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CN  10-1502/P

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A new approach for inversion of receiver function for crustal structure in the depth domain
TianYu Zheng, YuMei He, Yue Zhu
Recently Published, doi: 10.26464/epp2022008
[Abstract](264) [FullText HTML](6) [PDF 1705KB](13)
A method for reconstructing crustal velocity structure using the optimization of stacking receiver function amplitude in the depth domain, named common conversion amplitude (CCA) inversion, is presented. The conversion amplitude in the depth domain, which represents the impedance change in the medium, is obtained by assigning the receiver function amplitude to the corresponding conversion position where the P-to-S conversion occurred. Utilizing the conversion amplitude variation with depth as an optimization objective, imposing reliable prior constraints on the structural model frame and velocity range, and adopting a stepwise search inversion technique, this method efficiently weakens the tendency of easily falling into the local extremum in conventional receiver function inversion. Synthetic tests show that the CCA inversion can reconstruct complex crustal velocity structures well and is especially suitable for revealing crustal evolution by estimating diverse velocity distributions. Its performance in reconstructing crustal structure is superior to that of the conventional receiver function imaging method.
Application of deep learning to estimate stratospheric gravity wave potential energy
Yue Wu, Zheng Sheng, XinJie Zuo
Recently Published, doi: 10.26464/epp2022002
[Abstract](343) [FullText HTML](20) [PDF 2349KB](12)
One of the most important dynamic processes in the middle and upper atmosphere, gravity waves (GWs) play a key role in determining global atmospheric circulation. Gravity wave potential energy (GW Ep) is an important parameter that characterizes GW intensity, so it is critical to understand its global distribution. In this paper, a deep learning algorithm (DeepLab V3+) is used to estimate the stratospheric GW Ep. The deep learning model inputs are ERA5 reanalysis datasets and GMTED2010 terrain data. GW Ep averaged over 20−30 km from 60°S−60°N, calculated by COSMIC radio occultation (RO) data, is used as the measured value corresponding to the model output. The results show that (1) this method can effectively estimate the zonal trend of GW Ep. However, the errors between the estimated and measured value of Ep are larger in low-latitude regions than in mid-latitude regions, possibly due to the large number of convolution operations used in the deep learning model. Additionally, the measured Ep has errors associated with interpolation to the grid; this tends to be amplified in low-latitude regions because the GW Ep is larger and the RO data are relatively sparse, affecting the training accuracy. (2) The estimated Ep shows seasonal variations, which are stronger in the winter hemisphere and weaker in the summer hemisphere. (3) The effect of quasi-biennial oscillation (QBO) can be clearly observed in the monthly variation of estimated GW Ep, and its QBO amplitude may be less than that of the measured Ep.


The 600 keV electron injections in the Earth’s outer radiation belt: A statistical study
ChaoLing Tang, Xu Wang, BinBin Ni, ZhengPeng Su, and JiChun Zhang
Accepted Articles, doi: 10.26464/epp2022012
[Abstract](153) [PDF](3)
Relativistic electron injections are one of the mechanisms of enhancements of relativistic (≥ 0.5 MeV) electrons in the Earth’s outer radiation belt. In this study, we present the statistical observation of 600 keV electron injections in the outer radiation belt using the Van Allen Probes data. Based on the different injection characteristics, 600 keV electron injections in the outer radiation belt are divided into “pulsed electron injections” and “non-pulsed electron injections”. The 600 keV electron injections are observed at over 4.5 < L < 6.4 under the geomagnetic conditions of 450 nT < AE < 1450 nT. L ~ 4.5 is an inward limit for 600 keV electron injections. Before the electron injections, the flux negative L shell gradient for ≤ 0.6 MeV electrons or the low electron fluxes in the injected region are observed. For 600 keV electron injections at different L shells, the source populations from the Earth’s plasma sheet are different. For 600 keV electron injections at higher L shells, the source populations are higher energy electrons (~ 200 keV at X ~ –9 RE), while the source populations for 600 keV electron injections at lower L shells are lower energy electrons (~ 80 keV at X ~ –9 RE). These results are important for our further understanding of electron injections and rapid enhancements of 600 keV electrons in the Earth’s outer radiation belt.
