Regional Seismic Activity Research Articles

Regional Spectral Characteristics Derived Using the Generalized Inversion Technique and Applications to Stochastic Simulation of the 2021 Mw 6.1 Yangbi Earthquake

ABSTRACTIn this study, 334 accelerograms of 42 small-to-moderate earthquakes recorded at 36 strong-motion stations were used to investigate the ground-motion characteristics of the southwestern margin of the Sichuan–Yunnan rhombic block (SWM-SYRB), one of the most active seismic regions in China. The high-frequency attenuation parameter was estimated using the spectral decay method, and the site component (κ0) was fitted. The κ0 estimates decrease with the increasing time-averaged S-wave velocity of the uppermost 30 m (VS30). Using the generalized inversion technique, the source spectra, quality factor (Q), and site amplification were derived from the Fourier amplitude spectra (FAS). The obtained average stress drop for earthquakes occurred in SWM-SYRB was the second largest among various boundary areas of SYRB. The inverted Q model was Q(f)=115.1f0.616. The low Q structure that extends southwestward from the Songpan–Garze block to SWM-SYRB could be responsible for the strong regional attenuation of ground motion with distance. At frequencies above 10 Hz, the average site amplifications were influenced by the high-frequency attenuation effect. The site amplification of site class D reached a factor of 6 at 0.7 Hz. Moreover, it was observed that site amplification factors can be even higher when peak ground acceleration is larger than 0.8 m/s2. Finally, the obtained parameters were used in the stochastic finite-fault simulation method to reproduce the FAS, 5%-damped pseudospectral acceleration, and time series of the 2021 Mw 6.1 Yangbi earthquake. The simulated spectra properly matched the observations in a broad frequency band of 0.1–25 Hz. Furthermore, the simulated time series could generally represent the amplitude of the S-wave portion of observed ones.

Tsunami free oscillations excited by potential local earthquake sources in Bohai Sea, China

The Bohai Sea is the most seismic active region offshore mainland China. Its coastal region is highly populated. The present work proposed a comprehensive scheme for identifying and examining free oscillations with limited number of tide gauges in an irregular semi-enclosed basin. We combined modal analysis, spectral analysis, and multiple-scenario simulation to investigate tsunami free oscillations in the Bohai Sea Basin and three local bays including Liaodong Bay, Bohai Bay, and Laizhou Bay. The Bohai Sea had broadband periods of free oscillations ranging from 228 to 30 min. There existed both along-shore and cross-shore oscillations in most eigenmodes. The ultra-long dominant periods above 50 min were attributed to the shallow water depth (<20 m). Some free oscillation modes appeared in both the Bohai Sea and local bay domains. In addition, the spatial patterns of dominant eigenmodes were characterized and validated by performing spectral analysis for an Mw 7.8 tsunami scenario. The energy was mainly concentrated in the first three modes (171, 146, and 93 min). This is the first study to characterize the areas prone to tsunami hazards in the Bohai Sea, and the methodology can be further extended to other coastal regions.

Seismic Sequence Vulnerability of Low-Rise Special Moment-Resisting Frame Buildings with Brick Infills

When buildings are exposed to earthquake sequence, damage aggravation is expected to occur. Although several studies report seismic vulnerability of reinforced concrete (RC) buildings under the mainshock–aftershock sequence, indicating damage aggravation due to aftershock, none, to the best of our knowledge, quantifies seismic vulnerability of buildings under foreshock–mainshock–aftershock sequences. Since foreshock–mainshock–aftershock sequences are also expected in many active seismic regions, we aim to quantify the level of vulnerability under seismic sequences considering the seismically highly active Himalayan region as the case study location. Fragility functions are derived considering foreshock, foreshock–mainshock sequence, and foreshock–mainshock–aftershock sequence for a low-rise special moment-resisting frame (SMRF) building that represents a typical low-rise owner-built construction system in Nepal, one of the most active seismic regions in the world. The results highlight that the foreshock significantly increases seismic vulnerability of the structures with respect to the often-considered case of a mainshock–aftershock sequence.

