Seismological Data Research Articles

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.

Mapping InSAR deformation of low and moderate earthquakes

Abstract Low-to-moderate magnitude earthquakes often induce small ground displacement. For such events, ground deformation fields detected by SAR interferometry can be masked or not clearly discernible from the fringes distribution because of the presence of error sources, such as atmospheric artifacts and topographic residuals. We show two examples of low-moderate magnitude earthquakes, for which we adopted a new automatic tool for calculating the fringe pattern stemming from seismological data. The tool estimates the extent and the geographic position of the deformation by running a forward model of the seismic source, thus identifying the best SAR frame to be collected and the expected surface effects to better figure-out the outcomes of InSAR processing. We present the Mw 5.7 occurred in Greece and the Mw 5.4 occurred in Zagreb on 21 and 22 March 2020, respectively. For the Greek earthquake, the tool predicted a deformation close to the InSAR product, and gave evidences of atmospheric disturbances, thus providing information for inverse source modelling. The tool in the Zagreb event was used to infer the extent and location of the ground motion, that were used to identify the best SAR pair to be processed, being the SAR frames edge very close to the epicenter.

The Koryak Earthquake: an Example of a Successfully Predicted Seismic Event

In the border zone of Kamchatka krai and the Chukotka Autonomous Okrug, an earthquake occurred on January 9, 2020, with MS = 6.6 (MW = 6.4) and an intensity at the epicenter of 9–9.5. The seismotectonic position of the source and nature of its seismogenic movement have been preliminarily determined. The seismic history of the region is considered, and the features of the aftershock process are characterized. Seismotectonic and seismological data on this event were acquired. A conclusion is drawn about the propagation of seismic activity to the northeast along the Khatyr–Vyvenskaya active fault zone in the period of 1991–2020. The earthquake was well predicted in the medium term.

О ВОЗМОЖНОСТИ СОЗДАНИЯ СИНТЕТИЧЕСКОЙ S-СКОРОСТНОЙ МОДЕЛИ ПУТЕМ ПЕРЕСЧЕТА Р-СКОРОСТНОЙ МОДЕЛИ

The problem of limited and incomplete seismological data leads to significant distortions in design models based on primary seismological models. One of the reasons for the incompleteness of the primary seismological data is the significant predominance of these definitions of P phase arrivals of body waves in comparison with the registration of S-wave arrivals. In this regard, the question arises about the statistical proportionality (significance) and correspondence of the velocity models obtained from the arrivals of P- and S-waves. As part of the solution of this problem, the authors studied the possibility of recalculating the P-velocity model into a synthetic velocity model of S-waves to assess its similarity to the experimental model. The article shows that the deviations of the synthetic model of S-wave velocities from the measured curve are comparable with the accuracy of the methods by which these velocity models were obtained, therefore, the obtained synthetic model of S-waves can be used to construct structural models of the mantle. We proposed our own approach to solving the problem of obtaining a synthetic S-velocity model because of recalculation of the P-velocity model and showed that when recalculated, using the arithmetic mean of five reference seismological models of the Vp/Vs ratio, the synthetic S-velocity model has acceptable deviations comparable to the resolution ability of the method of obtaining them. As the results of determining the velocity errors when using the kinematic method of the Taylor approximation show, with errors in determining the maximum depth of the apparent velocity of the refracted ray ε0 (q) = 6; 7; 8 km, the calculated velocity errors are of the same order as the deviations for one-dimensional models, and significantly less than the error of ± 0.1 km / s, which is achieved by other seismic methods. The synthetic S-velocity model obtained because of the recalculation of the P-velocity model has the property of proportionality with the original model, which gives advantages in accuracy in the further joint use of two collinear models (P, S) when converting to physico-lithological models of the mantle.

