Simon McClusky

In 2018, four deadly (Mw 6.2 to 6.9) earthquakes struck the north coast of Lombok Island, on 28 July, 5August, and 19 August, distributed between the Flores back-arc thrust and the Rinjani-Samalas volcanic complex, causing hundreds of fatalities and extensive damage. We performed a comprehensive analysis of relocated aftershocks, static coulomb stress changes, and co-seismic and post-seismic deformation, to improve our understanding of this earthquake sequence. The fault geometries and slip distributions of the three mainshocks are modelled by inverting the co-seismic deformation imaged using an interferometric analysis of Sentinel-1 synthetic aperture radar (InSAR) measurements, based on rectangular dislocations embedded in a multi-layered elastic half-space. The earthquake sequence aftershocks were analysed using an unsupervised learning method (ST-DBSCAN) to cluster these relocated aftershocks so that we can identify the source of each aftershock. We used a time-series consisting of 658 descending and 370 ascending Sentinal-1 InSAR interferograms to investigate the time-dependent post-seismic deformation in the two years following the Lombok 2018 earthquake sequence, deriving a combined model that simulates the viscoelastic relaxation and afterslip simultaneously. The Coulomb stress change modelling based on the co-seismic and post-seismic rupture models indicates about 1 MPa of extensional stress change at 10 to 20 km of depth and 0.5 Mpa extensional stress change at 15 to 25 km of depth around the Barujari Crater region, respectively, which affects the open of the magma conduct, reflected as caldera-scale deflation and inflation. To quantify the influence of the earthquake sequence on the spatiotemporal deformation pattern of the volcano edifice, we extended our InSAR time-series range forward to the year 2014, just prior to the two eruptions that occurred on 25th October 2015 and 1st August 2016, and perform Principal Component Analysis to investigate the time-dependent inflation and deflation signals. We modelled the volume change and the location of the volcano pressure source for a better understanding of how changes in the magma body and magma movement may have been influenced by the 2018 Lombok earthquake sequence. A double-source compound model is used to invert the parameters of the magma chamber, including a shallow Moji point pressure source centred at 1.3 km north of the Barujari cone, and a deep source centred at 1.5 km northeast of the Rinjani cone, at ~3.9 km and ~3.5 km depth below the sea level respectively. We also used a uniform sill and dike combined model to interpret the co-eruptive signals surrounding the observed eruptive fissures. Our best-fit dike is nearly vertical, reaching a depth of 2 km below sea level with an opening of 8.5 cm, and the sill is at the depth of 3.1 km with a contraction of 40 cm.

The Indonesia‐Australia‐New Guinea collision zone comprises a complex system of tectonic blocks whose relative motion accommodates convergence of the Sunda Block, Pacific, Australian, and Philippine Sea plates. Previous studies have considered either the western or eastern ends of this system, in eastern Indonesia and Papua New Guinea, respectively. However, these studies had limited ability to characterize either the kinematics of the central part of the system or transitions in tectonic regime across it. In this study, we perform a simultaneous inversion of 492 earthquake slip vectors and 267 GPS velocities to quantify the block movement spanning the Sunda‐Banda Arc, Western New Guinea, and Papua New Guinea. Our best‐fitting kinematic block model comprises 23 elastic blocks, for which we estimate the rotation rates and block boundary slip rates. We show how the Cenderawasih Bay sphenochasm was likely formed by a combination of both rotations (2.82 ± 0.11°/Myr anticlockwise) of the Bird's Head Block and southwest‐directed convergence (39.9 ± 1.7 mm/yr) along the Lowlands fault. Our estimated relative slip vectors across the New Guinea Fold‐and‐Thrust Belt indicate a transition in the tectonic regime of the block boundary from predominately thrust faulting at its western segment, with a convergence rate up to 19.5 ± 0.6 mm/yr, to predominately sinistral motion in the center segment with slip rate ∼7 mm/yr, and returning to thrust in the eastern segment with a convergence rate up to 9.0 ± 0.5 mm/yr, implying the combined effect of multiple driving mechanisms.