Hydrochemical Characteristics of Hot Springs in the Intersection of the Red River Fault Zone and the Xiaojiang Fault Zone, Southwest Tibet Plateau

The coupling relationship between regional seismic activity and the hydrogeochemical field provides an important theoretical basis for regional earthquake precursor exploration. The intersection area of the Red River fault zone (RRF) and the Xiaojiang fault zone (XJF) in southeast Yunnan province has become the focus area of earthquake monitoring and prediction because of its special tectonic position in China. There were 20 hot springs that were sampled and analyzed in the laboratory for major elements, including trace elements, silica, stable isotopes (δ18O and δD), and strontium isotopes, from the years 2015 to 2019. (1) The meteoric water is the main source of recharge for thermal springs in the study area, and recharged elevations ranged from 1.1 to 2 km; (2) the geothermometer method was used to estimate the region of thermal storage temperature, and its temperature ranged between 64.3 to 162.7 °C, whereas the circulation depth ranged from 1.1 to 7.2 km. Hydrochemical types were mainly controlled by aquifer lithology, in which sodium bicarbonate and sulphuric acid water gathered mainly in the RRF, while calcium bicarbonate water gathered mainly in the XJF. According to the silicon–enthalpy equation method, the temperature range and cold water mixing ratio were 97–268 °C and 61–97%, respectively; (3) the circulation depth of the RRF was deeper than that of the XJF, and it was mainly concentrated in the second segment and the fourth segment on the RRF. Most of the hot spring water was immature with a weak water–rock reaction; (4) the hot water intersections of RRF and XJF were obviously controlled by the fault and the cutting depth of granite; (5) the relationship discussed between geothermal anomaly and earthquake activity had a good correspondence with regional seismicity. The intensity of the reaction between underground hot water and the surrounding rock may lead to the change of pore pressure, and the weakening effect of groundwater on fracture may change accordingly, followed by the change in the adjustment of tectonic stress. Eventually, the difference in seismic activity was shown, implying that deep fluid has an important control action on the regional seismicity.

Repair and Retrofit of RC Bridge Piers with Steel-Reinforced Grout Jackets: An Experimental Investigation

Many concrete bridges around the world have passed their service life and are at high risk of collapse, especially in active seismic regions. This is particularly true in many developing and underdeveloped countries, where deficient construction and underpreparedness would result in a major catastrophe should a major seismic event occur. Thus, there lies a need to introduce an efficient, cost-effective, and readily available method of bridge rehabilitation. In this study, steel-reinforced grout (SRG) jacketing is used as a seismic strengthening technique for reinforced concrete bridge piers. Two bridge piers, one seismically damaged and one seismically deficient, were jacketed with SRG and then tested to observe their seismic response. The findings from the experimental program revealed that SRG jacketing can rehabilitate/repair a damaged bridge pier, and restore its flexural capacity, energy dissipation capacity, and ductility. While strengthening a seismically deficient pier, results showed that the flexural capacity and energy dissipation capacity could be improved, and ductility could be enhanced.

Modelling of seismic assessment for large geological systems.

A new approach to seismic analysis has been introduced and demonstrated for a sequence of recent seismic events recorded in the Blackpool region of Lancashire, UK. The seismic activity, induced by an industrial hydraulic fracturing at a depth exceeding 2 km, had the extent of registered surface elastic vibrations reaching a distance exceeding 15 km. The analysis is based on the study of elastic fields, three-dimensional extrapolations of the landscape and the novel reconstruction of a three-dimensional digital model of seismic map boundaries and vertical profiles. The verification of the proposed approach is carried out via the comparison with published data of the Blackpool seismic events, combined with the new spectral analysis linked to the identified regions of seismic activity. The latter was accompanied by a finite-element simulation of vibrations for an elastic layer of variable thickness, approximating the test region. The analysis and numerical modelling have demonstrated consistency with the dynamic nature of structural stratification of the geological systems, and in addition, the predictive nature of the modelling work was demonstrated by the comparison of the model eigenmodes with the published parameters of registered earthquakes in the Blackpool area. This article is part of the theme issue 'Wave generation and transmission in multi-scale complex media and structured metamaterials (part 1)'.