The sun as a significant agent provoking earthquakes

In this paper we provide significant evidence that the sun is a principal agent provoking seismic activity. In particular the aim of the studies presented is to examine the possible relation of the coronal hole (CH) driven high speed solar wind streams (HSSs) with seismicity We performed several statistical studies of solar space and seismological data between 1980 and 2017 as well as a study for a long time interval from the year 1900 until the year 2017. (A1) Concerning the period 1980–2017 among other results we found that the earthquakes (EQs) with M ≥ 83 between 2010–2017 (including the catastrophic earthquakes of Japan 2011 (M91) Sumatra 2012 (M86) and Chile 2015 (M83)) occurred during times of large coronal holes as seen by the Solar Dynamics Observatory (SDO) satellite and were related with CH-driven HSSs observed by the ACE spacecraft several weeks or a few months before the EQ occurrences. (A2) Further research on the hypothesis of the possible HSS-EQ relationship revealed a surprising novel finding: a power spectrum analysis suggests that during the decay phase of the SCC22 and SC23 and at the maximum of SC23 the values of the global seismic (M ≥ 6) energy output shows a periodic variation of ~27 days, which is the mean rotational period of the Sun. (A3) Moderate (not strong) storms in general precede the great EQs. (B) The study of the data for the time interval 1900–2017 revealed that: (1) all of the giant (M ≥ 85) EQs occurred during the decay minimum and the rising phase of the solar cycle or in the maximum phase but at times of a strong reduction of the monthly averaged sunspot number: Chile M95 1960 EQ – Alaska M92 1964 EQ – Sumatra M91 2004 EQ (decay phase) Japan M91 2011 EQ (rising phase of the "strange" SC24) (2) the global energy release of all EQs with magnitudes M ≥ 55 show the highest values during the decay phase of the solar cycle and in particular three years after the solar maximum and (3) a very significant negative correlation (rS = −042p < 10−4) was found between the SSN and the number of earthquakes with M ≥ 7 during the period 1930–2010 during times of moderate and high amplitude solar cycles. (C) Another result of our study is that the comparison of the yearly numbers of great (M ≥ 7) EQs with the SSN fails to provide correct statistical results whereas this is possible for the global seismic energy or the giant EQs. (D) Finally we infer that the case and statistical studies presented in this paper strongly suggest a close relation between CH-associated HSSs and seismic activity. We present some observational evidence that most probably Alfvèn waves mediate the interaction of CH-driven HSSs with seismicity.

Seismotectonics of Armenia: main problems

The article describes the main problems in the existing researches on the seismotectonics of the territory of Armenia. A review was conducted on certain issues related to the accuracy of the initial seismological data, the problem of identifying possible focal zones of strong earthquakes, and the probabilistic assessment of the seismic hazard of the territory of Armenia. As the results of numerous studies show, the initial seismological data from catalogs and bulletins are characterized by their heterogeneity. Different observation systems and different methods of seismological data processing have led to the fact that when visualizing the spatial distribution of earthquake epicenters on the territory of Armenia, during the instrumental period of observations, an artificially lattice network of epicenters is obtained, which does not agree in any way with the fault tectonics of the territory. During the stud of distribution of earthquake hypocenters by depth, the discrete nature of their distribution was revealed. There is also a number of disadvantages in the area of allocation of focal zones of strong earthquakes. The main part of the conducted research was mainly based on data on previously recorded strong seismic events, as well as on data on historical earthquakes, as a result, for the most part of the studied territory, in fact, no research was conducted. In addition, there are relevant questions about the tectonic basis used in these studies. The last group of problems concerns the probabilistic assessment of the seismic hazard of the territory of Armenia. It is shown that in the existing works, there are quite a lot of unsubstantiated assumptions.Based on the analysis, it is concluded that in the field of seismotectonic studies of the territory of Armenia, there are a number of urgent tasks that require priority solutions.

Magma plumbing system and origin of the intraplate volcanoes in Mainland China: an overview of constraints from geophysical imaging

Abstract Volcanic eruptions, despite causing large-scale disasters, also provide important natural resources and are an effective way to understand the Earth's internal structure and its evolution. Herein, a comprehensive review is presented on recent progress in geophysical imaging of the structure and origin of intraplate volcanoes in Mainland China. We primarily focus on the Changbaishan, Wudalianchi, Tengchong, Hainan and Ashikule volcanoes as they are currently active and hence pose potential hazards during future eruptions, particularly the Changbaishan volcano. The Changbaishan and Wudalianchi volcanoes are widely believed to be caused by the dehydration of the stagnant Pacific slab in the mantle transition zone along with wet upwelling in the big mantle wedge. There are a number of different views regarding the formation mechanism of the Tengchong volcano. Some studies suggest that a big mantle wedge structure is also present under eastern Tibet, and the Tengchong volcano has a deep origin, similar to volcanism in NE China. Others suggest that the Tengchong volcano is caused by a local and shallow process. Most tomographic studies suggest that the Hainan volcano is a hotspot, and its track has been located in SE China by combining seismological, geochemical and numerical modelling data. A gap exists between the subducted Indian Plate and the Tarim lithosphere beneath the Ashikule volcano, which provides a channel for asthenospheric upwelling to give rise to intraplate volcanism in the Ashikule basin. The interactions of lithospheres may produce shear heating of the subcontinental lithospheric mantle, which can generate localized melting. This process has been proposed as an explanation for the intraplate volcanism in Ashikule.

