Gempa Nusantara: a database of 7380 macroseismic observations for 1200 historical earthquakes in Indonesia from 1546 to 1950

We present a new database called Gempa Nusantara, which is a collection of 7380 macro-seismic 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 cor-respond 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.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionStacey S. Martin and Aron J. Meltzner were financially supported by the Earth Observatory of Singapore and by the National Research Foundation (NRF) Singapore and the Singapore Ministry of Education (MoE) under the Research Centers of Excellence initiative. Stacey S. Martin was also supported by the Australian National University (ANU) Research Scholarship (738/2018). Aron J. Meltzner was also supported by the National Research Foundation (NRF) Singapore under its NRF Fellowship scheme (Award Number NRF-NRFF11-2019-0008)

Time reversal imaging of the tsunami source

In this paper, we apply time reversal imaging (TRI) to the problem of reconstructing the initial sea surface displacement that generates a tsunami. We discuss theoretical considerations in the application of TRI to the tsunami problem, including time reversibility and reciprocity of the shallow-water equations. Several numerical experiments are conducted to establish the efficacy of TRI in the tsunami context. TRI is applied to observations of the tsunami generated by the Tohoku earthquake on March 11, 2011, for which an unprecedented number of high-quality observations are available. Finally, we compare the findings of the TRI results with other, more conventional methods of source inversion. Results indicate that TRI is effective for imaging a tsunami source when a sufficient number of observations are available. Because it involves fewer assumptions about the nature of the tsunami source, in particular those regarding source location and fault geometry, we believe that TRI has the potential to improve our understanding of tsunami generation—for example, through detection of non-seismic components of the tsunami source.This research was supported by the Australian Research Council Discovery Project (DP110101983)

Earthquake Geology of the Lembang Fault, West Java, Indonesia

The Lembang Fault is a major fault in western Java that skirts the northern edge of Bandung, one of Indonesia's largest cities, just south of the active Tangkuban Perahu volcano. Although it has no recorded or historical large earthquakes, the Lembang Fault shows obvious geomorphic evidence of recent activity and has long been thought to be active. In this study, we use geomorphic analysis to unequivocally establish that the fault has a dominantly sinistral sense of movement with a slip rate of 1.95–3.45 mm/yr. This proves that the fault is accommodating trench parallel slip resulted from a slight obliquity in plate convergence at the JavaTrench. With a length of 29 km, this suggests that the Lembang Fault could produce a Mw 6.5–7.0 earthquake with a recurrence time of 170–670 years. We also conducted paleoseismological trenching of the Lembang Fault and found evidence for at least 3 earthquakes in the 15th century, 2300–60 BCE and 19,620–19,140 BP. The 2300–60 BCE earthquake had a measurable vertical displacement of 40 cm, which is consistence with a Mw 6.5 earthquake. This is the first mapping of a source of crustal earthquakes in Java, Indonesia, the world's most densely populated island in one of its most tectonically active areas. The Lembang Fault and other faults in Java are likely to pose substantial risk to not only Bandung but many of Java's major urban agglomerations

Accurate numerical simulation of the far-field tsunami caused by the 2011 Tohoku earthquake, including the effects of Boussinesq dispersion, seawater density stratification, elastic loading, and gravitational potential change

In this study, we considered the accurate calculation of far-field tsunami waveforms by using the shallow water equations and accounting for the effects of Boussinesq dispersion, seawater density stratification, elastic loading, and gravitational potential change in a finite difference scheme. By comparing numerical simulations that included and excluded each of these effects with the observed waveforms of the 2011 Tohoku tsunami, we found that all of these effects are significant and resolvable in the far field by the current generation of deep ocean-bottom pressure gauges. Our calculations using previously published, high-resolution models of the 2011 Tohoku tsunami source exhibited excellent agreement with the observed waveforms to a degree that has previously been possible only with near-field or regional observations. We suggest that the ability to model far-field tsunamis with high accuracy has important implications for tsunami source and hazard studies

Direct-seismogram inversion for receiver-side structure with uncertain source-time functions

