InSAR Measurements Research Articles

Decades of Ground Deformation in the Weihe Graben, Shaanxi Province, China, in Response to Various Land Processes, Observed by Radar Interferometry and Levelling

Ground deformation is usually used as direct evidence for early warning of geological hazards. The Weihe Graben, located in the southern margin of the Ordos Plateau, is surrounded by many active faults. Earthquakes (e.g., the 1556 Huaxian M 8 earthquake), mine accidents and ground fissures are the major hazards that pose great threats to this densely populated region. In order to characterise both tectonic and anthropogenic activities in the Weihe Graben, we use Envisat data from 2003 to 2010 and Sentinel-1 data from 2014 to 2021, combined with levelling data from 1970 to 2014, to investigate the long-term ground deformation. We generate four InSAR rate maps using the small-baseline subset (SBAS) algorithm. The uncertainties of the InSAR rates are 1–2 mm/year by calculating the differences between the InSAR and levelling measurements. From the deformation time series, we found that most of the faults surrounding the Weihe Graben move at a relatively slow rate (<3 mm/year). Elastic dislocation modelling based on the InSAR and levelling data yields a slip rate of 2.3 ± 0.3 mm/year for the Huashan Fault, the seismogenic fault for the 1556 Huaxian earthquake. Anthropogenic deformation is much stronger than the tectonic deformation. We identified localised subsidence of 12 mines with a deformation rate ranging from 5 to 17 mm/year. The cities of Xi’an and Xianyang also show evident subsidence, which is likely to be caused by groundwater extraction. Land subsidence in Xi’an has slowed down from an average rate of 10–20 mm/year between 2003 and 2010 to about 5–10 mm/year between 2017 and 2020, but in Xianyang, subsidence has increased dramatically in the past five years from 1 mm/year to 7 mm/year. This is because new industrial and urban development centres have gradually moved from Xi’an to Xianyang. We identified a region bounded by the Kouzhen-Guanshan and Fufeng-Liquan Faults with strong subsidence, as a result of excessive extraction of groundwater. To quantify the effects of crustal groundwater unloading on faults, we calculated the static Coulomb stress changes on the two faults and found that Coulomb stress changes are localised in the upper 5 km with a magnitude of 0.01–0.02 bar/year. The Coulomb stress changes might be large enough (0.1 bar) to affect local seismicity if such excessive extraction of groundwater continued for 10 years.

Joint exploitation of space-borne and ground-based multitemporal InSAR measurements for volcano monitoring: The Stromboli volcano case study

We present a joint exploitation of space-borne and ground-based Synthetic Aperture Radar Interferometry (InSAR) and Multi Temporal (MT) InSAR measurements for investigating the Stromboli volcano (Italy) deformation phenomena. In particular, we focus our analysis on three periods: a) the time interval following the 2014 flank eruption, b) the July–August 2019 eruption and c) the following post-eruptive phase. To do this, we take advantage from an unprecedented set of space-borne and ground-based SAR data collected from April 2015 up to November 2019 along two (one ascending and one descending) Sentinel-1 (S-1) tracks, as well as, in the same period, by two ground-based systems installed along the Sciara del Fuoco northern rim. Such data availability permitted us to first characterize the volcano long-term 3D deformation behavior of the pre-eruptive period (April 2015–June 2019), by jointly inverting the space-borne and ground-based InSAR measurements. Then, the GB-SAR measurements allowed us to investigate the sin-eruptive time span (3rd July 2019 – 30th August 2019) which revealed rapid deformation episodes (e.g. more than 30 mm/h just 2 min before the 3rd July 2019 explosion) associated with the eruptive activity, that cannot be detected with the weekly S-1 temporal sampling. Finally, the S-1 measurements permitted to better constrain the post 2019 eruption deformations (31st August 2019 – 5th November 2019), which are mainly located outside the GB-SAR sensed area. The presented results demonstrate the effectiveness of the joint exploitation of the InSAR measurements obtained through satellite and terrestrial SAR systems, highlighting their strong complementarity to map and interpret the deformation phenomena affecting volcanic areas.

