The Java‐Timor subduction‐collision zone is one of the most tectonically complicated structures along the strike of the Indonesia‐Australia convergence. This complexity makes it difficult to interpret observations of interseismic deformation. In this study, we simultaneously inverted 108 Global Positioning System velocities and 176 earthquake slip vectors to investigate the variation in interseismic coupling along the Java‐Timor megathrust and the Flores‐Wetar back‐arc thrust. Our estimated coupling indicates two relatively wide coupled zones in the Nusa Tenggara segment of the megathrust and back‐arc thrust of widths ∼198 and 90 km, respectively. Elsewhere, the widths of estimated coupled zones are narrower, but still indicate significant earthquake and tsunami hazards. The gaps in seismicity and estimated coupled zones along the Java‐Timor megathrust are not obviously correlated. The highly coupled Lombok segment illustrates consistency with the occurrence of the 2018 Lombok earthquake sequence.

A destructive shallow earthquake with a magnitude of 5.6 struck Cianjur, West Java, Indonesia on November 21, 2022. This earthquake resulted in 602 casualties and the collapse of over 67,504 residences. The day after the mainshock, we deployed 19 temporary seismic stations to monitor aftershocks for a period of 30 days. We manually picked arrival times for 4499 P-waves and 3419 S-waves and determined locations for 514 events. Following the velocity model update, phase refinement through waveform cross correlation, and relocation using double-difference methods, we were able to determine 442 well-defined hypocenters of the aftershocks. We identified two clusters of aftershocks: one in the NNW-SSE direction, with a length of about 8 km, and another in the WSW-ENE direction, with a length of around 6 km. The seismogenic zone of these clusters ranges from a depth of 3 to 13 km. Our interpretation suggests that these clusters may indicate a conjugate fault. It is possible that the mainshock (Mw5.6) Cianjur earthquake on November 21, 2022 occurred on the WSW-ENE direction with sinistral movement.

Bandung is one of the major cities of Indonesia, a country that often experiences damaging earthquakes. Geologically, Bandung is located in the Bandung Basin, which has a thick sedimentary fill that likely has a significant effect on seismic ground motion. In this study, a microtremor array experiment was conducted to estimate the near-surface S-wave velocity structure using 30 observation sites spread throughout the city, each involving a triangular array configuration. The Spatial Autocorrelation method was applied to obtain dispersion curves, before S-wave velocity structure was inverted for using a Genetic Algorithm. The results show that the engineering bedrock depth in the study area ranges between 100 and 320 m, with a gradual increase toward the south, while the time-averaged shear-wave velocity to 30 m depth (Vs30) is in the range 100–340 m/s, corresponding to site classes SE and SD. These parameters were considered in a seismic hazard analysis in order to determine the effects of site characteristics on ground motion. The results reveal a potentially strong influence, particularly on the amplification factor for the city of Bandung, both for peak ground acceleration and for response spectral acceleration at 0.2 and 1.0 s. Our findings reinforce the need for detailed imaging and analysis of near surface structure in order to produce robust seismic hazard assessments.

Indonesia is located in one of the most seismically active regions in the world and often experiences damaging earthquakes. In the past the housing sector has sustained higher earthquake related damage and losses than other sectors. This is often attributed to the fact that the most common houses in Indonesia are non-engineered, built with poor quality workmanship, poor quality materials and without resilient seismic design features. However little effort has been made to quantify how fragile Indonesian houses are, or how their fragility may vary according to the population density or relative wealth of a region. It is not possible to derive empirical fragility functions for Indonesia due to insufficient damage data. The aim of this study is to determine whether existing earthquake fragility functions can be applied to common house types in Indonesia. Scenario damage analyses simulating the 2006 Yogyakarta and 2009 Padang events were undertaken several times testing different fragility functions. The simulated damage results were then compared to the damage observed post event to determine whether an accurate damage prediction could be achieved. It was found that the common house types in Yogyakarta and Central Java vary according to age of construction, location and relative wealth of a region and can be reasonably well represented by existing fragility functions. However, the houses in Padang and surrounding West Sumatra did not vary in a predictable manner and are more fragile than anticipated. Therefore, the fragility of the most common house types in Indonesia differs between Central Java and West Sumatra. This has important implications for seismic damage and risk assessment undertaken in Indonesia.

