Current collision of the Yakutat block with N. America is accommodated over a large region of the... more Current collision of the Yakutat block with N. America is accommodated over a large region of the Canada-Alaska Northern Cordillera. We investigate this plate-boundary strain distribution using continuous and campaign Global Positioning System (GPS) data and deformation rates estimated from earthquake catalog statistics. Seismic deformation estimates have significant uncertainties, but generally agree with other estimates (GPS, geological, plate models) and provide useful first-order constraints on local long-term tectonics and seismic hazard. Our GPS data infer that the relative Yakutat/N. America motion is accommodated to the east by near-field right- lateral motion (~ 40 mm/yr), mainly on the Fairweather fault, and minor shortening (~ 6 mm/yr). To the north, collision is taken up by fold-and-thrust belt shortening (~ 31 mm/yr), with westward extrusion and possible counter-clockwise rotation of the Yakutat block and Alaskan forearc facilitated by ~ 23 mm/yr distributed dextral she...
Geophysical research at the School of Earth and Ocean Sciences, University of Victoria (UVic), is... more Geophysical research at the School of Earth and Ocean Sciences, University of Victoria (UVic), is carried out by faculty, students, and an active group of adjunct faculty (many at the Pacific Geoscience Centre of the Geological Survey of Canada, PGC/GSC). Research topics are diverse, but many focus on various aspects of earthquake seismology/hazard analysis, relevant topics given UVic’s location just landward of the Cascadia subduction zone, the most seismically active region in Canada. Several representative research programs are briefly described in this article.
Auxiliary Material for Paper 2007JB005456: Leonard, L.J., S. Mazzotti, and R.D. Hyndman (2008), D... more Auxiliary Material for Paper 2007JB005456: Leonard, L.J., S. Mazzotti, and R.D. Hyndman (2008), Deformation rates estimated from earthquakes in the northern Cordillera of Canada and eastern Alaska, J. Geophys. Res., B08406, doi:10.1029/2007JB005456. This auxiliary material contains earthquake incremental and cumulative frequency distribution plots (Figures S1-S21) for each region analyzed in the paper.
The northern Canadian Cordillera is very tectonically active with exceptionally high seismicity i... more The northern Canadian Cordillera is very tectonically active with exceptionally high seismicity in southwestern Yukon and neighboring Alaska, and surprisingly high seismicity 800 km to the northeast in the Mackenzie and Richardson mountains adjacent to the craton. The Yakutat block has been colliding with North America in the corner of the Gulf of Alaska for the last sim5 m.y. The oblique collision results in partitioning of strain into strike-slip (e.g., Denali Fault) and convergent components. The intense seismicity occurs in the collision zone, where deformation is taken up mainly by thrust and strike-slip faulting in the Chugach-Saint-Elias Mountains. Some of the convergent component of strain is transferred across the Cordillera to the northeast, where seismicity rates and Global Positioning System (GPS) vectors indicate that motion is taken up in a broad zone of the fold-and-thrust belt, with little internal deformation of the intervening Cordillera. Simple numerical models fo...
ABSTRACT Probabilistic seismic hazard analyses are principally based on frequency-magnitude stati... more ABSTRACT Probabilistic seismic hazard analyses are principally based on frequency-magnitude statistics of historical and instrumental earthquake catalogues. This method assumes that return periods of large damaging earthquakes (100s–1000s yr) can be extrapolated from 50-100 yr statistics of small and medium earthquakes. The method has obvious limitations when applied to areas of low-level seismicity where the earthquake statistics may be poorly constrained. In this study, we test an alternative approach to assess seismic hazard in Western Canada. We use horizontal velocities at ~250 Global Positioning System (GPS) sites in BC and Alberta to calculate strain rates and earthquake statistics within seismic source zones. GPS-based strain rates are converted to seismic moment, earthquake frequency-magnitude statistics, and seismic hazard using a logic-tree method. The GPS-based earthquake statistics and seismic hazard are then compared to those derived from the earthquake catalogue. In one zone (Puget Sound), the GPS seismic hazard estimates are in good agreement with those from earthquake statistics. In nearly all other zones (e.g., most of BC and Alberta), the GPS seismic hazard estimates are significantly larger than those from the earthquake catalogue by one or two orders of magnitude. This discrepancy could indicate that the earthquake catalogue significantly under predicts long-term seismic hazard (over 100s–1000s yr) in areas of low-level seismicity. Alternatively, significant aseismic deformation may occur over long time-scales, which would imply that the GPS strain rates over predict the true seismic hazard. We discuss the nature and limitations of both methods in light of our results for Western Canada, with the goal of defining a methodology to incorporate GPS strain rate data into probabilistic seismic hazard assessments.
