In 1996 we began investigating the paleoseismic history of the Cascadia margin based on turbidite records. The possibility that a good earthquake record existed along the margin had been suggested in the late 1960's, and proposed formally in 1990 by John Adams. We conducted our first major cruise to test this hypothesis in 1999 aboard the R/V Melville. We have published several papers since then that expore in detail the realtionship between great earthquakes and turbidite stratigraphy in Cascadia. This work has been supported by the National Science Foundation Earth Science and Ocean Science Divisions, and the USGS NEHRP Program since 1999.
A large number of co-investigators, students, technicians and ships crew have been involved in this project since the late 1990s, including: Chris Goldfinger, Hans Nelson Joel Johnson, Ann Morey, Julia Gutiérrez-Pastor, Eugene Karabanov, Andrew T. Eriksson, Jason Chaytor, Eulàlia Gràcia, Gita Dunhill, Jason Patton, Randy Enkin, Audrey Dallimore, Tracy Vallier Steve Wolf, Mike Hamer, Michaele Kashgarian, John Southon, Pete Kalk, Chris Moser, Bob Wilson, Jeff Beeson, Handoko Wibowo, Bran Black, Amy Garrett, and the officers and crews of the R/V Melville, R/V Thompson and R/V Revelle and the Shipboard Scientific Parties. See below for the whole list!
Blog from a recent Cascadia coring cruise
The Cascadia Turbidite Record
The Holocene stratigraphy of submarine channels along the Cascadia margin has long been known to include a turbidite sequence.
L. D. Kulm and his students in the late 1960's and 1970's investigated the nature, distribution, and timing of these turbidite events in considerable detail using piston cores from some of the channels. Although G. Griggs and L. Kulm thought that these events might represent earthquakes along the margin, the extremely low seismicity in Cascadia made such a hypothesis seem unlikely. Following the discovery of the first buried marsh sequences on land, Adams (1990) assessed the possibility that these cores contained a record of the Holocene great earthquake history of the Cascadia margin. Adams examined core logs from cores in Cascadia, Astoria, and Rogue channels on the abyssal plain and their associated canyons and tributaries on the continental slope.
Cores taken in Cascadia Basin nearly all contain a unique datable event, the ash layer from the eruption of Mount Mazama, at 6845 ±50 radiocarbon yr BP (Bacon, 1983). The ash was distributed to the channel system via the drainage basins of major rivers, similar to the distribution of Mt St. Helens ash following the 1980 eruption (Nelson
et al., 1968). Only channel cores contain the ash, indicating that airfall was not significant. Griggs and Kulm(1970) used the Mazama ash to calculate that the mean recurrence interval for the post-Mazama turbidites in Cascadia channel was 410-510 years.
Following the discovery of the first buried marsh sequences on land
, Adams(1990) used existing cores to test the possibility that the Cascadia cores contained a record of Holocene great earthquakes of the Cascadia margin. Adams examined core
logs for Cascadia Basin cores, and determined that nearly all of them had 13 turbidites overlying the Mazama ash which is dated at 7627 ± 150 cal yr BP (Zdanowicz et al., 1999). He found that cores along the length of Cascadia channel contain 13 turbidites and argued that these 13 turbidites correlate along the channel. Adams observed that cores from Juan de Fuca Canyon, and below the confluence of Willapa, Grays, and Quinault Canyons contain 13-16
turbidites above the Mazama ash. The correlative turbidites in Cascadia channel lie downstream of the confluence of these channels.If these events had been independently triggered events with more than a few hours separation in time, the channels below the confluence should contain the sum of the tributaries, from 26-31 turbidites, not 13 as observed (Figure 1). The importance of this simple observation is that it demonstrates synchronous triggering of turbidite events in tributaries, the headwaters of which are separated by 50-150 km. Similar inferences about regionally triggered synchronous turbidites in separate channels are reported in Pilkey (1988). This elegant relative dating technique and variants of it using isolated lower slope basins are used extensively in our Cascadia, Sumatra, and NSAF work.
Figure 1. Cascadia margin turbidite canyons, channels and 1999-2002 core locations. Major canyon/channel systems are outlined in blue. Bathymetric grid constructed from newly collected multibeam data in 1999, Gorda Plate swath bathymetry collected in 1997 (Dziak et al., 2001), and archive data available from NGDC. Primary core sites shown with yellow rim, all other 1999-2002 cores are grey. White core numbers preceded with cruise number “M9907”, collected on the RV Melville, Yellow symbols are preceded with the cruise designation “RR0207”, collected on the R/V Revelle. Core EW9504-16PC shown in red. Earlier OSU cores shown in grey. “PC” = Piston Core; “BC” = Box Core; “KC = Kasten core; “GC” = Gravity core; “TC” = Trigger core. Trigger cores omitted for clarity. Inset of Effingham Inlet shows collection site of Pacific Geoscience Centre (PGC) collected piston cores. (Click for larger image)
Figure 2. The movie below shows the Cascadia margin turbidite
pathways investigated in this sudy, and a regional view of synchonous initiation of turbidity
currents in all Cascadua canyon/channel systems during a full margin great earthquake.
Figure 3. Summary core logs and digital photographs of all sections of Cascadia Channel core M9907-25PC. Turbidite numbers shown in ovals.