Sensitivity analysis of input ground motion on surface motion parameters in high seismic regions: a case of Bhutan Himalaya

Abstract. Historical earthquakes demonstrate that strong motion characteristics and local soil condition, when coupled, significantly influence seismic site response. Interestingly, most of the Himalayan earthquakes depicted anomalous behavior per the site conditions historically. Being one of the most active seismic regions on earth, the eastern fringe of the Himalaya has observed many devastating earthquakes together with non-uniform damage scenarios. To quantify such anomalies, we evaluate surface motion parameters for a soft soil deposit located in the city of Phuentsholing in western Bhutan. Using one-dimensional site response analysis, the sensitivity of ground motion variation is estimated. This study accounts for the earthquakes of moment magnitudes 6.6 to 7.5 with a wide variation in peak ground acceleration (PGA). To dissect the characteristics of six inputted ground motions on eight local ground conditions, a sensitivity analysis is performed statistically. The statistical correlation of the response datasets and the linear regression model of the bedrock outcrop and the surface motion spectral acceleration along the stratified depth are examined to quantify the variation in surface motion parameters. The results highlight that the strong motions with PGA greater than 0.34 g demonstrate greater sensitivity, leading to some anomalies in response parameters, especially amplification. Similar results were obtained for the low PGA range (&lt;0.1 g).

Life Cycle Thinking–Based Decision Making for Bridges under Seismic Conditions. I: Methodology and Framework

Bridges are an integral part of a country’s transportation infrastructure system. Earthquake-induced damages to bridges could result in fatalities and injuries, disturbances to the transportation infrastructure system, and incur significant repair costs. Furthermore, bridges must stay functional following an earthquake to maintain the continuity of transportation. Thus, innovative high-performance materials in the critical regions of structures and/or structural systems must be used located in regions of high seismic activity to mitigate the impacts of earthquakes. Despite having a higher seismic resilience, these structures, in general, tend to require a greater capital investment as a result of utilizing expensive material and special workmanship. However, when accounting for the postearthquake repair and maintenance costs, these seismically enhanced bridges can deliver significant cost advantages over conventional structures in the long run. Even so, the outcomes depend on the frequency and severity of the seismic damage, the lifespan of the bridge, the cost of materials, and the embodied emissions of the structures. To ensure better overall results, the cost–benefit analysis should be done with a life-cycle thinking perspective when comparing and making a decision related to multiple bridge systems. In this regard, a decision framework is developed to evaluate the overall life cycle performance of such a novel bridge, using fuzzy logic to integrate uncertainty. The proposed framework will assist engineers and the construction industry as a whole in making informed decisions regarding bridge infrastructure planning under dynamic conditions.

Seismic risk in the State of Colima, México: Application of a simplified methodology of the seismic risk evaluation for the localities with low-rise, non-engineered housing

The study of seismic risk is important element in the natural hazard assessment, especially in the regions of high seismic activity and low quality of housing. The new technologies, developed to reduce risk against natural hazards in large modern cities, are expensive and need in continuous instrumental monitoring of strong motions. The main problem in estimation of the seismic risk for the localities, representing with the vulnerable low-rise, non-engineered (LRNE) housing in many states of Africa, Asia and Latin America, consists in a search of an optimal and low-cost tool for estimation of the extent of physical damage to property for different type of housing. This article proposes a simplified methodology of the seismic risk evaluation for the localities, where the vulnerable LRNE houses represent most residential constructions. The methodology is based on evaluation of the following sources of information: the internet-available catalogs of world-wide of recent and historical earthquakes and visual inspection and classification of the vulnerability level of residential LRNE houses as well as a visual macroseismic inspection of the residential house damage after destructive earthquakes. This methodology is applied to the localities of the State of Colima, México characterizing with the LRNE type of residential houses. The prognostic of damage probability during large earthquake for residential houses in Colima city, based on this methodology, was firstly proposed in 1999 and then was updated in 2007 after the analysis of damage distribution during the Mw 7.5 earthquake of 2003 (MM VII in Colima city). This article presents microzoning of the seismic risk for two localities of Colima State together with a probabilistic prognostic of the expected damage in housing and financial losses during the maximum intensity earthquakes at the localities.

