Surface and deep potential geophysical signals respond to the spatial redistribution of global mass variations, which may be monitored by geodetic observations. In this study, we analyze dense Global Positioning System (GPS) time series in the Eastern Tibetan Plateau using principal component analysis (PCA) and wavelet time-frequency spectra. The oscillations of interannual and residual signals are clearly identified in the common mode component (CMC) decomposed from the dense GPS time series from 2000 to 2018. The newly developed spherical harmonic coefficients of the Gravity Recovery and Climate Experiment Release-06 (GRACE RL06) are adopted to estimate the seasonal and interannual patterns in this region, revealing hydrologic and atmospheric/nontidal ocean loads. We stack the averaged elastic GRACE-derived loading displacements to identify the potential physical significance of the CMC in the GPS time series. Interannual nonlinear signals with a period of ~3 to ~4 years in the CMC (the scaled principal components from PC1 to PC3) are found to be predominantly related to hydrologic loading displacements, which respond to signals (El Niño/La Niña) of global climate change. We find an obvious signal with a period of ~6 yr on the vertical component that could be caused by mantle-inner core gravity coupling. Moreover, we evaluate the CMC’s effect on the GPS-derived velocities and confirm that removing the CMC can improve the recognition of nontectonic crustal deformation, especially on the vertical component. Furthermore, the effects of the CMC on the three-dimensional velocity and uncertainty are presented to reveal the significant crustal deformation and dynamic processes of the Eastern Tibetan Plateau.
Fault slip rates estimated from geodetic data are being integrated into seismic hazard models. The standard approach requires modeling velocities and relative (micro‐)plate motions, which is challenging for fault‐based models. We present a new approach to directly invert strain rates to solve for slip rates and distributed strain simultaneously. We generate velocity and strain rate fields over the southeastern Tibetan Plateau, utilizing Sentinel‐1 Interferometric Synthetic Aperture Radar data spanning 2014–2023. We derive slip rates using block modeling and by inverting strain rates. Our results show a partitioning between localized strain on faults and distributed deformation. The direct inversion of strain rates matches the geodetic data best when incorporating distributed moment sources, accounting for a similar proportion to on‐fault sources. The direct strain methodology also aligns best with the independent geological slip rates, especially near fault tips. As high‐resolution strain rate fields become increasingly available, we recommend direct inversion as the preferred practice.
Geodetic strain rates from increasing Global Positioning System data provide a promising approach for seismicity forecast. With the strain rate field presented in Zheng et al. (2017, https://doi.org/10.1002/2017JB014465) derived from the most complete and up‐to‐date Global Positioning System data set in the India‐Eurasia collision zone, we forecast the shallow seismicity of this region, and infer that about 11 Mw ≥ 7.5, 36 Mw ≥ 7.0, 109 Mw ≥ 6.5, and 326 Mw ≥ 6.0 earthquakes may occur here every 100 years. We indicate that shallow seismicity forecast may be able to help us distinguish between block and continuum models, and block model cannot well describe the kinematics of the Tibetan Plateau, Tien Shan, West Mongolia, North China, and Myanmar. We suggest that the regions with high forecasted earthquake rates but lack of historical earthquakes are undergoing high seismic risk, such as the west‐central Himalaya (overdue for Mw ≥ 7.5 earthquakes, possibly Mw ≥ 8.0) and the central Altyn Tagh fault (overdue for Mw ≥ 7.0 and Mw ≥ 7.5 earthquakes).
22 July 2019 in Journal of Geophysical Research: Space Physics
Two remarkable ionospheric irregularities named equatorial plasma bubble (EPB) and medium‐scale traveling ionospheric disturbance (MSTID) are verified by using multi‐instrumental observations, for example, the ground‐based GPS networks, ionosonde stations, and Swarm satellites, when the tropical cyclones Tembin and Hagibis approached Hong Kong on 26 August 2012 and 15 June 2014, respectively. The low‐latitude plasma bubble over an area of 105–120 °E in longitude was observed during 12:30–17:00 universal time (UT) on 26 August 2012. GPS observations from magnetically conjugate locations indicate that the nighttime bubble during 20:30–01:00 local time (LT = UT + 8h) on 26 August should be formed from the magnetically equatorial region rather than drifted from the west (eastward drift) or generated locally. Different from the EPB, during another cyclone on 15 June 2014, the northwest‐southeast aligned nighttime MSTID was verified in midlatitude regions at a mean horizontal velocity of 156 m/s southwestward propagation during 12:30–17:30 UT when Hagibis was near the mainland coast. By comparing the ionospheric observations during the two cyclones, differences are identified: small‐scale irregularities associated with plasma bubble cause obvious perturbations of the rate of total electron content index with the value of ~3.0 TECU/min; while the MSTID in midlatitude only cause relatively slight rate of total electron content index disturbances with the value of ~1.5 TECU/min. In addition, the magnetic conjugacy of EPB and MSTID in two hemispheres during the passage of tropical cyclones has been also discussed in this study.