Stratigraphic correlation of seismoturbidites and the integration of sediment cores with 3.5 kHz chirp subbottom data in southern Cascadia Public Deposited

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  • 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.
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