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• 1. [Article] Seismicity at the Cascadia plate boundary beneath the Oregon continental shelf

  Since 2003, 39 small earthquakes have been detected offshore central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seis- mic activity has been recorded in spite ...

  Citation × Citation Citation Abstract Copy Title: Seismicity at the Cascadia plate boundary beneath the Oregon continental shelf Author: Williams, Mark Charles Since 2003, 39 small earthquakes have been detected offshore central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seis- mic activity has been recorded in spite of a paleo-seismic record of great subduction events. Although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events (M[subscript w] 4.9 and M[subscript w] 4.7, which occurred in 2004), analysis by Tréhu et al. (2008) indicates that they were low angle thrust events that occurred on the plate boundary at depths of 9-11 and 16 km, respectively. Due to sparse onshore station coverage, most of the smaller events have large location uncertain- ties. Double-difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart; each cluster is associated with one of the two larger events. Within each cluster, relocation reduces the hypocenter depth spread from > 15 km to < 3 km, with uncertainties on the order of 0.1 km. The relocations, combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean bottom seismometers, suggest that the seismicity occurred at plate boundary depths, possibly on the Cascadia megathrust. This concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics, possibly caused by subducted topographic features, which can affect the propagation of a large megathrust rupture. Title: Seismicity at the Cascadia plate boundary beneath the Oregon continental shelf Author: Williams, Mark Charles Since 2003, 39 small earthquakes have been detected offshore central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seis- mic activity has been recorded in spite of a paleo-seismic record of great subduction events. Although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events (M[subscript w] 4.9 and M[subscript w] 4.7, which occurred in 2004), analysis by Tréhu et al. (2008) indicates that they were low angle thrust events that occurred on the plate boundary at depths of 9-11 and 16 km, respectively. Due to sparse onshore station coverage, most of the smaller events have large location uncertain- ties. Double-difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart; each cluster is associated with one of the two larger events. Within each cluster, relocation reduces the hypocenter depth spread from > 15 km to < 3 km, with uncertainties on the order of 0.1 km. The relocations, combined with independent absolute hypocenter locations for two 2008 events using a deployment of land and ocean bottom seismometers, suggest that the seismicity occurred at plate boundary depths, possibly on the Cascadia megathrust. This concentrated activity in the seismogenic zone may represent patches on the fault plane with anomalous frictional characteristics, possibly caused by subducted topographic features, which can affect the propagation of a large megathrust rupture. Close

• 2. [Article] Late Holocene Tsunami Deposits at Salt Creek, Washington, USA

  We interpret two thin sand layers in the estuarine marsh at Salt Creek, on the southern shore of the Strait of Juan de Fuca, as the products of tsunamis propagated by earthquakes at the Cascadia subduction ...

