On 14 August 2021, the moment magnitude (
) 7.2 Nippes earthquake in Haiti occurred within the same fault zone as its devastating 2010
7.0 predecessor, but struck the country when field access was ...limited by insecurity and conventional seismometers from the national network were inoperative. A network of citizen seismometers installed in 2019 provided near-field data critical to rapidly understand the mechanism of the mainshock and monitor its aftershock sequence. Their real-time data defined two aftershock clusters that coincide with two areas of coseismic slip derived from inversions of conventional seismological and geodetic data. Machine learning applied to data from the citizen seismometer closest to the mainshock allows us to forecast aftershocks as accurately as with the network-derived catalog. This shows the utility of citizen science contributing to our understanding of a major earthquake.
SUMMARY
Quito, the capital of Ecuador, with more than 2.5 M inhabitants, is exposed to a high seismic hazard due to its proximity to the Pacific subduction zone and active crustal faults, both ...capable of generating significant earthquakes. Furthermore, the city is located in an intermontane piggy-back basin prone to seismic wave amplification. To understand the basin’s seismic response and characterize its geological structure, 20 broad and medium frequency band seismic stations were deployed in Quito’s urban area between May 2016 and July 2018 that continuously recorded ambient seismic noise. We first compute horizontal-to-vertical spectral ratios to determine the resonant frequency distribution in the entire basin. Secondly, we cross-correlate seismic stations operating simultaneously to retrieve interstations surface-wave Green’s functions in the frequency range of 0.1–2 Hz. We find that Love waves travelling in the basin’s longitudinal direction (NNE–SSW) show much clearer correlograms than those from Rayleigh waves. We then compute Love wave phase-velocity dispersion curves and invert them in conjunction with the HVSR curves to obtain shear-wave velocity profiles throughout the city. The inversions highlight a clear difference in the basin’s structure between its northern and southern parts. In the centre and northern areas, the estimated basin depth and mean shear-wave velocity are about 200 m and 1800 ms−1, respectively, showing resonance frequency values between 0.6 and 0.7 Hz. On the contrary, the basement’s depth and shear-wave velocity in the southern part are about 900 m and 2500 ms−1, having a low resonance frequency value of around 0.3 Hz. This difference in structure between the centre-north and the south of the basin explains the spatial distribution of low-frequency seismic amplifications observed during the Mw 7.8 Pedernales earthquake in April 2016 in Quito.
A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure ...consists of a NW‐SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE‐SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km‐long fault segments, adjacent and slightly offset from one another.
On 2 October 2020, the Maritime Alps in southern France were struck by the devastating Storm Alex, which caused locally more than 600 mm of rain in less than 24 h. The extreme rainfall and flooding ...destroyed regional rain and stream gauges. That hinders our understanding of the spatial and temporal dynamics of rainfall–runoff processes during the storm. Here, we show that seismological observations from permanent seismic stations constrain these processes at a catchment scale. The analysis of seismic power, peak frequency, and the back azimuth provides us with the timing and velocity of the propagation of flash-flood waves associated with bedload-dominated phases of the flood on the Vésubie River. Moreover, the combined short-term average to long-term average ratio and template-matching earthquake detection reveal that 114 local earthquakes between local magnitude ML=-0.5 and ML=2 were triggered by the hydrological loading and/or the resulting in situ underground pore pressure increase. This study shows the impact of Storm Alex on the Earth's surface and deep-layer processes and paves the way for future works that can reveal further details of these processes.