Corrections to “Iterative Atmospheric Phase Screen Compensation for Near-Real-Time Ground-Based InSAR Measurements Over a Mountainous Slope”

Corrections to “Iterative Atmospheric Phase Screen Compensation for Near-Real-Time Ground-Based InSAR Measurements Over a Mountainous Slope”

Multitechnology characterization of an unusual surface rupturing intraplate earthquake: the ML 5.4 2019 Le Teil event in France

SUMMARY Metropolitan France is a region of slow tectonic deformation with sparse seismicity. On 11 November 2019, the ML 5.4 Le Teil earthquake became the largest seismic event recorded in the last 16 yr. This event was recorded by the national seismic networks and also by a wide variety of other geophysical techniques including infrasound and InSAR measurements. These complementary technologies offer the opportunity to investigate in detail the earthquake source characteristics and the associated ground motion attenuation. Both seismic waveform inversions and InSAR interferogram reveal a shallow rupture on a reverse fault with an associated moment magnitude of 4.8–4.9. Infrasound signals also provide fast evidences pointing towards the area of ground surface displacements, which coincides with La Rouvière fault, in the Cévennes fault system, known as a formerly active normal fault during the Oligocene. The very significant amount of seismic records also helps towards validating the GMPE laws available for the region. This multitechnology characterization documents the kinematics of this rare example of shallow intraplate fault reactivation.

Fluid-driven seismicity in relatively stable continental regions: Insights from the February 3rd, 2020 MS5.1 Qingbaijiang isolated earthquake

On February 3rd, 2020, an isolated MS5.1 earthquake occurred in the northern section of the Longquanshan fault zone. This study aims at defining the geometry of seismogenic structures of this earthquake. In detail, centroid moment tensor inversion results show that the earthquake is characterized by a focal depth of 3.8 ​km with no corresponding surface faults. The strike/dip/rake angles for the two nodal planes are 205°/54°/96° and 15°/36°/82°, respectively. With the analyses of coseismic deformation of the surface obtained from InSAR measurements, together with the information of relocated hypocenters for a small number of aftershocks, it is concluded that a northwest-dipping nodal plane corresponds well to the source fault. The fault is suggested to have a length of about 2.8 ​km and a depth range of 2–5 ​km, and the centroid of the earthquake is located at 104.48°E and 30.71°N. Furthermore, multiple pieces of evidence indicate that this earthquake is partly driven by the overpressure effect associated with the adjacent natural gas packets, which is similar to several other moderate natural earthquakes in Sichuan Basin.

Integration of Multiband InSAR and Leveling Measurements for Analyzing the Surface Subsidence of Shield Tunneling at Beijing‐Zhangzhou High‐Speed Railway

In modern life, metro and high‐speed rails have become indispensable transportation and have been playing an important role in many areas, especially in cities. The metro and high‐speed rails have varying degrees of impact on the surface and surrounding buildings, which must be carefully understood to minimize the risk of hazards. This work is aimed at investigating the deformation of ground surface and the surrounding structures, caused by the excavation of underground tunnels. Because of the spatiotemporal characteristics of the land subsidence induced by underground tunnel construction, the measurements obtained from InSAR (Synthetic Aperture Radar Interferometry) and leveling techniques are integrated to study the subsidence phenomenon of the ground surface above the shield tunnels and the surrounding buildings. The subsidence‐related parameters, including the advance angles, lag angles, and boundary angles, are derived from the deformation results. This study suggests that leveling and InSAR observations can provide technical support to study the subsidence of dense buildings on the surface of shield tunnel excavation and to protect the surrounding environment.

