Low-rise buildings are susceptible to high-frequency ground motion. The high-frequency ground motions at regional distances are mainly controlled by crustal Lg waves whose amplitudes are typically much larger than those of body waves. In this study, we develop a Lg-wave Q model for the Sichuan and Yunnan region in the frequency band of 0.3–2.0 Hz using regional seismic records of 1166 earthquakes recorded at 152 stations. Comparison between the observed pattern of ground motion from real earthquake and model prediction demonstrates the robustness and effectiveness of our Lg-Q model. Then, assuming that the Lg-wave Q structure is the main factor affecting the propagation of the high-frequency ground motions, we calculate the spatial distributions of high-frequency ground motions from scenario earthquakes at different locations in the region using the average Lg-wave attenuation model over the frequency band of 0.3–2.0 Hz. We also use the Lg-Q model to estimate the distribution of cumulative energy of high-frequency ground motions based on the historical seismicity of the Sichuan and Yunnan region. Results show that the Lg-Q model can be used effectively in estimating the spatial distribution of high-frequency seismic energies and thus can contribute to the assessment of seismic hazard to low-rise buildings.
Late at night on 17 June 2019, a magnitude 6.0 earthquake struck Shuanghe Town and its surrounding area in Changning County, Sichuan, China, becoming the largest earthquake recorded within the southern Sichuan Basin. A series of earthquakes with magnitudes up to 5.6 occurred during a short period after the mainshock, and we thus refer to these earthquakes as the Changning M6 earthquake sequence (or swarm). The mainshock was located very close to a salt mine, into which for ~3 decades fresh water had been extensively injected through several wells at a depth of 2.7–3 km. It was also near (within ~15 km) the epicenter of the 18 December 2018 M5.7 Xingwen earthquake, which is thought to have been induced by shale gas hydraulic fracturing (HF), prompting questions about the possible involvement of industrial activities in the M6 sequence. Following previous studies, this paper focuses on the relationship between injection and seismicity in the Shuanghe salt field and its adjacent Shangluo shale gas block. Except for a period of serious water loss after the start of cross-well injection in 2005–2006, the frequency of earthquakes shows a slightly increasing tendency. Overall, there is a good correlation between the event rate in the Shuanghe area and the loss of injected water. More than 400 M ≥ 3 earthquakes, including 40 M ≥ 4 and 5 M ≥ 5 events, had been observed by the end of August 2019. Meanwhile, in the Shangluo area, seismicity has increased during drilling and HF operations (mostly in vertical wells) since about 2009, and dramatically since the end of 2014, coincident with the start of systematic HF in the area. The event rate shows a progressively increasing background with some fluctuations, paralleling the increase in HF operations. More than 700 M ≥ 3 earthquakes, including 10 M ≥ 4 and 3 M ≥ 5 in spatially and temporally clustered seismic events, are correlated closely with active fracturing platforms. Well-resolved centroid moment tensor results for M ≥ 4 earthquakes were shown to occur at very shallow depths around shale formations with active HF, in agreement with some of the clusters, which occurred within the coverage area of temporary or new permanent monitoring stations and thus have been precisely located. After the Xingwen M5.7 earthquake, seismic activity in the salt well area increased significantly. The Xingwen earthquake may have created a unidirectional rupture to the NNW, with an end point close to the NW-trending fault of the Shuanghe earthquake. Thus, a fault in the Changning anticline might have terminated the fault rupture of the Xingwen earthquake, possibly giving the Xingwen earthquake a role in promoting the Changning M6 event.
Previous studies have shown that the ionospheric responses to a solar flare are significantly dependent on the solar zenith angle (SZA): the ionospheric responses are negatively related to the SZAs. The largest enhancement in electron density always occurs around the subsolar point. However, from 2001 to 2014, the global distribution of total electron content (TEC) responses showed no obvious relationship between the increases in TEC and the SZA during some solar flares. During these solar flares, the greatest enhancements in TEC did not appear around the subsolar point, but rather far away from the subsolar point. The distribution of TEC enhancements showed larger TEC enhancements along the same latitude. The distribution of anomalous ionospheric responses to the solar flares was not structured the same as traveling ionospheric disturbances. This anomaly distribution was also unrelated to the distribution of background neutral density. It could not be explained by changes in the photochemical process induced by the solar flares. Thus, the transport process could be one of the main reasons for the anomaly distribution of ionospheric responses to the solar flares. This anomaly distribution also suggests that not only the photochemical process but also the transport process could significantly affect the variation in ionospheric electron density during some solar flares.