Analysis of inversion error characteristics of stellar occultation simulation data
MingChen Sun, QingLin Zhu, Xiang Dong and JiaJi Wu
Accepted Articles, doi: 10.26464/epp2022013
[Abstract](87) [PDF](4)
Atmospheric stellar occultation observation technology is an advanced space-based detection technology that can measure the vertical distribution of trace gas composition, temperature, and aerosol content in a planet's atmosphere. In this study, an inversion algorithm of the onion-peeling method was constructed to invert the transmittance obtained from the forward mask. The method used three-dimensional ray tracing simulation to obtain the transmission path of the light in the Earth's atmosphere. The relevant parameters were then combined in the HITRAN database, and line-by-line integration was performed to calculate the atmospheric transmittance. The value of transmittance was then used as an input to calculate the vertical distribution of oxygen molecules using the single-wavelength inversion onion-peeling method. Finally, the oxygen molecule content was compared to the value attained by the Mass Spectrometer and Incoherent Scatter Radar Extended (MSISE00) atmospheric model to determine the relative error of our model. The maximum error was found to be 0.3%, which is low enough to verify the reliability of our algorithm. Using Global-scale Observations of the Limb and Disk (GOLD) measured data to invert the oxygen number density, we calculated its relative deviation from the published result to further verify the algorithm. The inversion result was affected by factors such as prior data, absorption spectral line type, the ellipticity of the earth, and orbit accuracy. Analysis of these error-influencing factors showed that the seasons and the Earth's ellipticity only effected the accuracy of the model by 0.001% and could therefore be ignored. However, latitude and solar activity had a greater impact on accuracy, in the order of 0.1%. The absorption line type affected the accuracy of the model by as much as 1%. All three of these factors therefore need to be considered during the inversion process.
Spatial distribution characteristics and mechanism of nonhydrological time-variable gravity in mainland China
Yue Shen, QiuYu Wang, WeiLong Rao, WenKe Sun
Accepted Articles, doi: 10.26464/epp2022009
[Abstract](189) [PDF](5)
The purpose of this study is to explore nonhydrological mass transfer in mainland China. For this purpose, obtained the spatial distribution of time variant gravity signals in mainland China using gravity recovery and climate experiment (GRACE) data. Then, according to the current hydrological model and other auxiliary hydrological data, a new combined hydrological model of mainland China was constructed. Finally, the time variant signals of the above combined hydrological model were deducted from the time variant gravity field of GRACE to obtain the nonhydrological mass transfer of mainland China, and its physical sources and mechanisms were discussed. In this study, according to the existing hydrological models, the hydrological components with the best correlation were selected through correlation calculation, analysis, and comparison, and a more reasonable combined hydrological model was created. This combined hydrological model includes soil water, snow water, vegetation canopy water, groundwater, lakes, glaciers, and other water components (mainly reservoir storage, swamps, rivers). The spatial distribution of the nonhydrological mass transfer signals in mainland China was obtained by deducting the combined hydrological model from the GRACE time-variable gravity field. The results show that the nonhydrological signals in mainland China were mainly positive signals, which were distributed in the Bohai Rim and the northern and eastern parts of the Tibetan Plateau. The above nonhydrological mass transfer signals were further studied and discussed. The results show that the nonhydrological mass migration signals in the Bohai Rim region primarily originate from sea-level change and marine sediment accumulation. The mass accumulation of Indian plate collision in the Tibetan Plateau was the main reason for the increase in the residual gravity field in the Tibetan Plateau.
Modeling of Kilometer-Scale Ionospheric Irregularities at Mars
ChunHua Jiang, Rong Tian, LeHui Wei, GuoBin Yang, ZhengYu Zhao
Accepted Articles, doi: 10.26464/epp2022011
[Abstract](158) [PDF](3)
Recently, kilometer-scale Martian ionospheric irregularities have been measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission (Fowler et al., 2020). In this study, we carried out a simulation of kilometer-scale ionospheric irregularities at Mars. A uniform zonal neutral wind was adopted in this model, and the seeding source is the cosinusoidal perturbation of the plasma density. Results show that the vertical electric field shear could be induced when the plasma density perturbation occurred. The vertical electric field shear causes the velocity shear of the plasma between the topside and bottomside ionosphere. Then, the velocity shear of the plasma between the topside and bottomside ionosphere could produce smaller-scale (kilometer-scale) ionospheric irregularities than the previous simulation (Jiang et al., 2021). The kilometer-scale variations in the plasma density and magnetic field profiles (along the altitude) are comparable with observations.