Earthquake surface ruptures on the altiplano and geomorphological evidence of normal faulting in the December 2016 (Mw 6.1) Parina earthquake, Peru

The 2016 Mw 6.1 Parina earthquake ruptured a shallow-crustal normal fault within the high Andes of south Peru. We use high-resolution DEMs and field mapping of the surface ruptures generated by the earthquake, in combination with co-seismic and post-seismic InSAR measurements, to investigate how different features of the geomorphology at Parina are generated by the earthquake cycle on the Parina Fault. We systematically mapped 12 km of NW-SE trending surface ruptures with up to ~27 cm vertical displacement and ~25 cm tensional opening along strike, separated by a gap with no observable surface ruptures. Co- and post-seismic InSAR measurements require slip below this gap in surface ruptures, implying that surface offsets observed in paleoseismic trenches may not necessarily be representative of slip at seismogenic depths, and will typically yield an underestimate of paleo-earthquake magnitudes. The surface ruptures developed along 10–20 m high cumulative scarps cutting through late Quaternary fluvio-glacial deposits and bedrock. The 2016 Parina earthquake did not rupture the full length of the late Quaternary scarps, implying that the Parina Fault does not slip in characteristic, repeat earthquakes. At Parina, and across most of the Peruvian Altiplano, normal faults are most-easily identified from recent scarps cutting late Quaternary moraine crests. In regions where there are no recently-deposited moraines, faults are difficult to identify and lack time constraints to quantify rates of fault slip. For this reason, current fault maps may underestimate the seismic hazard in the Altiplano.

Derivation of high-quality three-dimensional surface deformation velocities through multi-source point cloud fusion: Application to Kīlauea volcano

Kīlauea volcano, located in the southeast of Hawai’i, is one of the most active volcanoes in the world. Multi-source InSAR measurements acquired from ascending and descending high-resolution COSMO-SkyMed SAR imagery are used to monitor Kīlauea volcano’s deformations. In order to obtain identical observations from multi-source SAR data, the InSAR measurements acquired by different platforms are generally geocoded and resampled into a unified coordinate system. Such method is however inapplicable to the point clouds provided by high-resolution SAR data. In addition, the geocoding accuracy of high-resolution InSAR measurements would be greatly affected by external DEM error, orbit error, propagation delay, reference point parameter error. In this study, a method for estimating 3-D deformations on the basis of SAR point cloud fusion is proposed. Firstly, iterative closest point (ICP) method is introduced to fuse InSAR point clouds from ascending and descending COSMO-SkyMed datasets to reduce the influence of geocoding offsets. Then, the precise 3-D surface deformations of Kīlauea volcano are obtained by employing the homogeneous weighted observations in the local space based on the strain model. The maximum deformation rates between April 2012 and April 2013 of Kīlauea volcano are estimated as 32 mm/year and 42 mm/year in the east and up directions, respectively. The accuracies of the InSAR measurements are validated by the deformation rates obtained from the precise GNSS observations.

An approach for retrieving complete three-dimensional ground displacement components from two parallel-track InSAR measurements

In the investigation and evaluation of geological hazards such as those caused by co-seismic displacement, displacement due to underground mining, landslides, volcanic eruptions, it is important to retrieve the complete three-dimensional ground displacement components. However, both conventional D-InSAR (differential synthetic aperture radar interferometry) and MT-InSAR (multi-temporal InSAR) can only measure the displacement component along the radar LOS (line of sight) direction, which greatly limits the application of InSAR technique. Over the past decades, great endeavors have been made to overcome its limitation of the one-dimensional LOS measurements, and several methods have been proposed to retrieve complete three-dimensional displacement components from InSAR measurements. On the whole, the commonality of the existing methods is characterized by using both ascending and descending SAR data. In this paper, we put forward an approach for retrieving complete three-dimensional ground displacement components from two parallel-track InSAR measurements and present the analytical expressions for the vertical, the North–South, the East–West displacement components. It reveals that the vertical displacement component is just a function of the two parallel-track InSAR LOS displacements, i.e., the vertical displacement component can be retrieved from the two parallel-track InSAR LOS displacements, while the North–South or the East–West displacement component is a function of both the two parallel-track InSAR LOS displacements and the azimuthal displacement. What is more, the proposed approach can effectively improve the accuracy of the azimuthal measurement by using a redundant measurement. As a result, the accuracies of the derived North–South and East–West displacement components are improved as well, which can provide more reliable results for characterizing ground movement associated with geological hazards. In terms of their geographic coverage, there is a strong complementary relationship between the proposed approach and the existing methods. Thus, they can be integratedly utilized in the practical applications. Finally, the approach is illustrated through the retrieval of the complete three-dimensional co-seismic displacement components in an area to the west of Bam, Iran, where a strong earthquake occurred in 2003.

