Mosaicking Andean morphostructure and seismic cycle crustal deformation patterns using GNSS velocities and machine learning

We use unsupervised machine learning techniques to analyze continental-scale crustal motions in areas affected by the seismic cycle of large subduction earthquakes along the Chilean Trench. Specifically, we use the agglomerative clustering algorithm as an exploratory tool to investigate spatial patterns in GNSS regional velocities without the complexity of modeling a physical source. We present a continental-scale velocity field including all available GNSS data for two-time windows (pre-2014, 2018–2021) that represents two periods with different deformation patterns of the seismic cycle. We test two different pre-processing methodologies for the design of machine learning features from the GNSS-derived velocities. The first method uses the direction and magnitude of the secular rates as input features to the clustering algorithm. These results show a clustering spatially related to seismic cycle deformation, separating latitudinal segments with different velocities in the fore-arc and back-arc, as well as regions affected by postseismic relaxation. Thus, highlighting the effectiveness of this method for mapping first-order patterns of active deformation in a subduction zone, that are particularly related to variations on interplate coupling and postseismic transient deformation. In a more sophisticated approach, we use surface strain and rotational rates from GNSS velocities as features in the second methodology. Here, we develop a novel methodology to estimate strain and rotation rates accounting for the spatial heterogeneity of the GNSS-network. We determine the spatial scale at which these features are estimated by least squares inversions, by using a Bayesian model class selection method. The distribution of stations allows to identify heterogeneities in strain and rotation rates at spatial scales larger than 50 km, being particularly notorious the main features of regional deformation at scales > 100 km. Interestingly, the results show a spatial correlation between seismic segmentation in the fore-arc and geologic and structural domains in the arc and back-arc. Our results demonstrate the ability of the combination of inverse and machine learning methods to efficiently identify active deformation patterns and their relationship to the subduction seismic cycle and regional-scale geological structures. Furthermore, our analysis suggests that Andean geological structures influence the observed deformation field

Anomalously steep dips of earthquakes in the 2011 Tohoku-Oki source region and possible explanations

The 2011 M_w 9.1 Tohoku-Oki earthquake had unusually large slip (over 50 m) concentrated in a relatively small region, with local stress drop inferred to be 5–10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (M_w 5.5–7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the M_w 9.1 event had steeper dip angles (by 5–10°) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5–10°, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues

Anomalously steep dips of earthquakes in the 2011 Tohoku-Oki source region and possible explanations

Keywords: subduction zone the 2011 Tohoku-Oki earthquake focal mechanism fault geometry seamount fault zone structure a b s t r a c t The 2011 M w 9.1 Tohoku-Oki earthquake had unusually large slip (over 50 m) concentrated in a relatively small region, with local stress drop inferred to be 5-10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (M w 5.5-7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the M w 9.1 event had steeper dip angles (by 5-101) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5-101, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues

Spatiotemporal slip distribution associated with the 2012–2016 Tokai long-term slow slip event inverted from GNSS data

