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2
Date Added: Jan 5, 2022
Date Added: Jan 5, 2022
Surface wave imaging based on the normal modes extracted via ambient noise cross-correlation has become a popular seismological technique during the last two decades. In addition to normal modes, which occupy most seismic wave energy, leaking modes with much weaker energy can also be obtained from ambient noise cross-correlation. We report the observation of multimodal leaking mode dispersion curves retrieved from the cross-correlation of ambient noise recorded by the LArge-n Seismic Survey in Oklahoma array. The extracted leaking modes are closely related to the guided wave dispersion resulting from the layered P-wave velocity (Vp) structure; hence, these modes can be used to invert the Vp structure. Moreover, the joint inversion of leaking modes and normal modes can not only accurately acquire the Vp structure but also effectively improve the fitness of normal modes from the inversion of only normal modes.
3
Date Added: Jan 16, 2021
Date Added: Jan 16, 2021
Over the last few years, seismic activity in the Pollino area (a sector of the Calabro–Lucanian Apennines in southern Italy known as a seismic gap) has been very weak. However, in 2011 the seismicity gradually intensified, culminating in an earthquake of Mw 5.0 occurred on 25 October 2012. The depth of the 2011–2012 earthquake hypocenters ranges between 2 and 10 km; the seismicity results in two separate clusters and traces a north‐northwest–south‐southeast fracture more evident in the western sector. In this area, an MT station was installed on 26 September 2012 by the Institute of Methodologies for Environmental Analysis, National Research Council of Italy (IMAA‐CNR), Italy, at about 50 km from another MT station operating since 2003 in the Agri Valley (Tramutola, southern Italy). Such a seismic swarm occurred in the Pollino area (more than 3600 events in last two years with local magnitude ML≥0.1). It has provided a rare opportunity to study the earthquake‐related temporal patterns of electromagnetic (EM) signals, and is potentially informative about ongoing seismogenic processes. In this study, we present several cases of EM field variations associated with the passage of seismic waves. The maximum amplitude of the electrical signals registered at the two MT sites and the earthquake magnitude are related by an attenuation factor that depends on the distance between the hypocenter and the MT station. Furthermore, at the two MT sites the maximum electrical anomalies seem to be more appreciable predominantly in different directions, indicating a certain directivity in the propagation of the electric field. A deep analysis of EM time series recorded during the mainshock Mw 5.0 was performed. In particular, by applying time–frequency misfit criteria based on the continuous wavelet transform, we compared the electric field with seismic recordings, and we found a good waveform similarity between signals. Moreover, we also found an EM signal that significantly anticipates the theoretical first P‐wave arrival at the Tramutola MT station.
5
Date Added: Jan 16, 2021
Date Added: Jan 16, 2021
The purpose of this paper is to analyze the most significant events that occurred in the period 1996-2018 located under the South Atlantic Anomaly (SAA), where the earth’s magnetic field is weaker and the trapped particles during the geomagnetic storms suffer decay over time. The time examined corresponds to the Solar Cycles 23 and 24; the area covered is defined by the following coordinates: 0N, -50S, 40E, -90W. Some significant events in this region reported Very Low Frequency (VLF) and Ultra Low Frequency (ULF) waves before the event, varying from minutes, hours, and even weeks before the earthquakes. Our study searches for a mechanism to explain why the crust creates electromagnetic signs detected at the ionosphere. Piezoelectric currents flow on the crust, combining with the magnetic field lines, temporarily producing pre-seismic electromagnetic pulses that are detected at the ionosphere prior to the events. The mechanism that allows electromagnetic signs to be detected at the ionosphere is also elucidated. These signs are brief, and they will cease with the beginning of the shock or shortly thereafter. However, some features contribute to making the signs impossible to detect, involving crust materials, location, magnitude, and depths. We suggest that these ionospheric events could happen for other kinds of hazard events, such as Volcanos.