Tsunamis propagate over long distances and can cause widespread damage even after crossing ocean basins. Prediction of tsunamis in distant areas based on observations near their sources is critical to mitigating damage. In recent years, the accuracy of numerical models of trans-oceanic tsunami propagation has improved significantly due to the incorporation of effects such as the solid earth response to tsunami loading and wave dispersion. However, these models are computationally expensive and have not been fully utilized for real-time prediction. Here, we derive the adjoint operator for the linear set of equations describing deep-ocean tsunami propagation and show how a pre-computed database of adjoint states can achieve rapid synthesis of tsunami waveforms at target sites from nonpoint arbitrary tsunami sources. The adjoint synthesis method allows for an exhaustive parameter search for tsunami source estimation. A method for simultaneous inversion of fault geometry and slip distribution using adjoint synthesis with Sequential Monte Carlo method was proposed and applied to the 2012 Haida Gwaii earthquake tsunami. The influence of model accuracy and the amount of observed data on the estimation of tsunami sources and waveforms was examined. It was found that with a highly accurate propagation model, using only a limited amount of observed data produced source and waveform estimates very similar to the final models obtained with much larger data sets. The final inferred fault model involved megathrust slip distributed between the Haida Gwaii trench and the Queen Charlotte fault. The proposed method can also quantify the uncertainty of the waveform forecasts.

Eastern Queensland (Australia) was struck by a major earthquake at 04:14 a.m. local time on 7 June 1918. Most previous studies have suggested that the epicenter of this earthquake lies off the coast of Bundaberg, between the port cities of Gladstone and Rockhampton. This epicentral location was based upon instrumental observations from the Riverview College observatory in Sydney. However, this epicenter lies 250 km to the northeast of an inland region that experienced both the strongest shaking effects and numerous felt aftershocks. We revisited available macroseismic data from 224 geographic locations and surviving instrumental observations for the 1918 Queensland earthquake to show that the most likely epicentral location was inland at 24.93 degrees S and 150.88 degrees E in the Banana Shire and North Burnett region. The re-estimated instrumental magnitude of Mw 6.0 +/- 0.3 (1 sigma) makes it one of the largest onshore earthquakes in eastern Australia in the past century. Our observations also offer support for a viewpoint proposed in 1935 by an eminent Queensland geologist, Walter Heywood Bryan, that the 1918 earthquake was inland. Our study highlights the benefit of the critical evaluation of primary source materials, both archival and seismological, to study historical earthquakes in Australia that are relevant for modern seismic hazard analysis.

Given the projected population and economic growth in urban and regional Australia over the coming decades, a robust knowledge of past seismicity must be required for modern seismic hazard assessments. This is important because it influences the recurrence rates used in seismic source-rate models needed to characterize ground-motion hazard. Together with observations from a short modern instrumental record and a growing paleoseismic dataset, information on past seismicity in Australia is gleaned from records of historical earthquakes since the start of European colonization in the late 18th century. As with any scientific data, such documentary evidence extracted from newspapers and other written materials is subject to uncertainties and is prone to the repetition of misinterpreted incomplete data stemming from the absence of a modern re-examination of primary sources. We address these issues by consulting primary documentary sources at Australian libraries and archives to construct a dataset of over 4,000 uniformly assessed macroseismic observations (felt shaking effects) for earthquakes since 1788. These uniformly assessed data have led to the identification of discrepancies in the historical catalogue such as in the locations of the 1897 Beachport, 1918 Queensland and the 1954 Adelaide earthquakes for which catalogued epicentral locations are inconsistent with their macroseismic intensity distributions, felt aftershocks, environmental effects, and reviewed instrumental observations. Our study highlights the benefit of the re-evaluation of underutilized pre-instrumental documentary source materials to homogenize both archival and seismological observations, with modern observations, and to quantify uncertainties that are geological, numeric or societal. This will in turn improve our understanding of historical earthquakes in Australia and will benefit future assessments of modern seismic hazard in the country. We note further that the need to document, interpret, and analyse such underutilised pre-instrumental documentary source materials is not restricted to the analysis of historical earthquakes alone, but is also of benefit to the evaluation of other natural hazards not only in Australia but also around the world.