Inter-annual variations of 6.5-day planetary waves and their relations with QBO
Ying-Ying Huang, Jun Cui, Hui-Jun Li, Chong-Yin Li
Accepted Articles, doi: 10.26464/epp2022005
[Abstract](276) [PDF](21)
This paper studies inter-annual variations of 6.5-Day Waves (6.5DWs) in 20110 km between 52°S52°N during March 2002January 2021 and their relations with equatorial stratospheric Quasi-Biennial Oscillation (QBO). 6.5DWs’ amplitudes in temperature are calculated based on SABER/TIMED observations. QBO zonal winds are obtained from ERA5 reanalysis dataset. QBO phases are derived from Empirical Orthogonal Functions (EOF) method. Wavelet analysis of 6.5DW variations demonstrates obvious spectral maximums around 2838 months in 32°52°N and 2630 months in 32°52°S. In the Northern Hemisphere, peak periods get longer poleward, while they remain unchanged with latitude in the Southern Hemisphere. Residual 6.5DWs’ amplitudes are calculated by removing composite amplitudes from 6.5DWs’ amplitudes. Comparisons between QBO and the monthly maximum residual 6.5DWs’ amplitudes (A_Mmax) show clear relations between QBO and 6.5DWs in both hemispheres, especially in the NH. When A_Mmax is large in the NH, mean QBO profile is easterly at all levels from 70 to 5 hPa. When it’s weak, mean QBO wind is weak westerly below 30 hPa. Linear Pearson correlation coefficients between QBO phases and A_Mmax show large positive values in 60110 km between 20°52°N in April and around 64 km at 24°S in February, and large negative values from 80 to 110 km between 20°N50°N in August and at 96106 km between 20°S44°S in February. These results indicate quantitative relations between QBO and 6.5DWs and provide credible evidences for further studies of QBO modulations on long-term variations of 6.5DWs.
Resolving co- and early post-seismic slip variations of the 2021 MW 7.4 Maduo earthquake in east Bayan Har block with a block-wide distributed deformation mode from satellite synthetic aperture radar data
Shuai Wang, Chuang Song, ShanShan Li, Xing Li
Accepted Articles, doi: 10.26464/epp2022007
[Abstract](402) [PDF](47)
On 21 May 2021 (UTC), an MW 7.4 earthquake jolted the east Bayan Har block in the Tibetan Plateau. The earthquake received widespread attention as it is the largest event in the Tibetan Plateau and its surroundings since the 2008 Wenchuan earthquake and in proximity to the seismic gaps on the east Kunlun fault. Here we use satellite interferometric synthetic aperture radar data and subpixel offset observations along the range directions to characterize the coseismic deformation of the earthquake. Rang offset displacements depict clear surface ruptures with a total length of ~170 km involving two possible activated fault segments in the earthquake. Coseismic modeling results indicate that the earthquake is dominated by left-lateral strike-slip motions of up to 7 m within top 12 km of the crust. The well-resolved slip variations are characterized by five major slip patches along strike and 64% of shallow slip deficit, suggesting a young seismogenic structure. Spatial-temporal changes of postseismic deformation are mapped from the early 6-day and 24-day InSAR observations, and are well explained by time-dependent afterslip models. Analysis of GPS velocity profiles and strain rates suggests that the eastward extrusion of plateau is diffusely distributed across the east Bayan Har block, but showing significant lateral heterogeneities as evidenced by magnetotelluric observations. The block-wide distributed deformation of the east Bayan Har block along with the significant co- and post-seismic stress loading from the Maduo earthquake imply high seismic risks on regional faults, especially the Tuosuo Lake and Maqin-Maqu segments of the Kunlun fault that known as seismic gaps.