Using 54 new cores in Cascadia, we have confirmed and extended the event record temporally and spatially. Thirteen post-Mazama and 19 Holocene events are found along~ northern margin in the Cascadia, Barclay, Willapa, and Grays Canyon/Channel systems between latitudes 47N and 49N. The most recent event took place in 1700 AD(Satake et al.,1996; Nelson et al., 1995), and an additional 12 turbidite events have occurred during the preceeding 7200 years, yielding a mean recurrence time of ~575 years.
Triggering mechanisms: Are they
Are these events all triggered by earthquakes? Common sense suggests that such a scenario is absurdly simplistic, yet our Cascadia work has led us to the unlikely conclusion that Adams(1990) was correct. We now discuss the methods used to test the hypothesis and why it seems to work. Adams(1990)suggested four plausible mechanisms for turbid flow triggering: 1) storm wave loading; 2) great earthquakes; 3) tsunamis; and 4) sediment loading. To these we add 5) crustal earthquakes, 6) slab earthquakes, 7) hyperpycnal flow, and 8) gas hydrate destabilization. All of these mechanisms could trigger a
turbid event, but how often do they actually occur, and can
earthquake-triggered events be distinguished from other events? Two basic techniques can be used to
distinguish seismic from non seismic events:
Sedimentological determination of individual event origin.
Regional correlations that require synchronous triggering.
Individual event determination can in some cases distinguish seismic turbidites from storm, tsunami, and other deposits using several methods. Nakajima and Kanai (2000) and Shiki et al. (2000) report that seismo-turbidites can in some cases be distinguished sedimentologically. They observe that historically known seismically derived turbidites in the Japan Sea and Lake Biwa are distinguished by wide areal extent, multiple coarse fraction pulses, variable provenance, and
greater depositional volume than storm-generated events. These investigators traced known seismo-turbidites to multiple slump events in many parts of a canyon system, generating multiple pulses in an amalgamated turbidity current, some of which sampled different lithologies that are separable in the turbidite deposit. These turbidites are complex, with reverse grading, cutouts, and multiple pulses. Gorsline et al. (2000) make similar observations regarding areal extent and volume of seismoturbidites. In general, these investigators observe that known storm triggered events are thinner, finer grained and have simple normally graded Bouma sequences, although complexity is also a function of proximity to the source, and some reports reach different conclusions (Mulder and Syvitski, 1996). While there may yet be applicable global, regional or local criteria to make such distinctions, these are at present poorly developed and somewhat contradictory.
Thus far in Cascadia and the San Andreas, we have not attempted to distinguish between triggering mechanisms directly because the physiography, numerous tephra layers, and long historical records favorable to this method in Japan are not present on the west coast of North America. Favorable factors that are present favor regional correlation and determination of synchronous triggering. Determination of synchronous triggering can eliminate non-earthquake triggers with the possible exception of storm wave loading or multiple hyperpycnal flows for very large storms, and perhaps triggering by a tele-tsunami. West Coast physiography favors filtering of non-seismic events from the record because a wide shelf separates river sources from canyon heads. Hyperpycnal flow, or direct turbid injection from rivers, can produce turbid flows, and can even mimic earthquakes in that they may affect several rivers over a span of days. We have found that while this certainly occurred during the Pleistocene when lowered sea-level resulted in direct river-canyon connections resulting in a major increase in turbidite frequency at the Holocene Pleistocene boundary. Once sea level-rise in the Holocene isolated most west coast canyons from the rivers, sediment was spread across the shelf and not directly injected in the canyons (e.g. Sternberg, 1986). Some exceptions are the Eel river record, which probably contains storm events, and the Viscaino channel along the northern San Andreas.>Both of these occur where the shelf is very narrow, and river injection is possible. Deep canyon heads also prevent triggering by storm wave loading and distant tsunami, the last two non-earthquake sources. For example, storm wave loading is an unlikely trigger in Cascadia, where, although deep water storm waves are large, the canyon heads where sediment accumulation occurs are at water depths of 150-400 m, too deep for sustained disturbance by maximal storm waves of ~20 meters. Tsunamis may also conceivably act as a regional trigger, however the tsunami from the 1964 Alaska Mw 9.0 event did not trigger a turbidite observed in any of the cores, although it did serious damage along the Pacific coast (Adams, 1990). Crustal or slab earthquakes could also trigger slumps and turbid flows, though not regionally. To test for this, we resampled the location of a 1986 box core in Mendocino channel, where the uppermost event is suspected to be the 1906 San Andreas event. Since 1986, the Mw7.1 Petrolia earthquake occurred in 1992, with an epicentral distance of only a few km from the canyon head. We found no turbidite in the 1999 box core, suggesting that triggering at that site may require earthquakes larger than Mw 7.2. Conversely, the Loma Prieta earthquake apparently did trigger a turbid flow event in Monterey Canyon at a greater epicentral distance (Garfield et al., 1994), though it is not known whether a discernable turbidite record exists from this event. Japanese investigators have suggested a minimum magnitude of ~Mw=7.2 for turbidite triggering, though we suspect that this minimum value is site and event specific.