Paper
11
Date Added: Nov 14, 2020
Date Added: Nov 14, 2020
The aim of this paper is to continue analyzing the interactions in the three-body system made up of the Sun, the Moon, and the Earth. First, we review new details about Moon-Earth connections, with a special focus on mechanical forces. Following, we expand the study to consider the pair Sun-Earth, with calculations for electromagnetic forces. The objective in both cases is to know how mechanical and electromagnetic forces affect seismological events on Earth. Our calculations found that Solar Cycles have no direct interaction with earthquake variations. Instead, we established that there is an internal discrepancy for quakes below 35 km detected in some of the regions analyzed. The results indicate that geomagnetic variations must be studied next to understand their connections to earthquakes.
9
Date Added: Nov 14, 2020
Date Added: Nov 14, 2020
A number of papers have reported on deviations of daily values of the maximum electron concentration of the ionospheric F2 layer and/or total electron content (TEC) in the vicinity of an earthquake’s epicenter some time prior to the quake. Owing to the importance of this problem, a question of a “locality” of those effects is emerging. To study this issue we have developed a method based on the calculation of global electron content and of local electron content in “check-region” with low seismic activity. The effect of TEC day-to-day changes before strong earthquakes is analyzed in this work. It is shown that in some cases this effect might be a reflection of global changes of the ionization caused by the 27-day variations as well as other fast alterations due to solar and geomagnetic activity changes. We discuss the problem of certain data corrections that permit local changes to be distinguished from global ones.
12
Date Added: Jul 8, 2021
Date Added: Jul 8, 2021
We use the benefits of the full-resolution methodology for time-series decomposition singular spectrum analysis to assess the quantitative impact of orbital and, for the first time, millennial-scale Sun-related climate responses from EPICA records. The quantitative impact of the three Sun-related cycles (unnamed ~9.7-kyr; proposed ‘Heinrich-Bond’ ~6.0-kyr; Hallstatt ~2.5-kyr), cumulatively explain ~4.0% (δD), 2.9% (CO2), and 6.6% (CH4) in variance, demonstrating for the first time the minor role of solar activity in the regional budget of Earth's climate forcing. A cycle of ~3.6 kyr, which is little known in literature, results in a mean variance of 0.6% only, does not seem to be Sun-related, although a gravitational origin cannot be ruled out. According to the recurrence analysis of Heinrich events (6.03 ± 1.4 kyr) and their correlation with EPICA stack ~6.0-kyr cycle, it is proposed that this band of solar activity be named the ‘Heinrich-Bond cycle’. On these basis, it is deemed that the ‘Heinrich-Bond’ solar cycle may act on the ice-sheet as an external instability factor both related to excess ice leading to calving process and IRD-layers (‘cold-related’ Heinrich events), and surface heating with meltwater streams (‘warm-related’ Heinrich events). The Hallstatt cycle is found in a number of solar proxies, geomagnetic secular variations, paleoclimatic oscillations, combination tones of Milankovitch forcings and resonant planetary beats, indicating an apparent ‘multi-forcing’ origin possibly related to planetary beat hypothesis. The orbital components consistently reflects the post-Mid-Pleistocene transition nature of the EPICA records in which the short eccentricity results in most of the variance (51.6%), followed by obliquity (19.0%) and precession (8.4%). Beyond the Milankovitch theory, evidence is emerging of a multiple-forcing cosmoclimatic system with stochastic interactions between external (gravitational resonances, orbitals, solar activity) and Earth's internal (geodynamics, atmosphere composition, feedback mechanisms) climate components, each having a strong difference in terms of the relative quantitative impact on Earth's climate.
4
Date Added: Jul 8, 2021
Date Added: Jul 8, 2021
The Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) microsatellite-monitored data (IAP and ISL) were employed in investigating pre- (30 days) and post- (10 days) perturbations in ionospheric parameters associated with the M6.8 Eastern Honshu (Japan) earthquake (EQ) of July 23, 2008. The results constrained by synchronously monitoring geomagnetic indices data: Kernnifzer digit and disturbance storm time (Dst), revealed strong seismic event-induced disturbances three weeks to 5 days before the seismic event. The geomagnetic indices data were used in filtering normal geomagnetic disturbances from the seismic counterparts, thereby constraining the interpretations. The total ion density measured in per cubic centimeter (cm−3) recorded variations of 7.90, 4.51, and 5.92 on days-20, -19, and -16, respectively, from the earthquake day during the night time half orbit observations. Contemporaneously, perturbations of 8.81 were observed for electron temperature measured in Kelvin (K) five days afore the earthquake. The geomagnetically quiet state of the ionosphere during the pre-seismic days suggests that the observed disturbances are seismogenic. More researches should be encouraged in this area to deepen their applications in earthquake monitoring and prediction.
