We are investigating the structure of the Cascadia subduction zone using an extensive academic and industry dataset of single and multichannel seismic reflection profiles, multibeam bathymetry sidescan sonar and submersible observations

 

   

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 Regional Structural Mapping

 

Introduction

The Cascadia Subduction zone structure map was created using 30, 50 and 150 kHz sidescan sonar imagery, NOAA SeaBeam and BS3 swath bathymetry, DELTA submersible observations, and a dense network of single-channel (SCS) and multichannel seismic (MCS) surveys.  We used these data to map the Oregon-Washington continental margin using a Computer Aided Design (CAD)/ Geographic Information System (GIS) system in which the various datasets and navigation layers were brought in as layers and could be viewed in any combination of raster and vector data.  This system allowed us to work at the best scale for each data set without the limitations of a fixed-scale base map.

Download the Map

Data Collection Sources

The neotectonic map of the Oregon continental margin represents the compilation and interpretation of about 30,000 km of seismic reflection profiles, SeaBeam swath bathymetry, and side-scan sonar mosaics of the seafloor morphology. The primary data sets used in constructing the map include: (1) single-channel sparker and airgun reflection profiles shot by Oregon State University and the University of Washington; (2) single and multi-channel airgun profiles shot by the U. S. Geological Survey; (3) migrated multi-channel airgun reflection profiles shot by Digicon for Oregon State University; (4) single-channel sparker profiles made by Shell Oil Corporation; (5) single and multi-channel profiles acquired by Chevron Oil Co (mostly shot by Gulf Oil); (6) SeaBeam swath bathymetry obtained by NOAA/National Ocean Survey and Oregon State University; (7) GLORIA long range side-scan sonar acquired by the U.S. Geological Survey; (8) 50 kHz Klein sidescan sonar and associated DELTA submersible observations; and (9) SeaMARC 1A high-resolution side-scan sonar acquired by Oregon State University. Several of these data sets were used in conjunction with one another to identify and map the active structures of the continental margin (shelf and slope) and adjacent abyssal plain. Table A.1 lists the sources and navigation methods for the various data sets (see also discussion below). Plate 2 shows the locations of ships tracks for the data used in this study. Seismic reflection profiles made on the Oregon margin and abyssal plain were acquired using a variety of sources, including various sized air gun and sparker arrays, and a variety of digital and analog recording systems. The single- and multi-channel seismic reflection data vary widely in type of source signal, record quality, depth of penetration, and navigational accuracy. They include unmigrated single-channel sparker records navigated with Loran-A and Loran-C, as well as migrated 144-channel digital profiles navigated with GPS (Global Positioning System). Some multi-channel reflection profiles were navigated with GPS, and the position information from these lines is used as the datum for mapping that portion of the study area. Navigational accuracy was more variable with older single-channel seismic profiles. Loran-A navigated profiles have maximum errors about 1-3 kilometers, Loran-C errors are approximately 0-1.5 km, and Transit satellite errors range from near zero up to several hundred meters. An exception is the Shell Oil Company lines. Although these profiles were shot in 1961-62, their navigational accuracy rivals the TRANSIT navigated lines due to the use of a company SHORAN radio navigation system. Horizontal errors with this system are approximately 50 meters. The dense coverage of reflection profiles allowed readjustment of older lines where crossed by satellite-navigated lines.

 

Data Source Navigation System Approx. Nav. Error
U.S.G.S MCS/GLORIA Transit/Loran C 100-500m
OSU-U/W Sparker SCS Loran A 1000-3000M
OSU/Digicon MCS GPS C/A 100m
NOAA/NOS Multibeam ARGO 50 m
OSU Multibeam (pre 1996) TRANSIT/GPS/Loran C 100 m
OSU,other academic multibeam (post 1996) GPS P-Code, WAAS, or non-SA 5-10m
OSU 50-150 kHz sidescan GPS C/A 100m
OSU SeaMarc 1A TRANSIT/GPS/Loran C 100 m
Chevron Oil MCS Transit/Loran C 0-500m
Shell Oil Sparker SCS SHORAN 50 m

 

Table A.1. Data sources used in this study and approximate navigational accuracy. MCS = multichannel seismic profiles, SCS = single channel seismic profiles, Multibeam = SeaBeam swath bathymetry.