Среднесрочные прогнозы землетрясений методом LURR на Сахалине: обобщение ретроспективных исследований за 1997–2019 гг. и новые подходы

The work presents the results of a retrospective analysis of the seismicity of Sakhalin using the LURR method of mid-term earthquake prediction for 1997–2019. All previously performed computations are reduced to a single database of seismological data (catalogue) of the Sakhalin Branch of the Federal Research Center “United Geophysical Survey of RAS”. Similar to previous studies, the Sakhalin territory was scanned by applying computational areas in the form of circles with a radius of one degree; however, the resolution was increased. The entire territory was covered by such zones with a step of 0.5 degrees in latitude and longitude, with the grid being detailed down to 0.1 degrees in three most dangerous seismogenerating zones. As a result, the number of computational samples was increased, which allowed the omission of anomalies in the LURR parameter during computations to be avoided. Due to a reasonable binding of the lower bound of the magnitude for predicted events to the upper bound of the magnitude range of the computational sample (M = 5), the number of objects for the retrospective forecast was increased by 3 times for the study period. 323 computational samples (119 of which are basic and 204 ones are detailed) were processed on the territory of the island. 15 alarm periods were obtained, which geographically represent all zones of moderate and strong earthquake generation on Sakhalin Island. As a result, 17 out of 19 earthquakes with M ≥ 5 occurred in the areas with anomalies during the alarm periods not exceeding three years. Out of 15 periods, 4 turned to be false. Thus, 75 % of the alarms predicted 89 % of the earthquakes.

Transfer zones in Mediterranean back-arc regions and tear faults

Slab tearing induces localized deformations in the overriding plates of subduction zones and transfer zones accommodating differential retreat in back-arc regions. Because the space available for retreating slabs is limited in the Mediterranean realm, slab tearing during retreat has been a major ingredient of the evolution of this region since the end of the Eocene. The association of detailed seismic tomographic models and extensive field observations makes the Mediterranean an ideal natural laboratory to study these transfer zones. We review in this paper the various structures in back-arc regions differential retreat from the Alboran Sea to the Aegean-Anatolian region and discuss them with the help of 3D numerical models to better understand the partitioning of deformation between high-angle and low-angle faults, as well as the 3-D kinematics of deformation in the middle and lower crusts. Simple, archetypal, crustal-scale strike-slip faults are in fact rare in these contexts above slab tears. Transfer zones are in general instead wide deformation zones, from several tens to several hundred kilometers. A partitioning of deformation is observed between the upper and the lower crust with low-angle extensional shear zones at depth and complex association of transtensional basins at the surface. In the Western Mediterranean, between the Gulf of Lion and the Valencia basin, transtensional strike-slip faults are associated with syn-rift basins and lower crustal domes elongated in the direction of retreat (a-type domes), associated with massive magmatic intrusions in the lower crust and volcanism at the surface. On the northern side of the Alboran Sea, wide E-W trending strike-slip zones in the brittle field show partitioned thrusting and strike-slip faulting in the external zones of the Betics, and E-W trending metamorphic core complexes in the internal zones, parallel to the main retreat direction with a transition in time from ductile to brittle deformation. On the opposite, the southern margin of the Alboran Sea shows short en-échelon strike-slip faults. Deep structures are not known there. In the Aegean-Anatolian region, two main tear faults with different degrees of maturity are observed. Western Anatolia (Menderes Massif) and the Eastern Aegean Sea evolved above a major left-lateral tear in the Hellenic slab. In the crust, the differential retreat was accommodated mostly by low-angle shear zones with a constant direction of stretching and the formation of a-type high-temperature domes exhumed from the middle and lower crust. These low-angle shear zones evolve through time from ductile to brittle. On the opposite side of the Aegean region, the Corinth and Volos Rift as well as the Kephalonia fault offshore, accommodate the formation of a dextral tear fault. Here, only the brittle crust can be observed, but seismological data suggest low-angle shear zones at depth below the rifts. We discuss the rare occurrence of pure strike-slip faults in these contexts and propose that the high heat flow above the retreating slabs and more especially above slab tears favors a ductile behavior with distributed deformation of the crust and the formation of low-angle shear zones and high-temperature domes. While retreat proceeds, aided by tears, true strike-slip fault system may localize and propagate toward the retreating trench, ultimately leading to the formation of new plate boundary, as shown by the example of the North Anatolian Fault.

