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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] 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

• 3. [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

• 4. [Article] Deconvolving orbital surface waves for the source duration of large earthquakes and modeling the receiver functions for the earth structure beneath a broadband seismometer array in the Cascadia subduction zone

  Citation × Citation Citation Abstract Copy Title: Deconvolving orbital surface waves for the source duration of large earthquakes and modeling the receiver functions for the earth structure beneath a broadband seismometer array in the Cascadia subduction zone Author: Li, Xiao-qing, 1963- Title: Deconvolving orbital surface waves for the source duration of large earthquakes and modeling the receiver functions for the earth structure beneath a broadband seismometer array in the Cascadia subduction zone Author: Li, Xiao-qing, 1963- Close

• 5. [Article] Shear-wave anisotrophy across the Cascadia Subduction Zone from a linear seismograph array

  Citation × Citation Citation Abstract Copy Title: Shear-wave anisotrophy across the Cascadia Subduction Zone from a linear seismograph array Author: Fabritius, R. Axel Title: Shear-wave anisotrophy across the Cascadia Subduction Zone from a linear seismograph array Author: Fabritius, R. Axel Close

• 6. [Article] Surfing the Cascadia Subduction Zone: Risk perceptions and evacuation knowledge among Oregon surfers

  Citation × Citation Citation Abstract Copy Title: Surfing the Cascadia Subduction Zone: Risk perceptions and evacuation knowledge among Oregon surfers Author: Shelby, Daniel Title: Surfing the Cascadia Subduction Zone: Risk perceptions and evacuation knowledge among Oregon surfers Author: Shelby, Daniel Close

• 7. [Article] Rotation and plate locking at the southern Cascadia subduction zone

  Copyrighted by American Geophysical Union.

  Citation × Citation Citation Abstract Copy Title: Rotation and plate locking at the southern Cascadia subduction zone Author: Goldfinger, Chris, Nabelek, John L., Smith, Curt, Long, Maureen D., Johnson, Cheryl K., Zwick, Peter C., McCaffrey, Robert Copyrighted by American Geophysical Union. Title: Rotation and plate locking at the southern Cascadia subduction zone Author: Goldfinger, Chris, Nabelek, John L., Smith, Curt, Long, Maureen D., Johnson, Cheryl K., Zwick, Peter C., McCaffrey, Robert Copyrighted by American Geophysical Union. Close

• 8. [Article] Investigating Plate Boundaries through New High-Resolution Bathymetry and Seismic Data; 2 Case Studies from the Cascadia Subduction Zone and San Andreas Fault

  Contiential margins on plate boundaries are complex systems with morphologies and characteristics dictated by the interplay of sediment deposition and erosion, tectonic faulting, folding, and strong ground ...