The two main earthquakes that occurred in 2012 (May 20 and 29) in the Reggio-Emiliano region (Northern Italy) were relatively small (Mw 6.1 and Mw 5.9) but they generated unexpected damages in a ...large area around the epicenter. On some stations, the observed seismic levels exceeded design levels recommended by the EC8 seismic code for buildings and civil engineering works. The ground motions generated by the two mainshocks have specific characteristics: the waveforms are mainly controlled by surface waves generated by the deep sedimentary Po plain, by local site effects and also, on some stations, by non-linear behaviors. In this particular context, we test the ability of an empirical Green’s function (EGF) simulation approach to reproduce the recorded seismograms in a large frequency band without any knowledge of the underground medium. We focus on the possibility to reproduce the strong surface waves generated by the basin at distances between 25 and 90 km. We choose to work on the second mainshock of the sequence (Mw 5.9), which occurred on May 29, 2012, because it is better recorded by the seismological networks than the May 20th first mainshock. We use a k-2 kinematic source model to generate a set of 100 slip distributions on the fault plane and choose the recordings of a close-by Mw 3.9 event as EGF. We then generate a set of broad-band seismograms (from 0.2 to 35 Hz) and compare them to the mainshock signals at 15 stations (Seismograms, Fourier spectra, PGA, PGV, duration, Stockwell Transforms) at epicentral distances from 5 to 160 km. We find that the main specific features of the signals are very well reproduced for all the stations within and beyond the basin. Nevertheless, at nearby stations, the PGA values are over-evaluated, which could be explained by the fact that non- linear effects are not taken into account in the simulation process. A better fit was found for a position of the nucleation point to the bottom west of the fault, that suggest a directivity effect of the rupture process of the May 29th event towards the North–East.
Time-series bathymetric data acquired from 1967 to 2011 are used to evaluate the morphological evolution of the continental shelf and upper continental slope off the city of Nice (SE France, Ligurian ...Sea). Mapping in water depths of 0–300m was undertaken to identify the changing morphology of landslide scars and their erosive chutes. Quantitative Digital Terrain Model (DTM) comparisons reveal areas of erosion and deposition over intervals of 5–8years. Sediment remobilization events on the upper slope (above depths of 200m) are frequent and significant; landslide scars with volumes>25,000m3 can occur with frequencies of <8years. Shelf break retreat toward the coastline can reach 60m over 7–8years where the continental shelf is wider (>200m). Periods of quiescence (1980–1990 and 2006–2011) are seen to alternate with periods when rapid retrogressive failure increase sediment volumes eroded from the upper slope-shelf transition by an order of magnitude (1999–2006). Temporal variations in landslide activity were correlated to several potential triggering factors that individually would not induce failures, including earthquake activity, rapid deposition of fine-grained sediments on a steep slope, and rainfall leading to fresh groundwater circulation below the shelf. This 4D bathymetric study suggests that over the last 50years the most important factor triggering landslides offshore Nice is overpressure due to freshwater outflows.
•Repetitive bathymetries show that landslides are active over periods<8years.•Sediment budget involved in individual landslide is about 103 to 106m3.•Frequency is <25years for 105–106m3 landslides and <5years for landslides<105m3.•The conjunction of several minor-magnitude factors is able to induce landslides.•The dominant factor favoring slope failure on the Var delta is excess-pore pressure due to high sedimentation rates and to high groundwater levels in the Var aquifers.
•We characterise the aftershocks sequence of the 2016 Mw 7.8 Ecuador earthquake.•We observe 3 permanent bands of seismicity also observed in the interseismic period.•We observe a spatio-temporal ...dependency of aftershocks with afterslip.•Distribution of seismic/aseismic slip correlates with that of bathymetric anomalies.•We propose a bimodal slip mode mechanically controlled by subduction of oceanic relief.