Comparisons of hyperpycnal and earthquake generated turbidites
Taking advantage of favorable physiography, we have used spatial and temporal patterns of event correlations that are unlikely be the result of triggers other than earthquakes. We use multiple techniques to test for linkage between sites and thus test for synchroneity. Typically, paleoseismologic investigations use radiocarbon constraints to establish these linkages, but often are unable to determine synchronous event chronology due to the inherent limits in dating techniques. Relative dating techniques, if available, and if of sufficient resolution, are strongly preferred to test for synchroneity. The “confluence test” of Adams (1990) is powerful in that it requires synchronous triggering within a few hours. Comparisons of the number of events between time markers is a somewhat less powerful technique that can be applied in some cases. Recently we have begun to use direct physical property correlations, which are proving to be a powerful new method of testing for linkages (if present) between sites.
Using Adams “confluence test”, we concluded that the northern Cascadia margin contains 19 Holocene events, all of which pass this test of synchronous triggering. The southern Oregon Rogue site also contains a record of 19 Holocene and 13 post Mazama events as along the northern margin, but also contains a record of 22 thinner events limited to the southern margin.
Figure 4. Image of a single turbidite, shoing elements of the unit and annotated with sampling and dating methods.
The Rogue channel has no confluence with the other systems, and the correlation there is based on physical property correlation and radiocarbon ages (Goldfinger et al., 2008, 2010). Subsequent to these results, We have found it possible to correlate the Holocene events directly using physical properties of the turbidites themselves in the cores. This method is discussed more fully below. Using these correlations we believe it is possible to establish synchronous triggering for the northern and central parts of the margin between 42 and 48 degrees N for may turbidites and determine the ruture lengths of the causative earthquakes. In Cascadia, event synchroneity is established not only with radiocarbon ages, but with correlation techniques within a radiocarbon constrained framework. Correlations have been able to link northern and southern sites, a connection that radiocarbon evidence alone cannot establish with either onshore or offshore records.
Figure 5. Examples of physical property traces vs. grain size. A. Detailed stratigraphic diagram of T6 in Juan de Fuca Channel showing a typical proximal event. Note similarities and differences between density, MS, and grain size plots. B. Event T3 in Cascadia Channel, showing typical distal event, with generally closer correspondence between grain size and physical property plots. X ray, lithologic logs, and density and magnetic traces show both of these typical events to be composed of three fining upward sequences. The lower two sequences are truncated by the overlying ones. Grain size determinations are shown (Coulter laser counter method, 1cm interval), showing correlated relationship between gain size, density and magnetic susceptibility. No hemipelagic sediment exists between coarse sub-units, indicating the three coarse sub-units were deposited in a short time during a single current (a coring artifact may exist between upper and middle subunits in panel A). Only the last sub-unit has a fine tail, indicating final waning of the turbidity current. We interpret these signatures as resulting from a single multipulse turbidity current. P-Wave velocity traces shown in A and B are typically non-diagnostic of grain size for these lithologies. (Click for larger image)
Figure 6. Correlation details from two representative pairs of cores on the Cascadia margin. Panel A shows events 8-11 in cores from Juan de Fuca Channel (left) and Cascadia Channel (right). Left traces are raw gamma density, right traces are magnetic susceptibility. Lithologic logs are also shown. Note correspondence of size, spacing, number of peaks, and trends of physical property traces between these cores. Panel B similarly displays events T10-T14 in Juan de Fuca Channel (left) and T10d-T14 in Rogue Channel (right). Panel A cores are part of the same channel system, distance along channel = 475 km. Panel B cores are in channels that do not meet, separation distance = 500 km. Note that correlation of longer sections and 14C data show that T10d and T10 do not correlate in panel B. Similarly, Mazama ash appears in T14, not T13 in Rogue apron, see text for discussion.
Figure 7. Core Lithology and physical properties from the Hydrate Ridge and Rogue Apron core sites, central-southern Cascadia margin. Data from HR cores 56 PC-TC and Rocgue cores 31PC-TC summarized with a single core composite of piston and trigger cores, all 14C ages from each site plotted on the representative core. These two cores are “flattened” on all turbidite base horizons, with the Rogue core at true scale. Undated Rogue Apron core TT0909-01JC also shown flattened to 31 PC-TC. Correlation between sites based on stratigraphic methods described in text, 14C data, Mazama ash and Holocene/Pleistocene boundary datums. Lithologic and geophysical symbology is the same as previous site figures. This site correlation between two sites 250 km apart, and isolated from each other, strengthens the correlation of major events, and supports correlation of smaller events based on similar numbers, stratigraphic positioning, and calculated ages. (Click for larger image)
Alternative Explanations? Hyperpycnal flow
Hyperpycnal flow is the density driven underflow from storm discharge of rivers into marine or lacustrine systems, and has been proposed as a link to turbidity currents in a variety of settings. Documentation of hyperpycnal flows into lakes and shelf basins is abundant, however evidence of such flows entering canyons systems and moving into deep water is sparse. Most, if not all examples involve very short distances between the river mouth and canyon head, either during Pleistocene low-stand conditions, or in systems that have very narrow shelves during high stand conditions. Hyperpycnal flows extend further from river mouths with high discharge (Alexander and Mulder, 2002), but documentation is sparse. Wright et al. (2001) infer that hyperpycnal flow is strongly affected by ambient currents, and generally deliver sediment to the slope only upon relaxation of longshore currents. Most investigators cite Pleistocene examples when referring to flows reaching the abyssal plain or lower fan reaches (e.g. Mulder et al., 2003; Normark et al., 1998; Normark and Reid, 2002; Piper et al., 1999). Under low stand conditions, rivers and canyons are directly connected, and such flows are expected. These authors relate the deep water deposition of hyperpycnites to sea level control, or alternatively to climate shifts. An example of high stand hyperpycnal flow has been reported for the Var River, in which the canyon and river mouth are less than 1 km apart (Klaucke et al., 2000; Mulder et al., 1998a). Many large river systems deposit most of their load in river mouth bars, with lesser quantities making it past such bars in to a delta front slope (eg. Yellow River, Li et al., 1998). Whether hyperpycnal flows can reach the deep sea via canyon systems incised during the Pleistocene appears to be a function of shelf width, river peak storm discharge, Holocene aggradation of Pleistocene canyons, and the wave and current climate during peak storm discharge. However, the requirements for and evidence of hyperpycnal flows to the deep ocean under high-stand conditions (excepting very narrow shelves) remain unknown at best (Mulder et al., 2001).