Water vapor in the stratosphere makes a significant contribution to global climate change by altering the radiative energy budget of the Earth’s climate system. Although many previous studies have shown that the El Niño–Southern Oscillation (ENSO) has significant effects on the water vapor content of the stratosphere in terms of the annual or seasonal mean, a comprehensive analysis of the seasonal evolution of these effects is still required. Using reanalysis data and satellite observations, we carried out a composite analysis of the seasonal evolution of stratospheric water vapor during El Niño/La Niña peaks in winter and decays in spring. The ENSO has a distinct hysteresis effect on water vapor in the tropical lower stratosphere. The El Niño/La Niña events moisten/dry out the tropical lower stratosphere in both winter and spring, whereas this wetting/dehydration effect is more significant in spring. This pattern is due to a warmer temperature in the upper troposphere and lower stratosphere during the El Niño spring phase, which causes more water vapor to enter the stratosphere, and vice versa for La Niña. This delayed warming/cooling in the lower stratosphere during the El Niño/La Niña decay in spring leads to the seasonal evolution of ENSO effects on water vapor in the lower stratosphere.
Correlation lengths of ultra-low-frequency (ULF) waves around Mars were computed for the first time, using data from MEX (electron density from 2004 to 2015) and MAVEN (electron density and magnetic field from 2014 to 2016). Analysis of the MEX data found that, for the frequency range 8 to 50 mHz, correlation length in electron density varied between 13 and 17 seconds (temporal scale) and between 5.5×103 km and 6.8×103 km (spatial scale). For the MAVEN time interval, correlation length was found to vary between 11 and 16 seconds (temporal scale) and 2×103 – 4.5×103 km in spatial scale. In the magnetic field data, correlation lengths are observed to be between 8–15 seconds (temporal scale) and between 1×103 and 5×103 km (spatial scale) over the same frequency range. We observe that the cross sections of the plasma regions at the dayside of Mars are smaller than these correlation lengths in these regions in both analyses, where the correlation length derived from the MEX electron density data was between 5 and 25 times the size of the magnetosheath and the magnetic pile-up region (MPR), respectively. For MAVEN these ratios are about 4 (magnetosheath) and 11 (MPR) in electron density and between 1.5 and 5.5 for magnetic field data, respectively. These results indicate that waves at the magnetosheath/MPR can be related to oscillations in the upper ionosphere. In a local region, wave trains may cause resonance effects at the planetary ionopause, which consequently contributes to the enhanced ion escape from the atmosphere.
In this paper, we analyze one reconnection event observed by the Magnetospheric Multiscale (MMS) mission at the earth’s magnetopause. In this event, the spacecraft crossed the reconnection current sheet from the magnetospheric side to the magnetosheath side, and whistler waves were observed on both the magnetospheric and magnetosheath sides. On the magnetospheric side, the whistler waves propagated quasi-parallel to the magnetic field and toward the X-line, while on the magnetosheath side they propagated almost anti-parallel to the magnetic field and away from the X-line. Associated with the enhancement of the whistler waves, we find that the fluxes of energetic electrons are concentrated around the pitch angle 90° when their energies are higher than the minimum energy that is necessary for the resonant interactions between the energetic electrons and whistler waves. This observation provides in situ observational evidence of resonant interactions between energetic electrons and whistler waves in the magnetic reconnection.
The ion-to-electron temperature ratio is a good indicator of the processes involved in the plasma sheet. Observations have suggested that patchy reconnection and the resulting earthward bursty bulk flows (BBFs) transport may be involved in causing the lower temperature ratios at smaller radial distances during southward IMF periods. In this paper, we estimate theoretically how a patchy magnetic reconnection electric field can accelerate ions and electrons differently. If both ions and electrons are non-adiabatically accelerated only once within each reconnection, the temperature ratio would be preserved. However, when reconnection occurs closer to the Earth where magnetic field lines are shorter, particles mirrored back from the ionosphere can cross the reconnection region more than once within one reconnection; and electrons, moving faster than ions, can have more crossings than do ions, leading to electrons being accelerated more than ions. Thus as particles are transported from tail to the near-Earth by BBFs through multiple reconnection, electrons should be accelerated by the reconnection electric field more times than are ions, which can explain the lower temperature ratios observed closer to the Earth.
