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Whats New

Upcoming Talks and Other Things

National Academy of Science, Engineering and Medicine

Joint BESR/COSG Meeting - The Cascadia Subduction Zone: Science, Impacts, and Response

November 10-11, 2016, National Academy of Sciences Building
2101 Constitution Ave NW Washington DC 20418


City Club Earthquake Forum, Kells Pub Portland, November 1, 5:30 pm


Goldfinger Active Tectonics Lab wins 2016 Geological Society of America Kirk Bryan Award.


TEdx Mt. HoodTEDx Portland, June 18, 2016. Revolution Hall



Science Pub Corvallis "Shaking up the Northwest, the Cascadia Earthquake in our Future" Majestic Theatre, Covallis 6 pm. Science Pub at The Majestic @ The Majestic Theatre | Corvallis | Oregon | United States


New Yorker Festival, Manhattan, October 3, School of Visual Arts, Theatre 1, 10 am. App_icon

NWEA Workshop, Hood River Inn, October 2.

Oregon Coast Economic Summit, August 27, Grand Ronde.

The Really Big One: A Public Forum On Earthquake Hazards

and Preparedness in the PNW, University of Oregon, Eugene, August 6, 7 PM. 156 Straub Hall.

New Books

New Novel: Stick Slip from Chris Scholz! An entertaining read about a Cascadia great earthquake.

The Next Tsunami examines our short term memory about disasters, Los Angeles Times, March 21, 2014

New Papers

Second Paper detailing the marine paleoseismic record of the Northern Sumatra margin released.

Second New Canadian Study Corroborates and Extends Cascadia Marine Paleoseismic Record

New Thesis Released: Southern Cascadia Turbidites Traced with High-Resolution CHIRP Sub Bottom Profiles.

New Dissertation Released: Sumatra Paleoseismology

New Paper Released: Cascadia Segmented Rupture Tsunami Models

New Paper Released: Cascadia Tsunami Models

New Paper Released: Cascadia Great Earthquake Clustering

Coastwide Tsunami inundation Scenarios for Oregon Released

"Superquakes and Supercycles" released, Seismological Research Letters

New Paper Released: Cascadia Turbidites in Forearc Lakes?

Preliminary study of existing lake sedimentary records suggests a record of great earthquakes.

New Cascadia Paper Released: Segmented Ruptures Along the Southern Cascadia Margin

New core and high resolution reflection data illuminate thesouthern Cascadia paleoseismic record.

New Canadian Study Corroborates Cascadia Marine Paleoseismic Record

Seismically generated turbidites in Effingham Inlet, western Vancouver Island.

Second in Sumatra Paleoseismology Series Released

Other Stuff

Bayesian Spatial Analysis Team Wins Department of the Interior Partners in Conservation Award

Oregon Earthquake Resiliency Report Released

Oregon Tsunami Work Wins Award

Successful geophysical cruise aboard the Derek. M. Baylis completed with very low carbon footprint

Cascadia, the Movie! Animation of 10,000 year earthquake record from marine and coastal paleoseismic sites.

Initiative to Retrofit Schools in Portand for Earthquakes




Cascadia Super-Scale Submarine Slides




Using SeaBeam bathymetry and multichannel seismic reflection records on the southern Oregon continental margin, we have identified three large submarine landslides on the southern Oregon Cascadia margin.  The area enclosed by the three arcuate slide scarps is approximately 8,000 km2, and involves an estimated 12,000-16,000 km3 of the accretionary wedge.  The three arcuate slump escarpments are nearly coincident with the continental shelf edge on their landward margins, spanning the full width of the accretionary wedge.  Debris from the slides is buried or partially buried beneath the abyssal plain, covering a subsurface area of at least 8,000 km2.  The three major slides, called the Heceta, Coos Basin and Blanco slides, display morphologic and structural features typical of submarine landslides.  Bathymetry, sidescan sonar, and seismic reflection profiles reveal that regions of the continental slope enclosed by the scarps are chaotic, with poor penetration of seismic energy and numerous diffractions.  These regions show little structural coherence, in strong contrast to the fold thrust belt tectonics of the adjacent northern Oregon margin.  The bathymetric scarps correlate with listric detachment faults identified on reflection profiles that show large vertical separation and bathymetric relief.  Reflection profiles on the adjacent abyssal plain image buried debris packages extending 20-35 km seaward of the base of the continental slope.  In the case of the youngest slide, an intersection of slide debris and abyssal plain sediments, rather than a thrust fault, Cascadia subduction zone bathymetry showing thrust vergencemark the base of slope.  The ages of the three major slides decrease from south to north, indicated by the progressive northward shallowing of buried debris packages, increasing sharpness of morphologic expression, and southward increase in post-slide reformation of the accretionary wedge.  The ages of the events, derived from calculated sedimentation rates in overlying Pleistocene sediments, are approximately 110 ka, 450 ka, and 1,210 ka.  This series of slides traveled 25-70 km onto the abyssal plain in at least three probably catastrophic events, which may have been triggered by subduction earthquakes.  The lack of internal structure in the slide packages, and the considerable distance traveled suggests catastrophic rather than incremental slip, although there could have been multiple events.  The slides would have generated large tsunami in the Pacific basin, possibly larger than that generated by an earthquake alone.  We have identified a potential future slide off southern Oregon that may be released in a subduction earthquake.  The occurrence of the slides and subsequent subduction of the slide debris, along with evidence for margin subsidence implies that basal subduction erosion has occurred over at least the last 1 Ma.  The massive failure of the southern Oregon slope may have been the result of the collision of a seamount province or aseismic ridge with the margin, suggested by the age progression of the slides and evidence for subducted basement highs.  The lack of latitudinal offset between the oldest slide debris and the corresponding scarp on the continental slope implies that the forearc is translating northward at a substantial fraction of the margin-parallel convergence rate.