Comparisons of hyperpycnal and earthquake generated turbidites
In some proximal settings such as large lakes, shelf basins, and fjords, records of both earthquakes and storm deposits are found. In one of the best comparisons, St.-Onge et al (2004) show that details of both seismic and hyperpycnal deposition in the Saguenay Fjord in eastern Canada are diagnostic, and argue that hyperpycnal deposits are distinguished by reverse grading at the base, followed by normal grading. The diagnostic reverse-then-normal grading for hyperpycnal deposits has been widely reported, and is the result of waxing, then waning flow associated with the storm. In the Saguenay Fjord, six events have normal grading, and are inferred to be earthquake generated. Four others have similar basal units, but are topped by a reverse graded unit, and then a normally graded unit, with no evidence of hemipelagic sediment between these units. These events are interpreted as an earthquake, followed by a hyperpycnite that resulted from the breaching of a landslide dam caused by the original earthquake. The dam breaching is a variant of the more common hyperpycnal scenario involving waxing and waning depletive flow (Kneller, 1995), but would likely produce a similar flow hydrograph (St-Onge et al., 2004).
In another well studied example, a major river flood in the1969 El Nino input ~25 million tons of sediment (20X the yearly Columbia sediment load) to the Santa Ana River in Southern California over a 24 hour period, in close proximity to nearby canyon heads. Sediment from this extreme flood did not continue down canyons as hyperpycnal flow, but deposited as a distinct yellow unit on the shelf and upper slope. Over the next 10 years it moved downslope as turbid layer transport caused by storm wave resuspension, similar to that observed by Puig et al. (2004) for the Eel system, and deposited as yellow layers between varves of the Santa Barbara Basin (Drake et al., 1972).
Figure 8. A. Idealized stratigraphy resulting from hyperpycnal flow, characterized by a coarsening upward sequence followed by a fining upward sequence attributed to a waxing then waning hydrographic profile during a storm event. Other events with a similar hydrographs, such as a gradual dam breaching may produce similar stratigraphy (after Mulder, 2001). B. Typical stratigraphic sequence from a turbidite with multiple fining upward pulses from core M9907-12PC in Juan de Fuca Channel. This turbidite and nearly all others in the Holocene Cascadia Basin turbidite sequence exhibit multi-pulsed stratigraphy, with no waxing phase. Multiple fining upward sequences are capped by a fine mud tail signaling the final waning of the turbidity current.
Other examples of reverse-then-normal grading for hyperpycnites have been reported and compared to normally graded failure deposits in the Var system (Mulder et al., 2001), Lake Biwa (Shiki et al, 2000a), and modeled by Mulder et al. (1998b), Felix (2002) and others. The dynamics of longitudinal and temporal variability and their effects have been discussed in detail by Kneller and McCaffrey, 2003, and Mulder et al., 2003. Hyperpycnites are also commonly very organic rich as compared to seismic turbidites, having their sources in floods rather than in resuspension of older canyon wall material as in earthquake triggering (Mulder et al., 2001; Shiki et al., 2000; Nakajima et al, 2000). It has been suggested that this distinction may be used as a basis for distinguishing earthquake and storm deposits using OSL dating.
On the Cascadia margin, all of the canyon systems have Holocene shelf widths of 22-60 km, the exception is the Eel River, at which the shelf width is 12 km (Table 1). In the Pacific Northwest, large storms both increase discharge, and produce strong southerly winds that disperse sediment northward on the shelf via the Davidson current (Sternberg, 1986; Wheatcroft et al., 1997; Sommerfield and Nittrouer, 1999; Wolf et al., 1999) as well as hundreds of kilometers seaward as evidenced by satellite imagery (Wheatcroft et al., 1997). The Eel River system, with its narrow shelf is the best candidate in Cascadia for delivering hyperpycnal flows to the canyon head. However, Puig et al. (2004) saw no evidence of hyperpycnal flow into the Eel Canyon head in observations made during storms in the Canyon head at 120 m water depth. They did observe some cross-shelf sediment transport due to wave loading at this depth. Other observations made for depths of 280 m, and 900 m were also linked to storms, with wave loading the presumed mechanism, but no link to hyperpycnal discharge or to turbidity currents delivering material to the base of the slope and beyond was found (Puig et al., 2003, 2004). The width of the Cascadia shelf is somewhat exacerbated at the Astoria and Eel canyon systems where the canyon heads are somewhat to the south of the river mouths, inhibiting northward moving sediment flow associates with southerly storms (plumes or hyperpycnal) from entering the canyon. In all systems, the late Pleistocene transgression and subsequent aggradation have erased any topographic canyon or channel expression across the Cascadia shelf.