Multifault complex rupture and afterslip associated with the 2018 Mw 6.4 Hualien earthquake in northeastern Taiwan

SUMMARYWe investigate the coseismic and post-seismic deformation due to the 6 February 2018 Mw 6.4 Hualien earthquake to gain improved insights into the fault geometries and complex regional tectonics in this structural transition zone. We generate coseismic deformation fields using ascending and descending Sentinel-1A/B InSAR data and GPS data. Analysis of the aftershocks and InSAR measurements reveal complex multifault rupture during this event. We compare two fault model joint inversions of SAR, GPS and teleseismic body waves data to illuminate the involved seismogenic faults, coseismic slip distributions and rupture processes. Our preferred fault model suggests that both well-known active faults, the dominantly left-lateral Milun and Lingding faults, and previously unrecognized oblique-reverse west-dipping and north-dipping detachment faults, ruptured during this event. The maximum slip of ∼1.6 m occurred on the Milun fault at a depth of ∼2–5 km. We compute post-seismic displacement time series using the persistent scatterer method. The post-seismic range-change fields reveal large surface displacements mainly in the near-field of the Milun fault. Kinematic inversions constrained by cumulative InSAR displacements along two tracks indicate that the afterslip occurred on the Milun and Lingding faults and the west-dipping fault just to the east. The maximum cumulative afterslip of 0.4–0.6 m occurred along the Milun fault within ∼7 months of the main shock. The main shock-induced static Coulomb stress changes may have played an important role in driving the afterslip adjacent to coseismic high-slip zones on the Milun, Lingding and west-dipping faults.

Accuracy detection of Satellite and InSAR Technology in the Deformation Monitoring in Civil Engineering

This paper focuses on the application of Satellite Navigation Positioning System (such as GPS and BeiDou Navigation Satellite System) and InSAR technology in healthy monitoring in civil engineering. Considering the high cost of Satellite Positioning measurement and blind spot of InSAR measurement, it is reasonable to combine both technologies for accomplishing regionally area and focused points monitoring. During the study, the control network points were positioned with GPS measurement technique and height differences were supported with precise levelling measurements. As the result of measurement campaigns, the height differences were determined from the GPS measurements and InSAR measurements respectively. Later on, deformation analysis using the height differences according to provided data from the GPS and the data from the InSAR system were carried out separately. The comparison between GPS and InSAR results will be shown in the paper.

Regression Analysis of Subsidence in the Como Basin (Northern Italy): New Insights on Natural and Anthropic Drivers from InSAR Data

Natural and anthropogenic subsidence such as that in the Como urban area (northern Italy) can cause significant damage to structures and infrastructure, and expose the city’s lakefront to an increasing risk of inundation from Lake Como. This phenomenon affecting the Como basin has been studied by several researchers, and the major drivers of subsidence are known. However, the availability of historical InSAR data allowed us to reconsider the relationship between subsidence predisposing factors (i.e., the thicknesses of reworked and compressible layers, overburden stress, and the piezometric level) and ground surface displacements with higher precision over the entire basin. Benefiting from the deep knowledge of the hydromechanical setting of the Como basin and the availability of InSAR measurements from 1992 to 2010, in this paper we model subsidence-related movements using linear and nonlinear regression methods in order to determine the combination of natural and anthropic factors that have caused subsidence in the Como basin over the past decades. The results of this study highlight peculiar patterns of subsidence that suggest the influence of two further causes, namely tectonic control of the sedimentary architecture and diversion of local streams, which have never been considered before. This analysis aims to assess the spatial distribution of subsidence through InSAR analysis in order to enhance the knowledge and understanding of the phenomenon in the Como urban area. The interferometric data could be used to better plan urban risk management strategies.