On May 12, 2008, an Mw7.9 earthquake occurred in Wenchuan County, Sichuan Province, China. Movement of Yingxiu–Beichuan Fault in the Longmenshan Fault Zone was considered to be the main cause of the earthquake. Earthquakes are closely related to fault activities. Therefore, studying the strain distribution and evolution process around active fault zones is important to the understanding of seismic activities. In this study, we conduct laboratory experiments with uniaxial compression applied to marble sheets with intentionally fabricated cracks. The speckle patterns of the rock samples under different loading conditions are recorded in real time by a digital camera. To calculate the deformation fields of the deliberately cracked marble sheets during different stages of the loading processes, the recorded images are processed by the digital image correlation method. The distribution and variation of the displacement and strain are further analyzed in order to understand the strain localization of and observed damage in the experimental fracture zones. Finally, we compare these laboratory results with the GPS-observed coseismic displacements during the 2008 Wenchuan earthquake, to assess the consistency between our laboratory observations and the field observations of the earthquake, but also to suggest how laboratory results can improve thinking about how earthquake patterns do and do not reflect fault patterns.
Geomagnetic storms and substorms play a central role in both the daily life of mankind and in academic space physics. The profiles of storms, especially their initial phase morphology and the intensity of their substorms under different interplanetary conditions, have usually been ignored in previous studies. In this study, 97 intense geomagnetic storms (Dstmin ≤ –100 nT) between 1998 and 2018 were studied statistically using the double superposed epoch analysis (DSEA) and normalized superposed epoch analysis (NSEA) methods. These storms are categorized into two types according to different interplanetary magnetic field (IMF) Bz orientations: geomagnetic storms whose IMF is northward, both upstream and downstream relative to the interplanetary shock, and geomagnetic storms whose upstream and downstream IMF is consistently southward. We further divide these two types into two subsets, by different geomagnetic storm profiles: Type I/Type II — one/two-step geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock; Type III/TypeIV — one/two-step geomagnetic storms with southward IMF both upstream and downstream of the interplanetary shock. The results show that: (1) geomagnetic storms with northward IMF both upstream and downstream of the interplanetary shock have a clear initial phase; geomagnetic storms with southward IMF in both upstream and downstream of the interplanetary shock do not; (2) the IMF is an important controlling factor in affecting the intensity characteristics of substorms. When Bz is positive before and after the interplanetary shock arrival, the Auroral Electrojet (AE) index changes gently during the initial phase of geomagnetic storms, the median value of AE index is maintained at 500–1000 nT; (3) when Bz is negative before and after the interplanetary shock arrival, the AE index rises rapidly and reaches its maxmum value about one hour after storm sudden commencements (SSC), although the time is scaled between reference points and the maximum value of AE is usually greater than 1,000 nT, representing intense substorms; (4) for most cases, the Dst0 usually reaches its minimum at least one hour after Bz. These results are useful in improving contemporary space weather models, especially for those that address geomagnetic storms and substorms.
An extraordinary (X-mode) electromagnetic wave, injected into the ionosphere by the ground-based heating facility at Tromsø, Norway, was utilized to modify the ionosphere on November 6, 2017. The high-power high-frequency transmitter facility located at Tromsø belongs to the European Incoherent Scatter Scientific Association. In the experiment, stimulated electromagnetic emission (SEE) spectra were observed. A narrow continuum occurred under cold-start conditions and showed an overshoot effect lasting several seconds. Cascading peaks occurred on both sides of the heating frequency only in the preconditioned ionosphere and also showed an overshoot effect. These SEE features are probably related to the ponderomotive process in the X-mode heating experiment and are helpful for understanding the physical mechanism that generated them during the X-mode heating experiment. The features observed in the X-mode heating experiments are novel and require further investigation.
The cold tongue mode (CTM), which represents the out-of-phase relationship in sea surface temperature anomaly (SSTA) variability between the Pacific cold tongue region and elsewhere in the tropical Pacific, shows a long-term cooling trend in the eastern equatorial Pacific. In this study, we investigate how well the CTM is reproduced in historical simulations generated by the 20 models considered in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). Qualitatively, all 20 models roughly capture the cooling SSTA associated with the CTM. However, a quantitative assessment (i.e., Taylor diagrams and the ratio of the trend between the simulations and observations) shows that only five of these 20 models (i.e., CESM1-CAM5, CMCC-CM, FGOALS-g2, IPSL-CM5B-LR, and NorESM1-M) can reproduce with useful accuracy the spatial pattern and long-term trend of the CTM. We find that these five models generally simulate the main ocean dynamical process associated with the CTM. That is, these models adequately capture the long-term cooling trend in the vertical advection of the anomalous temperature by the mean upwelling. We conclude that the performance of these CMIP5 models, with respect to simulations of the long-term cooling trend associated with the vertical advection, and the related long-term decreasing trend of the vertical gradient of the oceanic temperature anomaly, can play an important role in successful reproduction of the CTM.