Rinjani volcano is a highly active volcano located on Lombok Island in eastern Indonesia which has experienced ten eruptions in the last 100 years. Between 2014 and 2020, this stratovolcano has erupted twice, on 25th October 2015; and on 1st August 2016. Both eruptions lasted approximately two months, with activity concentrated in the volcanoes central Barujari Crater region. In 2018, four deadly (Mw 6.2 to 6.9) earthquakes struck the north coast of Lombok Island on 28th July, 5th August, and 19th August, causing hundreds of fatalities and extensive damage. These earthquakes also resulted in the remobilization of ash deposits on the flanks of Rinjani volcano located on the north island as landslides. Our InSAR-based finite fault rupture modelling suggests the estimated maximum fault slip of 1.4 m, 2.3 m, and 2.5 m for the three mainshocks located on southward dipping fault planes to the northwest-northeast of the Rinjani volcano occurred at depths of ~15 km, 12 km, and 32 km, respectively. Coulomb stress change modelling based on the these rupture models indicates about 1 MPa of extensional stress change at 10 to 20 km of depth around the crater region was observed, which may promote opening of the magma conduit. The short distance between the peak slip region and the volcano, as well as the stress change, raises the question of whether the earthquake sequence may have influenced the spatio-temporal deformation pattern of the Rinjani volcano.We use an InSAR time-series, consisting of 658 descending and 370 ascending Sentinal-1 interferograms to investigate the time-dependent inflation and deflation signals around the crater region generated by the 2015, 2016 eruptions and the 2018 earthquakes. We analyse the average inflation/deflation rate and the cumulative displacements in different periods between 2014 and 2020 to quantify the volcano deformation before and after the 2018 earthquake sequence. Our preliminary results reveal that the crater region has undergone rapid inflation of up to 20 mm/yr through the 2014 to 2017 period, before significantly slowing to ~10 mm/yr over the 2017 to 2018 period. During the first three months following the 2018 earthquake sequence, a noticeable deflation of the edifice was detected, followed by gentle inflation lasting until late 2020. These results imply that the influence of the 2018 earthquakes acted to reduce the pressure in the reservoir, at least temporarily. We will present results from modelling the volume change and the location of the volcano pressure source for better understanding how changes in the magma body and magma movement may have been influenced by the 2018 Lombok earthquake sequence.

Several different basis functions have been used to represent the Earth's gravity field in order to generate estimates of mass variations on Earth from the analysis of data of the Gravity Recovery and Climate Experiment (grace) and its successor grace Follow‐On missions, including spherical harmonics, mass concentration elements (mascons) and slepian functions. Each approach depends inherently upon accurate modeling of the orbits of the pair of satellites as they revolve around the Earth, so that the observations of inter‐satellite changes in range (or, more specifically, range rate) can be exploited to identify mass variations. We have developed software using a classical orbit modeling approach, mascons and 24‐hr orbit integration, to estimate simultaneously corrections to orbital parameters and the temporal gravity field from grace data. Rather than using the range rate, we use the range acceleration as the inter‐satellite observable as it aids in localizing the mass variations. Level‐1 B range acceleration observations contain high levels of high‐frequency noise that inhibits their usefulness for this purpose. Instead, we generate range acceleration observations by numerical differentiation of the Level‐1B range rate prefit residuals. Simulations show that the gravity signal is not attenuated in this process. Our monthly estimates of mass anomalies from grace data (2003–2016) agree well with previous studies, both spatially and temporally. When converted to spherical harmonics our time series of C_2,0, derived from grace data alone, are close to the independent estimates from satellite laser ranging, but the overall solution is improved by substituting the SLR C_2,0.

