We analyze continuous GPS measurements in Nepal, southern side of the Himalaya, and compare GPS results with GRACE observations in this area. We find both GPS and GRACE show significant seasonal ...variations. Further comparison indicates that the observed seasonal GPS height variation and GRACE‐derived seasonal vertical displacement due to the changing hydrologic load exhibit very consistent results, for both amplitude and phase. For continuous GPS stations whose observation time span are longer than 3 years, the average WRMS reduction is ∼45% when we subtract GRACE‐derived vertical displacements from GPS observed time series. The comparison for annual amplitudes between GPS observed and GRACE‐derived seasonal displacements also shows consistent correlation. The good seasonal correlation between GPS and GRACE is due to the improved GPS processing strategies and also because of the strong seasonal hydrological variations in Nepal. Besides the seasonal signal, GRACE also indicates a long‐term mass loss in the Himalaya region, assuming no GIA effect. This mass loss therefore will lead to crustal uplift since the earth behaves as an elastic body. We model this effect and remove it from GPS observed vertical rates. With a 2D dislocation model, most GPS vertical rates, especially in the central part of Nepal, can be interpreted by interseismic strain from the Main Himalayan Thrust, and several exceptions may indicate the complexity of vertical motion in this region and some potential local effects.
Key Points
Both GPS and GRACE show very strong and consistent seasonal variation in Nepal
GRACE indicates a long‐term mass loss
Hydrology‐free vertical rates can be interpreted by interseismic strain from MHT
Establishing what controls the timing of earthquakes is fundamental to understanding the nature of the earthquake cycle and critical to determining time-dependent earthquake hazard. Seasonal loading ...provides a natural laboratory to explore the crustal response to a quantifiable transient force. In California, water storage deforms the crust as snow and water accumulates during the wet wintermonths. We used 9 years of global positioning system (GPS) vertical deformation time series to constrain models of monthly hydrospheric loading and the resulting stress changes on fault planes of small earthquakes. The seasonal loading analysis reveals earthquakes occurring more frequently during stress conditions that favor earthquake rupture. We infer that California seismicity rates are modestly modulated by natural hydrological loading cycles.
We compare vertical seasonal loading deformation observed by continuous GPS stations in southern Alaska and modeled vertical displacements due to seasonal hydrological loading inferred from GRACE. ...Seasonal displacements are significant, and GPS‐observed and GRACE‐modeled seasonal displacements are highly correlated. We define a measure called the WRMS Reduction Ratio to measure the fraction of the position variations at seasonal periods removed by correcting the GPS time series using a seasonal model based on GRACE. The median WRMS Reduction Ratio is 0.82 and the mean is 0.73 ± 0.26, with a value of 1.0 indicating perfect agreement of GPS and GRACE. The effects of atmosphere and non‐tidal ocean loading are important; we add the AOD1B de‐aliasing model to the GRACE solutions because the displacements due to these loads are present in the GPS data, and this improves the correlations between these two geodetic measurements. We find weak correlations for some stations located in areas where the magnitude of the load changes over a short distance, due to GRACE's limited spatial resolution. GRACE models can correct seasonal displacements for campaign GPS measurements as well.
Key Points
GPS and GRACE observe highly correlated seasonal loading deformation
The effects of atmosphere and non‐tidal ocean loading are important
GRACE models can correct seasonal displacements for campaign GPS
•Hydrospheric induced stress time series derived from GRACE solutions from 2002–2016.•Seasonal stress changes are used to assess the influence of surface loading on faults.•Seismicity rates are ...greatest 3 months after the reduction in mean normal stress.
Shallow (≤40 km), low magnitude (M≥2.0) seismicity in southern Alaska is examined for seasonal variations during the annual hydrological cycle. The seismicity is declustered with a spatio-temporal epidemic type aftershock sequence model. The removal of aftershock sequences allows detailed investigation of seismicity rate changes as water, snow and ice loads modulate crustal stresses throughout the year. The GRACE surface loads are obtained from the JPL global land and ocean mass concentration blocks (mascon) solutions. The stress changes at a depth of 10 km are calculated using a 1D spherical layered Earth model. To evaluate the induced seasonal stresses of ∼10 kPa, we use >30 yr of earthquake focal mechanisms to constrain the tectonic background stress field orientation and assess the seasonal stress change with respect to the principal stress orientations. The background stress field is assumed to control the preferred orientation of faulting, and stress perturbations are expected to increase or decrease seismic activity on the faults. The number of excess earthquakes is calculated with respect to the background seismicity rates for discrete stress intervals. The results indicate a ∼25% increase in regional seismicity rates that correlate with a ∼3-month time lag following failure-encouraging annual mean-normal-stress, differential stress, and least principal compressive stress. No immediate earthquake rate variations are observed in this region-wide analysis. The correlation with a 3-month time lag suggests increased mobility of preexisting fluids at seismogenic depths is varying the pore pressure within fault zones to modulate the seismicity rates throughout the seasonal loading cycle.
