Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting

Magnetars, isolated neutron stars with magnetic-field strengths typically ≳1014 G, exhibit distinctive months-long outburst epochs during which strong evolution of soft X-ray pulse profiles, along with nonthermal magnetospheric emission components, is often observed. Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile. Such peak merging has not been seen before for a magnetar. Our high-resolution phase-resolved spectroscopic analysis reveals no significant evolution of temperature despite the complex initial pulse shape, yet the inferred surface hot spots shrink during peak migration and outburst decay. We suggest two possible origins for this evolution. For internal heating of the surface, tectonic motion of the crust may be its underlying cause. The inferred speed of this crustal motion is ≲100 m day−1, constraining the density of the driving region to ρ ∼ 1010 g cm−3, at a depth of ∼200 m. Alternatively, the hot spots could be heated by particle bombardment from a twisted magnetosphere possessing flux tubes or ropes, somewhat resembling solar coronal loops, that untwist and dissipate on the 30–40 day timescale. The peak migration may then be due to a combination of field-line footpoint motion (necessarily driven by crustal motion) and evolving surface radiation beaming. This novel data set paints a vivid picture of the dynamics associated with magnetar outbursts, yet it also highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem.

Crustal Motion and Strain Rates in the Southern Basin and Range Province

New research teases out variations in strain rates and explores potential earthquake hazards across the southern Basin and Range and Colorado Plateau.

Seasonal and long-term vertical land motion in Southwest China determined using GPS, GRACE, and surface loading model

Owing to the intense tectonic activity and significant seasonal surface mass change, Southwest China is characterized by noticeable vertical land movement. We determined the vertical movement of Southwest China using 10 years of data from 41 continuous global positioning system (GPS) stations, gravity recovery and climate experiment (GRACE), and surface loading model (SLM). The annual variation in hydrological loading is the main factor causing the seasonal oscillation of surface deformation in Southwest China. Seasonal deformations captured by GPS, GRACE, and SLM are consistent to a certain extent, and the correlation coefficients between GPS/GRACE, and GPS/SLM are 0.82 and 0.81, respectively. After deducting the results yielded by GRACE and SLM from the GPS time series, the average reductions in root mean square were 41.3% and 38.0%, respectively. However, some systematic differences were observed among the annual amplitudes and phases of the seasonal deformations among the three products. For example, the average amplitudes estimated by GRACE and SLM were 7.4 mm and 6.1 mm, respectively, which were smaller than the amplitude deduced from GPS (9.7 mm). Furthermore, mean phase delays of 16, 22, and 6 days were observed between GPS/GRACE, GPS/SLM, and GRACE/SLM. The data processing errors and local geophysical signals in GPS and the underestimation of land water storage in GRACE and SLM were jointly responsible for the systemic differences. The simulated data show that the misestimating of hydrological loading can explain approximately 50%, 64%, and 83% of the phase delays between GPS/GRACE, GPS/SLM, and GRACE/SLM, respectively. In addition, we obtained long-term vertical crustal motion rates by subtracting the loading deformation rates estimated by GRACE from the linear rates of the GPS. The vertical crustal motion in this region is block-dependent. The Central Yunnan block and its eastern boundary are uplifted; meanwhile, the Southwest Yunnan block, which features stretching in the horizontal direction, appears to be subsiding. The aforementioned results can provide data support for the study of water resource utilization and geodynamics in Southwest China.

Glacial Isostatic Adjustment Modelling of the Coast Mountains of British Columbia and Southeastern Alaska