Radar Corner Reflectors (CR) are increasingly used as reference targets for land surface deformation measurements with the Interferometric Synthetic Aperture Radar (InSAR) technique. When co-located with ground-based Global Navigation Satellite Systems (GNSS) infrastructure, InSAR observations at CR can be used to integrate relative measurements of surface deformation into absolute reference frames defined by GNSS. However, CR are also a potential source of GNSS multipath effects and may therefore have a detrimental effect on the GNSS observations. In this study, we compare daily GNSS coordinate time series and 30-second signal-to-noise ratio (SNR) observations for periods before and after CR deployment at a GNSS site. We find that neither the site coordinates nor the SNR values are significantly affected by the CR deployment, with average changes being within 0.1 mm for site coordinates and within 1 % for SNR values. Furthermore, we generate empirical site models by spatially stacking GNSS observation residuals to visualise and compare the spatial pattern in the surroundings of GNSS sites. The resulting stacking maps indicate oscillating patterns at elevation angles above 60 degrees which can be attributed to the CR deployed at the analysed sites. The effect depends on the GNSS antenna used at a site with the magnitude of multipath patterns being around three times smaller for a high-quality choke ring antenna compared to a ground plane antenna without choke rings. In general, the CR-induced multipath is small compared to multipath effects at other GNSS sites located in a different environment (e. g. mounted on a building).

Improper modeling of horizontal tropospheric gradients in GPS analysis induces errors in estimated parameters, with the largest impact on heights and tropospheric zenith delays. The conventional two‐axis tilted plane model of horizontal gradients fails to provide an accurate representation of tropospheric gradients under weather conditions with asymmetric horizontal changes of refractivity. A new parametrization of tropospheric gradients whereby an arbitrary number of gradients are estimated as discrete directional wedges is shown via simulations to significantly improve the accuracy of recovered tropospheric zenith delays in asymmetric gradient scenarios. In a case study of an extreme rain event that occurred in September 2002 in southern France, the new directional parametrization is able to isolate the strong gradients in particular azimuths around the GPS stations consistent with the “V” shape spatial pattern of the observed precipitation. In another study of a network of GPS stations in the Sierra Nevada region where highly asymmetric tropospheric gradients are known to exist, the new directional model significantly improves the repeatabilities of the stations in asymmetric gradient situations while causing slightly degraded repeatabilities for the stations in normal symmetric gradient conditions. The average improvement over the entire network is ∼31%, while the improvement for one of the worst affected sites P631 is ∼49% (from 8.5 mm to 4.3 mm) in terms of weighted root‐mean‐square (WRMS) error and ∼82% (from −1.1 to −0.2) in terms of skewness. At the same station, the use of the directional model changes the estimates of zenith wet delay by 15 mm (∼25%).

We use Global Positioning System (GPS) measurements of surface deformation to show that the convergence between the Australian Plate and Sunda Block in eastern Indonesia is partitioned between the megathrust and a continuous zone of back‐arc thrusting extending 2000 km from east Java to north of Timor. Although deformation in this back‐arc region has been reported previously, its extent and the mechanism of convergence partitioning have hitherto been conjectural. GPS observations establish that partitioning occurs via a combination of anticlockwise rotation of an arc segment called the Sumba Block, and left‐lateral movement along a major NE‐SW strike‐slip fault west of Timor. We also identify a westward extension of the back‐arc thrust for 300 km onshore into East Java, accommodating slip of ∼6 mm/yr. These results highlight a major new seismic threat for East Java and draw attention to the pronounced seismic and tsunami threat to Bali, Lombok, Nusa Tenggara, and other coasts along the Flores Sea.

During the past century, a series of predominantly westward migratingM > 7 earthquakes broke an ~1000 km section of the North Anatolian Fault (NAF). The only major remaining “seismic gap” along the fault is under the Sea of Marmara (Main Marmara Fault (MMF)). We use 20 years of GPS observations to estimate strain accumulation on fault segments in the Marmara Sea seismic gap. We report the first direct observations of strain accumulation on the Princes' Islands segment of the MMF, constraining the slip deficit rate to 10–15 mm/yr. In contrast, the central segment of the MMF that was thought to be the most likely location for the anticipated gap‐filling earthquakes shows no evidence of strain accumulation, suggesting that fault motion is accommodated by fault creep. We conclude that the Princes' Islands segment is most likely to generate the nextM > 7 earthquake along the Sea of Marmara segment of the NAF.