  Citation × Citation Citation Abstract Copy Title: Late Holocene Tsunami Deposits at Salt Creek, Washington, USA Author: Hutchinson, Ian, Peterson, Curt D., Sterling, Sarah L. Year: 2013 We interpret two thin sand layers in the estuarine marsh at Salt Creek, on the southern shore of the Strait of Juan de Fuca, as the products of tsunamis propagated by earthquakes at the Cascadia subduction zone. The sand layers extend for about 60 m along the left bank of the creek about 800 m from the mouth, and can be traced to the base of a nearby upland area. One layer is exposed in the creek bank about 400 m further upstream, but they are only patchily distributed in the rest of the central area of the marsh. Both layers contain brackish-marine epipsammic diatoms. The lower sand layer marks a sharp contact between intertidal peaty mud and overlying mud, perhaps reflecting modest coseismic subsidence in association with tsunami deposition, but little or no change in the bracketing sediment occurs in association with the upper sand layer. The ages of the sand layers are not closely constrained, but were most likely deposited by tsunamis generated by great earthquakes at the Cascadia subduction zone about 1650 and 1300 years ago. The Cascadia great earthquake of AD1700 may have induced slight subsidence in the marsh, but no tsunami deposit was detected at the inferred contact. The absence of deposits from the marsh immediately inland of the 4 m-high barrier beach indicates that the largest tsunamis in the late Holocene at this site have not overtopped the barrier, which suggests that these tsunamis were likely only 2-3 m high. Title: Late Holocene Tsunami Deposits at Salt Creek, Washington, USA Author: Hutchinson, Ian, Peterson, Curt D., Sterling, Sarah L. Year: 2013 We interpret two thin sand layers in the estuarine marsh at Salt Creek, on the southern shore of the Strait of Juan de Fuca, as the products of tsunamis propagated by earthquakes at the Cascadia subduction zone. The sand layers extend for about 60 m along the left bank of the creek about 800 m from the mouth, and can be traced to the base of a nearby upland area. One layer is exposed in the creek bank about 400 m further upstream, but they are only patchily distributed in the rest of the central area of the marsh. Both layers contain brackish-marine epipsammic diatoms. The lower sand layer marks a sharp contact between intertidal peaty mud and overlying mud, perhaps reflecting modest coseismic subsidence in association with tsunami deposition, but little or no change in the bracketing sediment occurs in association with the upper sand layer. The ages of the sand layers are not closely constrained, but were most likely deposited by tsunamis generated by great earthquakes at the Cascadia subduction zone about 1650 and 1300 years ago. The Cascadia great earthquake of AD1700 may have induced slight subsidence in the marsh, but no tsunami deposit was detected at the inferred contact. The absence of deposits from the marsh immediately inland of the 4 m-high barrier beach indicates that the largest tsunamis in the late Holocene at this site have not overtopped the barrier, which suggests that these tsunamis were likely only 2-3 m high. Close

• 3. [Article] Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia

  In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia Author: Black, Bran In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in the upper tens of meters of the subsurface were collected. Reflectors imaged with the 3.5 kHz CHIRP subbottom data represent turbidite beds within the first 20-30 m of seafloor sediments that have sufficient impedance contrast and are separated by at least 20 cm vertically. Correlation of subbottom reflectors along both margins, constrained by core data at numerous sites along strike, revealed that many of these reflectors were contiguous over distances of hundreds of kilometers in southern Cascadia. The placement of subbottom reflectors coincided strongly with the configuration of many of the turbidite beds identified in nearby cores. A number of thin, low density beds evident in the cores are not imaged in the subbottom data in the northern part of the study area near Rogue Canyon, but become more robust southward where some are imaged as reflectors in the subbottom data. The independently derived core and subbottom based correlations of beds in southern Cascadia were found to be in fairly close agreement, and collectively support the continuity of beds over significant distances of up to 200 km along strike. Robustness of the beds between Trinidad and Rogue Canyons does not appear to be related to proximity to canyon mouths, suggesting that these beds and their transport are not exclusively related to canyon sources. The combined core and reflection data and the continuity of beds support a seismogenic source for Holocene beds correlated over long distances owing to the broad areal extent and lack of strong linkage to canyon systems. Data from paleoseismic studies worldwide were compiled in an effort to parameterize the likely triggering distances associated with turbidity currents initiated by earthquakes on the CSZ and NSAF. These ranges are estimated at 60-120 km for the CSZ and 40-75 km for the NSAF. A similar approach using MMI values associated with earthquakes linked to seismoturbidites yielded a threshold estimate for northern California of ~ MMI 5.5. A region from between Eel Canyon and Mendocino Channel in the north to the area north of Noyo Canyon in the south was identified as having a higher likelihood for recording at least a portion past seismicity from both the NSAF and CSZ in local sediments. The high frequency of turbidite beds in southernmost Cascadia could be the result of Cascadia and NSAF seismicity combined, though some input from hyperpycnal flows cannot be excluded. Further study of sediments in this zone may improve our understanding of potential stress triggering interactions between the CSZ and NSAF. Title: Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia Author: Black, Bran In order to evaluate the shallow stratigraphy along the southern Cascadia abyssal margin and northern California abyssal plain, CHIRP subbottom profiles capable of imaging individual turbidite beds in the upper tens of meters of the subsurface were collected. Reflectors imaged with the 3.5 kHz CHIRP subbottom data represent turbidite beds within the first 20-30 m of seafloor sediments that have sufficient impedance contrast and are separated by at least 20 cm vertically. Correlation of subbottom reflectors along both margins, constrained by core data at numerous sites along strike, revealed that many of these reflectors were contiguous over distances of hundreds of kilometers in southern Cascadia. The placement of subbottom reflectors coincided strongly with the configuration of many of the turbidite beds identified in nearby cores. A number of thin, low density beds evident in the cores are not imaged in the subbottom data in the northern part of the study area near Rogue Canyon, but become more robust southward where some are imaged as reflectors in the subbottom data. The independently derived core and subbottom based correlations of beds in southern Cascadia were found to be in fairly close agreement, and collectively support the continuity of beds over significant distances of up to 200 km along strike. Robustness of the beds between Trinidad and Rogue Canyons does not appear to be related to proximity to canyon mouths, suggesting that these beds and their transport are not exclusively related to canyon sources. The combined core and reflection data and the continuity of beds support a seismogenic source for Holocene beds correlated over long distances owing to the broad areal extent and lack of strong linkage to canyon systems. Data from paleoseismic studies worldwide were compiled in an effort to parameterize the likely triggering distances associated with turbidity currents initiated by earthquakes on the CSZ and NSAF. These ranges are estimated at 60-120 km for the CSZ and 40-75 km for the NSAF. A similar approach using MMI values associated with earthquakes linked to seismoturbidites yielded a threshold estimate for northern California of ~ MMI 5.5. A region from between Eel Canyon and Mendocino Channel in the north to the area north of Noyo Canyon in the south was identified as having a higher likelihood for recording at least a portion past seismicity from both the NSAF and CSZ in local sediments. The high frequency of turbidite beds in southernmost Cascadia could be the result of Cascadia and NSAF seismicity combined, though some input from hyperpycnal flows cannot be excluded. Further study of sediments in this zone may improve our understanding of potential stress triggering interactions between the CSZ and NSAF. Close