Cretaceous–Cenozoic regional stress field evolution from borehole imaging in the southern Jinzhou area, western Liaoning, North China Craton
ChengWei Yang, ChengHu Wang, GuiYun Gao, Pu Wang
Accepted Articles, doi: 10.26464/epp2022001
[Abstract](309) [PDF](2)
The Mesozoic Yanshanian Movement affected the tectonic evolution of the North China Craton (NCC). It is proposed that Mesozoic cratonic destruction peaked ~125 Ma, possibly influenced by subduction of the western Pacific Plate beneath the Euro-Asian Plate in the Early Cretaceous. The southern Jinzhou area in the eastern block of the NCC preserves records for the tectonic events and related geological resources. Studies of the regional stress field evolution from the Cretaceous to Cenozoic can enhance our understanding of the tectonics and dynamics of the NCC. Borehole image logging technology was used to identify and collect attitudes of tensile fractures from 11 boreholes, which were subdivided into four groups according to dip directions, i.e., NNW-SSE, NWW-SEE, W-E and NE-SW. Their development was mainly controlled by the regional tectonic stress field while temperature, lithology, and depth contributed to some extent. The area was characterized by NNW-SSE- and NWW-SEE-oriented extension in 136-125 Ma in the Early Cretaceous. Subsequently, it has successively undergone W-E- and NE-SW-oriented extension in 125-101 Ma and after 101 Ma. This counterclockwise trend has persisted to the present, probably related to oblique subduction of the Pacific Plate and is characterized by ongoing nearly N-S-oriented extension and NEE-SWW-oriented compression.
Shallow crustal velocity structures revealed by active source tomography and fault activities of the Mianning-Xichang segment of the Anninghe fault zone, SW China
XiHui Shao, HuaJian Yao, Ying Liu, HongFeng Yang, BaoFeng Tian, LiHua Fang
Accepted Articles, doi: 10.26464/epp2022010
[Abstract](171) [PDF](7)
The Anninghe fault is a large left-lateral strike-slip fault in southwestern China. It has controlled the deposition and magmatic activities since the Proterozoic, and has frequent seismic activities. The Mianning-Xichang segment of the Anninghe fault is a seismic gap and has been locked with high stress. Many studies suggest that this segment has a great potential for large earthquakes (magnitude>7). We obtained three vertical profiles of the Anninghe fault (between Mianning and Xichang) based on inversion of P-wave first arrival times. The travel time data were picked from seismograms generated by Methane Gaseous Source and recorded by three linearly distributed across-fault dense arrays. The inversion results show that the P-wave velocity structures at depths of 0-2 km corresponds well with the local lithology. The Quaternary sediments have low seismic velocities, while the igneous rocks, metamorphic rocks and bedrocks have high seismic velocities. Then we further discuss the fault activities of the two fault branches of Anninghe fault in the study region based on the small earthquakes (magnitude between M_L 0.5 and M_L 2.5) detected by the Xichang array. The eastern fault branch is more active than the western branch, and the fault activities in the eastern branch are different on the northern and southern segments at the border of 〖28〗^° 〖21〗^' N. The obtained high-resolution models are essential for future earthquake rupture simulation and hazard assessment of the Anninghe fault zone. Future studies of velocity models at deeper depths may further explain the complex fault activities in the study region.
Neutralized Solar Energetic Particles for SEP Forecasting: Feasibility Study of an Innovative Technique for Space Weather Applications
X.-D. Wang, B. Klecker, G. Nicolaou, S. Barabash, M. Wieser, P. Wurz, A. Galli, F. Cipriani, and Y. Futaana
Accepted Articles, doi: 10.26464/epp2022003
[Abstract](352) [PDF](10)
Energetic neutral atoms (ENAs) are produced by the neutralization of energetic ions formed by shock-accelerated gradual solar energetic particle events (SEP). These high-energy ENAs (HENAs) can reach the Earth earlier than the associated SEPs and provide information about the SEPs at the lower corona. The HENA properties observed at Earth depend on the properties of the coronal mass ejection (CME)-driven shocks that accelerate the SEPs. With a model of HENA production in a shock-accelerated SEP event, we semi-quantitatively investigate the energy-time spectrum of HENAs depending on the width, propagation speed, and direction of the shock, as well as the density and ion abundances of the lower corona. Compared to the baseline model parameters, the cases with a wider shock width angle or a higher coronal density would increase the HENA flux observed at the Earth, while the case with an Earth-propagating shock shows a softened HENA spectrum. The comparison of expected HENA fluxes in different cases with a flight-proven ENA instrument suggests that solar HENAs can feasibly be monitored with current technologies, which could provide a lead time of 2-3 hours for SEPs at a few MeV. We propose that monitoring of solar HENAs could provide a new method to forecast shock-driven SEP events that could have significant space weather impacts on the near-Earth environment.