•GNSS is used to infer the mean annual water-thickness change of 0.53 m in Taiwan.•Consistent spatio-temporal change of water cycle in geodetic and hydrological data.•Phase shifts among data sets ...reflect the complex nature of transient water storage.•Water storage variation is affected by infiltration rate, capacity, and landscape.
We systematically investigate the spatiotemporal water storage changes in Taiwan using geodetic (GNSS and GRACE) and hydrological (precipitation, GLDAS and LSDM assimilation models, and in-situ groundwater level) datasets. We use GNSS-observed vertical deformation to estimate water storage changes based on elastic loading theory and weighted least-squares inversion, correcting for contributions from global loads using GRACE. The mean annual water-thickness change inferred from GNSS across Taiwan is 0.53 ± 0.17 m and the largest seasonal change of up to 0.91 m is estimated in southwest Taiwan. Comparison of the geodetic and hydrological data shows that the spatial pattern of annual water storage change estimated from GNSS, GLDAS, and precipitation data are generally consistent, indicating significant seasonal water-load fluctuations in Taiwan. However, the GRACE solution significantly underestimates the amplitude of water mass change in Taiwan due to leakage effect, but temporally correlates well with GNSS estimates. Hydrological assimilation model GLDAS, dominated by shallow soil moisture variations, predicts that the average seasonal variation of water thickness is only about 17% of GNSS estimates. This value is about half of the mean annual LSDM water storage change of 0.18 m including an estimate of both soil moisture and surface water. The discrepancy suggests that the contribution of groundwater is substantial and the total water storage change in the hydrological assimilation model is underestimated in Taiwan. The spatiotemporal distributions derived using independent component analysis (ICA) are generally consistent between the geodetic and hydrological data. However, comparisons of seasonal amplitudes and phases between all data pairs reveal different response times to precipitation, reflecting the complex nature of transient water storage due to variable rainfall patterns, infiltration rate, soil saturation, and runoff. The peak rainfall occurs in June-July, which is one-to-two months before the peak GNSS subsidence. Water storage of the GLDAS model also reaches its maximum in August, suggesting the water storage is controlled by the infiltration rate and capacity and the total water recharge from rainfall is generally larger than discharge in the summer. The highest groundwater levels lag one and two months behind the peak GNSS subsidence in western and eastern Taiwan, respectively, indicating a higher infiltration rate in western Taiwan.
We identify and study an ongoing Slow Slip Event (SSE) in the southcentral Alaska subduction zone using GPS measurements. This is the second large SSE in this region since modern geodetic ...measurements became available in 1993. We divide the ongoing SSE into two phases according to their transient displacement time evolution; their slip distributions are similar to each other but slip rates are slightly different. This ongoing SSE occurs downdip of the main asperity that ruptured in the 1964 Alaska earthquake, on the same part of the subduction interface as the earlier 1998–2001 SSE. The average slip rate of this SSE is ∼4–5cm/yr, with a cumulative moment magnitude of Mw 7.5 (Mw 7.3 and Mw 7.1 For Phases I and II, respectively) through the end of 2012. The time and space dependence of the GPS displacements suggest that the slip area remained nearly the same during Phase I, while the slip rate increased with time. The SSEs occur on a transitional section of the subduction plate interface between the fully locked updip part and the freely slipping deeper part. During the 1964 earthquake, slip on the region of the SSE was much lower than slip in the updip region. Based on this observation and the repeated SSEs, we conclude that this part of the interface slips repeatedly in SSEs throughout the interseismic period and does not build up a large slip deficit to be released through large slip in earthquakes.
•We identify an ongoing SSE in the southcentral Alaska subduction zone with GPS data.•This SSE occurs at downdip of the main ruptured asperity of the 1964 earthquake.•The slip rate is ∼4–5cm/yr, with an accumulated moment magnitude of Mw 7.5.•It started ∼8yr after the previous 1998–2001 SSE in the same region.•The SSEs occur in a transitional area between upper seismogenic and lower slip zones.
Breast cancer is still one of the most common malignancies worldwide and remains a major clinical challenge. We previously reported that the anthelmintic drug flubendazole induced autophagy and ...apoptosis via upregulation of eva-1 homolog A (EVA1A) in triple-negative breast cancer (TNBC) and was repurposed as a novel anti-tumor agent. However, the detailed underlying mechanisms remain unclear and need further investigation. Here, we found that flubendazole impairs the permeability of the mitochondrial outer membrane and mitochondrial function in breast cancer. Meanwhile, flubendazole increased dynamin-related protein (DRP1) expression, leading to the accumulation of PTEN induced putative kinase 1 (PINK1) and subsequent mitochondrial translocation of Parkin, thereby promoting excessive mitophagy. The resultant excessive mitophagy contributed to mitochondrial damage and dysfunction induced by flubendazole, thus inhibiting breast cancer cells proliferation and migration. Moreover, we demonstrated that excessive DRP1-mediated mitophagy played a critical role in response to the anti-tumor effects of EVA1A in breast cancer. Taken together, our results provide new insights into the molecular mechanisms in relation to the anti-tumor activities of flubendazole, and may be conducive to its rational use in potential clinical applications.
