Thermal environment of the Southern Washington region of the Cascadia subduction zone

Abstract

Eleven recently collected multichannel seismic (MCS) profiles from the Cascadia Open‐Access Seismic Transects experiment offshore Washington State are used to characterize the distribution of bottom‐simulating reflectors (BSRs) from seaward of the deformation front onto the continental shelf of the Cascadia Subduction Zone. The 11 MCS lines consisted of nine lines perpendicular and two lines parallel to the Cascadia margin covering a 100 km along‐strike region of the accretionary wedge. From these MCS profiles we generated a 3‐D view of the Cascadia margin thermal structure by interpreting 40,232 individual BSR picks in terms of temperature and heat flow. Overall BSR‐derived heat flow values decrease from approximately 95 mW m−2 10 km east of the deformation front to approximately 60 mW m−2 located 60 km landward of the deformation front. Anomalously low heat flow values near 25 mW m−2 on a prominent midmargin terrace indicate recent sediment failure within the accretionary prism. Localized differences between BSR heat flow and numerical models reflect an estimated regional mean vertical fluid flow of +0.53 cm yr−1 for the survey area, with localized fluid flow approaching a maximum of +3.8 cm yr−1. Distinct finite element models for the nine MCS profiles perpendicular to the deformation front reproduce BSR heat flow values, producing an overall root‐mean‐square misfit of 10.2 mW m−2. At the deformation front, the incoming oceanic sediment/crust interface temperatures vary from 164°C to 179°C, indicating the updip limit of the Cascadia seismogenic zone.

Anomalous Concentration of Methane Emissions at the Continental Shelf Edge of the Northern Cascadia Margin

“…php?id=MGL1212) or from https:// catalyst.uw.edu/workspace/paulj/ 17643/363717. Line 5 of the Langseth COAST survey was commercially processed postcruise, and the details are described in Salmi et al (2017). NSF grant 1634095 was provided to H.P.J.…”

“…However, this scale of topographic relief on the lower accretionary wedge shows little seismic wave amplification in ocean bottom seismometer instrumentation (Gomberg, ). The same lower to middle‐slope region of the accretionary wedge contains well‐defined BSRs within the anticlinal ridges, but the area is not a major source of methane bubble streams (Figure ; Salmi et al, ). This suggests that enhanced sediment permeability at the seaward boundary of the continental shelf that results from the relationship between topographic slope, thickness of the accretionary wedge, and the frequency of seismic wave energy from distal earthquakes is a likely real phenomenon (Gomberg, ), but the exact mechanism and resulting impact on permeability is still undefined.…”

“…Langseth Line 5 data are publically available from the National Geophysical Data Center or GeoPRISM website (COAST 2012: http://www.marine-geo.org/tools/search/entry.php?id=MGL1212) or from https://catalyst.uw.edu/workspace/paulj/17643/363717. Line 5 of the Langseth COAST survey was commercially processed postcruise, and the details are described in Salmi et al (). NSF grant 1634095 was provided to H.P.J.…”

“…A recent revision of the thermal model positions the downdip limit of the fully locked/seismogenic zone along the décollement at the 350 °C isotherm, which for the Washington margin also places it directly beneath the continental shelf break (Salmi et al, ). In this thermal model, the interval between 350 °C and 450 °C for décollement temperatures represented the partially locked transition zone (Hyndman et al, ; Oleskevich et al, ), a region which Wang et al () also assign as the updip limit of the zone of afterslip in the full megathrust earthquake cycle.…”

“…Note the growth structure adjacent to the listric fault, indicating that movement on the extension feature is semicontinuous over time. Water column velocities for all MCS plots are assumed to be 1,450 m/s and 1,700 m/s for sediments on the upper margin (Salmi et al, ). (b) 1985‐WO‐4032.…”