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Land-surface subsidence is a major contributor to land loss in many river deltas. New approaches yielding high-resolution data are needed to parse the relevant driving forces. In 2016, we established a novel “subsidence superstation” ∼2 km from the Mississippi River in coastal Louisiana (USA) to measure compaction in a global reference frame as a function of depth in Holocene sediments and deeper subsidence. The site features three borehole optical fiber strainmeters to obtain continuous records of displacement between ∼1.3 m below the surface and depths of ∼11, 25, and 38 m. These data are complemented by an adjacent station providing hydrologic data and near-surface compaction. We also installed three GPS antennas, one of which is mounted to a rod cemented into the Pleistocene basement. A core from one of the boreholes provides insight into the sediment properties of the entire Holocene succession. Five years of records reveal the compaction rate in the material between 1.3 and 38 m is likely less than 0.25 mm/yr. The GPS records yield a subsidence rate of 2.5 mm/yr regardless of the depth of the anchor, thus corroborating the low compaction rates observed by the strainmeters. The new instrumental records show that current subsidence at this location is governed mostly by deformation of the Pleistocene or underlying strata rather than compaction of Holocene material, with the exception of the uppermost meter. The methodology represents an important new approach to mapping subsidence rates at varying depths, providing insight into the mechanisms governing delta subsidence.

Conventional tide gauges are usually housed along the coast. Satellite altimetry works well in the open ocean but poorly near the coast due to issues such as signal contamination by land returns. T...

Seasonal variability in the Greenland Ice Sheet's (GrIS) sliding speed is regulated by the response of the subglacial drainage system to meltwater inputs. However, the importance of channelization ...

Annual observations of first-year ice (FYI) and second-year ice (SYI) near Zhongshan Station, East Antarctica, were conducted for the first time from December 2011 to December 2012. Melt ponds appeared from early December 2011. Landfast ice partly broke in late January, 2012 after a strong cyclone. Open water was refrozen to form new ice cover in mid-February, and then FYI and SYI co-existed in March with a growth rate of 0.8 cm/d for FYI and a melting rate of 2.7 cm/d for SYI. This difference was due to the oceanic heat flux and the thickness of ice, with weaker heat flux through thicker ice. From May onward, FYI and SYI showed a similar growth by 0.5 cm/d. Their maximum thickness reached 160.5 cm and 167.0 cm, respectively, in late October. Drillings showed variations of FYI thickness to be generally less than 1.0 cm, but variations were up to 33.0 cm for SYI in March, suggesting that the SYI bottom was particularly uneven. Snow distribution was strongly affected by wind and surface roughness, leading to large thickness differences in the different sites. Snow and ice thickness in Nella Fjord had a similar “east thicker, west thinner” spatial distribution. Easterly prevailing wind and local topography led to this snow pattern. Superimposed ice induced by snow cover melting in summer thickened multi-year ice, causing it to be thicker than the snow-free SYI. The estimated monthly oceanic heat flux was ∼30.0 W/m2 in March-May, reducing to ∼10.0 W/m2 during July–October, and increasing to ∼15.0 W/m2 in November. The seasonal change and mean value of 15.6 W/m2 was similar to the findings of previous research. The results can be used to further our understanding of landfast ice for climate change study and Chinese Antarctic Expedition services.

Several autonomous phase-sensitive radio-echo sounders (ApRES) were deployed at Greenland glaciers to investigate ice deformation. Different attenuation settings were tested and it was observed that, in the presence of clipping of the deramped ApRES signal, each setting produced a different result. Specifically, higher levels of clipping associated with lower attenuation produced an apparent linear increase of diurnal vertical cumulative displacement with depth, and obscured the visibility of the basal reflector in the return amplitude. An example with a synthetic deramped signal confirmed that these types of artifacts result from the introduction of harmonics from square-wave-like features introduced by clipping. Apparent linear increase of vertical displacement with depth occurs when the vertical position of a near-surface internal reflector changes in time. Artifacts in the return amplitude may obscure returns from internal reflectors and the basal reflector, making it difficult to detect thickness evolution of the ice and to correctly estimate vertical velocities. Variations in surface melt during ApRES deployments can substantially modulate the received signal strength on short timescales, and we therefore recommend using higher attenuator settings for deployments in such locations.

