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TitleIntertidal biological indicators of coseismic subsidence during the Mw7.8 Haida Gwaii, Canada, earthquake
AuthorHaeussler, P J; Witter, R C; Wang, K
SourceBulletin of the Seismological Society of America vol. 105, no. 2B, 2015 p. 1-15,
Alt SeriesEarth Sciences Sector, Contribution Series 20140302
PublisherSeismological Society of America
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS103B; 103C
AreaHaida Gwaii; Queen Charlotte
Lat/Long WENS-132.0000 -130.7500 52.7500 52.0000
Subjectsgeophysics; earthquakes; earthquake magnitudes; earthquake mechanisms; subsidence; crustal movements
Illustrationslocation maps; photographs; tables; plots
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
AbstractThe 28 October 2012 Mw 7.8 Haida Gwaii earthquake was a megathrust earthquake along the very obliquely convergent Queen Charlotte margin of British Columbia, Canada. Coseismic deformation is not well constrained by geodesy, with only six Global Positioning System (GPS) sites and two tide gauge stations within 250 km of the rupture area. To better constrain vertical coseismic deformation, we measured the upper growth limits of two sessile intertidal organisms, which are controlled by physical conditions, relative to sea level at 25 sites 5 months after the earthquake. We measured the positions of rockweed (Fucus distichus, 617 observations) and the common acorn barnacle (Balanus balanoides, 686 observations). The study focused on the western side of the islands where rupture models indicated that the greatest amount of vertical displacement, but we also investigated sites well away from the inferred rupture area to provide a control on the upper limit of the organisms unaffected by vertical displacement. We also made 322 measurements of sea level to relate the growth limits to a tidal datum using the TPXO7.2 tidal model, rather than ellipsoid heights determined by GPS. Three methods of examining the data all indicate 0.4-0.6 m subsidence along the western coast of Moresby Island as a result of the 28 October 2012 Haida Gwaii earthquake. Our data are, within the errors, consistent with data from two campaign GPS sites along the west coast of Haida Gwaii and with rupture models that indicate megathrust rupture offshore, but not beneath, the islands.
Summary(Plain Language Summary, not published)
The 2012 M 7.8 Haida Gwaii earthquake is the second largest earthquake ever recorded in Canada, and understanding its rupture process helps to improve our efforts to prepare for future large earthquakes. In this work, coastal surveys of intertidal biological indicators (such as barnacles and rockweeds) five months after the earthquake are documented and analyzed. The results provide constraints of the amount of coastal subsidence (< 50 cm) caused by the earthquake. The results compensate for the shortage of coseismic geodetic observations.