Thermal Inertia at the MSL and InSight Mission Sites on Mars
D. Singh, S. Uttam
Accepted Articles, doi: 10.26464/epp2022004
[Abstract](223) [PDF](5)
For planetary surface materials, thermal inertia is the critical property that governs the surface’s daily thermal response and controls the diurnal and seasonal surface temperature variations. Here we use the ground measurements made by the MSL Curiosity rover and the InSight lander to determine the thermal inertia of two sites on Mars. This study compares the variation of thermal inertia during and after the Large Dust Storm (LDS) of Martian Year (MY) 34. We derive a simple approximation (using energy balance), which utilizes surface albedo, surface energy flux, and diurnal change in the surface temperature for the surface thermal inertia determination. The average thermal inertia in MY34 is about 39.2%, 3.7%, and 3.4% higher than MY35 average thermal inertia for the MSL, InSight (FOV1), and InSight (FOV2), respectively. The thermal inertia at the InSight (FOV1) is consistently lower by about 20 J m–2 s–1/2 K–1 than the InSight (FOV2) site for all scenarios, indicating notable variation in the region’s surface composition. The best-fit surface albedo in MY34 (determined using the KRC model) are about 0.08, 0.05, and 0.03 higher than MY35 surface albedo for the MSL, InSight (FOV1), and InSight (FOV2), respectively. An increase in both surface albedo and thermal inertia during the LDS indicates that the underlying surface is both more thermally resistant and more reflective than the overlying loose dust.
Study of Mars Magnetosheath Fluctuations using the Kurtosis Technique: Mars Express Observations
Franco, A. M.S., Echer, E., Bolzan, M. J. A., and Fraenz, M.
Accepted Articles, doi: 10.26464/epp2022006
[Abstract](225) [PDF](11)
Planetary magnetosheaths are characterized by a high plasma wave and turbulence activity. This is also observed for the induced magnetosphere of Mars where both upstream and locally generated plasma waves have been observed in the region between its bow shock and magnetic boundary layer, the magnetosheath. In this work, 12 years (2005-2016) of Mars Express (MEX) magnetosheath crossings have been used in order to conduct a statistical study of wave activity in the magnetosheath of Mars. Electron density and temperature measured by the electron spectrometer (ELS) of the plasma analyzer (ASPERA-3) experiment on board of MEX spacecraft were used in this study. The kurtosis parameter has been calculated for these plasma parameters. This value indicates intermittent behavior in the data when it is higher than 3 (the value for a normal or Gaussian distribution). The variation of wave activity occurrence has been analyzed in relation to solar cycle, Martian orbit and distance to the bow shock. It was observed non-Gaussian properties in the magnetosheath of Mars on all analyzed scales, especially in those near the proton gyrofrequency in the upstream region of the Martian magnetosphere. We also observed that it is higher on smaller scales (higher frequencies). A significant influence of the solar cycle was also observed, where the kurtosis parameter is higher during declining and solar maximum phases, where the presence of disturbed solar wind conditions, caused by large scale solar wind structures, increases. The kurtosis decreases with increasing distance from the bow shock, which indicates that the intermittence level is higher near the bow shock. In the electron temperature data the kurtosis is higher near the perihelion due to the higher incidence of EUV when the planet is closer to the Sun, which causes a more extended exosphere, and consequently increases the wave activity in the magnetosheath and its upstream region. The extended exosphere seems to play a lower effect in the electron density data.