Automatic Detection of Volcanic Surface Deformation Using Deep Learning

AbstractInterferometric Synthetic Aperture Radar (InSAR) provides subcentimetric measurements of surface displacements, which are key for characterizing and monitoring magmatic processes in volcanic regions. The abundant measurements of surface displacements in multitemporal InSAR data routinely acquired by SAR satellites can facilitate near real‐time volcano monitoring on a global basis. However, the presence of atmospheric signals in interferograms complicates the interpretation of those InSAR measurements, which can even lead to a misinterpretation of InSAR signals and volcanic unrest. Given the vast quantities of SAR data available, an automatic InSAR data processing and denoising approach is required to separate volcanic signals that are cause of concern from atmospheric signals and noise. In this study, we employ a deep learning strategy that directly removes atmospheric and other noise signals from time‐consecutive unwrapped surface displacements obtained through an InSAR time series approach using an end‐to‐end convolutional neural network (CNN) with an encoder‐decoder architecture, modified U‐net. The CNN is trained with simulated synthetic unwrapped surface displacement maps and is then applied to real InSAR data. Our proposed architecture is capable of detecting dynamic spatio‐temporal patterns of volcanic surface displacements. We find that an ensemble‐average strategy is recommended to stabilize detected results for varying deformation rates and signal‐to‐noise ratios (SNRs). A case study is also presented where this method is applied to InSAR data covering Masaya volcano, Nicaragua and the results are validated using continuous GPS data. The results confirm that our network can indeed efficiently suppress atmospheric and other noise to reveal the noise‐free surface deformation.

Regional-scale hydrological monitoring of wetlands with Sentinel-1 InSAR observations: Case study of the South Florida Everglades

Wetland is a very fragile ecosystem that provides important services to a large variety of flora and fauna species, as well as for humans. As wetland depends on water availability, protecting this important ecosystem requires careful hydrological monitoring. The Sentinel-1 mission, featuring a wide swath coverage, high temporal observations and open data policy, provides unprecedented opportunity for high spatio-temporal resolution water level change mapping over regional-wide wetland areas. In this study, we assess Sentinel-1 InSAR observations for routine water level change measurements over the entire south Florida Everglades wetlands. The study utilizes 91 Sentinel-1 images acquired over a three-year period (Sep 2016 to Nov 2019) and generates routine 12-days Interferograms and correspondingly 30 m spatial resolution water level change maps over the entire Everglades. The high spatial resolution interferograms detect hydrological signals induced by both natural- and human-induced flow, including tides, gate operations, and canal overflow; all these cannot be detected by terrestrial measurements. The large number of both InSAR and ground-based gauge observations allow us to quantify the overall accuracy of the Sentinel-1 InSAR measurements, which is 3.9 cm for the entire wetland area, but better for smaller hydrological units within the Everglades. Our study reveals that the tropospheric delay for individual interferograms can be very large, as much as 30 cm (~10 fringes). When applying tropospheric corrections to all three years of Sentinel-1 InSAR observations, the overall accuracy level improved by 13% to 3.4 cm. Although our study is focused on the Everglades, its implications in term of the suitability of Sentinel-1 observations for space-based hydrological monitoring of wetlands and the derived accuracy level are applicable to other wetlands with similar vegetation types, located all over the world.

Sentinel-1 InSAR measurements of deformation over discontinuous permafrost terrain, Northern Quebec, Canada

