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  • 1
    Publication Date: 2012-10-13
    Description: Knowledge of seismic properties in an earthquake rupture zone is essential for understanding the factors controlling rupture dynamics. We use data from aftershocks following the Maule earthquake to derive a three-dimensional seismic velocity model of the central Chile forearc. At 36°S, we find a high vp (〉7.0 km/s) and high vp/vs (∼1.89) anomaly lying along the megathrust at 25 km depth, which coincides with a strong forearc Bouguer gravity signal. We interpret this as a subducted topographic high, possibly a former seamount on the Nazca slab. The Maule earthquake nucleated at the anomaly's updip boundary; yet high co-seismic slip occurred where the megathrust is overlain by lower seismic velocities. Sparse aftershock seismicity occurs within this structure, suggesting that it disrupts normal interface seismogenesis. These findings imply that subducted structures can be conducive to the nucleation of large megathrust earthquakes, even if they subsequently hinder co-seismic slip and aftershock activity.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
    Publication Date: 2012-02-25
    Description: SUMMARY We present an upgraded version of a previously published 3-D density model of the Andean subduction zone between 18°S and 45°S. This model consists of 3-D bodies of constant density, which geometry is constrained by independent seismic data and is triangulated from vertical cross-sections. These bodies define the first-order morphology and internal structure of the subducted Nazca slab and South American Plate. The new version of the density model results after forward modelling the Bouguer anomaly as computed from the most recent version of the Earth Gravitational Model (EGM2008). The 3-D density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental Moho (CMH) and has a trench-parallel resolution doubling the resolution of the previous model. As an example of the potential utility of our model, we compare the geometry of the subducted slab and CMH against the corresponding global models Slab1.0 and Crust2.0, respectively. This exercise demonstrates that, although global models provide a good first-order representation of the slab and upper-plate crustal geometries, they show large discrepancies (up to ±40 km) with our upgraded model for some well-constrained areas. The geometries of the slab, lithosphere–asthenosphere boundary below the continent, CMH and intracrustal density discontinuity that we present here as Supporting Information can be used to study Andean geodynamic processes from a wide range of quantitative approaches.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
    Publication Date: 2019
    Description: Abstract Megathrust earthquakes are commonly accompanied by increased upper‐plate seismicity and occasionally triggered fault slip. In Chile, crustal faults slipped during and after the 2010 Maule (M8.8) earthquake. We studied the El Yolki fault (EYOF), a transtensional structure midways the Maule rupture not triggered in 2010. We mapped a Holocene coastal plain using LiDAR, which did not reveal surface ruptures. However, the inner‐edge and shoreline angles along the coastal plain as well as 4.3‐4.0 ka intertidal sediments are back‐tilted on the EYOF footwall block, documenting 10 m of vertical displacement. These deformed markers imply ~2 mm/yr throw rate and dislocation models a slip rate of 5.6 mm/yr for the EYOF. In a 5‐m‐deep trench, the Holocene intertidal sediments onlap to five erosive steps, interpreted as staircase wave‐cut landforms formed by discrete events of relative sea‐level drop. We tentatively associated these steps with coseismic uplift during EYOF earthquakes between 4.3 and 4.0 ka. The Maule earthquake rupture may be subdivided into three subsegments based on coseismic slip and gravity anomalies. Coulomb stress transfer models predict neutral stress changes at the EYOF during the Maule earthquake but positive changes for a synthetic slip distribution at the central subsegment. If EYOF earthquakes were triggered by megathrust events, their slip distribution was probably focused in the central subsegment. Our study highlights the millennial variability of crustal faulting and the megathrust earthquake cycle in Chile, with global implications for assessing the hazards posed by hidden but potentially seismogenic coastal faults along subduction zones.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2012-11-07
    Description: SUMMARY The space–time–size distributions of seismicity during the last four decades (1973–2010) are used to identify megathrust asperities, the degree of plate coupling (hence potential future large earthquakes) and slab dehydration below the Andean margin (15°–48°S). This seismicity displays globally a typical magnitude distribution with a b value of 1.04 ± 0.024, for a magnitude of completeness of 4.5 and without considering aftershock sequences. However, this unitary b value of the entire catalogue masks large variations (0.6–1.7) in space and time, interpreted as related to different tectonics contexts or different states of the seismic cycle. The study has been performed for different regions (upper continental plate, upper and lower parts of the subducting oceanic plate) with respect to a 3-D slab geometry model all along the Andean margin. Low b values (0.6–0.8) are observed in areas that were ruptured by large M w ≥ 7.5 megathrust earthquakes. A temporal analysis of these features show that b values where low before these earthquakes and remain low after the relaxation of the aftershock phase, suggesting that they are related to time-persistent asperities within the seismogenic zone of the megathrust. Interestingly, a persistently low b value was observed from 2001 to the end 2009 within the southern limit of the Maule M w = 8.8 earthquake, 2010 February 27. Our results also identify other low b asperities below the Chilean margin that have not ruptured during the last 40 yr and are places of potentially large megathrust earthquakes in the future. In contrast, large b values (1.1–1.7) are observed at seismicity clusters occurring inside the subducted slab beneath the Central Volcanic Zone (CVZ) of the Andes and below the forearc near the subduction point of aseismic ridges and fracture zones. Clusters below the CVZ are spatially related to relative large intermediate-depth earthquakes (like the M w = 7.8 Tarapaca 2005 earthquake) and may reveal extensive thermally-driven dehydration of the oceanic lithosphere that favours wet mantle melting and therefore magmagenesis feeding the volcanic chain.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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