Towards Closing the Polar Gap: New Marine Heat Flow Observations in Antarctica and the Arctic Ocean

The thermal state of the lithosphere and related geothermal heat flow (GHF) is a crucial parameter to understand a variety of processes related to cryospheric, geospheric, and/or biospheric interactions. Indirect estimates of GHF in polar regions from magnetic, seismological, or petrological data often show large discrepancies when compared to thermal in situ observations. Here, the lack of in situ data represents a fundamental limitation for both investigating thermal processes of the lithosphere and validating indirect heat flow estimates. During RV Polarstern expeditions PS86 and PS118, we obtained in situ thermal measurements and present the derived GHF in key regions, such as the Antarctic Peninsula and the Gakkel Ridge in the Arctic. By comparison with indirect models, our results indicate (1) elevated geothermal heat flow (75 ± 5 mW m−2 to 139 ± 26 mW m−2) to the west of the Antarctic Peninsula, which should be considered for future investigations of ice-sheet dynamics and the visco-elastic behavior of the crust. (2) The thermal signature of the Powell Basin characteristic for oceanic crust of an age between 32 and 18 Ma. Further, we propose (3) that at different heat sources at the slow-spreading Gakkel Ridge in the Aurora Vent Field region might explain the geothermal heat flow distribution. We conclude that in situ observations are urgently required to ground-truth and fine-tune existing models and that a multidisciplinary approach is of high importance for the scientific community’s understanding of this parameter.

Landslide hazard assessment of the Neom promising city, northwestern Saudi Arabia: An integrated approach

Landslides hazard as one of the major environmental threats of the Neom promising city in northwestern Saudi Arabia has been assessed through data integration of several sources including topographic, geologic, and seismologic data sets. Moreover, Landsat imagery and historical landslide data are introduced in assessing landslide susceptibility of the study area. All these data oriented to generate eight thematic layers representing eight landslide conditioning factors. These factors arranged according to their relative importance for landslide susceptibility mapping as follows: slope angle, elevation, curvature, aspect, earthquake density, fault density, drainage density, and lineaments density. These factors were assessed using GIS through a numerical rating scheme for thematic layers. The ranks and weights were given in association with intensive field studies and authors’ experiences. The estimated values of Landslide Potential Index (LPI) varies from 32 up to 107. Then, the resultant landslide susceptibility map classifies the landslide hazards into three relative classes of susceptibility: high, moderate, and low. Furthermore, this map was tested through several field checks and the extracted historical landslides within Neom area as well. This landslide susceptibility map will boost decision-making authority while executing the developing projects throughout the Neom promising area either avoiding strategic facilities or applying corrective geotechnical procedures to minimize the landslide hazardous impact for Neom city.

Шкала ML для западной части Евразийской Арктики

The local magnitude scale ML was refined for the western part of the Eurasian Arctic on the basis of data from seismic stations operating on the archipelagos of Svalbard, Franz Josef Land, and Severnaya Zemlya: –logA0(R)=1.5*log(R/100)+1.0*10(–4)*(R–100)+3.0. Refinement was carried out on the basis of a sample of 167 earthquakes and 612 amplitude values at 5 seismic stations. The sample covered earthquakes that occurred in the main seismically active zones of the Eurasian Arctic for the period from January 2016 to April 2019. The refined scale can be applied in wide ranges of epicentral distances and magnitudes. The ML scale with the corresponding station corrections will be introduced into the practice of daily processing of seismological data from the western part of the Eurasian Arctic.