Cascadia structure map navigation tracklines

Trackline maps showing navigation for all seismic reflection profiles used in this study.

(click for larger image). Navigation files are avaialble in the download section at the top of this page.

 

 

The SeaBeam bathymetric survey system is a 12 kHz multibeam echo sounder developed by the General Instrument Corporation to produce near-real-time high-resolution contoured swath charts of the sea floor morphology. Full coverage digital SeaBeam swath bathymetry was acquired on the abyssal plain and continental slope from water depths extending from about 3,000 m to about 200 m on the upper continental slope by NOAA/NOS. These bathymetry data were contoured at a 20 m interval and used as a base map for the Oregon neotectonic map. The bathymetry is accurate to within 1% of the water depth across the swath. The NOAA/NOS multibeam surveys were navigated with an ARGO system which was placed in towers positioned along the shoreline; navigational accuracy is 50 m. The partial coverage academic SeaBeam surveys completed in 1987 and 1988 were utilized initially in this study before the NOAA/NOS data were declassified by the U. S. Navy in 1991. The academic SeaBeam surveys were generally navigated with a combination of GPS and Transit satellite navigation, with Loran-C tracking used between satellite fixes. The regional seafloor morphology of the abyssal plain and continental slope was imaged with the GLORIA system, a side-scan sonar instrument that uses a frequency of 6.8 kHz on the port side array and 6.2 kHz on the starboard side array. It images a 45 km swath width (i.e., 22.5 km either side of the ship's track), with spatial resolution of about 50 m in the across-track direction and 125 m in the along-track direction (see EEZ-SCAN 84 Staff, 1986 for details). This relatively low-resolution GLORIA system is designed to image the relatively large-scale features of the seafloor, such as mid-ocean ridges, fracture zones, abyssal hills and ridges, deep-sea channels, and lineaments. Higher resolution side-scan sonar data were collected with a SeaMARC 1A deep-towed vehicle, which uses a frequency of 27 kHz on the port array and 30 kHz on the starboard array. It is capable of imaging a 2 km or a 5 km swath width (i.e., 1 km and 2.5 km on either side of the ship's track, respectively), with spatial resolutions of 1 and 2.5 meters, respectively (see Appelgate, 1988 for details). The side-scan sonar surveys were navigated with a combination of GPS and Transit satellite navigation, with Loran-C tracking used between satellite fixes. Navigation of the deep-towed SeaMARC-IA side-scan fish was done by the method described by Appelgate (1988). Where spatial misfits occur, we have adjusted the side-scan and single-channel seismic reflection data to best fit the TRANSIT and GPS navigated multi-channel seismic reflection lines or the SeaBeam bathymetry where appropriate. The highest resolution side-scan sonar data were collected with a Klein 50 kHz sidescan sonar unit on the Oregon Shelf in July/August 1992. This system imaged details of shelf faults with 30 cm resolution. Ten areas of particular interest were surveyed, and dives were made with the submersible DELTA on active shelf faults to ground truth the sonar data.

 

