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TitleA recent increase in megathrust locking in the southernmost rupture area of the giant 1960 Chile earthquake
AuthorLuo, H; Ambrosius, B; Russo, R M; Mocanu, V; Wang, KORCID logo; Bevis, M; Fernandes, RORCID logo
SourceEarth and Planetary Science Letters vol. 537, 116200, 2020 p. 1-11,
Alt SeriesNatural Resources Canada, Contribution Series 20190303
Mediapaper; on-line; digital
File formatpdf; html
Lat/Long WENS -76.5833 -71.0000 -43.8333 -47.0000
Subjectstectonics; geophysics; Science and Technology; Nature and Environment; Health and Safety; earthquakes; subduction zones; deformation; stress analyses; crustal movements; satellite geodesy; navigation satellites; modelling; models; displacement; 1960 Mw 9.5 Chile Earthquake; Nazca Plate; Antarctica Plate; South American Plate; global navigation satellite systems (GNSS)
Illustrationstime series; schematic cross-sections; geoscientific sketch maps; models
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2020 03 13
AbstractAfter a great subduction earthquake, viscoelastic stress relaxation causes prolonged seaward motion of inland areas of the upper plate, as was observed around the turn of the century in the area of the 1960 Mw 9.5 Chile earthquake with Global Navigation Satellite System (GNSS) measurements. However, recent GNSS observations during 2010-2019 indicate a systematic decrease in the velocity of the seaward motion over a region covering the latitudinal range of the southern half of the 1960 rupture. Data from the only long-lived continuous site in this region (COYQ since 1997), situated over 200 km away from the trench, suggest that the decrease in the seaward velocity (or increase in the landward velocity) occurred within a few years prior to 2010. This rapid and regional change cannot be explained by viscoelastic relaxation. We thus propose that the change was caused by a relatively sudden downdip widening of the zone of locking along the megathrust. Using three-dimensional finite element modelling, we find that the observed velocity change cannot be otherwise explained, although the amount of the increase in locking cannot be uniquely determined because of trade-offs between, and uncertainties in, the various parameters involved. For example, the degree of the increase in locking is affected by the value of coseismic slip in 1960 in the southernmost part of the rupture zone. A postseismic deformation model with greater coseismic slip in accordance with the most recent coseismic slip model in the literature better fits COYQ data prior to 2005 and requires greater locking increase afterwards. A model with less coseismic slip requires less locking increase but an additional long-term slow slip event prior to 2005. The rapid surface velocity change and the inferred increase in megathrust locking several decades after a great earthquake present new challenges to the understanding of fault mechanics and subduction zone dynamics.
Summary(Plain Language Summary, not published)
The locking state of the megathrust fault constrained by geodetic observations tells us how the fault may rupture in a future earthquake. It is commonly assumed that the locked zone of the fault is more or less fixed during the interseismic period. In this work, we report intriguing observations that indicate an increase in megathrust locking in the early 21st century in the southern area of the 1960 M=9.5 Chile earthquake. In this area, observations made during 1994 - 2005 showed seaward motion of GPS sites, reflecting prolonged postseismic deformation following the 1960 earthquake. But new observations since 2010 show a regional, systematic decrease in the velocities of the seaward motion that must have occurred during 2005 - 2010. The sudden regional change can only be explained by a sudden widening of the megathrust locked zone in the downdip direction. This finding suggests that we need to consider temporal changes in the locking state in hazard assessment.

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