TECTONOPHYSICAL STUDY OF THE VERKHOVOI FAULT ACTIVITY ON THE NORTHERN SLOPE OF THE KYRGIZ RIDGE

An active fault was identified on the northern slope of the Kyrgyz ridge in the area near Research Scientific RAS in Bishkek. It belongs to a large system of crustal faults of the ridge. In our study, tectonophysical methods were used to analyse the regularities of the tectonic stress field reconstructed from seismological data on earthquake focal mechanisms. The stress distribution pattern near the investigated fault suggests its activity either in the recent past or at the present time. This conclusion is supported by the fact that at the eastern and western terminations of the fault, crustal stretching zones are located in a crosswise pattern. The Coulomb stresses on the fault surface were analysed, and the analysis results give grounds to state that its long section crossing the Alamedin river valley should be viewed as potentially hazardous. In the field, we observed abundant broken rock fragments and rock falls in the zone where the fault crosses the Alamedin and Aksu river valleys. It is known that rock falls have occurred more often in the last 3–5 years. The study results show slow movements along the fault. These strike-slip displacements have been going on for at least 10–15 years. According to the modern concepts of the preparation stage of an earthquake source, slow displacements along a fault gradually accelerate several years before an earthquake. Therefore, the studied fault (we named it Verkhovoi) should be considered a potentially hazardous zone wherein earthquakes can occur in future. A magnitude from 6.5 to 7.5 may be expected, depending on whether only the eastern part or the entire fault (i.e. 20 or 50 km, respectively) will be involved in a future seismic event. Further studies of the Verkhovoi fault are needed to clarify a trend in the development of slow sliding along the fault, which will allow understanding whether this process precedes dynamic rock failure (i.e. an earthquake) or tends to gradually decrease.

STRONG SEISMIC EVENTS on the SOUTH-WESTERN FLANK of the BAIKAL RIFT in 2014: November 1, 2014, Кp=13.6, Mw=4.6, I0=7–8 URIK EARTHQUAKE and December 5, 2014, Кp=13.9, Mw=4.9, I0=7–8 HOVSGOL EARTHQUAKE

We consider two earthquakes occurred at the south-western flank of the Baikal rift zone (BRZ): Urik, November 1, 2014 (Mwreg=4.6) and Hovsgol, December 5, 2014 (Mwreg=4.9). First of them is localized within the area of the Main Sayan fault, the second one is located at the north of the Hovsgol Lake. Seismic moment tensors (focal mechanisms, scalar seismic moments, moment magnitudes and hypocentral depths) of the study seismic events were calculated based on surface wave amplitude spectra. Earthquake hypocenters were found to be situated in the middle crust (h=14–21 km). Both events occurred under the strike-slip stress-strain field. The strike-slip was combined with a normal fault component in the source of the Urik earthquake and with a thrust fault component in the source of the Hovsgol earthquake. In both cases, shaking intensity in the nearest settlements (=42–124 km) was less than 4–5. Analysis of historical seismicity, seismological data on the Urik and Hovsgol earthquakes and the tectonic position of their sources demonstrates that the considered events are typical for the south-western flank of the BRZ and confirms the existence of the transition zone from rift structures at the central parts of the BRZ to regional compression structures in Northern Mongolia.

Gravedad en la estructura andina segmento 18°-42° S

Nuevos datos del campo potencial de la gravedad en los Andes Centrales entre las latitudes 18°-42° S, asociados a datos geofísicos y geológicos existentes, permiten en parte clarificar la estructura cortical del cordón montañoso y su engrosamiento. Evidencias estructurales de la corteza son reconocidas mediante datos sísmicos, gravimétricos y proyecciones geológicas. Dicha estructura y su composición cambian significativamente de este a oeste, en la parte este del retroarco, la discontinuidad de Moho se reconoce claramente, sin embargo, las mediciones de la sísmica activa en el altiplano y cordillera occidental se observan solamente llegadas débiles del Moho, esto no sucede en el análisis de los datos de la sismología de banda ancha, los cuales indican una discontinuidad aproximada a los 70 km. Por otra parte, al oeste de esta estructura a lo largo de la costa, el Moho oceánico se desarrolla a una profundidad aproximada de 40 km, la cual disminuye de norte a sur de acuerdo a la altura y ancho de la estructura andina.Las propiedades petrológicas de las rocas que conforman la corteza profunda, pueden cambiar por diversos procesos que mediante el análisis, presentan variaciones de la clásica discontinuidad del Moho que no necesariamente corresponden a un límite petrológico corteza-manto. El acortamiento tectónico en el retroarco es el proceso dominante que contribuye a la formación de la raíz de los Andes Centrales. Adicionalmente, en el antearco y arco, la hidratación de la cuña del manto produce en parte un engrosamiento cortical lo cual se traduce en un cambio en el gradiente regional del campo potencial de la gravedad. &#x0D; Este gradiente se observa en la dirección este-oeste y de norte a sur, cuyos valores en la dirección este-oeste varían entre los 2.9x10-⁵ ms-2 por kilómetro en el sector de Arica-Iquique y 1.01x10-⁵ ms-2 por kilómetro en el sector de Valdivia- Puerto Montt. Este gradiente es gradual en el sentido norte-sur. Al mismo tiempo la anomalía de Bouguer varía en el mismo sentido, siendo las mayores variaciones de 380x10-⁵ ms-2 en el sector de Antofagasta y de -50x10-⁵ ms-2 en el sector de Puerto Montt.