Mapping Procedures

The Neotectonic map of the Oregon continental margin was constructed in a Geographical Information System (GIS) fashion with several layers consisting of the different data sets. The GLORIA side-scan sonar mosaics were used as a base layer at a scale of 1:500,000. Large features such as the major submarine channels, the Blanco Fracture Zone and the deformation front were mapped primarily from this data set. The GLORIA side-scan mosaics were selected for the base map because of its wide coverage of the Exclusive Economic Zone (EEZ) off the Pacific Northwest. At the time this mapping project was initiated in 1990, full SeaBeam coverage was not yet available for the Oregon margin from NOAA/NOS. (SeaBeam bathymetry coverage remains classified for the entire Washington margin and abyssal plain, where a new neotectonic map is being compiled by us in the same manner as described here.) Preliminary copies of the SeaBeam swath bathymetry charts were provided by NOAA/NOS as soon as they became available, and were scaled to 1:500,000 on transparent media. Together, these contour charts and GLORIA mosaics were used to map faults and fold axes between seismic reflection profiles, allowing nearly continuous mapping of individual structures from the abyssal plain to the outer continental shelf. The accurately navigated SeaBeam bathymetry provided another means of correcting position information from older Loran-A navigated seismic profiles, structural information from which was shifted to match the high-resolution bathymetry as needed. In the Coos Bay area, our mapping is modified from Clarke and others (1985), who mapped the structures of Coos Bay basin using closely spaced and well navigated single channel reflection profiles. We incorporated and modified this earlier mapping using SeaBeam bathymetry and industry reflection profiles. As a convention used in this map, we show offsets across faults only where they can be demonstrated with existing data. Other offsets that might be expected, but have not been demonstrated, are not shown. This may cause some ambiguity where seafloor features are most probably offset by a fault, but do not appear so on this map. We considered this preferable to the greater ambiguity caused by inferring offsets where insufficient data exist. We emphasize young structures (i.e., those that offset and/or deform the seafloor) on the neotectonic map in order to evaluate regional structural trends under the present oblique subduction environment. Young structures are also the easiest to map as they generally deform the seafloor or appear in the more easily interpretable uppermost portion of seismic reflection profiles. We used a color coding scheme to represent the estimated age of youngest demonstrated activity on folds and faults. Structures active in the latest Pleistocene and Holocene are shown in red, structures active in the Pliocene-Pleistocene are shown in purple, all older structures are shown in blue. The youngest estimated age of continuous structures may vary along strike, this is represented on the map by along-strike color changes. A color change on the map could indicate either an actual along-strike change in the motion history of a structure, or it could be an artifact created by the variability in quality or distribution of the seismic data. In some areas, younger structures can be seen to overlap older (usually NE trending) structures. This occurs where erosional unconformities vertically juxtapose structures of widely differing ages. In some cases the older structural trends have remained active despite unfavorable orientation in the present stress field. In a few cases, coeval structures cross each other. These occurrences are associated with active strike-slip faults cutting other, sometimes active structures. Some of these faults have associated flower structures and fault-parallel folding that is active while other folds of different orientations also remain active. This can occur because both sets of structures are compatible with a single greatest principal stress orientation. Age determinations for structures other than active structures that cut or deform the present sea floor generally have a large and undefined error bar, and should be considered relative ages for general use. Active structures are difficult to confirm in areas of older surface exposure, thus some active fault have probably been missed in these areas (i.e. inshore areas of southern Oregon south of Coos Bay and inshore areas between Cape Falcon and Cape Foulweather). Structures in these areas that are shown as active have generally been confirmed as active by sidescan sonar imaging or submersible dives. A number of other suspected active structures have been ground truthed in this manner. Ages are constrained by biostratigraphy in industry drill holes, Deep-Sea Drilling Program (DSDP) drill holes, dart cores and dredges from sedimentary rock outcrops on the seafloor, and piston and gravity cores from unconsolidated sediments.

 