Differential synthetic aperture radar interferometry (D-InSAR) has been applied in permafrost environments to detect surface deformation caused by freeze-thaw processes in the active layer and underlying permafrost. The effectiveness of Sentinel-1 InSAR in monitoring ground surface deformation over continuous permafrost terrain above the treeline has been proven. The heterogeneous landscape and developed vegetation cover increase the difficulty of applying D-InSAR in sub-Arctic discontinuous permafrost terrain. The potential of Sentinel-1 InSAR in such an environment has not been fully explored. In this study, we explore the capabilities and limitations of applying Sentinel-1 time series data for monitoring surface deformation over discontinuous permafrost terrain. The interferometric coherence time series from September 2016 to April 2018 were analyzed over typical landscapes in discontinuous permafrost environments and their thaw subsidence curves are revealed by the small-baseline subset (SBAS) InSAR technique. The seasonal thaw subsidence in the summer of 2017 was in the range of 15–80 mm in the study area. The land cover types with thaw subsidence magnitudes from low to high are exposed land, peatland, lichen–low shrub, lichen-dominated and wetland low vegetation. The difference in displacement pattern between lichen-dominated and wetland-low vegetation-dominated permafrost terrains is especially clear at the end of the thawing stage, in September and October. The differences in thaw subsidence magnitude and pattern reveal the influence of the soil water content in the active layer and permafrost properties on the thaw subsidence patterns. We also compared the Sentinel-1 retrieved cumulative displacement with the X-band TerraSAR-X and L-band ALOS PALSAR results. The difference of retrieved deformation magnitude using the three sensors is amplified when shrubs are more developed. The findings indicate that Sentinel-1 time series with a 6-day or 12-day span work well over discontinuous permafrost terrain above the tree line (i.e., tundra, tundra wetlands and less developed shrub-tundra environments) during the thawed season, but the results and accuracy are not promising over developed shrub-tundra and, especially forest-tundra environments.

Stick-slip vs. stable sliding fault behaviour: A case-study using a multidisciplinary approach in the volcanic region of Mt. Etna (Italy)

In active volcanic zones, fault dynamics is considerably fast but it is often difficult to separate the pattern of nearly continuous large-scale volcanic processes (inflation/deflation processes, flank instability) from impulsive episodes such as dyke intrusions or coseismic fault displacements. At Etna, multidisciplinary studies on active faults whose activity does not strictly depend on volcanic processes, are relatively few. Here we present the case-study of the San Leonardello fault, an active structure located in the eastern flank of Mt. Etna characterised by a well-known seismic history. This fault saw renewed activity in May 2009, when pre-seismic creeping along the southern segment preceded an MW 4.0 earthquake in the northern segment, followed by some twenty-five aftershocks. Later, in March–April 2016, creep events reactivated the southern section of the same fault. Both the seismic and aseismic phenomena were recorded by the seismic and GNSS networks of INGV-Osservatorio Etneo, and produced surface faulting that left a footprint in the pattern of ground deformation detected by the InSAR measurements. We demonstrate that the integration of multidisciplinary data collected for volcano surveillance may shed light on different aspects of fault dynamics, and allow understanding how coseismic slip and creep alternate in space and time along the strike. Moreover, we use findings from our independent datasets to propose a conceptual model of the San Leonardello fault, taking into account behaviour and previous constraints from fault-based seismic hazard analyses. Although the faulting mechanisms described here occur at a very small scale compared with those of a purely tectonic setting, this case-study may represent a perfect natural lab for improving knowledge of seismogenic processes, also in other fault zones characterised by stick slip vs. stable-sliding fault behaviour.

Reliability of InSAR satellite monitoring of buildings near inner city quay walls

Abstract. Amsterdam currently has a huge task of assessing and potentially upgrading its quay walls along the historic canals. Before replacement can take place, Amsterdam needs to determine the potential impact the replacement can have on the nearby buildings. The rate of vertical deformation of the adjacent buildings is used as indicator of potential foundation problems. To determine that rate, the current practice is to monitor the buildings by levelling for two years at least. This study shows that application of satellite measurements using Permanent Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) could reduce the monitoring period to a few months. The paper describes the statistical procedure that has been applied to levelling and satellite measurements to verify their reliability and determine the rate of vertical deformation of the buildings. The procedure was applied in three case studies. The rates of deformation observed in the InSAR measurements are in good agreement with the rates of deformation observed in the levelling in two of the case studies. The locally optimized InSAR data set with observations in the period 2014–2019 provides an almost 100 % coverage of reliable data points for all buildings in the case studies. More experience will need to be gained in the interpretation of InSAR measurements with respect to vulnerability of the buildings. Also, the procedure may be extended to include analysis of non-linear trends such as second order trends and seasonal effects.