Seismicity in the equatorial margin of Brazil reactivates the Precambrian basement fabric

The continental margin of Brazil in intraplate South America exhibits a seismicity rate 70% higher than the average stable continental regions. This margin encompasses Northeastern Brazil, where most of the seismicity and active faults concentrate along the coast. The current study presents an integration of high-resolution aeromagnetic, seismological, and geological data to investigate the relationship between the seismicity and basement fabric in two areas in the northern part of the Precambrian Borborema Province. Our investigation indicates a relation between the metamorphic basement foliation and two seismogenic faults. In the first area adjacent to the Transbrasiliano Lineament, the largest continental suture zone in the South American platform, the magnetic anomaly patterns indicate that the regional basement grain and structural elements are E-W-oriented, which coincides with the strike of the E-W-striking, strike-slip Riacho Fundo Seismogenic Fault. In the second area, the magnetic anomaly patterns also coincide with the NE-SW-striking, strike-slip Samambaia Fault, and a swarm of chalcedony-quartz veins. Euler deconvolution used to estimate the source depth of the magnetic anomalies indicates that the faults in both cases could be longer and deeper than segments illuminated by the present-day seismicity. Although these findings are difficult to use as predictive tools, they indicate that the basement fabric could be reactivated by seismicity in continental interiors, as previously discussed in several cases in Brazil.

Joint Geophysical‐Petrological Modeling on the Ivrea Geophysical Body Beneath Valsesia, Italy: Constraints on the Continental Lower Crust

AbstractOne of the few near‐complete continental crustal sections exposed on Earth's surface is the Ivrea‐Verbano Zone (Western Alps, Italy), which is considered as a petro‐geophysical reference of the continental lithosphere. Exposed peridotite slivers embedded in lower crustal rocks at the surface and large density, seismic velocity anomalies of the Ivrea Geophysical Body in the subsurface suggest that mantle‐like rocks are located as shallow as a few kilometers depth, but the actual composition of the rocks producing these anomalies is unknown. Here we investigate how the published seismological and new gravimetric data in the location of Valsesia could be reconciled with petrologic data and models of the Ivrea‐Verbano Zone. We use the Perple_X software to calculate densities and compressional wave velocities for a range of possible deep crustal rock types. We argue that amphibole gabbros (&lt;18 km depth) and pyroxene hornblendites (&gt;18 km depth) provide the best fit to the joint geophysical and petrologic constraints, whereas residual ultramafic rocks and anhydrous gabbros are inconsistent with the existing data. This indicates that the Ivrea Geophysical Body beneath the Valsesia area in the Ivrea‐Verbano Zone preserves the structure of an igneous complex formed during magmatic underplating from the crystallization of hydrous mafic magmas. This would imply melting of a damp mantle source that produced a continental crust of an original thickness of up to ∼48 km in the Permian, of which ∼30 km are exposed at Earth's surface today.