Могодын Хагарлаас Үүсэх Газар Хөдлөлийн Аюулыг Магадлалын Аргаар Үнэлэх нь

Mongolia is considered to be located at the seismic active region and is divided into severaltectonic zones. The Mogod earthquake (M=7.5, January 1967) which occurred at east of Hangai dome was one of the strongest earthquakes that occurred in the past century in Mongolia. The seismic event was closely studied by several researchers because seismicity of the Central Mongolian region was previously regarded as moderate.

Crust and Upper Mantle Structure Beneath the Eastern United States

AbstractThe Eastern United States (EUS) has a complex geological history and hosts several seismic active regions. We investigate the subsurface structure beneath the broader EUS. To produce reliable images of the subsurface, we simultaneously invert smoothed P‐wave receiver functions, Rayleigh‐wave phase and group velocity measurements, and Bouguer gravity observations for the 3D shear‐wave speed. Using surface‐wave observations (3–250 s) and spatially smoothed receiver functions, our velocity models are robust, reliable, and rich in detail. The shear‐wave velocity models fit all three types of observations well. The resulting velocity model for the eastern U.S. shows thinner crust beneath New England, the east coast, and the Mississippi Embayment (ME). A relatively thicker crust was found beneath the stable North America craton. A relatively slower upper mantle was imaged beneath New England, the east coast, and western ME. A comparison of crust thickness derived from our model against four recent published models shows first‐order consistency. A relatively small upper mantle low‐speed region correlates with a published P‐wave analysis that has associated the anomaly with a 75 Ma kimberlite volcanic site in Kentucky. We also explored the relationship between the subsurface structure and seismicity in the eastern U.S. We found that earthquakes often locate near regions with seismic velocity variations, but not universally. Not all regions of significant subsurface wave speed changes are loci of seismicity. A weak correlation between upper mantle shear velocity and earthquake focal mechanism has been observed.

Reversed cyclic tests on precast concrete shear walls with grouted corrugated metallic duct connections

The grouted corrugated metallic duct (GCMD) connections represent a feasible solution for precast concrete shear walls (PCSWs) thanks to their lowered construction difficulty, improved grouting reliability and reduced costs compared with the grouted steel sleeve splices (GSS). However, applications and related studies on PCSWs with GCMD are very limited, especially for their use in seismic active regions. Reversed cyclic test on six full scale shear wall specimens was conducted, considering both low and high axial compression ratios of 0.12 and 0.3. The test program includes two PCSWs with hybrid connections (GSS in the boundary elements and GCMD in the middle part of the walls), two PCSW with GCMD and two CIP reference specimens. Test results showed that all specimens failed by flexural-shear and the walls under high axial compression underwent earlier failure. The hysteretic curves of the PCSWs were found relatively wider than the CIP walls. The load carrying capacities of the PCSWs were very close to that of the CIP specimen. Under low axial compression, the PCSWs with GCMD, with hybrid connection and the CIP wall shared similar displacement ductility (4.11, 4.35 and 4.44, respectively). Under higher axial compression, the ductility of the PCSWs with GCMD (2.42) and with hybrid connections (2.36) was very close, which however was significantly higher than that of the CIP specimen (1.63). In addition, it was also revealed that precast specimens with GCMD had similar stiffness degradation characteristics to those of the PCSWs with hybrid connection and CIP specimen.

Fluid geochemistry and geothermal anomaly along the Yushu-Ganzi-Xianshuihe fault system, eastern Tibetan Plateau: Implications for regional seismic activity

