David Bekaert

A spatially variable power law tropospheric correction technique for InSAR data

Microwave signals traveling through the troposphere are subject to delays. These delays are mainly described by spatial and temporal variations in pressure, temperature, and relative humidity in the lower part of the troposphere, resulting in a spatially varying tropospheric signal in interferometric synthetic aperture radar (InSAR). Tropospheric correction techniques rely either on external data, often limited by spatial and temporal accuracy or can be estimated fromthe high-resolution interferometric phase itself. However, current phase-estimated correction techniques do not account for the spatial variability of the tropospheric properties and fail to capture tropospheric signals over larger regions. Here we propose and test a novel power law correction method that accounts for spatial variability in atmospheric properties and can be applied to interferograms containing topographically correlated deformation. The power law model has its reference fixed at the relative top of the troposphere and describes, through a power law relationship, how the phase delay varies with altitude. We find the power law model reduces tropospheric signals both locally (on average by ?0.45 cm for each kilometer of elevation in Mexico) and the long-wavelength components, leading to an improved fit to independent Global Navigation Satellite Systems data. The power law model can be applied in presence of deformation, over a range of different time periods and in different atmospheric conditions, and thus permits the detection of smaller-magnitude crustal deformation signals with InSAR.

Reassessing the 2006 Guerrero slow‐slip event, Mexico: Implications for large earthquakes in the Guerrero Gap

In Guerrero, Mexico, slow-slip events have been observed in a seismic gap, where no earthquakes have occurred since 1911. A rupture of the entire gap today could result in a Mw 8.2–8.4 earthquake. However, it remains unclear how slow-slip events change the stress field in the Guerrero seismic region and what their implications are for devastating earthquakes. Most earlier studies have relied on a sparse network of Global Navigation Satellite Systems measurements. Here we show that interferometric synthetic aperture radar can be used to improve the spatial resolution. We find that slip due to the 2006 slow-slip event enters the seismogenic zone and the Guerrero Gap, with ?5 cm slip reaching depths as shallow as 12 km. We show that slow slip is correlated with a highly coupled region and estimate that slow-slip events have decreased the total accumulated moment since the end of the 2001/2002 slow-slip event (4.7 years) by ?50%. Nevertheless, even accounting for slow slip, the moment deficit in the Guerrero Gap increases each year by Mw?6.8. The Guerrero Gap therefore still has the potential for a large earthquake, with a slip deficit equivalent to Mw?8.15 accumulated over the last century. Correlation between the slow-slip region and nonvolcanic tremor, and between slow slip and an ultraslow velocity layer, supports the hypothesis of a common source potentially related to high pore pressures.

Coseismic deformation and triggered landslides of the 2016 Mw 6.2 Amatrice earthquake in Italy

The Central Apennines in Italy have had multiple moderate-size but damaging shallow earthquakes. In this study, we optimize the fault geometry and invert for fault slip based on coseismic GPS and interferometric synthetic aperture radar (InSAR) for the 2016 Mw 6.2 Amatrice earthquake in Italy. Our results show that nearly all the fault slip occurred between 3 and 6 km depth but extends 20 km along strike. There was less than 4 cm static surface displacement at the town Amatrice where the most devastating damage occurred. Landslides triggered by earthquake ground shaking are not uncommon, but triggered landslides with submeter movement are challenging to be observed in the field. We find evidence of coseismically triggered deep-seated landslides northwest and northeast of the epicenter where coseismic peak ground acceleration was estimated >0.5 g. By combining ascending and descending InSAR data, we are able to estimate the landslide thickness as at least 100 and 80 m near Monte Vettore and west of Castelluccio, respectively. The landslide near Monte Vettore terminates on the preexisting fault Monte Vettore Fault (MVEF) scarp. Our results imply that the long-term fault slip rate of MVEF estimated based on paleoseismic studies could potentially have errors due to triggered landslides from nearby earthquake events.

