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TitleMapping tectonic stress at subduction zones with earthquake focal mechanisms: application to Cascadia, Japan, Nankai, Mexico, and northern Chile
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorPlourde, A PORCID logo; Cassidy, J FORCID logo
SourceGeological Survey of Canada, Open File 8925, 2022, 35 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatreadme
File formatpdf; csv
ProvinceBritish Columbia
NTS82E/03; 82E/04; 82E/05; 82E/06; 82E/11; 82E/12; 82E/13; 82E/14; 92B; 92C/01; 92C/02; 92C/07; 92C/08; 92C/09; 92C/10; 92C/15; 92C/16; 92F/01; 92F/02; 92F/07; 92F/08; 92F/09; 92F/10; 92F/15; 92F/16; 92G; 92H
AreaUnited States of America; Japan; Mexico; Chile; China
Lat/Long WENS-125.0000 -119.0000 51.0000 45.0000
Lat/Long WENS-125.0000 -119.0000 43.0000 39.0000
Lat/Long WENS 136.0000 148.0000 46.0000 34.0000
Lat/Long WENS 129.0000 139.0000 36.0000 29.0000
Lat/Long WENS-102.0000 -90.0000 20.0000 12.0000
Lat/Long WENS -72.0000 -68.0000 -19.0000 -24.0000
Subjectstectonics; Science and Technology; Nature and Environment; tectonic elements; subduction zones; earthquakes; stress analyses; Cascadia; Nankai
Illustrationslocation maps; cross-sections; tables
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 11 09
AbstractEarthquake focal mechanisms have contributed substantially to our understanding of modern tectonic stress regimes, perhaps more than any other data source. Studies generally group focal mechanisms by epicentral location to examine variations in stress across a region. However, stress variations with depth have rarely been considered, either due to data limitations or because they were believed to be negligible. This study presents 3D grids of tectonic stress tensors using existing focal mechanism catalogs from several subduction zones, including Cascadia, Japan, Nankai, Mexico, and northern Chile. We bin data into 50 x 50 x 10 km cells (north, east, vertical), with 50% overlap in all three directions. This resulted in 181380 stress inversions, with 90% of these in Japan (including Nankai). To the best of our knowledge, this is the first examination of stress changes with depth in several of these regions. The resulting maps and cross-sections of stress can help distinguish locked and creeping segments of the plate interface. Similarly, by dividing the focal mechanism catalog in northern Japan into those before and those >6 months after the 2011 Mw 9.1 Tohoku-Oki earthquake, we are able to produce detailed 3D maps of stress rotation, which is close to 90° near the areas of highest slip. These results could inform geodynamic rupture models of future megathrust earthquakes in order to more accurately estimate slip, shaking, and seismic hazard. Southern Cascadia and Nankai appear to have sharp stress discontinuities at ~20 km depth, and northern Cascadia may have a similar discontinuity at ~30 km depth. These stress boundaries may relate to rheological discontinuities in the forearc, and may help us unravel how forearc composition influences subduction zone behaviour and seismic hazard.
Summary(Plain Language Summary, not published)
The focal mechanism of an earthquake describes the orientation of its fault and the direction of slip. The direction of slip depends on the shape of tectonic stress in the earth, so with enough focal mechanism estimates in a small area (from a variety of fault orientations), seismologists can estimate the state of stress. We compile catalogs of earthquake focal mechanisms from western North America (Cascadia), Japan, Mexico, and northern Chile and use them to map tectonic stress in 3D. These regions are all active subduction zones, where one tectonic plate dives beneath another, and can potentially produce large earthquakes. The resulting maps can be used to determine what areas of the plate interface are locked (and are building up stress), and what areas are freely slipping (and pose less hazard). We observe stress boundaries in some upper, continental plates that may help us unravel how the geological composition of the upper plate affects subduction zone behaviour and seismic hazard.

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