Bulletin of the Seismological Society of America, Jun 2015
We describe near-field coseismic and tsunami evidence collected following the 28 October 2012 Mw ... more We describe near-field coseismic and tsunami evidence collected following the 28 October 2012 Mw 7.8 thrust earthquake that occurred offshore western Haida Gwaii, British Columbia, and discuss factors that influence its extent and preservation potential. Observations indicate minor geomorphic and sedimentological impacts on the rugged and unpopulated west coast of the islands, despite widespread coastal coseismic subsidence (∼0.5 m), triggered landslides, and tsunami waves exceeding 3 m in runup (maximum 13 m) along ∼230 km of coastline. Evidence left by the tsunami was minimal, likely because it occurred during a low tide that restricted its onshore reach, its flow depth, and its capacity to entrain and transport significant amounts of clastic sediment, sources of which are minimal or absent on the dominantly steep, rocky coastline. It is unlikely that subaerial evidence of coseismic subsidence and tsunami inundation will be recognizably recorded in the coastal stratigraphy of western Haida Gwaii, due to the relatively small magnitude of subsidence, a lack of suitable coastal environments such as tidal marshes to record paleoelevation differences, sedimentation rates that are too low to bury a paleoseismic or paleotsunami record, and long-term relative sea level fall leading to erosion and bioturbation. A higher preservation potential is likely for tsunami deposits in coastal lakes, ponds, and bogs, as well as coseismic slope failure deposits offshore, in lakes and in sheltered fjords. Our findings imply that large tsunamigenic earthquakes are likely undersampled in the paleoseismic record of Haida Gwaii and of other plate margins with similar characteristics.
Geological Survey of Canada Open File 7737, Dec 2014
Relative sea-level projections are provided for 59 locations in Canada and 10 in the adjacent mai... more Relative sea-level projections are provided for 59 locations in Canada and 10 in the adjacent mainland United States (New England and Washington State) through the 21st century, relative to 1986-2005. The projections are based on the Representative Concentration Pathway (RCP) scenarios of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). They include contributions from thermal expansion of the ocean (steric effect), land ice melting and discharge, and anthropogenic influences. The global mean sea-level projection for RCP8.5, the largest emissions scenario, at 2100 is 74 cm (5%-95% range is 54 to 98 cm). Global Positioning System (GPS) measurements of vertical land motion are incorporated into the relative sea-level projections. In the regions presented here, vertical land motion, largely arising from glacial isostatic adjustment, plays a prominent role in determining projected relative sea-level change. On the east coast, crustal subsidence, combined with dynamic oceanographic changes, generates relative sea-level projections that are similar to or larger than the global mean projections in large parts of Atlantic Canada and New England. On the west coast, most relative sea-level projections are smaller than the global means, although some sites in Washington State and southern British Columbia feature relative sea-level projections similar to the global values. The largest variation in projected relative sea-level rise occurs in the Arctic, owing to the very large spatial differences in present-day crustal uplift due to glacial isostatic adjustment. Here, projected relative sea-level at 2100 varies from around 1 m of sea-level fall (median values) where land is rising quickly on Hudson Bay, while it reaches about 70 cm of sea-level rise on the Beaufort coast where the land is subsiding. A scenario featuring partial collapse of a portion of the West Antarctic Ice Sheet provides an additional 65 cm of sea-level rise to RCP8.5, and may be appropriate to consider when tolerance to the risk of sea-level rise is low. The relative sea-level projections given here only provide a trajectory through this century, but the IPCC AR5 projects continued global sea-level rise in coming centuries. As understanding improves of the various components of sea-level rise, it will be necessary to update, on an occasional basis, the relative sea-level projections and re-evaluate the implications for infrastructure, habitat, and marine navigation.