We characterise the aftershock sequence following the 2016 Mw=7.8 Pedernales earthquake. More than 10,000 events were detected and located, with magnitudes up to 6.9. Most of the aftershock seismicity results from interplate thrust faulting, but we also observe a few normal and strike-slip mechanisms. Seismicity extends for more than 300 km along strike, and is constrained between the trench and the maximum depth of the coseismic rupture. The most striking feature is the presence of three seismicity bands, perpendicular to the trench, which are also observed during the interseismic period. Additionally, we observe a linear dependency between the temporal evolution of afterslip and aftershocks. We also find a temporal semi-logarithmic expansion of aftershock seismicity along strike and dip directions, further indicating that their occurrence is modulated by afterslip. Lastly, we observe that the spatial distribution of seismic and aseismic slip processes is correlated to the distribution of bathymetric anomalies associated with the northern flank of the Carnegie Ridge, suggesting that slip in the area could be influenced by the relief of the subducting seafloor. To explain our observations, we propose a conceptual model in which the Ecuadorian margin is subject to a bimodal slip mode, with distributed seismic and aseismic slip mechanically controlled by the subduction of a rough oceanic relief. Our study sheds new light on the mechanics of subduction, relevant for convergent margins with a complex and heterogeneous structure such as the Ecuadorian margin.
Slip distributions of the moderate magnitude (Mw 5.9), 1999 Athens earthquake, inverted from surface waves and interferometric Synthetic Aperture Radar (SAR) images, show very different ...characteristics. The robustness analysis proposed in this study, confirms the discrepancy between the well-constrained features of each individual solution. Irrespective of the hypotheses we made (data/modeling errors, slow deformation, post- or pre-seismic slip), the joint inversion of the two data sets led to a complex and heterogeneous rupture model. This model is characterized by a short rise time (<5 s) slip patch centred on the hypocentre, extending bilaterally up to 4 km depth and down to 17 km and releasing approximately 70 per cent of the total moment. Located further to the WNW and releasing the remaining 30 per cent of the total moment, a long rise time slip patch extends from 8 to 17 km depth. If the short rise time slip patch propagated above and below the brittle zone delineated by the aftershocks, the long rise time slip patch (slow deformation) appears to be mostly confined below the brittle zone. This unified model satisfies the analysis of the seismic and geodetic slip distributions as well as the location of the aftershock sequence and attests to the diversity of the crustal response even for moderate size faults.
A new seismically active zone is found in the southern part of the Ligurian basin, 80-km west of Corsica (western Mediterranean). The activity began in February 2011 with a foreshock (ML 4) and a ...mainshock (ML 5.3) 5days later, followed by numerous aftershocks. We first analyze the fore- and mainshock in detail. We compare the results obtained using classical methods (linear location in a 1D medium and focal mechanisms from P and S polarities) and new approaches (non-linear location in a 3D medium and waveform modeling for determining the seismic moment and the focal mechanism). Both methods provided similar results for location, depth (in the range of 6–13km) and focal mechanisms, which reveal reverse faulting with nodal planes oriented N–S and NE–SW. We then locate 27 of the aftershocks in the 3D model and find a 10-km-long NE–SW alignment with a depth between 7 and 16km. In 2012 and 2013, three other moderate-size events (ML 3.8, 4 and 4.5) occurred and confirm that this zone is still active. The epicentral area is located in the oceanic domain of the Ligurian Basin. From analysis of the bathymetry and high-resolution multi-channel seismic profiles, no morphologic anomaly at surface and no inherited fault in the shallow ~4km depth were imaged, which suggest that no significant deformation occurred in the area since 5Ma. Thus, the structure(s) activated during the 2011–2013 sequence remain unknown. In light of these results, we point out a notable difference on both sides of the Ligurian Basin: the northern margin, close to the alpine chain, suffered strong earthquakes and large cumulated deformation since 5Ma, while the southern margin, close to the Corsica–Sardinia continental block, is poorly deformed since 5Ma.
•A sequence of unusual, moderate-size earthquakes occurred in a seismically quiet area offshore western Corsica.•The sequence includes the largest earthquake ever instrumentally recorded in the area.•A reverse faulting mechanism is consistent with shortening between the Corsica–Sardinia block and the European continent.•Determination of a 6–13km focal depth from 3 different methods for the mainshock (2011/07/07, ML 5.3).•No fault is known in the epicentral area from MCS profiles.
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