Abstract We used Global Navigation Satellite System (GNSS) time series data to estimate the spatiotemporal slip distribution for a long-term slow slip event (L-SSE) that occurred in the Tokai region, central Japan, from 2012 to 2016. Since all the used GNSS data were affected by the postseismic deformation associated with the 2011 Mw9.0 Tohoku-Oki earthquake, we removed such postseismic signal from the time series of three components at each of the stations. The minimal time window for an inversion analysis was set to 0.5 years (6 months), taking into account the signal-to-noise ratio of displacements for each time window. In the horizontal displacement fields, displacements were observed in the south‒southeast and southeast directions on the west and east sides of Lake Hamana, respectively, with temporal changes in their amounts and directions. In the vertical displacement fields, uplift was observed on the east side of Lake Hamana. From these data, we estimated the L-SSE initiated in approximately 2012.5 and ended by 2017.0, indicating the duration time is 4.5 years and the duration was much longer than that obtained in a previous study. Using these data, we performed the inversion analysis, in which three a priori information were assumed, i.e., the spatial distribution of slip is smooth, slip mainly occurs in the direction of plate convergence, and the temporal variation in the slip is smooth, to obtain the spatiotemporal slip distribution on a plate boundary with 3-D geometry. As a result, we identified that the L-SSE consisted of two subevents. The first subevent initiated on the southwest side of Lake Hamana and expanded during the period from 2013.0 to 2014.5. The maximum slip velocity during the period from 2012.5 to 2017.0 was estimated to be approximately 3.5 cm/year there for 2013.5–2014.0. The second subevent took place on the west side of Lake Hamana gradually from 2015.0 to 2015.5, continued, and expanded from 2015.5 to 2016.5. From the cumulative slip distribution, we found that its shape spread in the dip direction and obtained a maximum slip of approximately 10.6 cm, a moment release of 2.7 × 1019 Nm, and an equivalent moment magnitude of 6.9. Comparing our results with the L-SSE that occurred in the Tokai region between 2000 and 2005, we found that the slip initiation location was almost the same, but the subsequent slip location was more southerly for the 2012–2016 Tokai L-SSE. Additionally, the maximum slip velocity and moment magnitude were smaller for the 2012–2016 L-SSE

Three-dimensional elastic wave speeds in the northern Chile subduction zone: variations in hydration in the supraslab mantle

International audienceWe use seismic tomography to investigate the state of the supraslab mantle beneath northern Chile, a part of the Nazca-South America Plate boundary known for frequent megathrust earthquakes and active volcanism. We performed a joint inversion of arrival times from earthquake generated body waves and phase delay times from ambient noise generated surface waves recorded by a combined 360 seismic stations deployed in northern Chile at various times over several decades. Our preferred model shows an increase in Vp/Vs by as much as 3 per cent from the subducting slab into the supraslab mantle throughout northern Chile. Combined with low values of both Vp and Vs at depths between 40 and 80 km, we attribute this increase in Vp/Vs to the serpentinization of the supraslab mantle in this depth range. The region of high Vp/Vs extends to 80–120 km depth within the supraslab mantle, but Vp and Vs both increase to normal to high values. This combination, along with the greater abundance of ambient seismicity and higher temperatures at these depths, suggest that conversion from basalt to eclogite in the slab accelerates and that the fluids expelled into the supraslab mantle contribute to partial melt. The corresponding maximum melt fraction is estimated to be about 1 per cent. Both the volume of the region affected by hydration and size of the wave speed contrasts are significantly larger north of ∼21°S. This latitude also delimits large coastal scarps and the eruption of ignimbrites in the north. Ambient seismicity is more abundant north of 21°S, and the seismic zone south of this latitude is offset to the east. The high Vp/Vs region in the north may extend along the slab interface to depths as shallow as 20 km, where it corresponds to a region of reduced seismic coupling and overlaps the rupture zone of the recent 2014 M8.2 Pisagua earthquake. A potential cause of these contrasts is enhanced hydration of the subducting oceanic lithosphere related to a string of seamounts located on the Iquique Ridge of the Nazca Plate

Shallow intraplate seismicity related to the Illapel 2015 Mw 8.4 earthquake: Implications from the seismic source

The September 16, 2015, Mw 8.4 Illapel, Chile earthquake is the first large event occurring in north-central Chile after the 1943 earthquake, filling a known seismic gap in the region. The earthquake took place in a complex tectonic region, nearby an area where transition from erosive to accretionary margin occurs due to the collision of Juan Fernandez Ridge (JFR) along the Chilean margin. We inverted the kinematic rupture process of the 2015 Mw 8.4 Illapel earthquake from the joint inversion of teleseismic body waves and near-field data. The relative weighting between datasets and the weighting of spatial/temporal constraints are objectively estimated by applying the Akaike's Bayesian Information Criterion. The coseismic slip model yields a total seismic moment of 4.92 × 1021 Nm occurred over ~120 s. The rupture shows both downdip and updip propagation with slip extending along the thrust interface from ~50 km depth to shallow near-trench depths (<15 km), with maximum slip of ~9 m located at shallow depths, where low average rupture speeds ~1.8–2 km/s are estimated. Outer-rise events, triggered within the oceanic Nazca plate after the mainshock, did not penetrate into the mantle and are related to preexisting faults due to both bending of Nazca plate and JFR uplift, which promotes a tensional stress regime in the surrounding area. This seismicity was triggered by static stress transfer from near-trench slip revealed by our source rupture modelling, suggesting outer-rise seismicity as a proxy for near-trench coseismic slip. Crustal seismicity within continental South American plate is observed prior to, and after, the mainshock, mainly related to extensional faulting within eroded and fractured wedge due to tectonic processes along erosive margins. We also show evidence of shallow seismicity after the mainshock associated with a long-lived crustal fault, which can represent a high seismic hazard for La Serena-Coquimbo conurbation