<p>The Coast Mountains in British Columbia and southeastern Alaska contain around 9040 km<sup>2 </sup>of glaciers and ice fields at present. While these glaciers have followed an overall trend of mass loss since the Little Ice Age (or LIA around 300 years before present), the past decade has seen a significant increase in melting rate that is likely to continue due to the effects of climate change. The region is home to a complex tectonic setting, having proximity to the Queen Charlotte-Fairweather transform plate boundary in the northern region and the Cascadia subduction zone (CSZ) in the southern region, which has an associated active volcanic arc underlying the glaciated area. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) glacier melt data collected between 2000 and 2019 represent a melt rate that is averaged between periods of relatively low mass loss (2000-2009) and high mass loss (2010-2019). As a preliminary test, this average melt rate was assumed to be constant back to the LIA. A history of gridded ice thicknesses was calculated to create an ice loading model for input to a series of forward modelling calculations to determine the crustal response. Predictions of vertical crustal motion are compared to available Global Navigation Satellite System (GNSS) measurements of uplift rate to constrain Earth rheology. The results using this simplified loading model favour a thin lithosphere (around 20-40 km thick) and asthenospheric viscosities on the order of 10<sup>19</sup> Pa s. These values are significantly lower than those of rheological profiles used in extant global GIA models, but are in general agreement with previous GIA modelling of the forearc region of the CSZ. To improve the glacial history model, the Open Global Glacier Model (OGGM), driven by historic climate data and statistically downscaled climate projections, is being employed to create a more accurate loading model and refine our estimates of Earth rheology and regional crustal motion. The best-fitting models will be employed to separate GIA and tectonic components of crustal motion and to generate improved regional sea-level projections.</p>

North American Crustal Motion and Glacial Isostatic Adjustment Model Predictions

<p>A suite of forward GIA model predictions, spanning a wide range of layered mantle viscosity and lithospheric thickness values, is compared to observed horizontal crustal motions in North America to discern optimal model parameters in order to minimize a root-mean-square (RMS) measure of the velocity residuals. To obtain the Earth model response, a combination of the full normal mode analysis and the collocation method is implemented. It provides a means to determine the surface loading response automatically and robustly to 1-dimensional (radially varying) Earth models, while retaining as much of the physics of the normal mode method as numerically feasible, given documented issues with singularities along the negative inverse-time axis in the Laplace transform domain. This method enables the exploration across a wide parameter range (for the lower mantle, transition zone, asthenosphere, and thickness of the elastic lithosphere) to find optimal combinations to explain horizontal crustal motion in North America. The analysis utilizes crustal motion rates from approximately 300 GNSS sites in central North America (Canada and United States) provided by the Nevada Geodetic Laboratory.  Preliminary results indicate that horizontal crustal motion predictions generated with a thin lithosphere, 40 – 60 km, produce horizontal motions that are strongly discrepant with the observations and have velocity residuals larger than the null model (modelled horizontal motion set to zero). As the lithospheric thickness increases, from 80 km to 240 km, the horizontal motion residuals gradually decrease with no minimum apparent for the thicknesses thus far considered. The residual velocities for the best-fitting models appear to carry a remaining signal, confirming previous inferences of limitations to spherically symmetric Earth models in modeling horizontal crustal motions in North America.</p>

GIA effects of Holocene rapid ice thinning on the observed geodetic signals along the coast of Lützow-Holm Bay in East Antarctica