In 2018, four deadly (Mw 6.2-6.9) earthquakes struck the north coast of Lombok Island on 28 July, 5 August, and 19 August. The slip distributions of the three mainshocks are modeled in this study by inverting the co-seismic deformation imaged using an interferometric analysis of Sentinel-1 synthetic aperture radar measurements (InSAR), based on rectangular dislocations embedded in a multi-layered elastic half-space. Our best-fit co-seismic slip model suggests the estimated maximum fault slips of 1.3 m, 2.2 m, and 2.5 m for the mainshocks from July to August, located at depths of 9.6 km, 13.6 km, and 22.2 km, respectively. We applied an unsupervised learning method (ST-DBSCAN) to cluster the relocated aftershocks so that we could identify the source of each aftershock. The clustered aftershocks are primarily distributed in the areas with increased Coulomb stress and are less abundant in the maximum slip patch on the three rupture faults, indicating consistency with our estimated co-seismic slip model. In addition, we use an InSAR time series, consisting of 337 descending and 177 ascending Sentinel-1 acquisitions to investigate the time-dependent, post-seismic deformation in the two years following the Lombok 2018 earthquake sequence, based on a pure afterslip model and a combined model that simulates viscoelastic relaxation and afterslip simultaneously. The best-fit combined model suggests a Maxwell viscosity of 1 x 10(18) Pa s for both the lower crust and asthenosphere, and it reveals that the maximum of the cumulative afterslip within two years is similar to 0.7 m, along the northwestward up-dip continuation of the co-seismic rupture area.

The seismically active Sumatra subduction zone has generated some of the largest earthquakes in the instrumental record, and both historical accounts and paleogeodetic coral studies suggest these were large enough to transfer stress to the surrounding region, including the Great Sumatran Fault (GSF). Therefore, evaluating the stress transfer from these large subduction earthquakes could delineate segments of elevated stress along the GSF where large earthquakes may potentially occur. In this study, we investigated eight megathrust earthquakes from 1797 to 2010 and resolved the accumulated Coulomb stress changes onto 18 segments along the GSF. Additionally, we also estimated the rate of tectonic stress on the GSF segments which experienced large earthquakes. We considered two cases, with: (1) no forearc sliver movement, and (2) the forearc sliver movement suggested by recent studies. Based on the historical stress changes of large earthquakes and the increase in tectonic stress rate, we analyzed the time evolution of stress changes on the GSF. The Coulomb stress changes on the GSF due to megathrust earthquakes between 1797 and 1907 increased the Coulomb stress mainly on the southern part of GSF, which was followed by four major GSF events during 1890–1943. The estimation of tectonic stress rates using case (1) produces a low rate of stress accumulation and long recurrence intervals, which would imply that megathrust earthquakes play an important role in promoting the occurrence of GSF earthquakes. When implementing the arc-parallel sliver movement of case (2), the tectonic stress rates are much higher than case (1), with an observed slip rate of 15–16 mm/yr at the GSF consistent with a recurrence interval for full-segment rupture of 100–200 years. The case (2) result suggests that the occurrence of GSF earthquakes is dominantly controlled by the rapid arc-parallel forearc sliver motion. Furthermore, the analysis of the evolution of stress changes with time shows that some segments such as Tripa (North and South), Angkola, Musi and Manna, which have experienced full-segment rupture and are therefore likely locked, appear to have returned to stress levels similar to those prior to previous historical events, suggesting elevated earthquake hazard along these GSF segments.

In the past decade, cross-correlations of ambient seismic noise have been exploited in various applications to model the shallow-to-deep structure of Earth’s interior through tomographic inversions. The stack of cross-correlations between a 2-station pair represents empirical Green’s function and comprises the information of the subsurface structure between those stations. In practice, noise correlation function (NCF) is analyzed to reconstruct surface wave group or phase velocity dispersion; then, the dispersion data is used to model shear-wave velocity (Vs). This study presents a case for temporary seismic networks deployed in the Jakarta Basin; we applied a two-step routine to obtain a representative 1D Vs profile beneath an array. First, we extracted our array’s average phase velocity dispersion based on the relationship between NCF’s spectra and the Bessel function. Then, we invert for the 1D depth profile of Vs using a transdimensional Bayesian inversion to allow for exploring a number of layers in parameterizations. We successfully generate a 1D Vs profile up to 5 km depth reflecting the regional stratigraphy of the Jakarta Basin. In general, a sedimentary basin fill covers the area reaching a depth of 650 m. We suggest that this simple routine can be undertaken for other ambient noise cross-correlation cases; such a 1D depth profile would be beneficial to be used as a reference model.

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.