This article documents the near-field effects of the largest tsunami of 2012 (globally), which oc... more This article documents the near-field effects of the largest tsunami of 2012 (globally), which occurred following Canada’s second-largest recorded earthquake, on a thrust fault offshore western Haida Gwaii on October 28 (UTC). Despite a lack of reported damaging waves on the coast of British Columbia (largest amplitudes were recorded in Hawaii), three field surveys in the following weeks and months reveal that much of the remote unpopulated, uninstrumented coastline of western Haida Gwaii was impacted by significant tsunami waves that reached up to 13 m above the state of tide. Runup exceeded 3 m at sites spanning ~200 km of the coastline. Greatest impacts were apparent at the heads of narrow inlets and bays on western Moresby Island, where natural and manmade debris with a clear oceanward origin was found on the forest floor and caught in tree branches, inferring flow depths up to 2.5 m. Bays that see regular exposure to storm waves were generally less affected; at these sites a storm origin cannot be ruled out for the debris surveyed. Logs disturbed from their apparent former footprints on the forest floor at the head of Pocket Inlet provide evidence of complex runup, backwash and oblique flow patterns, as noted in other tsunamis. Discontinuous muddy sediments were found at a few sites; sedimentation was not proportional to runup. Lessons learned from our study of the impacts of the Haida Gwaii tsunami may prove useful to future post-tsunami and paleotsunami surveys, as well as tsunami hazard assessments.
We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from loca... more We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from local and far-field, earthquake, and large submarine landslide sources. Analyses involve published historical, palaeotsunami and palaeoseismic data, modelling, and empirical relations between fault area, earthquake magnitude, and tsunami run-up. The cumulative estimated tsunami hazard for potentially damaging run-up (≥1.5 m) of the outer Pacific coastline is ~40–80 % in 50 years, respectively one and two orders of magnitude greater than the outer Atlantic (~1–15 %) and the Arctic (<1 %). For larger run-up with significant damage potential (≥3 m), Pacific hazard is ~10–30 % in 50 years, again much larger than both the Atlantic (~1–5 %) and Arctic (<1 %). For outer Pacific coastlines, the ≥1.5 m run-up hazard is dominated by far-field subduction zones, but the probability of run-up ≥3 m is highest for local megathrust sources, particularly the Cascadia subduction zone; thrust sources further north are also significant, as illustrated by the 2012 Haida Gwaii event. For Juan de Fuca and Georgia Straits, the Cascadia megathrust dominates the hazard at both levels. Tsunami hazard on the Atlantic coastline is dominated by poorly constrained far-field subduction sources; a lesser hazard is posed by near-field continental slope failures similar to the 1929 Grand Banks event. Tsunami hazard on the Arctic coastline is poorly constrained, but is likely dominated by continental slope failures; a hypothetical earthquake source beneath the Mackenzie delta requires further study. We highlight areas susceptible to locally damaging landslide-generated tsunamis, but do not quantify the hazard.
Probabilistic seismic hazard analyses (PSHA) are commonly based on frequency - magnitude statisti... more Probabilistic seismic hazard analyses (PSHA) are commonly based on frequency - magnitude statistics from 50-100 yearlong earthquake catalogs, assuming that these statistics are representative of the longer-term frequency of large earthquakes. We test an alternative PSHA approach in continental western Canada, including adjacent areas of northwestern U.S.A., using regional strain rates derived from 179 Global Positioning System (GPS) horizontal velocities. GPS strain rates are converted to earthquake statistics, seismic moment rates, and ground shaking probabilities in seismic source zones using a logic-tree method for uncertainty propagation. Median GPS-based moment rates and shaking estimates agree well with those derived from earthquake catalogs in only two zones (Puget Sound and mid-Vancouver Island). In most other zones, median GPS-based moment rates are 6-150 times larger than those derived from earthquake catalogs (shaking estimates 2-5 times larger), although the GPS-based and catalog estimates commonly agree within their 67% uncertainties. This discrepancy may represent an under-sampling of long-term moment rates and shaking by earthquake catalogs in some zones; however a systematic under-sampling is unlikely over our entire study area. Although not demonstrated with a high confidence level, long-term regional aseismic deformation may account for a significant part of the GPS/catalog discrepancy and, in some areas, represent as much as 90% of the total deformation budget. In order to integrate GPS strain rates in PSHA models, seismic versus aseismic partitioning of long-term deformation needs to be quantified and understood in terms of the underlying mechanical processes.
Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the C... more Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the Cascadia subduction zone. We compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust-triggered turbidites, and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions. Age-correlated marsh data are compatible with event rupture extents defined by the published turbidite record , and a 6500 yr mean recurrence interval that increases northward from ~230 to ~480 yr. Within the constraints of the marsh data, the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data. In southernmost Cascadia, where the model does not include the complex deformation near the Mendocino triple junction, the coastal data may be better fit by a model with an ~25% narrower rupture than that inferred from regional geophysical data. At each coastal marsh site, coseismic displacements are roughly similar from event to event, independent of the time since the previous event. Slip in the A.D. 1700 earthquake was consistent with the preceding interval of strain accumulation (~200 yr) only at the northern and southern ends of the margin, but it was apparently much higher in southern Washington and northern Oregon, possibly indicating postseismic contamination and/or catch-up coseismic slip to make up for a deficit in the preceding event. Overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions.
Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rup... more Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data; this paper tests the consistency of the models for the 1700 great Cascadia earthquake with compiled coastal coseismic subsidence as estimated from paleoelevation studies. Coastal estimates have large uncertainties but show a consistent pattern. Greatest coseismic subsidence (;1–2 m) occurred in northern Oregon/ southern Washington; subsidence elsewhere was ;0–1 m. Elastic dislocation models constrained by interseismic geodetic and thermal data are used to predict the coseismic subsidence for two likely strain accumulation periods of (i) 800 and (ii) 550 yr of plate convergence and for uniform megathrust slip of 10, 20, 30, and 50 m. The former two models provide a better and equally good fit; predicted subsidence is in broad agreement with marsh estimates. Discrepancies exist, however, at the ends of the subduction zone. In the south, misfit may be due to breakup of the Gorda plate. The discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there, consistent with long-term net uplift in excess of eustatic sea-level rise. The coseismic slip magnitude, estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates, is consistent with the M 9 earthquake indicated by Japanese tsunami records. The coseismic slip was greatest in northern Oregon/southern Washington, declining to the north and south.
Current collision of the Yakutat block with N. America is accommodated over a large region of the... more Current collision of the Yakutat block with N. America is accommodated over a large region of the Canada-Alaska Northern Cordillera. We investigate this plate-boundary strain distribution using continuous and campaign Global Positioning System (GPS) data and deformation rates estimated from earthquake catalog statistics. Seismic deformation estimates have significant uncertainties, but generally agree with other estimates (GPS, geological, plate models) and provide useful first-order constraints on local long-term tectonics and seismic hazard. Our GPS data infer that the relative Yakutat/N. America motion is accommodated to the east by near-field right- lateral motion (~ 40 mm/yr), mainly on the Fairweather fault, and minor shortening (~ 6 mm/yr). To the north, collision is taken up by fold-and-thrust belt shortening (~ 31 mm/yr), with westward extrusion and possible counter-clockwise rotation of the Yakutat block and Alaskan forearc facilitated by ~ 23 mm/yr distributed dextral she...
Geophysical research at the School of Earth and Ocean Sciences, University of Victoria (UVic), is... more Geophysical research at the School of Earth and Ocean Sciences, University of Victoria (UVic), is carried out by faculty, students, and an active group of adjunct faculty (many at the Pacific Geoscience Centre of the Geological Survey of Canada, PGC/GSC). Research topics are diverse, but many focus on various aspects of earthquake seismology/hazard analysis, relevant topics given UVic’s location just landward of the Cascadia subduction zone, the most seismically active region in Canada. Several representative research programs are briefly described in this article.
Auxiliary Material for Paper 2007JB005456: Leonard, L.J., S. Mazzotti, and R.D. Hyndman (2008), D... more Auxiliary Material for Paper 2007JB005456: Leonard, L.J., S. Mazzotti, and R.D. Hyndman (2008), Deformation rates estimated from earthquakes in the northern Cordillera of Canada and eastern Alaska, J. Geophys. Res., B08406, doi:10.1029/2007JB005456. This auxiliary material contains earthquake incremental and cumulative frequency distribution plots (Figures S1-S21) for each region analyzed in the paper.