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Preface to the special collection of Induced Earthquakes
HongFeng Yang, XiaoWei Chen, Rebecca Harrington, YaJing Liu
2021, 5(6): 483 -484. doi: 10.26464/epp2021057
Spatial-temporal evolution of injection-induced earthquakes in the Weiyuan Area determined by machine-learning phase picker and waveform cross-correlation
Wing Ching Jeremy Wong, JinPing Zi, HongFeng Yang, JinRong Su
2021, 5(6): 485 -500. doi: 10.26464/epp2021055
Anthropogenic induced seismicity has been widely reported and investigated in many regions, including the shale gas fields in the Sichuan basin, where the frequency of earthquakes has increased substantially since the commencement of fracking in late 2014. However, the details of how earthquakes are induced remain poorly understood, partly due to lack of high-resolution spatial-temporal data documenting the evolution of such seismic events. Most previous studies have been based on a diffusive earthquake catalog constructed by routine methods. Here, however, we have constructed a high resolution catalog using a machine learning detector and waveform cross-correlation. Despite limited data, this new approach has detected one-third more earthquakes and improves the magnitude completeness of the catalog, illuminating the comprehensive spatial-temporal migration of the emerging seismicity in the target area. One of the clusters clearly delineates a potential unmapped fault trace that may have led to the Mw 5.2 in September 2019, by far the largest earthquake recorded in the region. The migration of the seismicity also demonstrates a pore-pressure diffusion front, suggesting additional constraints on the inducing mechanism of the region. The patterns of the highly clustered seismicity reconcile the causal link between the emerging seismicity and the activity of hydraulic fracturing in the region, facilitating continued investigation of the mechanisms of seismic induction and their associated risks.
Machine-learning-facilitated earthquake and anthropogenic source detections near the Weiyuan Shale Gas Blocks, Sichuan, China
PengCheng Zhou, William L. Ellsworth, HongFeng Yang, Yen Joe Tan, Gregory C. Beroza, MinHan Sheng, RiSheng Chu
2021, 5(6): 501 -519. doi: 10.26464/epp2021053
Seismic hazard assessment and risk mitigation depend critically on rapid analysis and characterization of earthquake sequences. Increasing seismicity in shale gas blocks of the Sichuan Basin, China, has presented a serious challenge to monitoring and managing the seismicity itself. In this study, to detect events we apply a machine-learning-based phase picker (PhaseNet) to continuous seismic data collected between November 2015 and November 2016 from a temporary network covering the Weiyuan Shale Gas Blocks (SGB). Both P- and S-phases are picked and associated for location. We refine the velocity model by using detected explosions and earthquakes and then relocate the detected events using our new velocity model. Our detections and absolute relocations provide the basis for building a high-precision earthquake catalog. Our primary catalog contains about 60 times as many earthquakes as those in the catalog of the Chinese Earthquake Network Center (CENC), which used only the sparsely distributed permanent stations. We also measure the local magnitude and achieve magnitude completeness of ML0. We relocate clusters of events, showing sequential migration patterns overlapping with horizontal well branches around several well pads in the Wei202 and Wei204 blocks. Our results demonstrate the applicability of a machine-learning phase picker to a dense seismic network. The algorithms can facilitate rapid characterization of earthquake sequences.
Development of a new high resolution waveform migration location method and its applications to induced seismicity
SiYu Miao, HaiJiang Zhang, YuYang Tan, Ye Lin
2021, 5(6): 520 -531. doi: 10.26464/epp2021056
Locating seismic events is a central task for earthquake monitoring. Compared to arrival-based location methods, waveform-based location methods do not require picking phase arrivals and are more suitable for locating seismic events with noisy waveforms. Among waveform-based location methods, one approach is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times, and the maximum stacking values are assumed to indicate the correct event location and origin time. In this study, to obtain a high-resolution location image, we improve the waveform-based location method by applying a hybrid multiplicative imaging condition to characteristic functions of seismic waveforms. In our new stacking method, stations are divided into groups; characteristic functions of seismic waveforms recorded at stations in the same group are summed, and then multiplied among groups. We find that this approach can largely eliminate the cumulative effects of noise in the summation process and thus improve the resolution of location images. We test the new method and compare it to three other stacking methods, using both synthetic and real datasets that are related to induced seismicity occurring in petroleum/gas production. The test results confirm that the new stacking method can provide higher-resolution location images than those derived from currently used methods.