Cascadia turbidite characteristics
In our work, Cascadia turbidites show rare to no evidence of the characteristic reverse grading and high organic content observed for hyperpycnal deposits. We observe normally graded sequences with multiple amalgamated pulses, sharp erosional bases, and normal grading of individual pulses when present. The number and form of these multiple pulses persists over wide areas, and are the basis for individual event correlation. In further testing for event origin in our cores, we find no evidence of the1964 and 1995 extreme El Nino river flood events as hyperpycnal turbidite deposits in any core.
Storm events that may trigger hyperpycnal flow occur relatively frequently in settings where they have been observed. Such events occur every few years in Lake Baikal and Crater Lake yet there are only 4-6 Holocene turbidites in Lake Baikal and a few per thousand years in Crater Lake, far less than the hundreds Holocene hyperpycnal flow events. Similarly, the number of storm events in Cascadia that exceed 15m significant wave height and 100 knots windspeet is 2-3 per year, or 600-900 such storms since the AD 1700 earthquake. We see no evidence of any turbidites above the 1700 event except in the southermost part of Cascadia, where there are both proximal river sources, and more seismic sources. The three turbidites that overly the likely 1700 AD event there are fare fewer thatn the number of storms, but are as yet unexplained. We conclude that hyperpycnal flow is an important mechanism for delivery of sediments to the shelf or upper slope, but other triggering mechanisms (sediment overloading failures, storm wave loading, or earthquakes) must be responsible for the observed frequency of robust turbidites in lacustrine or marine turbidite systems.
Figure 9. Space time diagram for the Cascadia margin showing Holocene marine radiocarbon data and stratigraphic correlations. Filled symbols are marine 14C ages, smaller filled symbols are hemipelagic calculated ages. Marine data plotted as 2s midpoints and and 2s ranges. Plotted ages correspond to the Land-Marine Compilation tab in Appendix 1. Dashed lines show stratigraphic correlation of the turbidite data, which show deviations from the preferred age range where correlation overrules an individual 14C age. Up arrows shown for marine data where sitewide erosion suggests a maximum age. Marine error ranges are RMS 2s propagated errors. Smaller southern Cascadia events shown with thinner dashed lines. Green bars are best fitting offshore-onshore age trends for Cascadia earthquakes. Land data plotted as published, with some sites revised as discussed in text. Preference among land sites given to recent publications using well constrained ages. Down arrows indicate minimum ages as published (land only). Two sided arrows shown where maximum and minimum ages averaged (land sites only). Superscript numerals in legend keyed to publications cited in References Cited. Marine 14C data are given in Appendix 1; onshore data and numbered sources are given in Appendix 2. (Click for larger image)
Figure 10. A. Holocene rupture lengths of Cascadia great earthquakes from marine and onshore paleoseismology. Four panels showing rupture modes inferred from turbidite stratigraphic/14C correlation, supported by onshore radiocarbon data. Marine core sites controlling rupture length estimates shown as yellow dots. A. Full or nearly full rupture, represented at most sites by 19. B. Mid-Southern rupture, represented by 3-4 events. Northern extents of T8a, T8b uncertain, though one of them likely present at Astoria. T5b present at Astoria, but most likely does not reach JDF. C. Southern rupture from central Oregon southward represented by 10-12 events. Northern extents of Segment C events break into two groups, one terminating south of HR, indicated by dashed line. The second group extends north of HR but is not observed at Astoria. D. Southern Oregon/northern California events, represented by a minimum of 7-8 events. Northern extents of Segment D events break into two groups, one terminating south of Rogue, indicated by dashed line. The second group extends north of Rogue but is not observed at HR. Southern rupture limits are poorly known for all events indicated by query, limited by temporal coverage and probable non-seismic events in the early Holocene. See Table 8 for details of limiting criteria, and text for additional constraints applied. Recurrence intervals for each segment shown in left panel. Rupture terminations are approximately located between three forearc structural uplifts, Nehalem Bank (NB), Heceta Bank (HB) and Coquille Bank (CB). Approximate updip and downdip limits from Goldfinger et al. (1992; 1996; 2007c), Clarke and Carver (1992), Oleskevich et al. (1999), and Priest et al. (2009). Paleoseismic segmentation shown is also compatible with latitudinal boundaries of ETS (Episodic Tremor and Slip) events proposed for the downdip subduction interface (Brudzinski et al., 2007) and shown by white dashed lines. A northern segment proposed from ETS data at ~ 48N does not appear to have a paleoseismic equivalent. (Click for larger image)
Figure 11. A. Time series of Cascadia turbidite emplacement for the northern and central margin. Bars are scaled with height representing turbidite mass (taller bars are larger turbidites). Bar widths are the 2s error range from OxCal combines for each event. Turbidite mass calculated from gamma density data for cores 12 PC (Juan de Fuca Channel) and 23PC (Cascadia Channel), data given in Table 11. The time series suggests a history of clusters of earthquakes (average repeat times shown), separated by gaps of ~750-1150 years. Gaps appear to have a tendency to conclude with a large event. B. Four cluster model. C. Clustering dendrogram for the north central Cascadia turbidite time series, model B.
