ALBERT

Description: The constant increase of geodetic instrumentation over the past decades enables us to not only detect ever smaller tectonic signals but also to monitor their evolution in time and space. We present spatial and temporal slip variations observed on a fault affected by a large, intermediate-field earthquake: the 2015 Mw7.2 Sarez, Central Pamir, earthquake ruptured the sinistral, NE-trending Sarez-Karakul fault system. 120–170 km North of the main rupture, the thin-skinned, E-trending Pamir thrust system bounding the Pamir to the North was co-seismically activated. We derived co-seismic offsets and post-seismic rates observed by two dense, high-rate Global Positioning System (GPS) profiles crossing the Pamir thrust system at different longitudes. The continuous GPS observations of the western profile focus on the dextral, NW-striking Aramkungey fault segment that connects two thrust faults with opposite dip. We compare inter-, co- and post-seismic displacement rates by complementing the continuous data with survey-mode GPS data and East rates derived from satellite radar interferometric displacement time-series. All the GPS stations were shifted toward the epicenter against the direction of the interseismic load with an increased gradient in the Aramkungey fault segment. During the postseismic stage, the fault-parallel and fault-perpendicular rates were affected differently, suggesting gradual re-locking of the Aramkungey fault after its unlocking by right-lateral co-seismic slip.

Description: Long-term tide gauge records provide valuable insights to sea level variations, but interpretation requires an accurate determination of the associated vertical land motion. Within the Tide Gauge Benchmark Monitoring Working Group of the International GNSS Service, we performed a dedicated reprocessing (1994-2020) for GNSS stations co-located with tide gauges. Based on 341 stations the GFZ contribution to the third TIGA reprocessing provides vertical land motion rates for 230 stations at or close to recently active tide gauges. We limited the processing to GPS observations.

Description: As glaciers react to rising temperatures, the risk of glacial lake outburst floods (GLOFs) endangers downstream lives and infrastructures. To assess how C-band synthetic aperture radar data can be used to determine the timeframe of interest (TOI: includes a GLOF event), an intra-annual time-series analysis based on Sentinel-1 (S1) data was conducted covering six-month seasons (April – September) from 2017 – 2022 at the debris-covered Inylchek Glacier/Lake Merzbacher, Kyrgyzstan. In total, 90 Level-1 Ground Range Detected (High-Resolution, Dual-Pol) scenes were processed, and amplitude offset-tracking was executed. This way, 102 velocity maps (temporal baseline: 24 and 12 days) were generated. The spatio-temporal time-series analysis was performed in Geographic Information Systems with the maximum horizontal surface velocity (MSV) as the critical parameter. It is found that S1 data is suitable for detecting and quantifying horizontal glacier surface movement. The statistical analysis indicates that the MSV increased at the Inylchek Glacier during the observation period, e.g., median per season: Δ〈sub〉max.〈/sub〉 = 0.278 md〈sup〉-1〈/sup〉. For all seasons, TOIs were calculated. Optical imagery captured by an in-situ meteorological station was used for verification, indicating that 66.7 % of the TOIs are accurate. The GLOFs at Lake Merzbacher documented in this study continue the inter-annual trend of occurring earlier during ablation seasons. The MSV shows high potential to serve as a parameter for isolating GLOFs. Further studies on the glacier dynamics are recommended by using complementing parameters and multi-source remote sensing data to enhance precision and accuracy. Remote sensing techniques pose great potential for monitoring and assessing GLOF risk.

Description: The Laurentide ice sheet was the largest late Pleistocene ice mass and the largest contributor to Holocene pre-industrial sea-level rise. While glaciological dates suggest final ice sheet melting between 8 and 6 ka, inversion of sea-level data indicates deglaciation at ca. 7 ka. Here, we present new chronostratigraphic constraints on Laurentide ice sheet disappearance based on Holocene relative sea-level observations from the tectonically stable north coast of Java, Indonesia. Age-elevation data from the flat upper surfaces of 13 fossil intertidal corals (i.e., microatolls) indicate that the Java Sea experienced a relative sea level of 1.3 ± 0.7 m above present between 6.9 and 5.3 ka. To determine uncaptured relative sea-level trends within the observational uncertainties of this apparently constant highstand, we analyzed the internal structure of three sliced microatolls from the same site to produce a high-resolution data set. These data were used to statistically model relative sea-level rates and trends. Employing the data with the model provided evidence for a short-lived rise of relative sea level from 1.0 ± 0.3 m above present at 6.7 ± 0.1 ka to 1.9 ± 0.3 m above present at 6.4 ± 0.1 ka. The end of this rise likely represents the last input of meltwater from the vast Laurentide ice sheet, which, consequently, collapsed at least 400 yr later than assumed by some widely used models of glacial isostatic adjustment. Incorporating these new results into such predictive models will help to better understand the geographical variability of future sea-level rise as a result of global warming.