• 4. [Article] A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc

  Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench ...

  Citation × Citation Citation Abstract Copy Title: A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc Author: Kenyon, Christopher Bradford Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This segment of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hinted at large velocity variations of the overlying forearc crust immediately above the plate boundary. Approximately 18000 airgun shots along 4 profiles on the continental shelf and upper slope and a profile oblique to the coast that extends across the margin to the trench provided sources for this study. Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for most shots. We present velocity models that merge the new data with constraints from onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model for this segment of the Cascadia subduction zone, which appears to be a segment boundary in the paleoseismic record and currently exhibits anomalous locking behavior as indicated by geodetic data. Our approach to modeling involved starting with a 1D P-wave velocity model and using an inversion process to modify the model to beetter fit the observed first arrival times of the airgun shots. In a subsequent step, we used a 2D starting model that incorporated constraints from prior 2D experiments that extended farther seaward and landward of the 3D study region. Finally, a smoothed version of the model obtained from the 2D inversion was used as a starting model to see if the final model could be further refined. Our final preferred model used a 2D starting model and moderate smoothing. The RMS misfit of predicted travel times is reduced from 0.48s for the initial 2D model to 0.040s for the final preferred 3D model. This model includes interesting velocity structure including possible subducted seamounts on the subducting Juan de Fuca plate and a mafic dike related to Cape Perpetua volcanics within the upper plate. These results contribute to the broader goal of understanding the relationship between plate boundary behavior and crustal structure. Title: A 3-D Tomographic Model of the P-wave Velocity Structure of the Central Cascadia Forearc Author: Kenyon, Christopher Bradford Signals from the R/V Langseth’s tuned airgun array were recorded on an array of 33 EarthScope FlexArray seismometers deployed in the Oregon Coast Range from 43.5° to 45°N as part of the 2012 Ridge2Trench experiment to image the structure of the Juan de Fuca plate. This segment of the Juan de Fuca/North America plate boundary slipped in 2 moderate low-angle thrust earthquakes in 2004, and continuing seismicity has clustered around these events since that time. Previously acquired bathymetric, potential field, and seismic data indicate the presence of subducted seamounts in this region and hinted at large velocity variations of the overlying forearc crust immediately above the plate boundary. Approximately 18000 airgun shots along 4 profiles on the continental shelf and upper slope and a profile oblique to the coast that extends across the margin to the trench provided sources for this study. Data quality is excellent, with strong Pg and PmP arrivals observed at most stations for most shots. We present velocity models that merge the new data with constraints from onshore/offshore data acquired in 1989 and 1996 to improve the resolution and extend the velocity model for this segment of the Cascadia subduction zone, which appears to be a segment boundary in the paleoseismic record and currently exhibits anomalous locking behavior as indicated by geodetic data. Our approach to modeling involved starting with a 1D P-wave velocity model and using an inversion process to modify the model to beetter fit the observed first arrival times of the airgun shots. In a subsequent step, we used a 2D starting model that incorporated constraints from prior 2D experiments that extended farther seaward and landward of the 3D study region. Finally, a smoothed version of the model obtained from the 2D inversion was used as a starting model to see if the final model could be further refined. Our final preferred model used a 2D starting model and moderate smoothing. The RMS misfit of predicted travel times is reduced from 0.48s for the initial 2D model to 0.040s for the final preferred 3D model. This model includes interesting velocity structure including possible subducted seamounts on the subducting Juan de Fuca plate and a mafic dike related to Cape Perpetua volcanics within the upper plate. These results contribute to the broader goal of understanding the relationship between plate boundary behavior and crustal structure. Close