<p>Geodetic and geomorphological observations in the Antarctic coastal area generally indicate the uplift trend associated with the Antarctic Ice Sheet (AIS) change since the Last Glacial Maximum (LGM). The melting models of AIS derived from the comparisons between sea-level and geodetic observations and glacial isostatic adjustment (GIA) modeling show the monotonous retreat through the Holocene era (e.g., Whitehouse et al., 2012, <em>QSR</em>; Stuhne and Peltier, 2015, <em>JGR</em>). However, the observed crustal motion by GNSS in some regions of Antarctica cannot be explained as the deformation rates by only glacial rebound due to the last deglaciation of AIS (e.g., Bradley et al., 2015, <em>EPSL</em>). One reason for this mismatch is considered as the control of the uplift induced by the re-advance of AIS following a post-LGM maximum retreat, which was recently reported as the West AIS re-advance in the Ross and the Weddell Sea sectors (e.g., Kingslake et al., 2018, <em>Nature</em>).</p><p>On the other hand, the current crustal motion includes the elastic GIA component due to the present-day surface mass balance of AIS. To reveal the secular crustal movement induced by GIA, the separation of the elastic deformation induced by the current mass balance using GRACE data is essential. In the Lützow-Holm Bay, East Antarctica, GNSS observations have been carried out at several sites on the outcrop rocks since the 1990s to monitor recent crustal movements. Hattori et al. (2019, <em>SCAR</em>) precisely analyzed the GNSS data obtained from this area, which revealed the secular crustal movement by correcting the elastic deformation due to current mass balance. The results indicated the mismatch between secular current crustal motion and GIA calculations based on the previously published ice and viscosity models. Consequently, to represent the observed crustal deformation rates based on the GIA modeling, we must carefully investigate the numerical dependencies of various parameters such as local and global ice history in the AIS.</p><p>Recently, the study of glacial geomorphology and surface exposure dating (Kawamata et al., 2020, <em>QSR</em>) has suggested that the abrupt ice thinning and retreat occurred in Skarvsnes, located at the middle of the Lützow-Holm Bay, during 9 to 6 ka. We obtained the preliminary results related to the GIA effects induced by the abrupt thinning on the geodetic observations in this area. The numerical simulations that we examined are employed for a simple ice model with the thickness change by 400 m during 9 to 6 ka in this area based on the IJ05_R2 model grids (Ivins et al., 2013, <em>JGR</em>). The predictions based on the high-viscosity upper mantle (5x10<sup>20</sup> Pa s) show high uplift rates (~ +4.0 mm/yr), whereas the calculated uplift rates for the weaker viscosity (2x10<sup>20</sup> Pa s) show low value (~ +1.0 mm/yr). These results suggest that the viscoelastic relaxation due to the abrupt ice thinning in the mid-to-late Holocene may influence the current crustal motion and highly depend on the upper mantle viscosity profile. We will discuss the influences on the GIA-calculated crustal movement by AIS retreat history and mantle viscosity structure.</p>

Southern California’s Crustal Motion Tells of Earthquake Hazards

Precise measurements of the Earth’s vertical surface motion help to elucidate the hazards of faults in an earthquake-prone region.

Contemporary movement of the Earth's crust in the Northwestern Vietnam by continuous GPS data

The paper presents an estimation of the Earth’s crustal motion from the continuous GPS data at 6 stations (MTEV, MLAY, DBIV, TGIV, SMAV and SLAV) in the Northwestern and at PHUT (Hanoi) station using GAMIT/GLOBK software. The absolute displacements of the Earth’s crust at 7 stations in the IGS14 frame are respectively: 34.10±0.71 mm/yr (DBIV), 34.31±0.65 mm/yr (PHUT), 34.51±0.75 mm/yr (SMAV), 34.55±0.80 mm/yr (MLAY), 34.80±0.72 mm/yr (TGIV), 34.93±0.99 mm/yr (SLAV) and 35.59±0.73 mm/yr (MTEV), in the southeastward with the azimuth range 104-108o. The Son La fault is a right-lateral slip fault with a shear amplitude of ~1.5 mm/yr. The Lai Chau-Dien Bien fault is a left-lateral slip fault with a shear amplitude of ~1.9 mm/yr. Although the absolute velocities at the DBIV, SMAV, SLAV, TGIV and MLAY stations are evaluated with the error <1 mm/yr, the relative displacement on the Ma River fault is of ~0.5 mm/yr, and it seems that we still do not have a reliable assessment of the slip rate on the Ma River right-lateral slip fault.

GPS Constraints on the Active Deformation in Tunisia: Implications on the Geodynamics of the Western Mediterranean

&lt;p&gt;The plate boundary in the western Mediterranean includes the Tunisian Atlas Mountains. We study the active deformation of this area using GPS data collected from 2014 to 2018. WNW to NNW trending velocities express the crustal motion and geodetic strain field from the Sahara platform to the Tell Atlas, consistent with the African plate convergence. To the south, the velocities indicate a nearly WNW-ESE trending right-lateral motion of the Sahara fault-related fold belt with respect to the Sahara Platform. Further north and northeast, the significant decrease in velocities between the Eastern Platform and Central &amp;#8211; Tell Atlas marks the NNW trending shortening deformation associated with local ENE &amp;#8211; WSW extension visible in the Quaternary grabens. The velocity field and strain distribution associated with the active E-W trending right-lateral faulting and NE-SW fault-related folds sustain the existence of three main tectonic blocks and related transpression tectonics. The velocity field and pattern of active deformation in Tunisia document the oblique plate convergence of Africa towards Eurasia.&amp;#160;&lt;/p&gt;