The northern Canadian Cordillera is very tectonically active with exceptionally high seismicity i... more The northern Canadian Cordillera is very tectonically active with exceptionally high seismicity in southwestern Yukon and neighboring Alaska, and surprisingly high seismicity 800 km to the northeast in the Mackenzie and Richardson mountains adjacent to the craton. The Yakutat block has been colliding with North America in the corner of the Gulf of Alaska for the last sim5 m.y. The oblique collision results in partitioning of strain into strike-slip (e.g., Denali Fault) and convergent components. The intense seismicity occurs in the collision zone, where deformation is taken up mainly by thrust and strike-slip faulting in the Chugach-Saint-Elias Mountains. Some of the convergent component of strain is transferred across the Cordillera to the northeast, where seismicity rates and Global Positioning System (GPS) vectors indicate that motion is taken up in a broad zone of the fold-and-thrust belt, with little internal deformation of the intervening Cordillera. Simple numerical models fo...
ABSTRACT Probabilistic seismic hazard analyses are principally based on frequency-magnitude stati... more ABSTRACT Probabilistic seismic hazard analyses are principally based on frequency-magnitude statistics of historical and instrumental earthquake catalogues. This method assumes that return periods of large damaging earthquakes (100s–1000s yr) can be extrapolated from 50-100 yr statistics of small and medium earthquakes. The method has obvious limitations when applied to areas of low-level seismicity where the earthquake statistics may be poorly constrained. In this study, we test an alternative approach to assess seismic hazard in Western Canada. We use horizontal velocities at ~250 Global Positioning System (GPS) sites in BC and Alberta to calculate strain rates and earthquake statistics within seismic source zones. GPS-based strain rates are converted to seismic moment, earthquake frequency-magnitude statistics, and seismic hazard using a logic-tree method. The GPS-based earthquake statistics and seismic hazard are then compared to those derived from the earthquake catalogue. In one zone (Puget Sound), the GPS seismic hazard estimates are in good agreement with those from earthquake statistics. In nearly all other zones (e.g., most of BC and Alberta), the GPS seismic hazard estimates are significantly larger than those from the earthquake catalogue by one or two orders of magnitude. This discrepancy could indicate that the earthquake catalogue significantly under predicts long-term seismic hazard (over 100s–1000s yr) in areas of low-level seismicity. Alternatively, significant aseismic deformation may occur over long time-scales, which would imply that the GPS strain rates over predict the true seismic hazard. We discuss the nature and limitations of both methods in light of our results for Western Canada, with the goal of defining a methodology to incorporate GPS strain rate data into probabilistic seismic hazard assessments.
Bulletin of the Seismological Society of America, Jun 2015
We describe near-field coseismic and tsunami evidence collected following the 28 October 2012 Mw ... more We describe near-field coseismic and tsunami evidence collected following the 28 October 2012 Mw 7.8 thrust earthquake that occurred offshore western Haida Gwaii, British Columbia, and discuss factors that influence its extent and preservation potential. Observations indicate minor geomorphic and sedimentological impacts on the rugged and unpopulated west coast of the islands, despite widespread coastal coseismic subsidence (∼0.5 m), triggered landslides, and tsunami waves exceeding 3 m in runup (maximum 13 m) along ∼230 km of coastline. Evidence left by the tsunami was minimal, likely because it occurred during a low tide that restricted its onshore reach, its flow depth, and its capacity to entrain and transport significant amounts of clastic sediment, sources of which are minimal or absent on the dominantly steep, rocky coastline. It is unlikely that subaerial evidence of coseismic subsidence and tsunami inundation will be recognizably recorded in the coastal stratigraphy of western Haida Gwaii, due to the relatively small magnitude of subsidence, a lack of suitable coastal environments such as tidal marshes to record paleoelevation differences, sedimentation rates that are too low to bury a paleoseismic or paleotsunami record, and long-term relative sea level fall leading to erosion and bioturbation. A higher preservation potential is likely for tsunami deposits in coastal lakes, ponds, and bogs, as well as coseismic slope failure deposits offshore, in lakes and in sheltered fjords. Our findings imply that large tsunamigenic earthquakes are likely undersampled in the paleoseismic record of Haida Gwaii and of other plate margins with similar characteristics.
