RECURRENT VOICES: Tsunami and coseismic events – PART 2 of Evolving perspectives on the Pacific-North American plate boundary near Haida Gwaii, BC
July 11, 2016
Tsunami and coseismic events
By Sean Mullan
Shortly after the 2012 Haida Gwaii quake, the US National Oceanic and Atmospheric Administration’s issue of a Tsunami warning led to the coastal evacuation of over 100,000 people in Hawaii. Tsunami generating energy radiated perpendicularly from the Queen Charlotte Fault resulting in a considerably larger wave in Maui, Hawaii (1.52 m) than the height of only 0.52 m recorded at the nearest unobstructed tide gauge at Langara Island off the northern tip of Haida Gwaii, 135 km from the earthquake’s epicentre (Leonard and Bednarski, 2014). Along an approximately 230 km length of Haida Gwaii’s remote west coast, earthquake triggered events included: 1) widespread subsidence (~0.5 m); 2) landslides; and 3) tsunami waves with run-up estimates ranging from 3-13 m above the tide state (Leonard and Bednarski, 2015). Despite such effects, field surveys found only minor earthquake-induced geomorphic and sedimentological changes along the remote west coast of Haida Gwaii (Leonard and Bednarski, 2015).
Although relatively small tsunami wave heights were instrumentally measured, NRCan’s reconnaissance of Haida Gwaii (figure 2) suggests that the wave run-up reached up to 13 m above the state of tide, achieving overland flow depths up to 2.5 m (Leonard and Bednarski, 2014). The tsunami occurred during a relatively low tide height of about 1 m below mean sea level (Leonard and Bednarski, 2014). To leave evidence above the high tide line, wave run-up had to be generally greater than 3 meters at the head of inlets. A wave run-up must be even higher along the exposed outer coast in order to exceed the range inundated by past storm events (Leonard and Bednarski, 2014). Some persuasive indicators of inland wave penetration and flow depth were identified at inlet heads: natural and anthropogenic marine debris littered on the forest floor and caught in tree branches; and logs and other forest materials found to be disturbed or relocated (Leonard and Bednarski, 2014).
Only minimal inland sedimentation seems to have been induced by the tsunami, but marine-origin anthropogenic debris (garbage) accumulation was significant at several forested locations (Leonard and Bednarski, 2014). Sandy deposition was absent from all surveyed sites (Leonard and Bednarski, 2014). This is probably due to the regional scarcity of potential sand sources offshore and in channels, but the flow itself may not have had enough energy to facilitate sand transport (Leonard and Bednarski, 2014; Barrie et al., 2013). Discontinuous mud deposits up to 20 cm thick were discovered, but such deposits were generally found in isolated locations like deep tree wells and their distribution didn’t seem related to the amount of wave run-up (Leonard and Bednarski, 2014). The schematic in figure 3 shows subaerial evidence of the tsunami runup at Pocket Inlet.
Paleotsunami studies can benefit from the insights that tsunamis may penetrate further inland than their sedimentary record indicates and that tsunami deposits may be entirely absent from locations where run-up did occur (Leonard and Bednarski, 2014). It is unlikely that the subaerial coastal stratigraphy of Haida Gwaii will record much evidence of tsunami inundation for several reasons: 1) the small magnitude of coastal subsidence; 2) the absence of suitable coastal environments (e.g. tidal marshes); 3) low sedimentation rates resulting in poor burial potential for tsunami deposits; and 4) erosion and bioturbation due to long-term relative sea-level fall (Leonard and Bednarski, 2015). Marine geoscientists may have success in the search for records of past regional seismicity in Haida Gwaii’s fjords and nearby offshore regions (e.g. coseismic slope failure deposits).
Barrie, J. V., Conway, K. W., and Harris, P. T. (2013). The Queen Charlotte Fault, British Columbia: Seafloor anatomy of a transform fault and its influence on sediment processes. Geo-Marine Letters. 33. pp. 311 – 318.
Leonard, L.J. and Bednarski, J.M. (2014). Field survey following the 28 October 2012 Haida Gwaii tsunami. Pure and Applied Geophysics. 171. pp. 3467-3482.
Leonard, L.J. and Bednarski, J.M. (2015). The Preservation Potential of Coastal Coseismic and Tsunami Evidence Observed Following the 2012 Mw 7.8 Haida Gwaii Thrust
*Part 2 of 4
**article originally released in the Geological Association of Canada (GAC) Marine Geoscience newsletter