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Detection of seismic events at or below the noise level is enabled by the use of dense arrays of receivers and corresponding advances in data analysis methods. It is not only important to detect tectonic events, but also events from man-made, non-earthquake sources and events that originate from coupling between the solid Earth and the atmosphere. In urban environments with high ambient noise levels the effectiveness of event detection methods is unclear, particularly when deployment restrictions result in an irregular receiver array geometry. Here, we deploy a dense nodal array for 1 month in the highly populated city state of Singapore. We develop a new detection method based on image processing that we call spectrogram stacking, which detects anomalous, coherent spectral energy across the array. It simultaneously detects multiple classes of signal with differing spectral content and aids event classification, so it is particularly useful for signal exploration when signal characteristics are unknown. Our approach detects more local events compared to the traditional short-term average over long-term average and waveform similarity methods, while all methods detect similar numbers of teleseismic and regional earthquakes. Local events are principally man-made non-earthquake sources, with several events from the same location exhibiting repeating waveforms. The closest earthquake occurs in peninsular Malaysia, in an area where no earthquakes have previously been detected. We also detect ground motion over a wide frequency range from discrete thunder events that show complex coupling between acoustic and elastic wavefield propagation. We suggest that care should be taken deciphering local high-frequency tectonic events in areas prone to thunder storms. [ABSTRACT FROM AUTHOR]

Abstract: We estimated the crustal thickness and velocity structure beneath the five stations comprising the Republic of Singapore’s seismic network. Our data set was composed of 697 teleseismic receiver functions and 7months of broad-band data that was cross-correlated to produce inter-station Green’s functions. Surface wave group velocities were extracted from the Green’s functions to obtain dispersion data for a path from central Sumatra to Singapore in order to provide a complimentary data set to the receiver functions. Crustal thickness was estimated via an H − k stacking technique, and high-resolution 1D P-wave velocity profiles were generated beneath each station by jointly inverting receiver function stacks and the group velocity data using a linearised time-domain inversion scheme. Crustal thickness beneath four stations was found to be between 28.0km and 32.0km, while one station in the northeast of Singapore indicates 24.0km thick crust. This implies a significant crustal thinning beneath Singapore over the lateral extent of 50.0km. Inversion results exhibit several crustal features that are observable in the derived models at all five stations, indicating that they exist across Singapore as a whole. There appears to be an upper-crustal high-velocity zone beneath Singapore, underlain by a velocity inversion. Station NTU shows slower near-surface velocities than the other stations, consistent with its situation above the sedimentary Jurong formation. These results expand the available global velocity data set, as well as being useful for assessing the seismic hazard in Singapore. [Copyright &y& Elsevier]

Gede Volcano is one of the active volcanoes in Indonesia located about 60km from the capital Jakarta. Due to its proximity to the large population around Gede, Center for Volcanology and Geological Hazard Mitigation (CVGHM) closely monitored this volcano since 1985. It is considered active; however, there has not been any eruption in the past 60 years. Historical records suggest that in the past few centuries, Gede have had frequent VEI 1-2 sometimes Vulcanian-type eruption and sometimes small-volume extrusion of viscous lava. Since the first installation of seismometer and observation post, Gede is observed to exhibit seismic swarm every few years with only minor visible degassing from its central craters. In collaboration between Earth Observatory of Singapore (EOS) and CVGHM, we gradually upgraded seismic, tilt, geochemical and hydrological monitoring network on Gede Volcano and the neighbouring volcano Salak to further investigate its potential activity and hazard associated with its eruption. Our seismic network recorded swarms consists of tens to a couple hundred Volcano Tectonic (VT) earthquakes in few days to a few weeks period. The initial study of swarms of earthquakes and tilt shed light into possible periods of magma intrusion that did not reach the surface with some control of the tectonic forces around Gede Volcano. However, another later on study suggests since seismic swarms associated with no deformation and no seismic migration, the swarms probably caused by interaction of meteoric water and existing remain of magmatic bodies under the volcano. Further evaluations suggest that earthquakes swarms consist of not only VT type earthquakes but also long-period (LP) earthquakes that occurred at shallow depths. We conducted three-dimensional seismic P-wave travel time tomography to image the magma sources beneath Gede Volcano. We used travel time arrivals from > 700 VT earthquakes that occurred beneath the volcano over the period 2011-2018. Combining results from the tomography and long-period earthquakes analyses, we aim is to further find the magmatic body under the volcano at shallow depths (< 2 km) up to about 8km. [ABSTRACT FROM AUTHOR]