Age Control

The Cascadia continental shelf was subjected to multiple Pleistocene transgressive/regressive cycles during the sea-level fluctuations caused by glacial advance and retreat.  The last transgressive/regressive cycle left a widespread unconformity over which a thin Holocene sequence of transgressive sand was deposited on the middle to inner shelf, and a hemipelagic mud deposited on the middle to outer shelf (Kulm and others, 1975; Peterson and others, 1984). The age of the underlying strata ranges from Pleistocene (conformable in some locations on the middle to outer shelf) to Eocene and older on the southern Oregon inner shelf (Kulm and Fowler, 1974) .  This unconformity represents a relatively low-relief seaward-dipping surface, and thus serves as an effective strain marker for latest Pleistocene and Holocene deformation.  This erosional event is time-transgressive over the shelf.  The last sea-level minimum of 70-130 m below modern sea-level occurred approximately 18,000 years ago, with sea-level rising to within a few meters of present level by about 6000 years ago (Curray, 1965; Blackwelder and others, 1979; Chappel and Shackleton, 1986;  Fairbanks, 1989; Matthews, 1990) .  Thus tectonic activity that deforms this surface has a maximum age of about 18,000 years.  Deformation of the Holocene shelf sand or mud on the middle to outer shelf has a maximum age of about 6000 years.  Deformation of these sediments on the inner shelf is less common, as water depths less than about 150 m are subject to active erosion and sediment transport by bottom currents and storm waves (Komar and others, 1972) .   In some areas of the inner shelf where sediment supply is low, recent sediments are thin and patchy or altogether absent.  Deformation mapped in these older rocks is difficult to evaluate without younger sediments, however faults can be evaluated in terms of late Quaternary deformation by satisfying one of two possible criteria:  1) The fault can be traced seaward into deep enough water that a Holocene scarp in unconsolidated sand or mud is preserved along the same structure, or 2) the fault can be correlated to a known onshore fault that offsets late Quaternary deposits. 
The Pleistocene-Holocene transition can easily be distinguished visually in cores and in outcrop over much of the Cascadia shelf.   The Holocene hemipelagic muds are olive green in color, and very poorly consolidated.  A sharp transition to more consolidated gray silty clay marks the transition into Pleistocene sediments (Barnard and McManus, 1973) .  The color change is due to the abrupt upsection decrease in the terrigenous sediment fraction, and thus a relative increase in organic content, at the end of the Pleistocene.   This occurrence was time-transgressive, decreasing in age from the abyssal plain to the continental slope, and occurred at about 12 ka on the upper slope after application of a reservoir correction to the radiocarbon ages (Barnard and McManus, 1973) .  A shift from foraminiferan dominated to radiolarian dominated biostratigraphy lagged somewhat behind the color change, occurring at approximately 9-12 ka on the lower continental slope, and 8 ka on the upper slope (Barnard and McManus, 1973; Goldfinger et a., 2011).  The color change was found in cores and by direct submersible observations on the Oregon-Washington continental margin.  Change in color is the result of the sharp reduction in the terrigenous sediment fraction (gray) at the close of glaciation, while the organic (green-brown) fraction was relatively constant.

 

Discussion of Selected Map Features and Symbols

 

Regional Physiography

The physiography of the Cascadia margin and abyssal plain is shown in Figure 4.8 to show the relationship of the structures to the main physiographic features of the region. The continental shelf extends from the shoreline to the shelf break (i.e., a significant increase in bottom slope), at a water depth from 180 to 200 m. The shelf break is highly sinuous in Oregon and its distance from the coast varies from 25 to 75 km. The widest shelf areas correspond to Nehalem, Heceta, and Coquille submarine banks. The continental slope extends from the shelf break to a water depth of about 3,000 m, seaward of which is the relatively flat abyssal plain of the Juan de Fuca Plate. In northern and central Oregon, the slope is further subdivided morphologically into an upper and lower slope based upon the occurrence of upper and lower structural terraces, respectively (see below). Various named and unnamed submarine channels are indicated on the continental slope and abyssal plain. On the abyssal plain, most of these channels are distributary channels of the Astoria submarine fan, the apex of which is at the mouth of Astoria canyon in the northwestern quadrant of the map. The Blanco Fracture Zone, a transform fault, connects the southern end of the spreading Juan de Fuca Ridge with the northern end of the spreading Gorda Ridge. The Juan de Fuca plate is converging with the North American plate off central Oregon along a vector oriented at 062° and at a rate of 40 mm/yr (calculated from the poles of DeMets and others (1990) at 45° 00' N latitude along the deformation front). The neotectonic structures of the Cascadia margin, abyssal plain and Blanco Fracture Zone are shown on the accompanying map. A brief description of the structure is presented here to acquaint the reader with the main structural elements. Additional background information on the structural and tectonic setting of this portion of the NE Pacific Ocean is given in Cited and Related References.