3
Date Added: Jul 8, 2021
Date Added: Jul 8, 2021
Despite decades of observational, laboratory and theoretical studies, the processes leading to large earthquake generation remain enigmatic. However, recent observations provide new promising perspectives that advance knowledge. Here, we review data on the initiation processes of large earthquakes and show that they are multiscale and diverse, involving localization of deformation, fault heterogeneities and variable local loading rate effects. Analyses of seismic and geodetic data reveal evidence for regional weakening by earthquake-induced rock damage and progressive localization of deformation around the eventual rupture zones a few years before some large earthquakes. The final phase of deformation localization includes, depending on conditions, a mixture of slow slip transients and foreshocks at multiple spatial and temporal scales. The evolution of slip on large, localized faults shows a step-like increase that might reflect stress loading by previous failures, which can produce larger dynamic slip, in contrast to the smooth acceleration expected for a growing aseismic nucleation phase. We propose an integrated model to explain the diversity of large earthquake generation from progressive volumetric deformation to localized slip, which motivates future near-fault seismic and geodetic studies with dense sensor networks and improved analysis techniques that can resolve multiscale processes.
4
Date Added: May 31, 2021
Date Added: May 31, 2021
During 2017–2018, the National Seismic Hazard Model for the conterminous United States was updated as follows: (1) an updated seismicity catalog was incorporated, which includes new earthquakes that occurred from 2013 to 2017; (2) in the central and eastern United States (CEUS), new ground motion models were updated that incorporate updated median estimates, modified assessments of the associated epistemic uncertainties and aleatory variabilities, and new soil amplification factors; (3) in the western United States (WUS), amplified shaking estimates of long-period ground motions at sites overlying deep sedimentary basins in the Los Angeles, San Francisco, Seattle, and Salt Lake City areas were incorporated; and (4) in the conterminous United States, seismic hazard is calculated for 22 periods (from 0.01 to 10 s) and 8 uniform VS30 maps (ranging from 1500 to 150 m/s). We also include a description of updated computer codes and modeling details. Results show increased ground shaking in many (but not all) locations across the CEUS (up to ~30%), as well as near the four urban areas overlying deep sedimentary basins in the WUS (up to ~50%). Due to population growth and these increased hazard estimates, more people live or work in areas of high or moderate seismic hazard than ever before, leading to higher risk of undesirable consequences from forecasted future ground shaking.
4
Date Added: Apr 6, 2021
Date Added: Apr 6, 2021
The studies completed to-date on a relation of the Earth’s seismicity and solar processes provided the fuzzy and contradictory results. The main problem of this research is a lack of physical explanation of a mechanism of earthquake triggering by strong variations of space weather conditions. Based on results obtained in the field and laboratory experiments on earthquake triggering by DC pulses injection into the Earth crust we may assume that the similar triggering phenomena may occur after the strong electromagnetic impact to the earthquake source due to solar flares or geomagnetic storms. Numerical estimations demonstrated that telluric currents induced by geomagnetic pulsations generated by solar flare have the similar density at the depth of earthquake source location (10-6 A/m2) in comparison with the current density generated by artificial power sources (10-7 – 10-8 A/m2) resulted in observed spatiotemporal redistribution of seismic activity in the regions of Pamirs and Northern Tien Shan. For supporting the idea of a possible earthquake triggering by solar flares we carried out a statistical analysis of global and regional (Greece) seismicity behavior during the solar flare of X9.3 class occurred on September 6, 2017 (the strongest flare over the past thirteen years). We have discovered a new evidence of earthquake triggering due to the Sun-Earth interaction by simple comparison of a number of earthquakes before and after the strong solar flare. The global number of earthquakes (USGS catalog, M ≥ 4) for time window of ±11 days after the solar flare has increased by 68%, and the regional seismicity (Greece, EMSC catalog, M ≥ 3) has increased by 120%.
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