The wind and temperature fields at 20 to 55 km above the Antigua launch site (17°N, 61°W) were analyzed by using sounding rocket data published by the research organization on Stratosphere-Troposphere Processes and their Role in Climate (SPARC). The results showed distinct variations in the wind and temperature fields at different heights from the 1960s to the 1990s. The overall zonal wind speed showed a significant increasing trend with the year, and the overall change in meridional wind speed showed a falling trend from 1976 to 1990, whereas the temperature field showed a significant cooling trend from 1964 to 1990. The times the trends mutated varied at different levels. By taking the altitudes at 20, 35, and 50 km as representatives, we found that the zonal wind speed trend had mutated in 1988, 1986, and 1986, respectively; that the meridional wind speed trend had mutated in 1990, 1986, and 1990, respectively; and that the temperature trend had mutated separately in 1977, 1973, and 1967, respectively. Characteristics of the periodic wind and temperature field variations at different heights were also analyzed, and obvious differences were found in time scale variations across the different layers. The zonal and meridional wind fields were basically characterized as having a significant periodic variation of 5 years across the three layers, and each level was characterized as having a periodic variation of less than 5 years. Temperature field variation at the three levels was basically characterized as occurring in 10-year and 5-year cycles.
There are many active faults in the southeast margin of Tibetan Plateau, where three large active faults zones, the Longmenshan, Xianshuihe and Anninghe, merge to form a "Y" shape. Strong crustal deformation and a complicated fault distribution accompany strong earthquake activity in this zone. In this paper, we investigate a multi-scale gravity anomaly in the southeastern margin of the Tibetan Plateau using the wavelet transform; we find that the pattern of the gravity field is closely related to the fault system in the study area. Analyzing the characteristics of this Bouguer gravity anomaly at different orders indicates that the eastern Himalayan syntaxis has produced a strong eastward push during its northward movement, resulting in a shortening of the crust from west to east and a rapid uplift of the Tibetan Plateau. The Songpan–Garzê and Sichuan–Yunnan blocks have been forced to slip and extrude southward and eastward laterally. The distributions of seven large earthquakes from 1970 to 2018 reflects the relationship between large earthquakes and characteristics of the gravity anomaly. Comparing the tectonic backgrounds of several earthquakes reveals that the large earthquakes occur usually in the high gravity anomaly gradient zone, which corresponds in general to the boundary zones of the blocks. We infer that large earthquakes occur primarily in high Bouguer gravity anomaly zones in the upper crust, while low Bouguer gravity anomalies encompass the lower crust and the uppermost mantle.
Twenty-seven FHDZ-M15 combined geomagnetic observation systems (each of which is equipped with a fluxgate magnetometer and a proton magnetometer) had been installed in the China geomagnetic network before the 2008 Wenchuan earthquake, during which coseismic disturbances were recorded by 26 fluxgate magnetometer observatories. The geomagnetic disturbances have similar spatial and temporal patterns to seismic waves, except for various delays. Six proton magnetometer observatories recorded coseismic disturbances with very small amplitudes. In addition, fluxgate magnetometers registered large-amplitude disturbances that are likely to have included responses to seismic waves. However, two problems remain unresolved. First, why do these geomagnetic disturbances always arrive later than P waves? Second, why do the geomagnetic disturbances have spatial and temporal directivity similar to the main rupture direction of the earthquake? Solving these two problems may be crucial to find the mechanism responsible for generating these geomagnetic anomalies.
P-wave waveforms in the distance range between 12° and 30° were analyzed to investigate upper-mantle P velocity structures beneath the Tibetan Plateau and surrounding areas. The waveform data from 504 earthquakes with magnitudes larger than 5.0 between 1990 and 2005 that occurred within 30° from the center of the Plateau were modelled. We divided the study area into 6 regions and modeled upper-mantle-distance P waveforms with turning points beneath each region separately. The results show that the upper-mantle P-wave velocity structures beneath India, the Himalayas, and the Lhasa Terrane are similar and contain a high-velocity lid about 250 km thick. The upper-mantle velocities down to 200 km beneath the Qiangtang Terrane, the Tarim Basin, and especially the Songpan-Garzê Terrane are lower than those in the south. The 410-km discontinuity beneath these two terranes is elevated by about 20 km. High-velocity anomalies are found in the transition zone below 500 km under the Lhasa and Qiangtang Terranes. The results suggest that the Tibetan Plateau was generated by thrusting of the Indian mantle lithosphere under the southern part of Tibet. Portions of the thickened Eurasian mantle lithosphere were delaminated; they are now sitting in the transition zone beneath southern Tibet and atop of the 410-km discontinuity underneath northern Tibet.