Figure 1.  Onshore-offshore shaded relief image of the Cascadia subduction zone, Oregon, USA.  This image was compiled from onshore USGS DEM's onshore, offshore NOAA SeaBeam and BSSS swath bathymetry, and interpolated surfaces generated from digitized contours where swath bathymetry was unavailable.  The image resolution is 100 meters.  Relief image (and those to follow) shown without contours and only minimal depth shading to emphasize morphology.  Note morphologic contrast of the lower slope between the southern segment (42° 17’N-44° 14’N) the fold thrust belt of the northern Oregon segment (45°N-46°N), and the transitional zone between these two provinces. Landward-vergent (LV) and seaward-vergent (SV) segments indicated at left.

(Click image for larger image)


Although the Oregon convergent margin is frequently cited as a type example of a seaward-vergent accretionary wedge, this characterization is only applicable to a small part of the northern Oregon margin.  Taken as a whole, the Oregon margin is better characterized by significant along-strike variability in structural style and wedge morphology (Figure 1).  The northern Oregon and Washington accretionary wedge is a broad landward-vergent thrust system with widely spaced folds, and a decollement stepping down to the basement, with virtually all incoming sediment being frontally accreted (Flueh et al., 1996; MacKay, 1995; Goldfinger, 1994; MacKay et al., 1992; Snavely and McClellan, 1987; Silver, 1972).  This low-taper wedge is composed primarily of the Pleistocene Astoria and Nitinat Fans, which have been accreting outboard of a narrow, older Cenozoic accretionary complex.  The older complex in turn lies outboard of an outer arc high and Cenozoic forearc basin.

The steep, narrow, southern margin is poorly known, but as we will present here, it is characterized by massive slope failures that dominate the structure and morphology of the continental slope between 42° and 44° N latitude. We have identified arcuate escarpments on the Oregon continental slope enclosing regions of hummocky topography, and underlain by detachment surfaces that delineate at least three failure zones encompassing much of the southern Oregon continental slope (Figure 2).   Beneath the abyssal plain, we have found widespread subsurface and partially buried debris aprons.  We first present evidence for buried slump debris beneath the abyssal plain, then discuss morphologic and structural evidence for the slides themselves.  We then determine the age of the slides, and discuss possible mechanisms for the apparent massive collapse of the margin,  implications for earthquake and tsunami hazards, and forearc deformation.    


Cascadia subduction zone bathymetry showing thrust vergence and megaslide locations      Much of the continental slope off southern Oregon has a distinctive morphology that can be easily differentiated from the northern Oregon, Washington and northern California margins, and from the adjacent uppermost slope (Figures 1 and 2).  Four morphologic features distinguish the southern Oregon slope: 1) Lack of or poor definition of accretionary wedge fold-thrust ridges and slope basins; 2) Hummocky surface morphology; 3) Pervasive closely-spaced linear trends; 4) Large arcuate scarps enclosing domains characterized by the first three morphologic elements.  Examination of shaded relief bathymetry (Figure 2) reveals three large arcuate scarps, the northernmost of which is both the largest and best defined.  The upper slope scarp morphology is distinct in the northern area, becoming progressively less so to the south.  From north to south, these three large scarps are 76, 68 and 65 km in length, and 33, 30, and 34 km in width.  They enclose 2874 km2, 2304 km2, and 2713 km2 respectively, totaling 7890 km2 in area.  We have named these features the Heceta slide, the Coos Basin slide, and the Blanco slide respectively after adjacent features on the margin and nearby coast.  
In addition to the northward increasing definition of the slump scar morphology, the three other distinguishing morphologic elements (poor fold-thrust definition, hummocky topography, and closely spaced linear trends) also exhibit a north-south variation.




