Interseismic coupling, segmentation and mechanical behavior of the Central Chile subduction zone.

International audienceGlobal Positioning System (GPS) measurements carried out in Chile over the last two decades showed that an entire portion of the Nazca-South America subduction zone (38°S ␣ 24°S) was locked over this period of time. The induced accumulation of elastic deformation in the upper-plate was not released until the recent Maule earthquake of 27 February 2010 (Mw 8.8) that ruptured the southern part of this section. Locking or coupling between the two plates varies both with depth and along strike. Here we use our own GPS data (an updated solution of our extended network in central Chile), combined with other published data sets, to quantify the spatial variations of the coupling that prevailed before the Maule earthquake. Using a simple elastic model based on the back-slip assumption, we show that coupling variations on the subduction plane are sufficient to explain the observed surface deformation, with no need of a sliver in central Chile. We identify four segments characterized by higher coupling and separated by narrow areas of lower coupling. This segmentation is in good agreement with historical and recent seismicity in Chile. In particular the narrow zones of lower coupling seem to have stopped most large seismic ruptures, including Maule's. These zones are often associated with irregular bathymetric or coastal features (fracture zones or peninsulas). Finally, coseismic and early post-seismic slip distribution of the Maule earthquake, occurring either in previously highly or weakly coupled zones, map a complex distribution of velocity-weakening and velocity-strengthening patches on the subduction interface

Seismological analyses of the 2010 March 11, Pichilemu, Chile M_w 7.0 and M_w 6.9 coastal intraplate earthquakes

On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around M_w 7.0 and M_w 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, M_w 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances (Δ > 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k^(−2) earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude M_w 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, M_w 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event

Andean structural control on interseismic coupling in the North Chile subduction zone

Segmentation can influence the extent of earthquake rupture and event magnitude: large megathrust earthquakes result from total rupture of relatively continuous segments of the subduction interface. Segmentation is attributed to variations in the frictional properties of the seismogenic zone or to topographic features on the down-going plate. Structures in the overriding plate may also influence segmentation but their importance has been dismissed. Here, we investigate the links between interface segmentation at the North Chile seismic gap and a crustal-scale fault structure in the overriding plate that forms a coastal scarp of about 1 km in height. We use satellite interferometric synthetic aperture radar (InSAR) and Global Positioning System (GPS) data to measure interseismic surface deformation between 2003 and 2009 and compare the deformation with rupture extent during well-documented earthquakes. From these data we infer the degree of coupling and segmentation at depth. We find that along a 500-km-long segment, the base of the strongly coupled seismogenic zone correlates with the line of the surface coastal scarp and follows the outline of the Mejillones Peninsula. This correlation implies that large-scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth. We therefore suggest that the occurrence of megathrust earthquakes in northern Chile is controlled by the surface structures that build Andean topography

Slow slip detection with deep learning in multi-station raw geodetic time series validated against tremors in Cascadia

Slow slip events (SSEs) originate from a slow slippage on faults that lasts from a few days to years. A systematic and complete mapping of SSEs is key to characterizing the slip spectrum and understanding its link with coeval seismological signals. Yet, SSE catalogues are sparse and usually remain limited to the largest events, because the deformation transients are often concealed in the noise of the geodetic data. Here we present the first multi-station deep learning SSE detector applied blindly to multiple raw geodetic time series. Its power lies in an ultra-realistic synthetic training set, and in the combination of convolutional and attention-based neural networks. Applied to real data in Cascadia over the period 2007-2022, it detects 78 SSEs, that compare well to existing independent benchmarks: 87.5% of previously catalogued SSEs are retrieved, each detection falling within a peak of tremor activity. Our method also provides useful proxies on the SSE duration and may help illuminate relationships between tremor chatter and the nucleation of the slow rupture. We find an average day-long time lag between the slow deformation and the tremor chatter both at a global- and local-temporal scale, suggesting that slow slip may drive the rupture of nearby small asperities

Detection of slow slip events along the southern Peru - northern Chile subduction zone

Detections of slow slip events (SSEs) are now common along most plate boundary fault systems at the global scale. However, no such event has been described in the south Peru - north Chile subduction zone so far, except for the early preparatory phase of the 2014 Iquique earthquake. We use geodetic template matching on GNSS-derived time series of surface motion in Northern Chile to extract SSEs hidden within the geodetic noise. We detect 33 events with durations ranging from 9 to 40 days and magnitudes from Mw 5.6 to 6.2. The moment released by these aseismic events seems to scale with the cube of their duration, suggesting a dynamic comparable to that of earthquakes. We compare the distribution of SSEs with the distribution of coupling along the megathrust derived using Bayesian inference on GNSS- and InSAR-derived interseismic velocities. From this comparison, we obtain that most SSEs occur in regions of intermediate coupling where the megathrust transitions from locked to creeping or where geometrical complexities of the interplate region have been proposed. We finally discuss the potential role of fluids as a triggering mechanism for SSEs in the area

Observation of a Synchronicity between Shallow and Deep Seismic Activities during the Foreshock Crisis Preceding the Iquique Megathrust Earthquake

We analyze at a broad spatial scale the slab seismicity during one of the longest and best recorded foreshock sequence of a subduction earthquake to date: the M8.1 2014 Iquique earthquake in Chile.  We observe the synchronisation of this sequence with seismic events occurring in the deep slab (depth ~100km). This synchronisation supports the existence of long-range seismic bursts already observed in the Japan Trench subduction. It suggests that, like for the 2011 Tohoku earthquake, the deep slab was involved in the nucleation process of the Iquique earthquake.  We interpret these observations by the presence of pressure pulses propagating in transient fluid channels linking the deep slab where dehydration occurs to the shallow seismogenic zone before the earthquake. These observations may seem surprising but they are in line with the short-lived pulse-like channelized water escape from the dehydration zone predicted by recent studies in slab mineralogy and geochemistry

Relation Between Oceanic Plate Structure, Patterns of Interplate Locking and Microseismicity in the 1922 Atacama Seismic Gap

We deployed a dense geodetic and seismological network in the Atacama seismic gap in Chile. We derive a microseismicity catalog of >30,000 events, time series from 70 GNSS stations, and utilize a transdimensional Bayesian inversion to estimate interplate locking. We identify two highly locked regions of different sizes whose geometries appear to control seismicity patterns. Interface seismicity concentrates beneath the coastline, just downdip of the highest locking. A region with lower locking (27.5°S–27.7°S) coincides with higher seismicity levels, a high number of repeating earthquakes and events extending toward the trench. This area is situated where the Copiapó Ridge is subducted and has shown previous indications of both seismic and aseismic slip, including an earthquake sequence in 2020. While these findings suggest that the structure of the downgoing oceanic plate prescribes patterns of interplate locking and seismicity, we note that the Taltal Ridge further north lacks a similar signature

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics