of GMPEs against the recordings from three of the largest magnitude inland earthquakes that occurred in the Korean Peninsula, which belongs to an intraplate region with low seismicity. In this context, we select a total of 14 GMPEs developed for active shallow crustal zones (Next Generation Attenuation-West2 [NGA-West2] project), stable continental regions, and Korea. Three statistical approaches, including the classical residual, log-likelihood (LLH), and Euclidean distance-based ranking (EDR) methods, are used to evaluate the performance of the GMPEs. The residual analyses show that for the very short spectral period (T≤0.1 s), regionally developed GMPEs perform the best, whereas the NGA-West2 GMPEs outperform other equations for short (0.2≤T≤0.5 s) and medium to long periods (T≥0.75 s). The LLH approach is shown to favor a stable continental region GMPE that has the highest standard deviation. The EDR method, which can account for both aleatory uncertainties and model bias, is found to favor the NGA-West2 and Korean GMPEs. NGA-West2 GMPEs show the lowest model bias, whereas the Korean GMPEs exhibit the lowest residual bias. Five GMPEs selected based on the EDR method are recommended for a probabilistic seismic hazard analysis in Korea. For deterministic hazard assessment, using the Korean GMPEs for the very short spectral period and NGA-West2 GMPEs for short and medium to long periods is recommended. Overall, the stable continental region GMPEs are demonstrated to perform poorly when tested against the earthquakes recorded in Korea.

The 1885 M 6.9 Earthquake in the French Guiana–Brazil Border: The Largest Midplate Event in the Nineteenth Century in South America

In 4 August 1885, 06:30 local time, a strong earthquake (reported intensities up to VI–VII modified Mercalli intensity [MMI]) was felt in the French Guiana, causing slight damage. Recently discovered newspaper records show that this event was also felt as far as Georgetown (British Guyana), Belém, and several other localities along the Amazon River toward Manaus (Brazil). The distribution of intensities and the radius of the felt area indicate a magnitude around Mw 6.9, which makes it the largest known earthquake in the stable continental region of South America, since the nineteenth century. The epicenter, determined with four different attenuation relations, lies onshore near the border between the French Guiana and Brazil, although an epicenter offshore in the continental slope cannot be ruled out with 95% confidence. The epicenter (03.4° N, 52.9°W±100 km) likely lies in the Transamazonian (2.2–2.0 Ga) geochronological province in the Guyana shield of the Amazon craton. No nearby failed rift is known onshore near the epicenter, which would place this event in the ∼30% class of nonextended stable continental crust. Other nearby smaller earthquakes (both historical and instrumental) with magnitudes up to mb 5.2, indicate a cluster of seismicity in the region of the 1885 earthquake, possibly delineating an onshore seismic zone separate from the sparse seismicity along the continental shelf. This large midplate earthquake will likely affect future reevaluations of seismic hazard in midplate South America.