 

Plate boundary and Deformation Front

The primary structural feature is the North American-Juan de Fuca plate boundary at about 3,000 m water depth. The deformation front is characterized by a seaward-vergent thrust fault from the Gorda plate off northern California north to 44° 51' N latitude off southern and south-central Oregon. North of 44° 51' N latitude into Washington, the basal thrust is landward-vergent, with one minor exception (Goldfinger and others, 1994; 1997). The plate boundary is complex in detail, highly sinuous in many seaward-vergent areas, offset by oblique structures, and commonly distributed over many splay thrusts. Many small and several large slumps are mapped on the lowermost slope and abyssal plain, notably a very large debris pile and arcuate scarp at 44° 00' N and a smaller one at 45° 21' N latitude.

 

Lower Slope Structures

In central and northern Oregon, the continental slope is characterized by upper and lower terraces separated by a major landward-dipping thrust and a coincident break in slope (labeled SB for slope break on the map) at about 1,000 meters water depth. Seaward of this fault, thrusts and folds of the accretionary wedge trend north-south, sub-parallel to the continental margin. Landward of the fault, folds of the upper slope and shelf trend mostly north-northwest to west-northwest, oblique to the margin. In southern Oregon, the terraces become one steep escarpment and this boundary becomes indistinguishable as a bathymetric feature. The two domains of structural orientations remain distinct south into California (Clarke and Carver, 1992). Thrust faulting within the accretionary wedge occurs in both landward and seaward vergent styles, with landward vergence common off northern Oregon and rare off southern Oregon. Out of sequence thrusting follows a similar pattern, being common, if not typical, in the north and less so in the south. Many second order features of particular tectonic significance are also apparent on the map.

 

Transverse Strike Slip Faults (link to more information)

At least nine WNW-trending left-lateral strike-slip faults have been mapped on the Oregon-Washington continental margin using sidescan sonar, seismic reflection, and bathymetric data, augmented by submersible observations.  The faults range in length from 33 to 115 km, and cross much of the continental slope.  Five faults offset both the Juan de Fuca plate and North American plates, and cross the plate boundary with little or no offset by the frontal thrust.  Left-lateral separation of channels, folds, and Holocene sediments indicate active slip during the Holocene and late Pleistocene.  Offset of surficial features ranges from 120 to 900 m,  and displaced subsurface piercing points at the seaward ends of the faults indicate a minimum of 2.2 to 5.5 km of total slip.  Near their western tips, fault ages range from 300 ka to 650 ka, yielding late Pleistocene-Holocene slip rates of 5.5 ± 2 to 8.5 ± 2 mm/yr.  The geometry and slip direction of these faults implies clockwise rotation of fault bounded blocks about vertical axes within the Cascadia forearc.  Structural relationships indicate that some of the faults probably originate in the Juan de Fuca plate and propagate into the overlying forearc.  The basement involved faults may originate as shears antithetic to a dextral shear couple within the slab, as plate coupling forces are probably insufficient to rupture the oceanic lithosphere.  The set of sinistral faults are consistent with a model of regional deformation of the submarine forearc (defined to include the deforming slab) by right simple shear driven by oblique subduction of the Juan de Fuca plate.  

 

Folding and Flexural-Slip Faulting

            In numerous seismic reflection profiles on the Oregon shelf we found varying degrees of late Quaternary deformation on folds that had been truncated by the Pleistocene transgressive/regressive episodes. This post-Pleistocene deformation is most commonly expressed as warping of the seafloor and offsets of the seafloor by flexural-slip faults, or bedding plane faults (Yeats, 1986)  on which dip-slip or oblique-slip motion occurs as folds grow (Fig. 4.2).  These structures are common on the Oregon shelf, but are

 

 

Figure 4.2.  Single channel airgun seismic record (OSU line SP-118) showing an active syncline and flexural-slip faults.  The flexural-slip faults offset the sea floor (latest Pleistocene abrasion platform) and Holocene cover.  Location is west of Tillamook Bay, Oregon.  