Geological Survey of Canada Open File 7737, Dec 2014
Relative sea-level projections are provided for 59 locations in Canada and 10 in the adjacent mai... more Relative sea-level projections are provided for 59 locations in Canada and 10 in the adjacent mainland United States (New England and Washington State) through the 21st century, relative to 1986-2005. The projections are based on the Representative Concentration Pathway (RCP) scenarios of the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). They include contributions from thermal expansion of the ocean (steric effect), land ice melting and discharge, and anthropogenic influences. The global mean sea-level projection for RCP8.5, the largest emissions scenario, at 2100 is 74 cm (5%-95% range is 54 to 98 cm). Global Positioning System (GPS) measurements of vertical land motion are incorporated into the relative sea-level projections. In the regions presented here, vertical land motion, largely arising from glacial isostatic adjustment, plays a prominent role in determining projected relative sea-level change. On the east coast, crustal subsidence, combined with dynamic oceanographic changes, generates relative sea-level projections that are similar to or larger than the global mean projections in large parts of Atlantic Canada and New England. On the west coast, most relative sea-level projections are smaller than the global means, although some sites in Washington State and southern British Columbia feature relative sea-level projections similar to the global values. The largest variation in projected relative sea-level rise occurs in the Arctic, owing to the very large spatial differences in present-day crustal uplift due to glacial isostatic adjustment. Here, projected relative sea-level at 2100 varies from around 1 m of sea-level fall (median values) where land is rising quickly on Hudson Bay, while it reaches about 70 cm of sea-level rise on the Beaufort coast where the land is subsiding. A scenario featuring partial collapse of a portion of the West Antarctic Ice Sheet provides an additional 65 cm of sea-level rise to RCP8.5, and may be appropriate to consider when tolerance to the risk of sea-level rise is low. The relative sea-level projections given here only provide a trajectory through this century, but the IPCC AR5 projects continued global sea-level rise in coming centuries. As understanding improves of the various components of sea-level rise, it will be necessary to update, on an occasional basis, the relative sea-level projections and re-evaluate the implications for infrastructure, habitat, and marine navigation.
This article documents the near-field effects of the largest tsunami of 2012 (globally), which oc... more This article documents the near-field effects of the largest tsunami of 2012 (globally), which occurred following Canada’s second-largest recorded earthquake, on a thrust fault offshore western Haida Gwaii on October 28 (UTC). Despite a lack of reported damaging waves on the coast of British Columbia (largest amplitudes were recorded in Hawaii), three field surveys in the following weeks and months reveal that much of the remote unpopulated, uninstrumented coastline of western Haida Gwaii was impacted by significant tsunami waves that reached up to 13 m above the state of tide. Runup exceeded 3 m at sites spanning ~200 km of the coastline. Greatest impacts were apparent at the heads of narrow inlets and bays on western Moresby Island, where natural and manmade debris with a clear oceanward origin was found on the forest floor and caught in tree branches, inferring flow depths up to 2.5 m. Bays that see regular exposure to storm waves were generally less affected; at these sites a storm origin cannot be ruled out for the debris surveyed. Logs disturbed from their apparent former footprints on the forest floor at the head of Pocket Inlet provide evidence of complex runup, backwash and oblique flow patterns, as noted in other tsunamis. Discontinuous muddy sediments were found at a few sites; sedimentation was not proportional to runup. Lessons learned from our study of the impacts of the Haida Gwaii tsunami may prove useful to future post-tsunami and paleotsunami surveys, as well as tsunami hazard assessments.
We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from loca... more We present a preliminary probabilistic tsunami hazard assessment of Canadian coastlines from local and far-field, earthquake, and large submarine landslide sources. Analyses involve published historical, palaeotsunami and palaeoseismic data, modelling, and empirical relations between fault area, earthquake magnitude, and tsunami run-up. The cumulative estimated tsunami hazard for potentially damaging run-up (≥1.5 m) of the outer Pacific coastline is ~40–80 % in 50 years, respectively one and two orders of magnitude greater than the outer Atlantic (~1–15 %) and the Arctic (<1 %). For larger run-up with significant damage potential (≥3 m), Pacific hazard is ~10–30 % in 50 years, again much larger than both the Atlantic (~1–5 %) and Arctic (<1 %). For outer Pacific coastlines, the ≥1.5 m run-up hazard is dominated by far-field subduction zones, but the probability of run-up ≥3 m is highest for local megathrust sources, particularly the Cascadia subduction zone; thrust sources further north are also significant, as illustrated by the 2012 Haida Gwaii event. For Juan de Fuca and Georgia Straits, the Cascadia megathrust dominates the hazard at both levels. Tsunami hazard on the Atlantic coastline is dominated by poorly constrained far-field subduction sources; a lesser hazard is posed by near-field continental slope failures similar to the 1929 Grand Banks event. Tsunami hazard on the Arctic coastline is poorly constrained, but is likely dominated by continental slope failures; a hypothetical earthquake source beneath the Mackenzie delta requires further study. We highlight areas susceptible to locally damaging landslide-generated tsunamis, but do not quantify the hazard.