Interseismic Loading of Subduction Megathrust Drives Long‐Term Uplift in Northern Chile

AbstractLarge earthquakes are the product of elastic stress that has accumulated over decades to centuries along segments of active faults. Assuming an elastic crust, one can roughly estimate the location and rate of accumulation of elastic stress. However, this general framework does not account for inelastic, irrecoverable deformation, which results in large‐scale topography. We do not know over which part of the earthquake cycle such deformation occurs. Using InSAR and GNSS measurements, we report on a potential correlation between long‐term, inelastic vertical rate and short‐term, interseismic vertical rate in northern Chile. Approximately 4% to 8% of the geodetically derived interseismic vertical rates translate into permanent deformation, suggesting that topography of the forearc builds up during the interseismic period. This observation provides a quantitative basis for an improved understanding of the interplay between short‐term and long‐term dynamics along convergent plate boundaries.

Did the Flores backarc thrust rupture offshore during the 2018 Lombok earthquake sequence in Indonesia?

SUMMARYThe Flores thrust forms the west segment (∼450 km) of a very active, ∼E–W striking, ∼800-km-long backarc thrust along the east Sunda Arc. In 2018, a deadly earthquake sequence composed of ~110 M4+ events rattled the Indonesian island of Lombok near the Flores thrust and caused tremendous damage on the island, however what is the nature of rupturing during this earthquake sequence remains unknown. Here, using a total of 2120 km of high-resolution seismic profiles covering ∼300 km of the Flores thrust off Bali, Lombok and Sumbawa, in addition to earthquake data and InSAR measurements, we investigated the active thrusting during this earthquake sequence. Our seismic interpretation and structural mapping show that offshore north of Lombok and Bali, the remarkable Flores thrust is essentially blind, deforming the seabed by folds, not faults. The Lombok earthquakes were all shallow thrust events with depth <40 km and occurred within ∼35 km north of the Rinjani volcano beneath the Lombok Island and its northern extremity. The InSAR measurements suggested that the most of the crustal deformation caused by these earthquakes occurred the north the and northeast of the island. The maximum vertical deformation was ∼36 cm near the northwest margin of the island, caused by the 5th August Mw 6.9 event. These observations combined with the presence of blind thrusts off Lombok suggest that the offshore portion of Flores thrust did not rupture during the 2018 Lombok earthquake sequence; the most coseismic slip must have occurred along a deep-rooted, north-verging basal fault and a range of imbricate thrusts beneath the north of the island, not along the buried thrusts offshore. Despite being blind off Lombok and Bali, the Flores thrust can still pose tsunami threats to the adjacent population centres by rupturing the seafloor during future large earthquakes (M > 7) that occur directly on the offshore blind thrusts, not beneath the island like the Lombok sequence. The proximity of the Rinjani volcano and thrust earthquakes suggests a possible role of volcanic activity (e.g. magmatic fluids and gas migration, stress change induced by pressurized magma chamber) in inducing the Lombok earthquakes.

Rapid localized flank inflation and implications for potential slope instability at Tungurahua volcano, Ecuador

High rates of volcano surface deformation can be indicative of a forthcoming eruption, but can also relate to slope instability and possible flank collapse. Tungurahua volcano, Ecuador, has been persistently active since 1999 and has previously experienced catastrophic flank failures. During the ongoing eruptive activity, significant surface deformation has been observed, with the highest rates contained within the amphitheatre-shaped scar from the 3000-year-old failure on the west flank. However, the cause of this asymmetric deformation and how it might relate to slope stability has not been assessed. Here, for the first time, we present a range of models to test physical processes that might produce asymmetric deformation, which are then applied to slope stability. Our models are informed by InSAR measurements of a deformation episode in November 2015, which show a maximum displacement of ∼3.5 cm over a period of ∼3 weeks, during which time the volcano also experienced multiple explosions and heightened seismicity. Asymmetric flank material properties, from the rebuilding of the cone, cannot explain the full magnitude and spatial footprint of the observed west flank deformation. The inflation is inferred to be primarily caused by shallow, short-term, pre-eruptive magma storage that preferentially exploits the 3 ka flank collapse surface. Shallow and rapid pressurization from this inclined deformation source can generate shear stress along the collapse surface, which increases with greater volumes of magma. This may contribute to slope instability during future unrest episodes and promote flank failure, with general application to other volcanoes worldwide displaying asymmetric deformation patterns.