Surface load variations from large water bodies alter the state of stress on faults and potentially affect earthquake nucleation processes. We report observational evidence for seasonal variations of ...seismicity on the border faults of the western branch in the East African Rift System. Relying on water level changes of Lake Victoria and the surrounding rift lakes and soil moisture from the Global Land Data Assimilation System model, we calculate the crustal stress changes using a finite element model. Our results suggest the seasonal hydrological variations produce a 0.2–0.8 kPa Coulomb stress change on the border faults. The primary annual peak in seismicity is coincident with the peak Coulomb stress with an insignificant time lag. The correlation suggests seasonal variations of seismicity rates in the western branch of East African Rift System are modulated by the hydrological loading from Lake Victoria and the large rift lakes in the surrounding region.
Plain Language
Lake Victoria and the surrounding lakes of the East African Rift System (EARS) are the largest lakes (by surface area) in Africa and show annual water level variations concurrent with the regional dry and wet seasons. We use numerical models to calculate crustal stress changes from lake water and soil moisture surface loading and investigate if the hydrological cycle encourages or discourages earthquake activity in the western branch of the EARS. Our results show seasonal hydrological loading produces stress changes at seismogenic depths that have good spatiotemporal correlation with seismicity rate variations in the western branch in the EARS. Hydrological loading from water level changes in Victoria Lake and the rift lakes weakly modulates seismicity rates in the western branch with the most pronounced results in the central Kivu rift and southern Tanganyika rift.
Key Points
Seismicity rates in the East Africa Rift System exhibit seasonal variations
Hydrological loading from Lake Victoria and rift lakes modulates the stress on the faults of the western branch of East Africa Rift System
Hydrological loading Coulomb stress changes correlate with earthquake rate variations in the southern sectors of the western branch
Stresses in the lithosphere arise from multiple natural loading sources that include both surface and body forces. The largest surface loads include near‐surface water storage, snow and ice, ...atmosphere pressure, ocean loading, and temperature changes. The solid Earth also deforms from celestial body interactions and variations in Earth's rotation. We model the seasonal stress changes in California from 2006 through 2014 for seven different loading sources with annual periods to produce an aggregate stressing history for faults in the study area. Our modeling shows that the annual water loading, atmosphere, temperature, and Earth pole tides are the largest loading sources and should each be evaluated to fully describe seasonal stress changes. In California we find that the hydrological loads are the largest source of seasonal stresses. We explore the seasonal stresses with respect to the background principal stress orientation constrained with regional focal mechanisms and analyze the modulation of seismicity. Our results do not suggest a resolvable seasonal variation for the ambient stress orientation in the shallow crust. When projecting the seasonal stresses into the background stress orientation we find that the timing of microseismicity modestly increases from an ~8 kPa seasonal mean‐normal‐stress perturbation. The results suggest that faults in California are optimally oriented with the background stress field and respond to subsurface pressure changes, possibly due to processes we have not considered in this study. At any time a population of faults are near failure as evident from earthquakes triggered by these slight seasonal stress perturbations.
Key Points
Mechanical models are developed to quantify stress changes for the regional fault geometry induced by seven sources of annual loading
Annual hydrospheric loading in California produces the largest Coulomb stress changes of 0.5–2 kPa on regional fault systems
Annual stress changes of ≤5 kPa in the principal orientation of the background stress field modulate low‐magnitude seismicity
We apply a Kalman filter‐based time‐dependent slip inversion method to model a long‐term Slow Slip Event (SSE) in the southcentral Alaska subduction zone from 2008 to 2013. This event occurred ...downdip of the asperity that ruptured in the 1964 earthquake, the same part of plate interface that slipped during a previous SSE between 1998 and 2001. Most of the slip deficit that accumulated during the steady period between 2001 and 2008 (8 years total) in the SSE source region was released by this SSE. Our results indicate both lateral and downdip propagation during this event. The SSE started at the end of 2008 at the upper section of the slip patch, and gradually propagated to the east and to the deeper part of the interface. Our results indicate no connection between this SSE in Upper Cook Inlet and another SSE in Lower Cook Inlet that started in 2010. Analysis of the earthquake catalog in the southcentral Alaska subduction zone shows a clear increase in seismicity associated with the 2008–2013 SSE. With the data from a newly available continuous GPS site, we now can better constrain the start time of the 1998–2001 SSE as ∼1998.58.
Key Points:
The 2008–2013 SSE released most slip deficit accumulated in 2001–2008 steady period
The 2008–2013 SSE shows both lateral and downdip propagation during this event
Seismicity rate increased from 2009, coinciding with the SSE
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