  Citation × Citation Citation Abstract Copy Title: Investigating Plate Boundaries through New High-Resolution Bathymetry and Seismic Data; 2 Case Studies from the Cascadia Subduction Zone and San Andreas Fault Author: Beeson, Jeffrey Watson Contiential margins on plate boundaries are complex systems with morphologies and characteristics dictated by the interplay of sediment deposition and erosion, tectonic faulting, folding, and strong ground motion generating mass wasting events. With ever increasing advances in high-resolution remote sensing techniques these systems are increasingly becoming illuminated. A ~120 km offshore portion of the northern San Andreas Fault (SAF) between Point Arena and Point Delgada was mapped using closely spaced seismic-reflection profiles, high-resolution multibeam bathymetry and marine magnetics data. This new dataset documents SAF location and continuity, associated tectonic geomorphology, shallow stratigraphy and deformation. Variable deformation patterns in the generally narrow (~1-km-wide) fault zone are largely associated with fault trend, and with transtensional and transpressional fault bends. We divide this unique transtensional portion of the offshore SAF into six sections along and adjacent to the SAF based on fault trend, deformation styles, seismic stratigraphy, and seafloor bathymetry. This southern region of the SAF includes a 10-km-long zone characterized by two active, parallel fault strands in which the SAF is evolving into a straighter orientation via migrating fault releasing and restraining bends. The SAF in the northern region of the survey area passes through two acute fault bends (~9° (right), and ~8° (left)), resulting in both an asymmetric “Lazy Z” sedimentary basin (“Noyo Basin”) and an uplifted rocky shoal (“Tolo Bank”). Noyo Basin subsidence and tilt rates, as well as SAF lateral slip rates, were determined based on seismic-stratigraphic sequences and unconformities correlated with the previous 4 major Quaternary sea-level lowstands. Progressively steeper erosional surfaces record basin tilting of ~0.6° per 100,000 years. Migration of the basin depocenter indicates a lateral slip rate on the San Andreas Fault of 10 to 19 mm/yr for the past 350,000 years. Data collected west of the SAF on the south flank of Cape Mendocino rule out the previously postulated presence of an offshore fault strand that connects the SAF with the Mendocino Triple Junction. Instead, the SAF passes on land at Point Delgada, where the SAF plate boundary transitions to the Kings Range thrust. Utilizing new high resolution multibeam bathymetric data, chirp sub-bottom and multichannel seismic reflection profiles, we identify and describe submarine channels, submarine landslides, and three “new” erosional features on the toe of the Cascadia accretionary wedge near Willapa Canyon, offshore Washington, USA. Bathymetric data was compiled from the Cascadia Open-Access Seismic Transects (COAST) cruise and from the site survey cruise for the Cascadia Initiative. This new high-resolution dataset has illuminated geomorphic features that suggest this section of the margin underwent radical erosion in the latest-Pleistocene. Three “new” and peculiar features were imaged that superficially resemble slope failures of the frontal thrust, but are distinguished from such failures by 1) incision of the crest of the frontal thrust and anticlinal ridge, and piggyback basin; 2) they have floors below the level of the abyssal plain, and have excavated deeply into the frontal anticline 3) The features are connected to the main Willapa Channel by inactive paleo channels. The features were likely formed during the latest Pleistocene based on post event deposition, cross-cutting by the modern Juan de Fuca and Willapa Channel levees, and post- event slip on the frontal thrust of the Cascadia accretionary prism. Based on morphology, dissimilarity with other submarine features on the Cascadia and other margins, and available age constraints, we infer that these features were most likely formed by massive turbidity currents associated with the glacial lake outpourings in the Pacific Northwest known as the Missoula floods. Title: Investigating Plate Boundaries through New High-Resolution Bathymetry and Seismic Data; 2 Case Studies from the Cascadia Subduction Zone and San Andreas Fault Author: Beeson, Jeffrey Watson Contiential margins on plate boundaries are complex systems with morphologies and characteristics dictated by the interplay of sediment deposition and erosion, tectonic faulting, folding, and strong ground motion generating mass wasting events. With ever increasing advances in high-resolution remote sensing techniques these systems are increasingly becoming illuminated. A ~120 km offshore portion of the northern San Andreas Fault (SAF) between Point Arena and Point Delgada was mapped using closely spaced seismic-reflection profiles, high-resolution multibeam bathymetry and marine magnetics data. This new dataset documents SAF location and continuity, associated tectonic geomorphology, shallow stratigraphy and deformation. Variable deformation patterns in the generally narrow (~1-km-wide) fault zone are largely associated with fault trend, and with transtensional and transpressional fault bends. We divide this unique transtensional portion of the offshore SAF into six sections along and adjacent to the SAF based on fault trend, deformation styles, seismic stratigraphy, and seafloor bathymetry. This southern region of the SAF includes a 10-km-long zone characterized by two active, parallel fault strands in which the SAF is evolving into a straighter orientation via migrating fault releasing and restraining bends. The SAF in the northern region of the survey area passes through two acute fault bends (~9° (right), and ~8° (left)), resulting in both an asymmetric “Lazy Z” sedimentary basin (“Noyo Basin”) and an uplifted rocky shoal (“Tolo Bank”). Noyo Basin subsidence and tilt rates, as well as SAF lateral slip rates, were determined based on seismic-stratigraphic sequences and unconformities correlated with the previous 4 major Quaternary sea-level lowstands. Progressively steeper erosional surfaces record basin tilting of ~0.6° per 100,000 years. Migration of the basin depocenter indicates a lateral slip rate on the San Andreas Fault of 10 to 19 mm/yr for the past 350,000 years. Data collected west of the SAF on the south flank of Cape Mendocino rule out the previously postulated presence of an offshore fault strand that connects the SAF with the Mendocino Triple Junction. Instead, the SAF passes on land at Point Delgada, where the SAF plate boundary transitions to the Kings Range thrust. Utilizing new high resolution multibeam bathymetric data, chirp sub-bottom and multichannel seismic reflection profiles, we identify and describe submarine channels, submarine landslides, and three “new” erosional features on the toe of the Cascadia accretionary wedge near Willapa Canyon, offshore Washington, USA. Bathymetric data was compiled from the Cascadia Open-Access Seismic Transects (COAST) cruise and from the site survey cruise for the Cascadia Initiative. This new high-resolution dataset has illuminated geomorphic features that suggest this section of the margin underwent radical erosion in the latest-Pleistocene. Three “new” and peculiar features were imaged that superficially resemble slope failures of the frontal thrust, but are distinguished from such failures by 1) incision of the crest of the frontal thrust and anticlinal ridge, and piggyback basin; 2) they have floors below the level of the abyssal plain, and have excavated deeply into the frontal anticline 3) The features are connected to the main Willapa Channel by inactive paleo channels. The features were likely formed during the latest Pleistocene based on post event deposition, cross-cutting by the modern Juan de Fuca and Willapa Channel levees, and post- event slip on the frontal thrust of the Cascadia accretionary prism. Based on morphology, dissimilarity with other submarine features on the Cascadia and other margins, and available age constraints, we infer that these features were most likely formed by massive turbidity currents associated with the glacial lake outpourings in the Pacific Northwest known as the Missoula floods. Close

• 9. [Article] Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon

  This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination ...