Description: The 2015 M7.2 Sarez (Pamir) earthquake occurred at the north-west margin of the Tibetan Plateau. We use Sentinel-1 and ALOS-2 Synthetic Aperture Radar and Global Navigation Satellite System data to investigate coseismic and postseismic deformation due to the Sarez earthquake. Kinematic inversions show that the earthquake ruptured a ∼80 km long, sub-vertical fault producing the maximum surface offset of 3–4 m on the south-west and central fault segments. In contrast, the largest postseismic displacements are observed at the north-east end of the earthquake rupture, predominantly on the west (hanging wall) side of the fault with an average rate of 20–30 mm/yr in the satellite line of sight. We use the derived coseismic and postseismic slip models to investigate mechanisms of time-dependent relaxation, stress transfer and possible triggering relationships between the Sarez earthquake and a sequence of strong M6+ events that occurred within ∼100 km of the 2015 earthquake. We find that the near-field postseismic displacements are best explained by shallow afterslip driven by the coseismic stress changes. The data also allow some contribution from poroelastic rebound, but do not show a clear signature of viscoelastic relaxation in the lower crust and upper mantle during the observation period, suggesting a lower bound on the effective viscosity of ∼1019 Pa s. A pair of M6+ events that occurred within 100 km and several months of the 2015 mainshock have experienced near-zero and in some cases negative static Coulomb stress changes, suggesting either delayed dynamic triggering, or no relation to the mainshock.

Description: Satellite altimetry has continuously monitored global and regional sea level since the early 1990s. These measurement are essential for the monitoring of climate change and as well for; the validation of climate models. Over the past 30 years, there have been constant improvements in measurement and data processing techniques. In particular, recent missions have deployed Synthetic Aperture Radar (SAR) altimeters, which offer higher accuracy and spatial resolution compared to conventional altimeters. However, the new instruments could potentially introduce inconsistencies in the sea level record. The Sentinel-6 Michael Freilich mission, launched at the end of 2020, has become the new reference mission for all active altimeter missions. To ensure the continuity of the 30-year altimetric sea level record, Sentinel-6MF performs both conventional and SAR measurements in parallel. We conducted an assessment of the accuracy and precision of sea level measurements derived from conventional and SAR altimeter measurements in coastal waters using five GNSS-controlled tide gauges situated in the German Bight (southeastern North Sea). Our analysis examines the differences and drifts between the two types of measurements. It includes a comparison with corresponding findings obtained from the Jason-1/2/3 missions and a discussion of potential implications of the technology switch for the long-term sea level record.

Description: Long-term tide gauge records provide valuable insights to sea level variations but interpretation requires an accurate determination of associated vertical land motion. Within the Tide Gauge Benchmark Monitoring Pilot Project of the International GNSS Service dedicated reprocessing campaigns are performed for GNSS stations co-located with tide gauges. Based on 341 stations the GFZ contribution to the third TIGA reprocessing provides vertical land motion rates for 230 stations at or close to recently active tide gauges. GNSS station coordinate time series determined by using a network approach and a conventional time series analysis show mean repeatabilities of 2.9, 3.3, and 5.6 mm for north, east, and up coordinates. The derived vertical velocity pattern is analyzed but also compared to the ALTIGAPS and the ULR6a solutions showing mean differences of 0.04 mm yr−1 and −0.1 mm yr−1, respectively. By correcting tide gauge records available via PSMSL for the individual vertical station velocity including eventually velocity changes geocentric sea level changes are determined. Compared to AVISO’s multi-mission altimetric trend map a difference of −0.7 mm yr−1 is determined.

Description: Lake Issyk Kul in Kyrgyzstan is a large high-altitude inland lake but ice-free the year around. Due to its E-W extension of ~170km all active radar altimetry missions cross this lake. Since 2016 CAIAG (Kyrgyzstan) and GFZ Potsdam (Germany) installed five GNSS-controlled tide gauges and climate monitoring stations at the lake. The lake surface is neither influenced by ocean tides nor by inverse barometric effects. Tide Gauge data is sampled every minute and climate data, such as wind or air pressure, every five minutes. Starting in 2017 we performed ship-based lake surveys with GNSS/radar for profiling the lake surface along the satellite passages. With the connection to the gauges, we are able to reconstruct the instantaneous lake profile in ITRF for any altimeter passage. We analyzed the 20Hz GDR Level 2 data of Jason-3 and Sentinel-6 (STC/NTC both LRM/SAR). 2022 we performed surveys along the 35-day (ERS-1, ERS-2, ENVISAT, AltiKa) and GFO-1 orbits. Along each pass stretching up to ~60km we testing all retracker available. We estimated the individual retracker offsets and the internal accuracy. For, e.g., Sentinel-6 the SAR-OCOG retracker performs best (revision F02 onward) after applying an offset of more than 33cm in respect to the GNSS reference pass. For Jason-3 (Rev. F) the OCOG also performs best with an offset of ~19cm. Our analyses show the capability of the Lake Issyk Kul observatory for the short-latency monitoring of radar altimetry missions as well as for the performance control of all altimeters starting with ERS-1 in 1992.