Segmented Rupture of Southern Cascadia
A series of 23 thin, mostly mud-silt turbidites are found interspersed between larger, well-dated and regionally correlated paleoseismic sandy turbidites that extend along most of the Cascadia margin, northwestern United States. Investigation of the structure, distribution, and sedimentology of these thin mud-silt units supports the interpretation of these units as turbidites originating in shallow water. Interpretation of mud turbidites is inhibited by bioturbation and lower response to analytical and imaging techniques, nevertheless most of the 23 interpreted beds exhibit most of the characteristics of coarser turbidites. These characteristics include sharp bases, fining upward sequences, darker color, increased gamma and CT density and magnetic susceptibility relative to the hemipelagic background, sparse microfossils, high lithic content, and evidence of transport from marine sources on the continental slope. New core data from sites south of Rogue Apron indicate that sandy and muddy turbidites may be correlated at least 150 km south to Trinidad Plunge Pool for the period ~ 4800 years BP to present. Many of the mud turbidites initially described at Rogue Apron coarsen southward, becoming sandy turbidites. High-resolution Chirp seismic profiles reveal that turbidite stratigraphy along the base of the southern Cascadia continental slope is continuous, with little variation for at least 240 kilometers along strike. The Chirp data show that turbidites along the Cascadia base of slope are ubiquitous, and likely not sourced solely from submarine canyon mouths, but may also have been delivered to the proximal abyssal plain as sheet flows from the open continental slope. The thin mud turbidites cannot be imaged directly, however the accommodation space they fill maintains its consistency along strike, providing independent, though circumstantial evidence of regional dispersal and stratigraphic continuity. Regional stratigraphy reveals that hemipelagic sedimentation rates and total Holocene turbidite thickness and mass are similar at widely separated sites, yet the total thickness of the Holocene section is greater by a factor of two in southern Cascadia. This difference is primarily due to the presence of the 23 mud turbidites. We conclude that the Cascadia mud turbidites are ubiquitous along southern Cascadia only, with only one possible example of a correlated turbidite limited to the northern margin. Correlation of mud turbidites between widely separated sites, including an isolated lower slope basin and possible correlative turbidites in both coastal lakes and marshes and coast range lakes supports a regional and synchronous triggering mechanism, most likely subduction earthquakes. Eight onshore sites, three marsh sites and five lakes with variable record lengths include likely correlatives of the southern Cascadia turbidites. In all, sixteen onshore sites may have recorded 88% of the offshore events during the period 0-6500 years ago. Slope stability calculations suggest that earthquakes of Mw=7.0 or greater are expected to provide ground accelerations sufficient to destabilize open slopes and canyon heads with or without excess pore fluid pressure. Estimates of Mw for segmented ruptures are in the range of 7.4-8.7, exceeding the slope stability criteria for typical slopes by at least a factor of ~ four.
Figure 12. A. Detailed physical property and CT data from correlated events T5, T5a, T5b, and T5c in a transect from Rogue Apron to the Trinidad Plunge Pool, flattened to regional event T5. This transect shows the southward increase in thickness, density and grain size for regional event T5 and three southern Cascadia beds (T5a-c). B. A similar transect for events interpreted as T9, T9a, and T9b. Examples are approximately flattened to correlated event T9, shown by heavy red line. These data are shown in context in Figure 8. Mean grain size shown for cores with sandier turbidites and large contrast between turbidites and hemipelagic. Grain size mode shown for Smith and Klamath apron cores which have poorly developed channel systems, and reduced average grain size profiles. Mean and mode shown for 34PC for comparison. In these cores, the grain-size variation is commonly dominated by large biogenic forms. Log-transformed grain size data shown in green for 31 PC which has the finest grain size profile for the Rogue mud turbidites.. See figure 1 for core locations. Age control, and density and magnetic scales shown in Figure 13.
Figure 13. Turbidite correlation along ~130km of the southern Cascadia margin from Rogue Apron to the Trinidad Plunge Pool. Turbidites with sandy bases shown in solid lines, silty and mud turbidites shown in dashed lines. Three turbidites, T5, T10, and T11 are color coded to match the corresponding seismic reflectors in Figure 14. Depth of key reflectors is compatible with the trends in the seismic section (though definitive correlation is not possible) after velocity correction: deepening at Smith, then shallowing slightly before deepening significantly near Trinidad Plunge Pool. Many of the Rogue mud turbidites appear to thicken and coarsen southward, as do most of the margin wide turbidites, with the exception of T4, T6, T7 and T8, which thin southward (interpreted T numbers assigned to beds after final correlation). Tentative correlations unsupported by age control shown in gray. Modified with additional data after Goldfinger et al., 2012, their Fig. 44b.