Historical and Modern Seismicity of the Semipalatinsk Test Site

According to the current maps of general seismic zoning of the Republic of Kazakhstan (2006), the territory of the Semipalatinsk Test Site (STS) is considered an aseismic region. However, recent investigations and an analysis of archived data have shown that the Test Site territory and its vicinity have experienced and still experience tectonic and induced earthquakes. The maximum magnitude of recorded earthquakes is 5–5.9. An analysis of global seismological bulletins and data on historical seismicity from literature sources is performed to solve the problem of seismicity in the STS region. Historic analog seismograms of earthquakes have been collected since 1925; the earthquake parameters are estimated more precisely. Modern instrumental seismic data of the Kazakhstan monitoring network since 1994 are processed, as is the data of temporary networks of seismic stations operating on the territory of the test site in 2005–2010 in the region of the Sary-Uzen, Balapan, and Degelen test sites. These works resulted in the creation of a common earthquake catalogue for the STS territory and its vicinity from 1783 up to the present. Macroseismic data of the felt earthquakes have been collected; strong motion records have been analyzed. Deep faults dividing Earth’s crust blocks have been marked out. Tectonic elements are located by decoding the Landsat space images and using materials of geologic and topographic surveys. The calculations show that the STS territory can experience events with intensity 6 based on the MSK-64 scale.

Active Tectonics in the Kvarner Region (External Dinarides, Croatia)—An Alternative Approach Based on Focused Geological Mapping, 3D Seismological, and Shallow Seismic Imaging Data

Active tectonics in long-lived orogenic belts usually manifests on the preexisting inherited structures. In the Kvarner region of the External Dinarides, an area with low-to-moderate seismicity related to the Adriatic microplate (Adria) northward movement, we deal with faults in predominantly carbonate rocks within tectonically complex NW-SE striking fold-and-thrust belt, which makes the identification and parametrization of the active structures challenging. Moreover, anthropogenic modifications greatly complicate access to the surface geological and geomorphological data. This paper demonstrates results of focused multidisciplinary research, from surface geological mapping and offshore shallow seismic surveys to earthquake focal mechanisms, as an active fault identification and parametrization kit, with a final goal to produce an across-methodological integrated model of the identified features in the future. Reverse, normal, and strike-slip orogen-parallel (longitudinal) to transverse faults were identified during geological mapping, but there is no clear evidence of their mutual relations and possible recent activity. The focal mechanisms calculated from the instrumental record include weak-to-moderate earthquakes and show solutions for all faulting types in the upper crust, compatible with the NE-SW oriented principal stress direction, with the stronger events favoring reverse and strike-slip faulting. The 3D spatial and temporal distribution of recent earthquake hypocenters indicate their clustering along predominantly subvertical transversal and steeply NE-dipping longitudinal planes. High-resolution shallow seismic geoacoustical survey (subbottom profiler) of the Quaternary sediments in the Rijeka Bay revealed local tectonic deformations of the stratified Late Pleistocene deposits that, along with overlaying mass-transport deposits, could imply prehistorical strong earthquake effects. Neotectonic faults onshore are tentatively recognized as highly fractured zones characterized by enhanced weathering, but there is no evidence for its recent activity. Thus, it seems that the active faults are blind and situated below the thin-skinned and highly deformed early-orogenic tectonic cover of the Adria. A strain accumulating deeper in the crust is probably irregularly redistributed near the surface along the preexisting fault network formed during the earlier phases of the Dinaric orogenesis. The results indicate a need for further multidisciplinary research that will contribute to a better seismic hazard assessment in the densely populated region that is also covered by strategic infrastructure.

ATC Mw7.1 Puebla–Morelos earthquake reconnaissance observations: Seismological, geotechnical, ground motions, site effects, and GIS mapping

This article presents the Applied Technology Council (ATC) team’s observations following the 2017 Mw7.1 Puebla–Morelos, Mexico earthquake. The team was deployed in Mexico City to collect seismological, geotechnical, structural, and overall performance information. The focus was on non-ductile concrete structures, to support implementation of recently published FEMA P-2018 procedures and to identify study buildings for incorporation into NIST-funded ATC-134 ongoing project. This article presents seismological data with 71 strong-motion records processed and ready for use in engineering analysis, geotechnical observations, and characterization of sites visited. Analyses of the response of representative soil profiles are presented in the form of acceleration response spectra and seismic amplification at the ground surface. A comparison of the same analysis using records from the 1985 Michoacán Ms8.1 (approximately Mw8.0) earthquake is also discussed. The ATC team composed GIS maps with structural and geotechnical characteristics of the inspected sites, including color-coded damage of inspected buildings and estimated soil fundamental period to correlate observed behavior with potential soil–structure interaction resonance effects. Recommendations on further detailed studies based on this comprehensive set of case histories are proposed.