• 5. [Article] Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA

  A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in ...

  Citation × Citation Citation Abstract Copy Title: Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA Author: Peterson, Curt D., Kristensen, Kurt, Minor, Rick Year: 2014 A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems. Title: Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA Author: Peterson, Curt D., Kristensen, Kurt, Minor, Rick Year: 2014 A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems. Close

• 6. [Article] Multi-hazard Damage and Loss Assessment of Bridge Networks

  The overarching objective of this study is to examine and compare the vulnerabilities of bridges combining effects of earthquake-induced ground shaking, ground failure (e.g., landslides and lateral spreading), ...

  Citation × Citation Citation Abstract Copy Title: Multi-hazard Damage and Loss Assessment of Bridge Networks Author: Burns, Patrick O. The overarching objective of this study is to examine and compare the vulnerabilities of bridges combining effects of earthquake-induced ground shaking, ground failure (e.g., landslides and lateral spreading), and tsunami inundation. A parametric study is performed to understand the sensitivity of economic loss and traffic capacity with respect to the demand, resistance, and dispersion of the fragility functions of bridges for the combined hazards. The predicted damage varies substantially based on the selected fragility functions for the bridge structure. Additionally, when ground failure models are considered, there is an overwhelming increase in bridge damage and economic loss estimates, reinforcing the need for more refined ground failure analyses to reduce epistemic uncertainty. The loss estimates are significantly sensitive to fragility parameters, particularly the resistance models, emphasizing the need to develop more specific and systematic fragility functions suitable for an entire bridge inventory. Economic loss estimates based on these cascading hazards are ultimately provided with the M9.0 Cascadia Subduction Earthquake Zone scenario. Title: Multi-hazard Damage and Loss Assessment of Bridge Networks Author: Burns, Patrick O. The overarching objective of this study is to examine and compare the vulnerabilities of bridges combining effects of earthquake-induced ground shaking, ground failure (e.g., landslides and lateral spreading), and tsunami inundation. A parametric study is performed to understand the sensitivity of economic loss and traffic capacity with respect to the demand, resistance, and dispersion of the fragility functions of bridges for the combined hazards. The predicted damage varies substantially based on the selected fragility functions for the bridge structure. Additionally, when ground failure models are considered, there is an overwhelming increase in bridge damage and economic loss estimates, reinforcing the need for more refined ground failure analyses to reduce epistemic uncertainty. The loss estimates are significantly sensitive to fragility parameters, particularly the resistance models, emphasizing the need to develop more specific and systematic fragility functions suitable for an entire bridge inventory. Economic loss estimates based on these cascading hazards are ultimately provided with the M9.0 Cascadia Subduction Earthquake Zone scenario. Close

• 7. [Article] Sedimentology of Seismo-Turbidites off the Cascadia and Northern California Active Tectonic Continental Margins, Northwest Pacific Ocean

  This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/marine-geology/....