Figure 14. Compilation of ~ 100 km of high-resolution Chirp reflection profiles parallel to the Cascadia margin. Chirp system center frequency was 4 kHz, sweeping through 2-6 kHz. Correlated horizons correspond to margin wide larger turbidites with thicker sand bases observed in Rogue Apron cores as shown in the inset at center with core TN0909-01JC/TC and representative seismic section. Correlation ties to nearby core M990731PC/TC with age control shown at right. Margin parallel profiles image the entire Holocene section and include the late degalcial, and show the consistency of the turbidite stratigraphy and accommodation space required for several sequences of thin mud turbidites along strike.
Cascadia Basin contains a variety of types and scales of turbidite systems located on the continental margin from Vancouver Island, Canada to Cape Mendocino California, USA. These systems include multiple canyon sources on the Washington margin that funnel turbidites into Cascadia Channel (1,000 km length); Astoria Canyon on the northern Oregon margin that feeds Astoria Fan (300 km diameter) containing channel splays with depositional lobes; Rogue, Smith, and Klamath Aprons on the southern Oregon and northern California margins that feed small (<5 km) base-of-slope aprons; and Trinidad, Eel, and Mendocino canyons (30-100 km length) on the northern California margin that feed into plunge pools, sediment wave fields, and channels.
Cascadia Basin turbidite systems are an ideal place to develop a turbidite paleoseismologic method and record because: (A) a single Cascadia subduction zone fault underlies the margin, (B) multiple tributary canyons and a variety of turbidite systems and sedimentary sources and basins exist to test for synchronous triggering or turbidity currents, (C) the presence of an excellent Mazama ash marker provide a stratigraphic anchor in the northern two thirds of the basin, (D) During high stands of sea-level, Cascadia margin physiography exert a strong control on sediment input to canyon heads, limiting most storm/river input except for those localities with narrow shelves; and (E) The Cascadia trench is filled, thus channel systems flow away from the margin, remaining isolated rather than merging in the trench.
Detailed swath bathymetric data and core sampling procedures verify that key turbidite channel pathways of Cascadia Basin are open and provide a good turbidite event record. Proximal canyon mouth and inner fan channel areas have erratic turbidite event records because of extensive cut and fill episodes in turbidity currents; however, even in these difficult locations, complete records can be found in some point bars, terraces and canyon walls that are slightly elevated above the channel thalweg. The most consistent turbidite event records occur in distal locations of continuous deep-sea channel systems such as Cascadia Channel.
The similarity of the turbidite time series and good stratigraphic correlation of the turbidite event record along the northern two thirds of the Cascadia subduction zone is best explained by paleoseismic triggering of great earthquakes. Turbidites in this region pass several tests of synchronous triggering; including the “confluence test” that requires passage of multiple source turbidites past a channel confluence in a span of a few hours 19 consecutive times during the Holocene. Stratigraphic correlation of individual event signatures, correlation of series characteristics such as mass and number of coarse fraction pulses, as well as 14C dates further support synchronous triggering. Sediment supply to canyon sources appears not to be a significant controlling factor in the Holocene, partly because high-stand deposition is concentrated on the shelf, and because strong ground shaking is probably sufficient to overcome variability in sediment input to the canyons.
The mismatch between the frequency of tele-tsunami and local storms, as well as the good match in frequency and dates of earthquake and tsunami evidence onshore also support the conclusion that the Holocene Cascadia turbidite record primarily records earthquakes. The lack of turbidites overlying the most recent turbidite, dated to within a decade of the A.D. 1700 Cascadia earthquake indicates that no other triggering mechanism has produced and observable turbidite in the last 300 years, except in some of the northern California channels adjacent to narrow shelves. Several other sites in southern Cascadia may record a mixed storm and earthquake signal in their early Holocene sections due to lowered sea-level. The lack of turbidite triggering in Cascadia Basin by historic El Nino storm and flood events (1964, 1998-1999), and the 1964 Alaskan earthquake tsunami suggest that storm events and tsunami, whether or not sediment is transported to canyon heads, do not generally result in correlative abyssal plain turbidites except where the shelf is narrow or sea-level is considerably lower than modern. A small number of uncorrelated turbidites may represent crustal earthquakes or other sources.
The mean AMS age of 270 (170-390) cal yr B.P. from three channel systems for the youngest turbidite event in Cascadia Channel T1 differs by only 15-20 years from (1) the coastal paleoseismic dates that consistently center about 250 cal yr B.P. (A.D. 1700; Nelson and others, 1995) and (2) the Japanese tsunami evidence showing a date of Jan. 26, 1700 for the youngest great earthquake on the Cascadia subduction zone (Satake and others, 1996; 2003). This further validates the synchronous turbidite event record and associated high resolution AMS radiocarbon dates as a method to provide a long-term paleoseismic record. The temporal correspondence between the onshore and offshore paleoseismic records along the Cascadia margin is quite good, despite a variety of methods and lines of evidence onshore. Within the time ranges that the two records overlap, there are few significant discrepancies. The ties between onshore and offshore paleoseismic data remain limited to radiocarbon timing for all sites except Effingham Inlet on Vancouver Island, which contains turbidites with possible stratigraphic correlatives offshore, and Bradley Lake, which appears to have a reasonable correlation based on event size characteristics in addition to radiocarbon evidence.