 


best expressed in areas landward of the major submarine banks discussed in more detail below (see Plate 1).  In the central parts of the major banks, post-Pleistocene folding is  more difficult to evaluate due to the thinning or absence of late Quaternary sediment.  In seismic reflection profiles, the origin of the flexural slip faults is clear and unambiguous, since they commonly occur as outward facing pairs of scarps symmetric about the fold axis, and fault dips are parallel to the dip of the underlying beds.  Individual faults in plunging folds can be traced on sidescan images along single bedding planes from one limb of the syncline, through the fold axis, to a corresponding fault on the other limb.  Vertical separation at the seafloor ranges from a few centimeters to several meters, based on reflection data and corresponding observations from  DELTA. The effect of this type of faulting on the seafloor topography depends on the style of the underlying fold.  If the folding is gentle, growth of a set of flexural slip faults produces a topography that mimics the underlying fold, that is, the seafloor is topographically lowest at the synclinal axes, highest at the anticlinal axes.  With tighter folding,  an interesting topographic inversion occurs.  With tighter folds, more displacement occurs on the interior bedding plane faults of the synclines, creating a topographic high that coincides with the synclinal axis.  This is analogous to cutting an onion in half along the long axis, then squeezing perpendicular to the long axis.  The layer-parallel slip causes the center sections to pop out. This type of topographic inversion is a temporary function of the interaction of the growing folds with the flat Pleistocene erosion surface, and would not persist through continued long term fold growth.  Sidescan sonar images of these growing folds reveal that, in plan view, the flexural slip faults converge or diverge from the synclinal axes depending on the plunge direction of the fold.  Submersible observations of the seafloor scarps from several localities indicate that these submarine features are better preserved than their land counterparts.  We observed overhanging scarps in several locations, and mole tracks in several others, both geomorphic features that would have very short life spans on land.  Both mole tracks and high-angle scarps were observed to deform both the late Pleistocene gray clay, and the overlying olive-gray Holocene unconsolidated silt, indicating movement younger than 6,000 yrs.  In several cases, colonization of the fault scarps by burrowing and attaching marine organisms decreased toward the bottom of the scarp, suggesting that uplift had occurred in multiple stages of fault movement.  In several instances, the lowermost tens of centimeters were devoid of burrows, but we were unable to determine if these scarps were being kept free of marine growth by bottom currents and bottom fish that hide at the scarp bases, or if the faults had moved so recently that they were not yet colonized.  As might be expected, we were unable to observe any indicators of slip direction on the exposed fault surfaces due to the lack of consolidation of the sediments.  The lack of slip indicators prevented  testing the hypothesis that such structures might accommodate part of the arc-parallel component of oblique convergence by oblique slip as suggested by Goldfinger and others (1992; 1994; 1997). 

 

Relationship Between Shelf Structures and Coastal Bays

 

Geophysical data from the offshore Cascadia forearc reveal many Quaternary upper-plate faults and folds. Most active structures are within the accretionary wedge, but significant deformation is also found on the continental shelf. Several faults and synclines project into adjacent coastal bays where deformation of Pleistocene marine terraces is reported. Rapidly buried marsh deposits and drowned forests in these coastal lowlands are interpreted to record coseismic deformation by prehistoric subduction zone earthquakes. The extent and amount of such coastal subsidence has been used to infer characteristic magnitudes and recurrence intervals. However, the record may incorporate both elastic strain release on the subduction zone and localized permanent upper-plate deformation. Movement on upper-plate structures may be triggered by a subduction zone earthquake, as observed in the Nankai and Alaskan forearcs. Alternatively, they may deform independently of subduction zone earthquakes. Regardless of which style of deformation predominates, the record of coseismic subsidence is likely to be affected. Crustal deformation may also contribute to the preservation of subsided marshes. Modelling of subduction zone earthquake characteristics based on coastal marsh stratigraphy is likely to be inaccurate in terms of the amount of subsidence, and may influence calculated locked zone postions and earthquake magnitudes. Most of these shelf and coastal structures respond to N-S compression, in contrast to convergence-related northeasterly compression in the accretionary prism, but in agreement with the regional stress field. Despite low historical coastal and continental shelf seismicity, upper-plate faults may also pose an independent seismic hazard.

Willapa Bay synclinde, Cascadia margin

North-South MCS profile across the North Nitinal Fault on hte inner shelf off Grays Harbor, Washington. The structure is NNW striking, dips steeply to the north, and has a component of thrust motion that offsets the modern seafloor. (Click for larger image).