International audienceThis paper presents direct-seismogram inversion (DSI) for receiver-side structure which treats the source signal incident from below (the effective source-time function-STF) as a vector of unknown parameters in a Bayesian framework. As a result, the DSI method developed here does not require deconvolution by observed seismogram components as typically applied in receiver-function inversion and avoids the problematic issue of choosing subjective tuning parameters in this deconvolution. This results in more meaningful inversion results and uncertainty estimation compared to classic receiver-function inversion. A rigorous derivation is presented of the likelihood function required for unbiased inversion results. The STF is efficiently inferred by a maximum-likelihood closed-form expression that does not require deconvolution by noisy waveforms. Rather, deconvolution is only by predicted impulse responses for the unknown environment (considered to be a 1-D, horizontally stratified medium). For a given realization of the parameter vector which describes the medium below the station, data predictions are computed as the convolution of the impulse response and the maximum-likelihood source estimate for that medium. Therefore, the assumption of a Gaussian pulse with specified parameters, typical for the prediction of receiver functions, is not required. Directly inverting seismogram components has important consequences for the noise on the data. Since the signal processing does not require filtering and deconvolution, data errors are less correlated and more straightforward to model than those for receiver functions. This results in better inversion results (parameter values and uncertainties), since assumptions made in the derivation of the likelihood function are more likely to be met by the inversion process. The DSI method is demonstrated for simulated waveforms and then applied to data for station Hyderabad on the Indian craton. The measured data are inverted with both the new DSI and traditional receiver-function inversion. All inversions are carried out for a trans-dimensional model that treats the number of layers in the model as unknown. Results for DSI are consistent with previous studies for the same location. The DSI has clear advantages in trans-dimensional inversion. Uncertainty estimates appear more realistic (larger) in both model complexity (number of layers) and in terms of seismic velocity profiles. Receiver-function inversion results in more complex profiles (highly-layered structure) and suggests unreasonably small uncertainties. This effect is likely also significant when the parametrization is considered to be fixed but exacerbated for the trans-dimensional model: If hierarchical errors are poorly estimated, trans-dimensional models overestimate the structure which produces unfavourable results for the receiver-function inversion

Simple deformation modelling using GNSS GPS data at Minahasa subduction

North Sulawesi Trench or Minahasa subduction area is a subduction zone between the oceanic crust of Sulawesi Sea and the North Sulawesi Arm located at the triple junction in Eastern Indonesia. This subduction activity causes the North Arm of Sulawesi as an earthquake-prone area. Tectonic activities in the region can be studied through geodetic monitoring using GNSS GPS observations and by physical modelling from the rate of geodetic geometric results. Yearly GNSS GPS campaign have been conducted in the region from 1997 to 2008 and continuously observed by BIG from 2008 to 2016 using permanent GNSS GPS stasion. The coordinates of monitoring stations realized in ITRF-2008 provide residual RMS values of 3.13 mm, 4.15 mm and 7.26 mm for the northern, eastern and vertical components, where this indicates a high degree of accuracy. A simple estimation profile using GNSS GPS data based on the Okada elastic equation for the subduction zone shows a subduction movement ranging from 4 to 5 cm/yr with a locking depth of about 50 km, a dip 300 and ending in the post-seismic phase due to the sequence of earthquakes occurring in Minahasa since January 1, 1996 Mw 7.9 to 16 June 2002 Mw 5.9.</p

Numerical Experiments of Shear Deformation with Frictional Heating

We develop a thermal-mechanical model for describing the formation of shear zones. We consider shear deformation of a two-dimensional rectangular region composed of a viscous fluid under a constant velocity at the boundary. Viscosity of the material is assumed to depend only on temperature for simplicity. In order to enhance the shear deformation, we included a small inclusion whose viscosity differs from that of the surrounding material. We carried out time-marching simulations and monitored the evolution of temperature and strain around the inclusion. Our results show that the deformation localizes in a narrow region when sufficient heat is generated by viscous dissipation. In the zone of localized deformation, temperature increases by several hundred degrees owing to strong dissipative heating. We found that the zone of localized deformation develops in the region where the greatest heat is generated at the initial stage of evolution. The zone of localized deformation pierces the inclusion when the inclusion is weaker than the surrounding material, while it develops away from the inclusion when the inclusion is stiffer than the surrounding material. These findings, though qualitatively, suggest that heating by viscous dissipation may play an important role in the formation of decollements around subducted seamounts