The Eastern California shear zone in the Mojave Desert, California, accommodates nearly a quarter of Pacific–North America plate motion. In south-central Mojave, the shear zone consists of six active faults, with the central Calico fault having the fastest slip rate. However, faults to the east of the Calico fault have larger total offsets. We explain this pattern of slip rate and total offset with a model involving a crustal block (the Mojave Block) that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the Coachella segment of the San Andreas fault (SAF), southwest of the transpressive “big bend” in the SAF. Both the shear zone and the Garlock fault are assumed to be a direct result of this restraining bend, and consequent strain redistribution. The model explains several aspects of local and regional tectonics, may apply to other transpressive continental plate boundary zones, and may improve seismic hazard estimates in these zones.

Ice velocity variations near the terminus of Jakobshavn Isbræ, Greenland, were observed with a terrestrial radar interferometer (TRI) during three summer campaigns in 2012, 2015, and 2016. We estimate a ∼ 1 km wide floating zone near the calving front in early summer of 2015 and 2016, where ice moves in phase with ocean tides. Digital elevation models (DEMs) generated by the TRI show that the glacier front here was much thinner (within 1 km of the glacier front, average ice surface is ∼ 100 and ∼ 110 m above local sea level in 2015 and 2016, respectively) than ice upstream (average ice surface is > 150 m above local sea level at 2–3 km to the glacier front in 2015 and 2016). However, in late summer 2012, there is no evidence of a floating ice tongue in the TRI observations. Average ice surface elevation near the glacier front was also higher, ∼ 125 m above local sea level within 1 km of the glacier front. We hypothesize that during Jakobshavn Isbræ's recent calving seasons the ice front advances ∼ 3 km from winter to spring, forming a > 1 km long floating ice tongue. During the subsequent calving season in mid- and late summer, the glacier retreats by losing its floating portion through a sequence of calving events. By late summer, the entire glacier is likely grounded. In addition to ice velocity variation driven by tides, we also observed a velocity variation in the mélange and floating ice front that is non-parallel to long-term ice flow motion. This cross-flow-line signal is in phase with the first time derivative of tidal height and is likely associated with tidal currents or bed topography.

Due to limit of coverage in TOPEX/Poseidon (T/P) satellite and sparseness of in-situ tide gauges around Antarctica, the accuracy of global ocean tide models in Antarctic seas is relatively poorer than in low- and mid-latitude regions. To better understand ocean tides in Prydz Bay, east Antarctica, a GPS receiver was deployed on floating sea ice to measure tide-induced ice motion in multiple campaigns. Four online Precise Point Positioning (PPP) services are used to process the GPS data in the kinematic PPP mode, and UTide software is used to separate the major tidal constituents. Comparison between results from different processing methods (relative processing solutions from Track, kinematic PPP solutions from online services) and with bottom pressure gauge (BPG) shows that, high-accuracy tidal information can be obtained from GPS observations on floating sea ice, the root-sum-square (RSS) for the eight major constituents (O1, K1, P1, Q1, M2, S2, N2, K2) is below 4 cm. We have also studied the imp...

Iceberg calving is a major contributor to Greenland’s ice mass loss. Pro-glacial mélange (a mixture of sea ice, icebergs, and snow) may be tightly packed in the long, narrow fjords that front many marine-terminating glaciers and can reduce calving by buttressing. However, data limitations have hampered a quantitative understanding. We develop a new radar-based approach to estimate time-varying elevations near the mélange-glacier interface, generating a factor of three or more improvement in elevation precision. We apply the technique to Jakobshavn Isbræ, Greenland’s major outlet glacier. Over a one-month period in early summer 2016, the glacier experienced essentially no calving, and was buttressed by an unusually thick mélange wedge that increased in thickness towards the glacier front. The extent and thickness of the wedge gradually decreased, with large-scale calving starting once the mélange mass within 7 km of the glacier front had decreased by >40%., Observation systems are not sufficient to determine the relationship between mélange strength and calving frequency. Here the authors used the derivation of digital elevation models from radar interferometry data to study Jakobshavn Isbræ and show an inverse correlation between mélange thickness and calving rate.