 

 

Structure map from the Colummbia River to Grays Harbor, Washington, showing the Willapa Bay syncline, and the controlling North Nitinat fault (near A' symbol), trending NNW, transverse to the regional strucural trend. This structure appears to control that location of Willapa Bay, and likely creates accomodation space for sediments to accumulate in Willapa Bay through permanent deformation, superimposed on the elastic seismic cycles.

 

 

North-South 3.5 kHz reflection profile acriss the North Nitinal Fault off Grays Harbor on the inner shelf. The Willapa bay Syncline is visible on the right (south). Click for larger image.

 

Acknowledgemts

We thank Richard Perry and Steve Matula, National Ocean Service, National Ocean & Atmospheric Administration, Rockville, MD for preliminary copies of the Oregon multibeam swath bathymetry used in mapping the continental margin structures. Chris Fox, NOAA Marine Resources Research Division, Hatfield Marine Science Center, Newport, OR, produced several colored contour maps of the multibeam bathymetry from the Oregon State University and NOAA/NOS digital databases. He also supplied digital data multibeam bathymetry data for computer analysis and visualization at Oregon State University. His collective efforts in this project are greatly appreciated. Bruce Appelgate, also at NOAA Newport (now at School of Earth Sciences and Technology, University of Hawaii, Honolulu, HI) processed the SeaMarc sidescan data and produced spectacular mosaics of the plate boundary near 45° N. Mary MacKay and Guy Cochrane processed a large set of 144 channel seismic profiles used in this study at University of Hawaii under the direction of Greg Moore. We thank Sam Clarke, Parke Snavely Jr., and Monty Hampton, U.S. Geological Survey Branch of Pacific Marine Geology Menlo Park, CA for providing U.S.G.S. seismic records. We had fruitful discussions on the geology of the Cascadia margin with many investigators interested in Cascadia, including Sam Clarke, Parke Snavely Jr., Ray Wells, Ray Weldon, Paul Komar, Harvey Kelsey, Curt Peterson, Brian Atwater, Dan Orange, Bruce Appelgate, Mary MacKay, Guy Cochrane, Casey Moore, Harold Tobin, and Greg Moore. Thanks also to Nathan Potter, OSU, for showing us how to deal with digital SeaBeam data, and to Margaret Mumford, OSU, for carefully digitizing the many structures on the map. Special thanks go to the crews of the research vessels R.V. Atlantis II, DSV ALVIN, Digicon M.V. Geotide, R.V. Wecoma, R.V. Jolly Roger, and DELTA, and to the sidescan techs at Williamson and Associates of Seattle, WA. This research was supported by NSF grants OCE-8812731 (OSU ) and OCE-8821577 (UH), by the National Earthquake Hazards Reduction Program, U.S. Geological Survey, Department of Interior, under award 14-08-001-G1800 (OSU), and by the National Undersea Research Program, National Oceanic and Atmospheric Administration. Publication and compilation of this map was supported by United States Geological Survey Cooperative Agreement Number 14-08-0001-A0512 under the auspices of the National Earthquake Hazards Reduction Program.

 

Cited and Related References

Appelgate, T.B., 1988, Tectonic and volcanic structures of the southern flank of Axial volcano, Juan de Fuca Ridge: results from a SeaMARC 1 side-scan sonar survey: Corvallis, Oregon State University, M.S. thesis, 161 p.

Appelgate, T.B., Goldfinger, C., Kulm, L. D., MacKay, M., Fox, C.G., Embley, R.W., and Meis, P.J., 1992, A left-lateral strike-slip fault seaward of the central Oregon convergent margin: Tectonics, v. 11, p. 465-477.

Carlson, P.R., 1967, Marine geology of the Astoria submarine canyon: Corvallis, Oregon State University, Ph.D. dissertation, 259 p.

Carlson, P.R., and Nelson, H.C., 1987, Marine geology and resource potential of Cascadia Basin, in Scholl, D. W., Grantz, A., and Vedder, J.G., eds., Geology and resource potential of the continental margin of western North America and adjacent ocean basins-Beaufort sea to Baja California: Houston, Circum-Pacific Council for Energy and Mineral Resources, p. 523-535.

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