Probabilistic seismic hazard analyses (PSHA) are commonly based on frequency - magnitude statisti... more Probabilistic seismic hazard analyses (PSHA) are commonly based on frequency - magnitude statistics from 50-100 yearlong earthquake catalogs, assuming that these statistics are representative of the longer-term frequency of large earthquakes. We test an alternative PSHA approach in continental western Canada, including adjacent areas of northwestern U.S.A., using regional strain rates derived from 179 Global Positioning System (GPS) horizontal velocities. GPS strain rates are converted to earthquake statistics, seismic moment rates, and ground shaking probabilities in seismic source zones using a logic-tree method for uncertainty propagation. Median GPS-based moment rates and shaking estimates agree well with those derived from earthquake catalogs in only two zones (Puget Sound and mid-Vancouver Island). In most other zones, median GPS-based moment rates are 6-150 times larger than those derived from earthquake catalogs (shaking estimates 2-5 times larger), although the GPS-based and catalog estimates commonly agree within their 67% uncertainties. This discrepancy may represent an under-sampling of long-term moment rates and shaking by earthquake catalogs in some zones; however a systematic under-sampling is unlikely over our entire study area. Although not demonstrated with a high confidence level, long-term regional aseismic deformation may account for a significant part of the GPS/catalog discrepancy and, in some areas, represent as much as 90% of the total deformation budget. In order to integrate GPS strain rates in PSHA models, seismic versus aseismic partitioning of long-term deformation needs to be quantified and understood in terms of the underlying mechanical processes.
Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the C... more Coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the Cascadia subduction zone. We compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust-triggered turbidites, and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions. Age-correlated marsh data are compatible with event rupture extents defined by the published turbidite record , and a 6500 yr mean recurrence interval that increases northward from ~230 to ~480 yr. Within the constraints of the marsh data, the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data. In southernmost Cascadia, where the model does not include the complex deformation near the Mendocino triple junction, the coastal data may be better fit by a model with an ~25% narrower rupture than that inferred from regional geophysical data. At each coastal marsh site, coseismic displacements are roughly similar from event to event, independent of the time since the previous event. Slip in the A.D. 1700 earthquake was consistent with the preceding interval of strain accumulation (~200 yr) only at the northern and southern ends of the margin, but it was apparently much higher in southern Washington and northern Oregon, possibly indicating postseismic contamination and/or catch-up coseismic slip to make up for a deficit in the preceding event. Overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions.
Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rup... more Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data; this paper tests the consistency of the models for the 1700 great Cascadia earthquake with compiled coastal coseismic subsidence as estimated from paleoelevation studies. Coastal estimates have large uncertainties but show a consistent pattern. Greatest coseismic subsidence (;1–2 m) occurred in northern Oregon/ southern Washington; subsidence elsewhere was ;0–1 m. Elastic dislocation models constrained by interseismic geodetic and thermal data are used to predict the coseismic subsidence for two likely strain accumulation periods of (i) 800 and (ii) 550 yr of plate convergence and for uniform megathrust slip of 10, 20, 30, and 50 m. The former two models provide a better and equally good fit; predicted subsidence is in broad agreement with marsh estimates. Discrepancies exist, however, at the ends of the subduction zone. In the south, misfit may be due to breakup of the Gorda plate. The discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there, consistent with long-term net uplift in excess of eustatic sea-level rise. The coseismic slip magnitude, estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates, is consistent with the M 9 earthquake indicated by Japanese tsunami records. The coseismic slip was greatest in northern Oregon/southern Washington, declining to the north and south.
Uploads