Hydrothermal activity and seismicity are abundant along the Yushu-Ganzi-Xianshuihe fault system (YGXFS) in the eastern Tibetan Plateau. However, the processes and factors that control hydrothermal fluid circulation at a regional scale remain less constrained, and the relationship between geothermal anomaly and seismic activity is poorly known. In this study, we report an integrated dataset of thermal water chemistry (δ18OH2O, δDH2O, and δ13C of dissolved inorganic carbon (DIC)) to identify the origin, circulation path, and reservoir properties of hydrothermal fluids and then reveal the relationship between fluid circulation and seismic activity. The thermal waters are mainly classified as Na-HCO3 type with minor Ca-Na-HCO3 and Na-HCO3-Cl types. The δ18OH2O (−18.7‰ to −14.5‰) and δDH2O (−142‰ to −110‰) values indicate recharge of meteoric waters, which infiltrated along the YGXFS to 2–6 km depths and underwent mixing with deep fluids prior to their discharge at the surface. A mass balance model based on δ13CDIC values (−4.3‰ to 5.9‰) and DIC contents of thermal waters reveals the involvement of CO2-bearing fluids from mantle and thermo-metamorphic decarbonation of crustal marine limestones in tectonic CO2 outgassing from the active fault. The estimated reservoir temperatures (85–336 °C) based on silica geothermometer of the thermal waters decrease from southeast to northwest along the YGXFS. Notably, the epicenters of strong earthquakes (Ms ≥ 6) are mostly located in the transition zone of geothermal anomalies according to spatial distribution of the reservoir temperatures. Combined with geological and geophysical information, we suggest that the heterogeneity of crustal thermal structure and the upwelling of deep fluids may play an essential role in the distribution of thermal springs and the high frequency of earthquakes along the YGXFS.

Local time dependence of earthquakes occurrence and its possible connection with geomagnetic diurnal variations

ABSTRACT Occurrence of earthquakes is not only controlled by internal processes such as the tectonic processes, but also by external ones as Sun-Earth system processes. The aim of the current study is to examine the distribution of earthquakes occurrence at some active seismic regions in order to determine the dominant period of earthquakes occurrence with respect to local time and its possible connection with geomagnetic diurnal variations. Analyzing long-term datasets from 1986 to 2020 demonstrated that the occurrence of earthquakes at the studied regions is strongly depending on local time where earthquakes tend to occur more often during noon local time. This behavior is in somehow similar to the pattern of geomagnetic diurnal variations. Therefore, the study was extended to investigate the probable correlation between both of them. Diurnal variation of North-South geomagnetic component (X-component) was examined and compared with the local time distribution of earthquakes at each region. Data analysis revealed a significant correlation between the geomagnetic diurnal variations and earthquakes occurrence. Thus, the results support the idea of existence a dominating time interval of earthquakes occurrence and also a relationship between local time dependence of earthquakes and geomagnetic diurnal variations that may help in earthquakes prediction in the future.

Extradosed cable‐stayed Pelješac Bridge in Croatia – Fabrication of steel box girders and on‐site installation

AbstractThe new Pelješac Bridge and access road project, which links Mainland Croatia to the Pelješac Peninsula, is the most significant social infrastructure project in Croatia under construction at present. The bridge site lies in the highly active seismic region with ground acceleration ag = 0.34g and a maximum average ten‐minute wind speed above 33.7 m/s. As a result, the execution class for the steel structure was judged to be EXC4 based on the recommended matrix in standard EN 1090‐2, which defines execution classes according to consequence class, service category, and production category. The correspondingly determined quality level was class B+, which equates to quality class B under ISO 5817 with supplementary requirements for bridge decks from EN 1090‐2. This article describes the whole fabrication process in the workshop, the long‐distance transportation and the installation on the bridge site. To satisfy such high‐quality technical requirements, the heavy steel structures were fabricated using a wide variety of mechanized equipment and automatic welding techniques. This helped to ensure a more efficient fabrication process and more consistent product quality. The length of the large side span segments was designed to suit the capabilities of the floating cranes and transit barges, thereby making on‐site installation more efficient. Against the backdrop of the global Covid‐19 pandemic, the contractor CRBC overcame seemingly unsurmountable obstacles beyond their control to successfully complete this challenging assignment in two years.