Figure 2.  Shaded relief image of the southern Oregon continental margin.  Beneath the abyssal plain between 42° 16.55' N and 44° 13.75' N, reflection reveal several thick intervals of hummocky, chaotic reflectors (Figures 3-8,13; profile locations A-H shown here.  R/V Farnella two-channel reflection profiles on abyssal plan shown with time ticks.  Buried packages at three stratigraphic levels, shown by superposed patterns. Upper Blanco slide is the smaller polygon within the Blanco Slide polygon.  The patterned polygons represent the minimum distribution based on available reflection data, and are dashed where inferred.  Interpreted slump scarps indicated by white arrows.  Burial of the frontal thrust by the Heceta slide can be seen at the arrow point for the label "Deformation Front" at upper left.  Estimated pre-slide deformation front shown by dashed white line.   



Cascadia subduction zone seismic reflection profile showing Heceta slide














Figure 3.  Industry multichannel reflection profile (A-A' in Figures 2 and 6)across the Cascadia plate boundary showing the northernmost slump debris on the continental slope, shallowly buried debris beneath the abyssal plain, and recent slump debris on the abyssal plain.  Coherent planar abyssal plain reflectors can be traced at least 11 km landward beneath the base of the slope.  We suggest that no the tectonic plate boundary is further east, and no subduction thrust is present on this profile.  Velocity pull-up accounts for the abrupt rise in abyssal plain reflectors beneath the slope.  Corresponds to YELLOW polygon in Figure 2.



Cascadia subduction zone bathymetry showing thrust vergence and slide detailsFigure 4.  Detail of the northernmost slump area off central Oregon.  This is the best expressed and largest of the proposed slope failures.  Abyssal plain debris is the shallowest in the sedimentary section opposite this feature, suggesting it is the most recent event.  The tectonic deformation front is buried by the debris slide.  The top of the debris pile is covered by 30-80 m of sediment (Figure 3), based on a sediment velocity of 1650 m/s.  An estimated sedimentation rate of 600 cm/1000 yrs (based on data in Nelson, 1968) suggests an age of approximately 50-130 ka for this event.  The slump scar has been buried in several locations by progradational lobes ("PL"), presumably deposited during Pleistocene sea level low-stands.  These lobes have themselves failed in secondary slumps ( "SF"), superimposing smaller slump piles on the surface of the larger detached block.  A small recent slump at the deformation front ( "RS") has deposited debris blocks at the surface of the abyssal plain.  Surficial morphological indicators of massive slope failure include:   1)  Lack of coherent fold and thrust belt typical of accretionary wedges;  2)  Chaotic surficial morphology of area enclosed by scar;   3)  Sharply contrasting surface morphology across the scar;  4)  Blocky, convoluted base of slope;  5)  Lack of an identifiable thrust fault at base of slope.  Dashed white line shows projected tectonic deformation front beneath the debris pile.  Colored lines show locations of seismic sections A-A' and F-F', keyed to boxes of the same color surrounding the figures.



Cascadia subduction zone sidescan showing heceta slide deformation front debris





Figure 5.  SeaMarc 1A sidescan sonar image of part of the base of the continental slope.  Area of figure indicated by label SS in Figure 6.  The base of the continental slope from about 43 15' to the northern limit of the slumped area at 44deg 13.75' is characterized by irregular, blocky material that we interpret as the onlapping of abyssal plain sediments on the top of the slumped debris pile.  We see no evidence of a thrust fault along this part of the margin in seismic, bathymetric, or sidescan data.  The sidescan coverage is nearly complete along the deformation front between these latitudes.