SUMMARYSituated on the northern coast of the Indonesian island of Java, Jakarta and its metropolitan area (Greater Jakarta) are subject to significant earthquake hazards from a subduction zone south of Java and nearby active crustal faults. The seismic risk may be even higher because Greater Jakarta resides on a sedimentary basin filled with thick Pliocene–Pleistocene sediments. A comprehensive study of Jakarta Basin's properties and geometry is important for creating robust seismic hazard and risk assessments. The main objective of this study is to develop a 3-D model of Jakarta Basin's shallow shear-wave velocity (VS ) structure and improve on previous models that did not cover the basin edge due to the extent of data coverage. Between April and October 2018, we deployed a new temporary seismic network to extend the spatial coverage beyond that of a previous deployment in 2013, and sampled 143 locations through sequential deployments of 30 broad-band sensors covering Jakarta and its adjacent areas. We conducted a 2-stage transdimensional Bayesian inversion of Rayleigh wave phase velocity dispersion curves derived from seismic noise. To begin, we applied tomography and constructed 2-D phase velocity maps for periods 1–5 s. Then, at each point in a regular grid defined on these maps, we invert each dispersion curve into 1-D depth profiles of VS . Finally, these profiles at gridpoints with ∼2 km spacing are interpolated to form a pseudo-3-D VS model. Our results reveal the edge of the Pliocene–Pleistocene sediments along the south. Also, we resolve a basement offset across south Jakarta that we suggest may be related to the western extension of the Baribis Fault (alternatively, the West Java Backarc Thrust). We recommend using this 3-D model of the Jakarta Basin for scenario earthquake ground motion simulations. Such simulations would help establish how important it might be to re-assess seismic hazard and risk in Greater Jakarta so that basin resonance and amplification are considered.

<p>In 2018, four deadly (Mw 6.2 to 6.9) earthquakes struck the north coast of Lombok Island, on 28 July, 5<sup> </sup>August, 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 25<sup>th</sup> October 2015 and 1<sup>st</sup> 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.</p>

ABSTRACT We present a new database called Gempa Nusantara, which is a collection of 7380 macroseismic observations for 1200 historical earthquakes in Indonesia between 1546 and 1950 C.E. using the European Macroseismic Scale (1998). Scrutinizing preserved original, first-hand, private, and official documentation from the colonial period in Indonesia, we could examine the completeness of this written record based on the gradual expansion of European influence in the Indonesian Archipelago. As the largest database of uniformly assessed macroseismic intensities ever assembled for Indonesia, our database can correct errors and fill gaps in other contemporary studies of historical Indonesian earthquakes, as well as paleoseismic studies such as the coral paleogeodetic record from Sumatra. Remarkably, given the presence of several major active faults, conclusive evidence of coseismic surface ruptures during the colonial period was limited to just two events in 1909 and 1933. Our reliance on original materials also allowed us to document extreme coseismic ground failure in Sumatra in 1936 with striking similarities to those observed on Sulawesi in 2018. From the perspective of seismic hazard in a rapidly urbanizing nation, we show that the frequencies of observed intensities over the duration of our database correspond with modern seismic hazard curves from recent publications by other authors for 12 Indonesian cities, including Jakarta, with some notable exceptions such as Ambon and Yogyakarta. In summary, our work on Gempa Nusantara demonstrates how a carefully vetted and well-documented historical record not only compliments studies of seismic hazard but is also itself an important standalone tool for the study of earthquake hazards in Indonesia.

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.

This paper presents a post-earthquake damage assessment of the 6.5 Mw earthquake in Pidie Jaya, Indonesia which occurred on December, 7th 2016. The earthquake killed 112 people and caused the collapse of about 3000 buildings. The assessment started on the day after the earthquake. The survey covered nine sub-districts of Pidie Jaya regency in the Province of Aceh, northern Sumatra. The post-event damage assessment was conducted by a team of researchers from Syiah Kuala University and local authorities. The damage survey assessed the damage levels of residential houses, school buildings and markets. The most common construction damages reported in this article are failures of reinforced concrete, confined masonry and timber structures. The ground motion field was modeled using Openquake application and the site response employed Vs30 values inferred from topography. This calculated ground motion was compared to data from post-earthquake field assessment reports. The highest percentage of heavy damage was recorded in the area of the fault rupture and along the shorelines area. The types of failure observed are summarized in the context of Indonesian construction practice, with particular reference to the seismic resilience of the Rumoh Aceh traditional wood construction.