Displacement response spectra for active seismic regions in India

Displacement response spectra for active seismic regions in India

Analysis of confined AAC walls under monotonic compression

Confined walls are structures frequently encountered in seismic zones. They have been put to numerous tests over time, including entire buildings being tested under cyclic loading. However, the observed trend is that confined masonry structures are also built in areas of little or no seismic activity. The draft 2nd edition of Eurocode 6 even brings about calculation procedures for confined walls erected outside active seismic regions. That said, an insufficient number of tests have still been performed on confined masonry structures under monotonic load, a situation which has a considerable negative impact on the verification process regarding standard recommendations. This paper presents results obtained from tests carried out on 20 walls, both non-confined and with reinforced concrete confinement. The full-scale walls put to tests were built of autoclaved aerated concrete (AAC). They were subjected to one loading cycle until failure. The testing procedure involved numerical simulations and calculations based on data obtained from the strut-and-tie models, as well as calculations carried out in accordance with the procedures laid down in the draft 2nd edition of Eurocode 6. It was demonstrated during the tests that wall confinement has a significant impact on the masonry resistance under monotonic load. The numerical models were used to determine the distribution of stresses in the test walls and to select the effective strut-and-tie model. The authors of the paper also verified the standard recommendations.

Possible depth and source localization of seismic anisotropy beneath Shillong Plateau and Himalayan foredeep region: An implication towards deformation mechanisms

Estimation of source localization and central depth of anisotropy beneath an active seismic region plays an important role in understanding the deformation mechanism and current configuration of tectonics. Though shear wave splitting measurements are capable of deciphering the mantle dynamics and deformation patterns, they are constrained from analysing the actual depth of anisotropy or heterogeneity, which may have large influences in interpreting the geodynamics of a particular region. The present study fills this gap of estimating the source localization and depth of seismic anisotropy beneath the Shillong Plateau and Himalayan foredeep region through a well‐established spatial coherency analysis of splitting parameters based upon the Fresnel zone concept. The model suggests a central depth of heterogeneity at around 100 km beneath the Shillong Plateau mass, coinciding with the lithosphere‐asthenosphere boundary and strengthens our implication that the asthenospheric drag at the base of the Indian lithosphere is responsible for absolute plate motion of India, is the major source of anisotropy and controls the deformation patterns of the Shillong mass. In a similar manner, the spatial coherency analysis predicts a central depth of anisotropy at around 150 km beneath the Himalayan foredeep region. The asthenospheric force corresponding to this particular depth is suggested as the major source of anisotropy for this region.

Usage of fiber optics as an underground deployable radon gas detector

Radon gas was previously presented to be a good tool as a proxy for pre-seismic precursory before earthquakes, especially when the detector is deployed a few meters underground in regions of high seismic activity. In this paper, we present a fiber optic-based detector that can be deployed underground and assist in the measurement of radon gas temporal concentration variations. The sensitivity of the fiber-based sensor is enhanced due to Fabry-Perot resonator realized within the fiber. The sensing principle is related to the impact of the alpha particles released from the surrounding radon gas on the optical transmission parameters of the fiber. By incorporation of WDM filters along the fiber sensor, the dispersion of the radon's radiation damage along the deployed fiber can be allocated.

Morpho-tectonic Assessment of the Abu-Dabbab Area, Eastern Desert, Egypt: Insights from Remote Sensing and Geospatial Analysis

The Abu-Dabbab area, located in the central part of the Egyptian Eastern Desert, is an active seismic region where micro-earthquakes (≈ML &lt; 2.0) are recorded regularly. Earthquake epicenters are concentrated along an ENE–WSW trending pattern. In this study, we used morphological indexes, including the valley floor width-to-valley floor height ratio (Vf), mountain front sinuosity (Smf), the asymmetry factor index (Af), the drainage basin shape index (Bs), the stream length–gradient index (SL), hypsometric integral (Hi) water drainage systems, and a digital elevation model analysis, to identify the role of tectonics. These indexes were used to define the relative tectonic activity index (RTAI), which can be utilized to distinguish low (RTAI &lt; 1.26), moderate (RTAI = 1.26–1.73), and high (RTAI &gt; 1.73) tectonic activity signals all over the study area. Firstly, our results indicate low to medium tectonic activity and general anomaly patterns detected along the major tectonic zones of the study area. Secondly, based on most of the low to medium tectonic activity distributed in the study area and the detected anomalies, we discuss two potential drivers of the seismicity in the Abu-Dabbab area, which are fault-controlled and deep-rooted activities.