This tectonic style of the southern Oregon margin differs sharply from northern Oregon, Washington and northern California.  In northern Oregon and Washington, the continental margin is clearly accretionary, with young, well defined thrust ridges and faults characterizing a youthful wedge that is largely Pleistocene in age.  The accretionary wedge is widest in Washington, ~100 km, and narrows southward to 30-50 km off southern Oregon.  Reflection profiles show that much of the slump debris is presently being subducted.  The subduction decollement  is seaward vergent south of 44  50', and landward vergent from that point northward to Vancouver Island.  The decollement in the landward vergent section of the margin offscrapes much of not all of the sedimentary section (Mackay at al., 1992; 1995), whereas the seaward vergent thrusts in southern Oregon override much of the sedimentary section and embedded slump debris.  The extreme narrowness of the margin, seaward vergence of the subduction decollement, and mass wasting of the southern Oregon margin suggest that the southern Oregon margin is undergoing basal subduction erosion and simultaneous frontal accretion.  There may be several causes for the shift from an accreting margin in Washington and northern Oregon to an eroding margin in southern Oregon.  In the north the sediment supply is much greater, with the large Pleistocene Astoria and Nitinat submarine fans dominating the sedimentary section.  Off southern Oregon, despite the presence of the relatively high topography of the Klamath mountains, the sediment supply is relatively low.  The rapid deposition of the large submarine fans contributes to their subsequent accretion in that high fluid pressures generated in the section by rapid deposition tend to favor landward vergent thrusting at the deformation front (Seely, 1977;  Mackay et al., 1992; 1995).   Landward vergence in turn promotes accretion because the decollement
frequently steps down to near the basement, offscraping the entire incoming sedimentary package, where seaward vergence permits subduction of more of the incoming section.   A possible mechanism for destabilizing the southern Oregon accretionary wedge may have been increased fluid pressures generated during rapid sediment deposition during the Pleistocene.  A consequent reduction in basal shear stress on the megathrust may have led  to both landward vergent accretion of the fans in the north, and destabilization of the southern margin.  If the accretionary wedge was at a critical taper angle, this would have brought the wedge to a super-critical (i.e. oversteepened) condition.  The oversteepened wedge may then have failed by gravity-driven detachment to re-establish a critical taper angle.   This hypothesis does not explain the obvious age progression, younger in the north, that is observed, in fact a reverse age progression might be expected based on sediment progradation from northern sources during the Pleistocene.  A better explanation may be simple basal erosion of the wedge by seamounts on the subducting plate.  There are presently a number of seamounts near the deformation front, buried by abyssal plain sediment.  These features were imaged by chance with 2 channel seismic reflection data collected during the GLORIA/Farnella cruises of the US EEZ.



Super-scale slumping of the southern Oregon Cascadia margin has been an important tectonic process operating in late Quaternary time.  At least three mega-slides have occurred, involving much of the accretionary prism.  The massive nature of slump debris buried in the abyssal plain, and the considerable distance the debris traveled, suggest that the slides were probably single catastrophic events, although we presently cannot exclude multiple events.  The evidence of extensive deep-seated slope failure, subsidence and tilting of adjacent submarine banks, and the apparent subduction of slide debris suggest that the southern Oregon margin is undergoing basal tectonic erosion.  This does not preclude frontal accretion, which is occurring simultaneously.  We cannot presently determine whether there is net erosion or accretion of material at the southern Oregon margin.  In contrast, the northern Oregon and Washington accretionary wedge is currently accreting and outbuilding as Pleistocene submarine fans are rafted landward on the subducting Juan de Fuca plate. 

The Oregon mega-slides may have multiple driving mechanisms.  Lowered Pleistocene basal shear stress on the megathrust, seamount/ridge subduction, and arc-parallel extension may all play a role in the tectonics of this segment of the margin.  Subduction of a basement topographic feature(s) offers the best explanation for the observed south to north age progression of the three megaslides, although the preferred NNW orientation of the inferred ridge does not correspond to known basement trends.  A seamount province that is potentially related to this process has been identified on the adjacent abyssal plain with seismic reflection data, and may continue beneath the wedge based on gravity and magnetics analysis.   

Southern Oregon can be defined as an area of greater tsunami hazard relative to other margin segments by virtue of its failure mode in great slides, and due to the presence of a large incipient slump that maybe released in a future earthquake.  The relative lack of displacement between the oldest slump debris and its corresponding scarp on the continental slope suggests little margin-parallel motion between them, despite oblique subduction.  This lack of relative motion suggests that the southern Cascadia forearc may be translating northward at or near the full margin-parallel component of the plate rate.     



Goldfinger, C., Kulm, L.D., McNeill, L.C., and Watts, P., 2000, Super-scale failure of the southern Oregon Cascadia margin: Pure and Applied Geophysics, v. 157,. p. 1189-1226.



Supported by National Science Foundation Grants OCE-8812731 and OCE-8821577, NOAA National Undersea Research Program Award UAF 96-0060, and by the National Earthquake Hazards Reduction Program, U.S. Geological Survey, Department of Interior, under award 14-08-001-G1800.  We gratefully acknowledge the use of an extensive collection of single and multichannel proprietary seismic data.  As part of a standard use agreement, company names and detailed trackline navigation are omitted.  The manuscript was substantially improved by reviews by Roland von Huene, Gregory Moore, and an anonymous reviewer.