We document remarkably efficient thermo-mechanical erosion by a small-volume lava flow. Downcutting by a basaltic-andesite lava flow on the steep-sided Momotombo volcano, Nicaragua, occurred at 100 times the rate commonly reported for thermal erosion in lava flow fields, even though this flow was small-volume (0.02 km3) and effused at a low rate for

Ice velocity variations near the terminus of Jakobshavn Isbræ, Greenland were observed with a terrestrial radar interferometer (TRI) during three summer campaigns in 2012, 2015, and 2016. Ice velocity variations appear to be largely modulated by ocean tides. We estimate a ∼ 1 km wide floating zone near the calving front in early summer of 2015 and 2016, where ice moves in phase with ocean tides. Digital Elevation Models (DEMs) generated by the TRI show that the glacier front here is thin (ice surface is 140 m above local sea level within a very short distance (

Accurate estimation of fault slip rate is fundamental to seismic hazard assessment. Previous work suggested a discrepancy between short-term geodetic and long-term geologic slip rates in the Mojave Desert section of the Eastern California Shear Zone (ECSZ). Understanding the origin of this discrepancy can improve understanding of earthquake hazard and fault evolution. We measured offsets in alluvial fans along the Calico Fault near Newberry Springs, California, and used several techniques to date the offset landforms and determine a slip rate. Our preferred slip rate estimate is 3.2 ± 0.4 mm/yr, representing an average over the last few hundred thousand years, faster than previous estimates. Seismic hazard associated with this fault may therefore be higher than previously assumed. We discuss possible biases in the various slip rate estimates and discuss possible reasons for the rate discrepancy. We suggest that the ECSZ discrepancy is an artefact of limited data, and represents a combination of faster slip on the Calico Fault, off-fault deformation, unmapped fault strands, and uncertainties in the geologic rates that have been underestimated. Assuming our new rate estimate is correct and a fair amount (40%) of off-fault deformation occurs on major ECSZ faults, the summed geologic rate estimate across the Mojave section of the ECSZ is 10.5 ± 3.1 mm/yr, which is equivalent within uncertainties to the geodetic rate estimate.

Volcanoes with rugged terrain remain a challenging target for generating high-resolution digital elevation models (DEMs), especially in tropical areas with frequent cloud cover. Using Nevado del Ruiz volcano as an example, we combined DEMs from the TanDEM-X (TDX) satellite mission, terrestrial radar interferometry (TRI), and Structure from Motion (SfM), to generate a new DEM with 10-m spatial resolution. This is the first study combining satellite radar, ground-based radar, photography, and freely available global DEMs to generate a high-resolution DEM without data gaps. TDX data from ascending and descending orbits were combined to generate the base DEM. Instead of using a raster format to fuse DEMs generated from different data sets with different resolutions, we developed a methodology based on 3-D point clouds: 1) re-georeference the 5-m TRI and ~1-m SfM DEMs to the 10-m TDX DEM using the iterative closest point (ICP) algorithm to minimize the horizontal and vertical discrepancy between DEMs; then 2) merge the multiple point clouds to generate a final DEM without data gaps using an adaptive algorithm that uses two search distances to smooth the transition at the edges of different data sets. We assess the new 10-m DEM by comparing simulated inundation zones obtained with two volcano flow models, LaharZ (for lahars) and VolcFlow (for pyroclastic flows), and find significant differences with respect to the 30-m SRTM DEM. Our LaharZ simulation over the new DEM shows a longer lahar run-out distance. For pyroclastic flows, the VolcFlow simulation over the new DEM produces highly channelized flows over the steep portions of a river channel and gives a larger extent of thicker deposits compared to those obtained with the 30-m SRTM DEM. Quantitative and qualitative geomorphic analysis suggests that up-to-date DEMs with high spatial resolution (~ 10 m or even better) need to be generated to improve volcano hazard assessment for active volcanoes.