  Citation × Citation Citation Abstract Copy Title: Sedimentology of Seismo-Turbidites off the Cascadia and Northern California Active Tectonic Continental Margins, Northwest Pacific Ocean Author: Gutierrez-Pastor, Julia, Escutia, Carlota, Nelson, C. Hans, Goldfinger, Chris This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/marine-geology/. Title: Sedimentology of Seismo-Turbidites off the Cascadia and Northern California Active Tectonic Continental Margins, Northwest Pacific Ocean Author: Gutierrez-Pastor, Julia, Escutia, Carlota, Nelson, C. Hans, Goldfinger, Chris This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/marine-geology/. Close

• 8. [Article] Catastrophic Post-disaster Long-term Recovery Planning: A Capacity and Needs Assessment of the Oregon Coast

  Project committee: Andre Leduc, chair, Dr. Jean Stockard, Krista Dillon

  Citation × Citation Citation Abstract Copy Title: Catastrophic Post-disaster Long-term Recovery Planning: A Capacity and Needs Assessment of the Oregon Coast Author: Pearce, Jennifer E. Year: 2009, 2008 Project committee: Andre Leduc, chair, Dr. Jean Stockard, Krista Dillon Title: Catastrophic Post-disaster Long-term Recovery Planning: A Capacity and Needs Assessment of the Oregon Coast Author: Pearce, Jennifer E. Year: 2009, 2008 Project committee: Andre Leduc, chair, Dr. Jean Stockard, Krista Dillon Close

• 9. [Article] Tsunami-induced Soil Instability

  Recent tsunami field surveys from the 2011 Great East Japan Earthquake and Tsunami have recorded numerous examples of tsunami-induced soil instability: significant scour around foundations, foundation ...