Anthropogenic and geologic influences on subsidence in the vicinity of New Orleans, Louisiana

New measurements of ongoing subsidence of land proximal to the city of New Orleans, Louisiana, and including areas around the communities of Norco and Lutcher upriver along the Mississippi are reported. The rates of vertical motion are derived from interferometric synthetic aperture radar (InSAR) applied to Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) data acquired on 16 June 2009 and 2 July 2012. The subsidence trends are similar to those reported for 2002–2004 in parts of New Orleans where observations overlap, in particular in Michoud, the 9th Ward, and Chalmette, but are measured at much higher spatial resolution (6 m). The spatial associations of cumulative surface movements suggest that the most likely drivers of subsidence are groundwater withdrawal and surficial drainage/dewatering activities. High subsidence rates are observed localized around some major industrial facilities and can affect nearby flood control infrastructure. Substantial subsidence is observed to occur rapidly from shallow compaction in highly localized areas, which is why it could be missed in subsidence surveys relying on point measurements at limited locations.

Geodetic observations of postseismic creep in the decade after the 1999 Izmit earthquake, Turkey: Implications for a shallow slip deficit

The relationship between aseismic slip and tectonic loading is important for understanding both the pattern of strain accumulation along a fault and its ability to generate large earthquakes. We investigate the spatial distribution and temporal evolution of aseismic creep on the western North Anatolian Fault (NAF) using time series analysis of Envisat interferometric synthetic aperture radar (InSAR) data, covering the full extent of the 1999 Izmit and Duzce earthquake ruptures and spanning 2002–2010. Discontinuities in the line-of-sight velocity across the fault imply that fault creep reaches the Earths surface at an average fault-parallel rate of ∼5 mm/yr along an ∼80 km section of the NAF. By combining InSAR and published GPS velocities, we are able to extract the vertical and east-west components of motion and show that the Adapazari basin is subsiding at a rate of ∼6 mm/yr. Vertical motions have biased previous estimates of creep in this region. The displacement time series close to the fault is consistent with an afterslip model based on rate-and-state friction, which predicts a rapid deceleration in fault creep rate after the Izmit earthquake to a near-steady state ∼5 mm/yr after 5 years. Projecting our model 200 years into the future we find that the cumulative displacement of 1–1.3 m is insufficient to account for the shallow coseismic slip deficit observed in previous studies. Distributed off-fault deformation in the shallow crust or transient episodes of faster slip are likely required to release some of the long-term strain during the earthquake cycle.

Interseismic strain accumulation across the central North Anatolian Fault from iteratively unwrapped InSAR measurements

The North Anatolian Fault (NAF) is a major tectonic feature in the Middle East and is the most active fault in Turkey. The central portion of the NAF is a region of Global Navigation Satellite Systems (GNSS) scarcity. Previous studies of interseismic deformation have focused on the aseismic creep near the town of Ismetpasa using radar data acquired in a single line-of-sight direction, requiring several modeling assumptions. We have measured interseismic deformation across the NAF using both ascending and descending data from the Envisat satellite mission acquired between 2003 and 2010. Rather than rejecting incorrectly unwrapped areas in the interferograms, we develop a new iterative unwrapping procedure for small baseline interferometric synthetic aperture radar (InSAR) processing that expands the spatial coverage. Our method corrects unwrapping errors iteratively and increases the robustness of the unwrapping procedure. We remove long wavelength trends from the InSAR data using GNSS observations and deconvolve the InSAR velocities into fault-parallel motion. Profiles of fault-parallel velocity reveal a systematic eastward decrease in fault slip rate from 30 mm/yr (25–34, 95% confidence interval (CI)) to 21 mm/yr (14–27, 95% CI) over a distance of ∼200 km. Direct offset measurements across the fault reveal fault creep along a ∼130 km section of the central NAF, with an average creep rate of 8 ± 2 mm/yr and a maximum creep rate of 14 ± 2 mm/yr located ∼30 km east of Ismetpasa. As fault creep is releasing only 30–40% of the long-term strain in the shallow crust, the fault is still capable of producing large, damaging earthquakes in this region.

Temporal changes in rock uplift rates of folds in the foreland of the Tian Shan and the Pamir from geodetic and geologic data

Understanding the evolution of continental deformation zones relies on quantifying spatial and temporal changes in deformation rates of tectonic structures. Along the eastern boundary of the Pamir‐Tian Shan collision zone, we constrain secular variations of rock uplift rates for a series of five Quaternary detachment‐ and fault‐related folds from their initiation to the modern day. When combined with GPS data, decomposition of interferometric synthetic aperture radar time series constrains the spatial pattern of surface and rock uplift on the folds deforming at decadal rates of 1–5 mm/yr. These data confirm the previously proposed basinward propagation of structures during the Quaternary. By fitting our geodetic rates and previously published geologic uplift rates with piecewise linear functions, we find that gradual rate changes over >100 kyr can explain the interferometric synthetic aperture radar observations where changes in average uplift rates are greater than ~1 mm/yr among different time intervals (~10¹, 10⁴‾⁵, and 10⁵‾⁶ years).