3-D thermal structure and dehydration near the Chile Triple Junction and its relation to slab window, tectonic tremors, and volcanoes

Abstract The southern Chile subduction zone is a complex tectonic environment, where the Chile Ridge, the Nazca (NZ) and Antarctic (AN) plates subduct underneath the South American (SA) plate. The intersection between the NZ, AN and SA plates is referred to as the Chile Triple Junction (CTJ). In this region, a gap, often referred to as a slab window, has been formed between the NZ and AN slabs due to the divergence in their plate motion velocities, with volcanoes existing mainly above the subducted NZ and AN plates. In this study, we constructed a three-dimensional thermomechanical model associated with simultaneous subduction of the NZ and AN plates near the CTJ. The results show that the current temperature distributions on the upper surface of the slabs are higher closer to the Chile Ridge, and the AN plate has a distribution of elevated temperatures relative to the NZ plate at the same depth due to the northward migration of the CTJ and the slower convergence rate of the AN plate. Moreover, we calculated the water content and dehydration gradient from the temperature distribution near the upper surface of the slab and discussed their relationship to the distribution of volcanoes. In the northern part of the model domain, high dehydration gradients were obtained below the volcanic chain. Therefore, we suggest that the water released from the slab and the mantle wedge decreased the melting point of the mantle wedge just above the slab and produced melts, which may have contributed to form the overlying volcanoes

Geometry of Geyser Plumbing Inferred From Ground Deformation

Broadband seismic data were recorded on the ground surface around an exceptionally regular eruptive system, geyser El Jefe, in the El Tatio geyser field, Chile. We identify two stages in the eruption, recharge and discharge, characterized by a radial expansion and contraction, respectively, of the surface around the geyser. We model the deformation with spherical sources that vary in size, location, and pressure, constrained by pressure observations inside the conduit that are highly correlated with deformation signals. We find that in order to fit the data, the subsurface pressure sources must be laterally offset from the geyser vent during the recharge phase and that they must migrate upward toward the vent during the eruption phase. This pattern is consistent with models in which ascending fluids accumulate and then are released from a bubble trap that is horizontally offset from the shallow conduit of the geyser

An 8month slow slip event triggers progressive nucleation of the 2014 Chile megathrust

The mechanisms leading to large earthquakes are poorly understood and documented. Here we characterize the long-term precursory phase of the 1 April 2014 M(w)8.1 North Chile megathrust. We show that a group of coastal GPS stations accelerated westward 8months before the main shock, corresponding to a M(w)6.5 slow slip event on the subduction interface, 80% of which was aseismic. Concurrent interface foreshocks underwent a diminution of their radiation at high frequency, as shown by the temporal evolution of Fourier spectra and residuals with respect to ground motions predicted by recent subduction models. Such ground motions change suggests that in response to the slow sliding of the subduction interface, seismic ruptures are progressively becoming smoother and/or slower. The gradual propagation of seismic ruptures beyond seismic asperities into surrounding metastable areas could explain these observations and might be the precursory mechanism eventually leading to the main shock.LabeX OSUG SMINGUE IRD AO-Sud INSU-Aleas grants CONICYT through "Becas Chile" PhD fellowships Proyecto Fondecyt 11140904 PNTS-2014-0