Rockfalls generate seismic signals that can be used to detect and monitor rockfall activity. Event locations can be estimated on the basis of arrival times, amplitudes or polarization of these seismic signals. However, surface topography variations can significantly influence seismic wave propagation and hence compromise results. Here, we specifically use the signature of topography on the seismic signal to better constrain the source location. Seismic impulse responses are predicted using Spectral Element based simulation of 3D wave propagation in realistic geological media. Subsequently, rockfalls are located by minimizing the misfit between simulated and observed inter-station energy ratios. The method is tested on rockfalls at Dolomieu crater, Piton de la Fournaise volcano, Reunion Island. Both single boulder impacts and distributed granular flows are successfully located, tracking the complete rockfall trajectories by analyzing the signals in sliding time windows. Results from the highest frequency band (here 13-17\,Hz) yield the best spatial resolution, making it possible to distinguish detachment positions less than 100\,m apart. By taking into account surface topography, both vertical and horizontal signal components can be used. Limitations and the noise robustness of the location method are assessed using synthetic signals. Precise representation of the topography controls the location resolution, which is not significantly affected by the assumed impact direction. Tests on the network geometry reveal best resolution when the seismometers triangulate the source. We conclude that this method can improve the monitoring of rockfall activity in real time once a simulated database for the region of interest is created.

Low-frequency earthquakes are peculiar energy-release events mostly occurring at the transition between the seismogenic and the freely creeping zones of a subducting slab. The source characterization of these events is of fundamental importance to understand physical processes that govern the slow out of equilibrium evolution of the subduction interface that may lead to the generation of large, destructive earthquakes. Nevertheless, their source mechanisms still remain unclear. Here, we estimate the source parameters of ~23,000 low-frequency earthquakes continuously detected from 2013 to 2015 in Shikoku, Japan. We show that a cubic moment-duration scaling characterizes these events, suggesting a self-similar process as for regular earthquakes. However, their high-frequency fall-off suggests an omega-cube decay in contrast to the omega-squared model of earthquakes. Source characteristics do not change when low-frequency earthquake bursts occur during the analyzed three years. On the other hand, we observe a coherent along-strike variation of the product of stress drop and the cube of rupture velocity, possibly related to a weaker behavior of tremor patches in central Shikoku. Secondary microseismic noise and network-dependent completeness magnitude lead to missing event detections that do not allow discriminating between Gutenberg-Richter event size distribution and any deviation from it. Our findings suggest that the same observational limits might affect worldwide detection of low-frequency earthquakes.

The 2014 Iquique seismic crisis (Chile), culminating with a Mw 8.1 earthquake, April 1st, highlights a complex unlocking of the North Chile subduction interface which has been considered as a seismic gap since 1877. During the year preceding this event, at least three seismic clusters were observed: in July 2013 and January and March 2014. These clusters possibly indicate aseismic slip transients accompanying the progressive destabilization of the plate contact. Recent studies have proposed large-scale slab deformation as a potential trigger for the megathrust earthquake; However, no evidence of gradual unlocking of the interface or transient deformation has yet been found in the seismic rate. To address this question, we develop a dense earthquake catalog during the fifteen months preceding the mainshock from the continuous waveform dataset recorded by the Integrated Plate Boundary Observatory Chile (IPOC) and Iquique Local Network (ILN) networks. After declustering the seismicity, a space-time analysis highlights a large-scale acceleration of the seismicity along the interface while it decelerates at intermediate-depths. We then demonstrate the existence of a seismic quiescence down-dip of the mainshock rupture before the July 2013 cluster. We propose that this seismic quiescence is related to fluid circulation and/or aseismic motion along upper-plate crustal fault(s).