  Citation × Citation Citation Abstract Copy Title: Tsunami-induced Soil Instability Author: Fischer, Rachael Marie Recent tsunami field surveys from the 2011 Great East Japan Earthquake and Tsunami have recorded numerous examples of tsunami-induced soil instability: significant scour around foundations, foundation failure of piles, and other damage caused by liquefaction. From the observations of soil instability leading to the failure of critical coastal structures, it is apparent that the behavior of tsunami waves has a unique effect on the soil bed. Previous research in the area of soil instability has predominantly focused on storm-generated waves, and a complete understanding of soil hazards from tsunami events remains rudimentary. In an effort to expand understanding of tsunami-induced soil instability, the primary objective of this thesis is to evaluate the mechanisms of sediment response and provide an analytical framework for evaluating soil hazards related to tsunamis. Evaluating tsunami-induced soil instability is particularly important in the Pacific Northwest, due to the presence of the Cascadia Subduction Zone. Soil instability and sediment transport were estimated at Seaside, Oregon for a hypothetical "500-year" tsunami, using the predicted flow height and velocity time series taken from the Park et al (2014) ComMIT/MOST inundation model. Two onshore locations of interest were selected from the model for evaluation of soil instability and sediment transport potential. Additionally, sand was sampled from the beach at Seaside to characterize the sand's fabric. Significant sediment transport and enhanced scour caused by the large bed shear stress during tsunami drawdown is expected based on Shields parameter and Rouse number estimates. A modified Shields parameter from Yeh and Mason (2014) was applied to account for the gradient of excess pore water pressure head that develops in the sand during tsunami inundation and drawdown. Based on initial estimates of drawdown, it is expected that soil instability caused by momentary liquefaction would be minor at Seaside, but soil instability caused by enhanced scour could be significant. To reach an improved understanding of the mechanisms behind tsunami-induced soil instability, a complete analytical model was developed to examine the influence of a tsunami wave on soils from runup to drawdown. The Carrier et al. (2003) shallow water wave solution for tsunami runup on a plane beach was combined with analytical solutions for pore pressure gradients and sediment transport. Within a probabilistic model of tsunami-induced soil instability, a sensitivity analysis was conducted on both the wave and soil parameters using a Monte Carlo approach. Threshold behaviors of wave forces and resulting soil instability were discussed under general site conditions. The height and the velocity of the inundating wave have a predominant influence on the response of soils. The beach slope is found to affect the response of pore pressure gradients, and ultimately, the momentary liquefaction risk. The entire range of the coefficient of consolidation produced cases of enhanced scour and momentary liquefaction, indicating potential tsunami-induced soil instability for the majority of fine sands. Analysis of soil behavior at critical thresholds remains important for developing structures that are resistant to wave forces and soil instability failure. Title: Tsunami-induced Soil Instability Author: Fischer, Rachael Marie Recent tsunami field surveys from the 2011 Great East Japan Earthquake and Tsunami have recorded numerous examples of tsunami-induced soil instability: significant scour around foundations, foundation failure of piles, and other damage caused by liquefaction. From the observations of soil instability leading to the failure of critical coastal structures, it is apparent that the behavior of tsunami waves has a unique effect on the soil bed. Previous research in the area of soil instability has predominantly focused on storm-generated waves, and a complete understanding of soil hazards from tsunami events remains rudimentary. In an effort to expand understanding of tsunami-induced soil instability, the primary objective of this thesis is to evaluate the mechanisms of sediment response and provide an analytical framework for evaluating soil hazards related to tsunamis. Evaluating tsunami-induced soil instability is particularly important in the Pacific Northwest, due to the presence of the Cascadia Subduction Zone. Soil instability and sediment transport were estimated at Seaside, Oregon for a hypothetical "500-year" tsunami, using the predicted flow height and velocity time series taken from the Park et al (2014) ComMIT/MOST inundation model. Two onshore locations of interest were selected from the model for evaluation of soil instability and sediment transport potential. Additionally, sand was sampled from the beach at Seaside to characterize the sand's fabric. Significant sediment transport and enhanced scour caused by the large bed shear stress during tsunami drawdown is expected based on Shields parameter and Rouse number estimates. A modified Shields parameter from Yeh and Mason (2014) was applied to account for the gradient of excess pore water pressure head that develops in the sand during tsunami inundation and drawdown. Based on initial estimates of drawdown, it is expected that soil instability caused by momentary liquefaction would be minor at Seaside, but soil instability caused by enhanced scour could be significant. To reach an improved understanding of the mechanisms behind tsunami-induced soil instability, a complete analytical model was developed to examine the influence of a tsunami wave on soils from runup to drawdown. The Carrier et al. (2003) shallow water wave solution for tsunami runup on a plane beach was combined with analytical solutions for pore pressure gradients and sediment transport. Within a probabilistic model of tsunami-induced soil instability, a sensitivity analysis was conducted on both the wave and soil parameters using a Monte Carlo approach. Threshold behaviors of wave forces and resulting soil instability were discussed under general site conditions. The height and the velocity of the inundating wave have a predominant influence on the response of soils. The beach slope is found to affect the response of pore pressure gradients, and ultimately, the momentary liquefaction risk. The entire range of the coefficient of consolidation produced cases of enhanced scour and momentary liquefaction, indicating potential tsunami-induced soil instability for the majority of fine sands. Analysis of soil behavior at critical thresholds remains important for developing structures that are resistant to wave forces and soil instability failure. Close

• 10. [Article] Method to Determine Locations of Tsunami Vertical Evacuation Shelters

  This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://www.springer.com/earth+sciences+and+geogr...

  Citation × Citation Citation Abstract Copy Title: Method to Determine Locations of Tsunami Vertical Evacuation Shelters Author: Park, Sangki, Gupta, Rakesh, van de Lindt, John W., Cox, Daniel This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://www.springer.com/earth+sciences+and+geography/natural+hazards/journal/11069. Title: Method to Determine Locations of Tsunami Vertical Evacuation Shelters Author: Park, Sangki, Gupta, Rakesh, van de Lindt, John W., Cox, Daniel This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Springer and can be found at: http://www.springer.com/earth+sciences+and+geography/natural+hazards/journal/11069. Close