Determination of the local magnitudes of small earthquakes using a dense seismic array in the Changning−Zhaotong Shale Gas Field, Southern Sichuan Basin
Wen Yang, GuoYi Chen, LingYuan Meng, Yang Zang, HaiJiang Zhang, JunLun Li
2021, 5(6): 532 -546. doi: 10.26464/epp2021026
With the development of unconventional shale gas in the southern Sichuan Basin, seismicity in the region has increased significantly in recent years. Though the existing sparse regional seismic stations can capture most earthquakes with \begin{document}$ {M}_{\mathrm{L}}\ge 2.5 $\end{document}, a great number of smaller earthquakes are often omitted due to limited detection capacity. With the advent of portable seismic nodes, many dense arrays for monitoring seismicity in the unconventional oil and gas fields have been deployed, and the magnitudes of those earthquakes are key to understand the local fault reactivation and seismic potentials. However, the current national standard for determining the local magnitudes was not specifically designed for monitoring stations in close proximity, utilizing a calibration function with a minimal resolution of 5 km in the epicentral distance. That is, the current national standard tends to overestimate the local magnitudes for stations within short epicentral distances, and can result in discrepancies for dense arrays. In this study, we propose a new local magnitude formula which corrects the overestimated magnitudes for shorter distances, yielding accurate event magnitudes for small earthquakes in the Changning−Zhaotong shale gas field in the southern Sichuan Basin, monitored by dense seismic arrays in close proximity. The formula is used to determine the local magnitudes of 7,500 events monitored by a two-phased dense array with several hundred 5 Hz 3C nodes deployed from the end of February 2019 to early May 2019 in the Changning−Zhaotong shale gas field. The magnitude of completeness (\begin{document}$ {M}_{\mathrm{C}} $\end{document}) using the dense array is −0.1, compared to \begin{document}$ {M}_{\mathrm{C}} $\end{document} 1.1 by the sparser Chinese Seismic Network (CSN). In addition, using a machine learning detection and picking procedure, we successfully identify and process some 14,000 earthquakes from the continuous waveforms, a ten-fold increase over the catalog recorded by CSN for the same period, and the \begin{document}$ {M}_{\mathrm{C}} $\end{document} is further reduced to −0.3 from −0.1 compared to the catalog obtained via manual processing using the same dense array. The proposed local magnitude formula can be adopted for calculating accurate local magnitudes of future earthquakes using dense arrays in the shale gas fields of the Sichuan Basin. This will help to better characterize the local seismic risks and potentials.
Teleseismic waves reveal anisotropic poroelastic response of wastewater disposal reservoir
Andrew J Barbour, Nicholas M Beeler
2021, 5(6): 547 -558. doi: 10.26464/epp2021034
Connecting earthquake nucleation in basement rock to fluid injection in basal, sedimentary reservoirs, depends heavily on choices related to the poroelastic properties of the fluid-rock system, thermo-chemical effects notwithstanding. Direct constraints on these parameters outside of laboratory settings are rare, and it is commonly assumed that the rock layers are isotropic. With the Arbuckle wastewater disposal reservoir in Osage County, Oklahoma, high-frequency formation pressure changes and collocated broadband ground velocities measured during the passing of large teleseismic waves show a poroelastic response of the reservoir that is both azimuthally variable and anisotropic; this includes evidence of static shifts in pressure that presumably relate to changes in local permeability. The azimuthal dependence in both the static response and shear coupling appears related to tectonic stress and strain indicators such as the orientations of the maximum horizontal stress and faults and fractures. Using dynamic strains from a nearby borehole strainmeter, we show that the ratio of shear to volumetric strain coupling is ~0.41 which implies a mean Skempton's coefficient of \begin{document}$ A = 0.24 $\end{document} over the plausible range of the undrained Poisson's ratio. Since these observations are made at relatively low confining pressure and differential stress, we suggest that the hydraulically conductive fracture network is a primary control on the coupling between pore pressure diffusion and elastic stresses in response to natural or anthropogenic sources.