SUMMARY Offshore Probabilistic Tsunami Hazard Assessments (offshore PTHAs) provide large-scale analyses of earthquake-tsunami frequencies and uncertainties in the deep ocean, but do not provide high-resolution onshore tsunami hazard information as required for many risk-management applications. To understand the implications of an offshore PTHA for the onshore hazard at any site, in principle the tsunami inundation should be simulated locally for every earthquake scenario in the offshore PTHA. In practice this is rarely feasible due to the computational expense of inundation models, and the large number of scenarios in offshore PTHAs. Monte Carlo methods offer a practical and rigorous alternative for approximating the onshore hazard, using a random subset of scenarios. The resulting Monte Carlo errors can be quantified and controlled, enabling high-resolution onshore PTHAs to be implemented at a fraction of the computational cost. This study develops efficient Monte Carlo approaches for offshore-to-onshore PTHA. Modelled offshore PTHA wave heights are used to preferentially sample scenarios that have large offshore waves near an onshore site of interest. By appropriately weighting the scenarios, the Monte Carlo errors are reduced without introducing bias. The techniques are demonstrated in a high-resolution onshore PTHA for the island of Tongatapu in Tonga, using the 2018 Australian PTHA as the offshore PTHA, while considering only thrust earthquake sources on the Kermadec-Tonga trench. The efficiency improvements are equivalent to using 4–18 times more random scenarios, as compared with stratified-sampling by magnitude, which is commonly used for onshore PTHA. The greatest efficiency improvements are for rare, large tsunamis, and for calculations that represent epistemic uncertainties in the tsunami hazard. To facilitate the control of Monte Carlo errors in practical applications, this study also provides analytical techniques for estimating the errors both before and after inundation simulations are conducted. Before inundation simulation, this enables a proposed Monte Carlo sampling scheme to be checked, and potentially improved, at minimal computational cost. After inundation simulation, it enables the remaining Monte Carlo errors to be quantified at onshore sites, without additional inundation simulations. In combination these techniques enable offshore PTHAs to be rigorously transformed into onshore PTHAs, with quantification of epistemic uncertainties, while controlling Monte Carlo errors.

We have successfully conducted the first ambient noise tomography on the island of Lombok, Indonesia using local waveform data observed at 20 temporary stations. Ambient noise tomography was used to delineate the seismic velocity structure in the upper crust. The waveform data were recorded from August 3rd to September 9th, 2018, using short-period and broadband sensors. There are 185 Rayleigh waves retrieved from cross-correlating the vertical components of the seismograms. We used frequency-time analysis (FTAN) to acquire the interstation group velocity from the dispersion curves. Group velocity was obtained for the period range of 1 s to 6 s. The group velocity maps were generated using the subspace inversion method and Fast Marching Method (FMM) to trace ray-paths of the surface waves through a heterogeneous medium. To extract the shear wave velocity (Vs) from the Rayleigh wave group velocity maps, we utilize the Neighborhood Algorithm (NA) method. The 2-D tomographic maps provide good resolution in the center and eastern parts of Lombok. The tomograms show prominent features with a low shear velocity that appears up to 4 km depth beneath Rinjani Volcano, Northern Lombok, and Eastern Lombok. We suggest these low velocity anomalies are associated with Quaternary volcanic products, including the Holocene pyroclastic deposits of Samalas Volcano (the ancient Rinjani Volcano) which erupted in 1257. The northeast of Rinjani Volcano is characterized by higher Vs, and we suggest this may be due to the presence of igneous intrusive rock at depth.

Coseismic changes in principal stress orientation in the northern Sumatra subduction zone due to two giant megathrust earthquakes there in 2004 and 2005 are estimated to investigate the in-situ stress. The two megathrust earthquakes, the 2004 Sumatra-Andaman and the 2005 Nias-Simeulue events, are both among the 11 largest earthquakes ever recorded. Previous studies have shown that these giant earthquakes perturbed the stress field in the Sumatra subduction zone enough to alter the principal stress directions there, and here we investigate whether these changes can be used to better understand spatial variations in stress along the subduction zone. We used 330 previously published focal mechanisms to estimate pre- and post-mainshock principal stress orientations in 3 outer forearc segments and assessed whether orientation differences were resolved and what they imply about the pre- and post-mainshock stress fields. Our results agree with previous studies in establishing that coseismic changes in stress orientation in the forearc are resolvable, and consistent with a low level of stress in the outer Sumatran forearc before the earthquake, with almost all the shear stress on the megathrust relieved in the 2004 and 2005 earthquakes. In this study, we reveal that both the stress orientations and coseismic changes in them exhibit along-strike variations, with a decrease in both the pre-mainshock stress and stress drop found in the rupture area of 2005 relative to that of the 2004 earthquake. The forearc segment between the 2004 and 2005 rupture areas, which coincides with a well-known megathrust rupture barrier beneath the island of Simeulue is observed to have a characteristic signature, with lower shear stress relative to the pre-mainshock stress field and higher shear stress relative to the post-mainshock stress field in the adjacent segments.