Presently available AMS radiocarbon dates down core for individual turbidite events show that the average recurrence interval for full margin paleoseismic events (900-1100 km in length) is ~500-530 years, with a variance ranging from ~200 to 1,200 years. A series of smaller ruptures represented by thinner turbidites of lesser areal extent can be correlated among southern Cascadia cores, and show moderately good correspondence with the presence of events of limited extent at coastal paleoseismic sites. These smaller events define three other margin segments that have recurrence intervals of 410-500, 300-380, and 220-240 years for segments with northern terminations at approximately 46° N (Nehalem Bank), 44° N (Heceta Bank), and 43° N (Coquille Bank) respectively. For full margin ruptures, the Holocene time series implies a probability during the next 50 years of 7-11 percent of a Cascadia earthquake using either Poisson or time-dependent calculation. Conditional probabilities for the next 50 years are similar. Using failure analysis statistics, the probability of having an earthquake by the year 2060, beginning at the last event in A.D. 1700 increases to ~25 percent. For the southern segment with a recurrence of ~240 years, probability in the next 50 years rises to 18 percent for a Poisson distribution, and 32-43 percent for a time-dependent model. Failure analysis indicates that by the year 2060, ~85 percent of recurrence intervals will have been exceeded along the southern margin. It is also highly likely that the next event will be a southern margin event.
We find a strong correspondence between turbidite mass among separate margin sites, suggesting that mass of the turbidites may crudely represent earthquake magnitude and shaking. The long paleoseismic record also indicates a repeating pattern of clustered earthquakes that includes four Holocene cycles of two to five earthquakes separated unusually long intervals of 700-1200 years. Two of the four cycles terminated with what were likely very large earthquakes.
We find that the pattern of long recurrence intervals and long ruptures along northern and central Cascadia margin is consistent with the thick sediment supply along that part of the margin. Where sediment supply thins along the southern margin, recurrence intervals and rupture lengths shorten, consistent with a model of greater interaction between lower plate and forearc structures in those areas, providing barriers to rupture propagation, as well as points of nucleation not present along most of the northern margin.
Segmentation of the margin is supported by new cores and CHIRP seisic reflection profiles that have allowed a better integration of the more complex southern Cascadia margin data. The combined dataset show that many Holocene Cascadia regional turbidites can be traced individually for at least 230 km along the southern margin, supported by radiocarbon ages and core subsurface correlation. Thin beds first reported in southern Cascadia as mud turbidites, thicken and coarsen southward from Rogue Canyon toward Trinidad and Eel Canyons. These spatailly limited beds do not exist along the northern Cascadia margin, and most likley represent segmented ruptures.
Finally, Cascadia Basin investigations establish new paleoseismic techniques utilizing marine turbidite event stratigraphy during sea level highstands, and can be applied in other specific settings worldwide where an extensive fault traverses a continental margin with several active turbidite systems and favorable physiography.
We thank the investigators who have contributed to this research in the following ways: Richard Boettcher who analyzed and picked planktic foraminifera for radiocarbon dates, and Steve Wolf and Mike Hamer who compiled seismic profiles for channel pathway analysis. Steve Wolf and Mike Hamer compiled seismic profiles for channel pathway analysis. We thank Michaele Kashgarian and John Southon for running the majority of the AMS radiocarbon dates and their assistance with many radiocarbon issues. We thank Rob Wheatcroft for running 210Pb samples. We also are grateful for the following students, graduate students and their sponsoring universities, whose assistance was invaluable for the success of the 1999 and 2002 cruises: Mike Winkler, Pete Kalk, Antonio Camarero, Clara Morri, Gita Dunhill, Luis Ramos, Alex Raab, Nick Pisias jr., Mark Pourmanoutscheri, David Van Rooij, Lawrence Amy, Churn-Chi "Charles" Liu, Chris Moser, Devin Etheridge, Heidi Stenner, Chris Popham, Claire McKee, Duncan McMillan, Chris Crosby, Susanne Schmid, Eulalia Gracia, Suzanne Lovelady, Chris Romsos, Vincent Rinterknecht, Rondi Robison, David Casas, Francois Charlet, Britta Hinrichsen, Jeremiah Oxford, Miquel Marin, Marta Mas, Sergio Montes, Raquel Villalonga, Alexis Vizcaino, Santiago Jimenez, Mayte Pedrosa, Silvia Perez, Jorge Perez, Andreu Turra, David Lamas, Himar Falcon, and Andres Baranco. Captain Tom Desjardins, and the officers and crews of the R.V. Melville and R.V. Revelle provided excellent ship handling and coring logistics for the field work. The coring techniques of Pete Kalk, Chris Moser, Bob Wilson and Chuen-Chi “Charles” Liu provided the superb piston cores that are necessary for paleoseismic analysis. We also thank Kat Crane, Suzanne Lovelady, Beth Myers, Abi Stephen, Cheryl Hummon, Trent Carmichael, Kaileen Amish, Morgan Erhardt, Jeff Beeson, Amy Garrett and Rachel King for help with graphics grain size, and a variety of things.
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