A Network Inversion Filter combining GNSS and InSAR for tectonic slip modeling

Studies of the earthquake cycle benefit from long-term time-dependent slip modeling, as it can be a powerful means to improve our understanding on the interaction of earthquake cycle processes such as interseismic, coseismic, postseismic, and aseismic slip. Observations from Interferometric Synthetic Aperture Radar (InSAR) allow us to model slip at depth with a higher spatial resolution than when using GNSS alone. While the temporal resolution of InSAR has typically been limited, the recent fleet of SAR satellites including Sentinel-1, COSMO-SkyMED, and RADARSAT-2 permits the use of InSAR for time-dependent slip modeling, at intervals of a few days when combined. With the vast amount of SAR data available, simultaneous data inversion of all epochs becomes challenging. Here, we expanded the original Network Inversion Filter to include InSAR observations of surface displacements in addition to GNSS. In the NIF framework, geodetic observations are limited to those of a given epoch, with a stochastic model describing slip evolution over time. The combination of the Kalman forward filtering and backward smoothing allows all geodetic observations to constrain the complete observation period. Combining GNSS and InSAR allows modeling of time-dependent slip at unprecedented spatial resolution. We validate the approach with a simulation of the 2006 Guerrero slow slip event. We highlight the importance of including InSAR covariance information, and demonstrate that InSAR provides an additional constraint on the spatial extent of the slow slip.

Decomposing DInSAR Time-Series into 3-D in Combination with GPS in the Case of Low Strain Rates: An Application to the Hyblean Plateau, Sicily, Italy

Differential Interferometric SAR (DInSAR) time-series techniques can be used to derive surface displacement rates with accuracies of 1 mm/year, by measuring the one-dimensional distance change between a satellite and the surface over time. However, the slanted direction of the measurements complicates interpretation of the signal, especially in regions that are subject to multiple deformation processes. The Simultaneous and Integrated Strain Tensor Estimation from Geodetic and Satellite Deformation Measurements (SISTEM) algorithm enables decomposition into a three-dimensional velocity field through joint inversion with GNSS measurements, but has never been applied to interseismic deformation where strain rates are low. Here, we apply SISTEM for the first time to detect tectonic deformation on the Hyblean Foreland Plateau in South-East Sicily. In order to increase the signal-to-noise ratio of the DInSAR data beforehand, we reduce atmospheric InSAR noise using a weather model and combine it with a multi-directional spatial filtering technique. The resultant three-dimensional velocity field allows identification of anthropogenic, as well as tectonic deformation, with sub-centimeter accuracies in areas of sufficient GPS coverage. Our enhanced method allows for a more detailed view of ongoing deformation processes as compared to the single use of either GNSS or DInSAR only and thus is suited to improve assessments of regional seismic hazard.

Constant strain accumulation rate between major earthquakes on the North Anatolian Fault

Earthquakes are caused by the release of tectonic strain accumulated between events. Recent advances in satellite geodesy mean we can now measure this interseismic strain accumulation with a high degree of accuracy. But it remains unclear how to interpret short-term geodetic observations, measured over decades, when estimating the seismic hazard of faults accumulating strain over centuries. Here, we show that strain accumulation rates calculated from geodetic measurements around a major transform fault are constant for its entire 250-year interseismic period, except in the ~10 years following an earthquake. The shear strain rate history requires a weak fault zone embedded within a strong lower crust with viscosity greater than ~1020 Pa s. The results support the notion that short-term geodetic observations can directly contribute to long-term seismic hazard assessment and suggest that lower-crustal viscosities derived from postseismic studies are not representative of the lower crust at all spatial and temporal scales.Accumulation of interseismic strain may now be constrained by satellite observations. Here, the authors show that strain accumulation rates on the North Anatolian Fault are constant for the interseismic period indicating that lower-crustal viscosities from postseismic studies are not representative.