3D geomechanical modeling of the response of the Wilzetta Fault to saltwater disposal
Behzad Hemami, Shahla Feizi Masouleh, Ahmad Ghassemi
2021, 5(6): 559 -580. doi: 10.26464/epp2021054
From 2009 to 2017, parts of Central America experienced marked increase in the number of small to moderate-sized earthquakes. For example, three significant earthquakes (~Mw 5) occurred near Prague, Oklahoma, in the U. S. in 2011. On 6 Nov 2011, an Mw 5.7 earthquake occurred in Prague, central Oklahoma with a sequence of aftershocks. The seismic activity has been attributed to slip on the Wilzetta fault system. This study provides a 3D fully coupled poroelastic analysis (using FLAC3D) of the Wilzetta fault system and its response to saltwater injection in the underpressured subsurface layers, especially the Arbuckle group and the basement, to evaluate the conditions that might have led to the increased seismicity. Given the data-limited nature of the problem, we have considered multiple plausible scenarios, and use the available data to evaluate the hydromechanical response of the faults of interest in the study area. Numerical simulations show that the injection of large volumes of fluid into the Arbuckle group tends to bring the part of the Wilzetta faults in Arbuckle group and basement into near-critical conditions.
Non-storm erosion of MeV electron outer radiation belt down to L* < 4.0 associated with successive enhancements of solar wind density
Ying Xiong, Lun Xie, SuiYan Fu, BinBin Ni, ZuYin Pu
2021, 5(6): 581 -591. doi: 10.26464/epp2021051
We report an unusual non-storm erosion event of outer zone MeV electron distribution during three successive solar wind number density enhancements (SWDEs) on November 27−30, 2015. Loss of MeV electrons and energy-dependent narrowing of electron pitch angle distributions (PAD) first developed at L* = 5.5 and then moved down to L* < 4. According to the evolution of the electron phase space density (PSD) profile, losses of electrons with small pitch angles at L* > 4 during SWDE1 are mainly due to outward radial diffusion. However during SWDE2&3, scattering loss due to EMIC waves is dominant at 4 < L* < 5. As for electrons with large pitch angles, outward radial diffusion is the primary loss mechanism throughout all SWDEs which is consistent with the incursion of the Last Closed Drift Shell (LCDS). The inner edge of EMIC wave activity moved from L* ~5 to L* ~4 and from L ~6.4 to L ~4.2 from SWDE1 to SWDE2&3, respectively, observed by Van Allen Probes and by ground stations. This is consistent with the inward penetration of anisotropic energetic protons from L* = 4.5 to L* = 3.5, suggesting that the inward extension of EMIC waves may be driven by the inward injection of anisotropic energetic protons from the dense plasma sheet.
Study on electron stochastic motions in the magnetosonic wave field: Test particle simulations
Kai Fan, XinLiang Gao, QuanMing Lu, Shui Wang
2021, 5(6): 592 -600. doi: 10.26464/epp2021052
Using the test particle simulation method, we investigate the stochastic motion of electrons with energy of 300 keV in a monochromatic magnetosonic (MS) wave field. This study is motivated by the violation of the quasi-linear theory assumption, when strong MS waves (amplitude up to ~1 nT) are present in the Earth’s magnetosphere. First, electron motion can become stochastic when the wave amplitude exceeds a certain threshold. If an electron initially resonates with the MS wave via bounce resonance, as the bounce resonance order increases, the amplitude threshold of electron stochastic motion increases until it reaches the peak at about the 11th order in our study, then the amplitude threshold slowly declines. Further, we find that the coexistence of bounce and Landau resonances between electrons and MS waves will significantly reduce the amplitude threshold. In some cases, the electron motion can become stochastic in the field of an MS wave with amplitudes below 1 nT. Regardless, if neither the bounce nor Landau resonance condition is satisfied initially, then the amplitude threshold of stochastic motion shows an increasing trend for lower frequencies and a decreasing trend for higher frequencies, even though the amplitude threshold is always very large (> 5 nT). Our study suggests that electron stochastic motion should also be considered when modeling electron dynamics regulated by intense MS waves in the Earth’s magnetosphere.
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