  Citation × Citation Citation Abstract Copy Title: Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon Author: Punke, Michele Leigh This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination of the stratigraphic record can reveal depositional signatures that provide insights into the geomorphic and tectonic forces active within coastal river basins. Three case studies from the southern Oregon coast illustrate the complex relationship between tectonics and geomorphic processes along an active margin coast such as Oregon's Cascadia subduction zone. This work illustrates that the differential preservation of late Pleistocene-age terrestrial deposits in Oregon's coastal landscape, and the early cultural sites they may contain, is not random but can be closely related to larger tectonogeomorphic processes operating at local and regional scales. Detailed subsurface investigation of one case study site, the lower Sixes River valley, reveals a complex history of depositional environment evolution in relation to geomorphic and tectonic forces. Litho- and biostratigraphic data sets are used to develop a depositional environment reconstruction for the lower Sixes River site through time. This reconstruction of the depositional environment from the late Pleistocene to the present indicates a transgressive evolution that differs from models of transgressive coastal facies and from other studied Northwest coast estuarine life histories. Factors such as eustatic sea level rise, regional and local tectonic alteration of the landscape, sediment supply, or valley morphology may have played roles in the creation and preservation of this atypical depositional sequence. Litho- and biostratigraphic evidence of six Cascadia subduction zone earthquakes younger than 6200 cal yr BP is recorded in a long sediment core from the Sixes River valley. All six of these events correlate with events previously reported for the area by Kelsey et al. (2002). At least five additional plate boundary earthquakes lowered tidal marshes and freshwater wetlands prior to 6200 cal yr BP. The presence within the lower Sixes River valley of an intertidal environment capable of recording Cascadia subduction zone earthquakes dating to the late Pleistocene and early Holocene has not been found at any other location on the Northwest coast. Title: Paleoenvironmental reconstruction of an active margin coast from the Pleistocene to the present : examples from southwestern Oregon Author: Punke, Michele Leigh This study illustrates geoarchaeological and paleoenvironmental approaches to the investigation of an active margin coastal setting and provides examples of how information gleaned through examination of the stratigraphic record can reveal depositional signatures that provide insights into the geomorphic and tectonic forces active within coastal river basins. Three case studies from the southern Oregon coast illustrate the complex relationship between tectonics and geomorphic processes along an active margin coast such as Oregon's Cascadia subduction zone. This work illustrates that the differential preservation of late Pleistocene-age terrestrial deposits in Oregon's coastal landscape, and the early cultural sites they may contain, is not random but can be closely related to larger tectonogeomorphic processes operating at local and regional scales. Detailed subsurface investigation of one case study site, the lower Sixes River valley, reveals a complex history of depositional environment evolution in relation to geomorphic and tectonic forces. Litho- and biostratigraphic data sets are used to develop a depositional environment reconstruction for the lower Sixes River site through time. This reconstruction of the depositional environment from the late Pleistocene to the present indicates a transgressive evolution that differs from models of transgressive coastal facies and from other studied Northwest coast estuarine life histories. Factors such as eustatic sea level rise, regional and local tectonic alteration of the landscape, sediment supply, or valley morphology may have played roles in the creation and preservation of this atypical depositional sequence. Litho- and biostratigraphic evidence of six Cascadia subduction zone earthquakes younger than 6200 cal yr BP is recorded in a long sediment core from the Sixes River valley. All six of these events correlate with events previously reported for the area by Kelsey et al. (2002). At least five additional plate boundary earthquakes lowered tidal marshes and freshwater wetlands prior to 6200 cal yr BP. The presence within the lower Sixes River valley of an intertidal environment capable of recording Cascadia subduction zone earthquakes dating to the late Pleistocene and early Holocene has not been found at any other location on the Northwest coast. Close

• 10. [Article] The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones

  This is the publisher’s final pdf. The published article is copyrighted by the Oceanography Society and can be found at: http://www.tos.org/oceanography/index.html.

  Citation × Citation Citation Abstract Copy Title: The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones Author: Toomey, Douglas R., Dziak, Robert P., Allen, Richard M., et al., Tréhu, Anne M., Barclay, Andrew H. This is the publisher’s final pdf. The published article is copyrighted by the Oceanography Society and can be found at: http://www.tos.org/oceanography/index.html. Title: The Cascadia Initiative: A Sea Change In Seismological Studies of Subduction Zones Author: Toomey, Douglas R., Dziak, Robert P., Allen, Richard M., et al., Tréhu, Anne M., Barclay, Andrew H. This is the publisher’s final pdf. The published article is copyrighted by the Oceanography Society and can be found at: http://www.tos.org/oceanography/index.html. Close