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The results of this study are presented in four parts including (1) regional compilation of subsidence data for northern Oregon, (2) test of tide level indicators from bays within the study area, (3) a compilation of the 300 yr. paleosubsidence estimate database and (4) potential flooding hazard in two estuaries. The regional compilation consists of existing core and cutbank data showing positive or negative evidence of episodic subsidence in Oregon. Tests of tide level indicators include (1) presence of plant macrofossils, (2) measurement of organic content, and (3) analyses of diatoms assemblages in modern tidal settings. The compilation of the 300 yr. subsidence event consists of tide level indicators used to estimate the amount of subsidence along the Washington and Oregon coast. These subsidence estimates are then applied to predicted flooding elevations to establish potential flood hazards from coseismic subsidence at two representative localities, Aberdeen, Washington and Seaside, Oregon.

A compilation of existing and new data for the occurrence of regional subsidence is described below. The description includes an introduction to the data sources, the interpretation of buried deposits, and the organization of critical attributes for the regional database. A compilation of existing core data is important for Oregon as cutbank exposures are rare by comparison to the number observed in southern Washington (Atwater, 1987; Atwater and Yamaguchi, 1991).

A database of published and unpublished core logs and cutbank measured sections from 16 Oregon estuaries is presented in Appendix A. Data sources include contributions from Darienzo (1991), Nelson (1992), Briggs (1994), Peterson and Priest (1995), and Peterson and others (1996). The number of cores and cutbanks sites from these studies ranges from less than five to over fifty per bay. A database for Oregon is important as most of the buried wetland data are observed in core as diking has reduced the ability to observe lateral exposures in cutbanks observed in southern Washington.

Visual changes in lithology or plant macrofossils that are interpreted as an abrupt change in relative sea level due to subsidence include (1) muddy peat overlain by slightly peaty mud, (2) rooted mud or barren mud, (3) peaty mud overlain by rooted mud or barren mud, and peat overlain by peaty mud, slightly peaty mud, rooted mud, or barren mud. The interpretations of subsidence for northern Oregon sites reflect distinct changes in relative peat development (Table 2). Visual analysis of relative peat development varies between different authors, e.g.; a barren mud (Darienzo, 1991) may be termed a rooted mud (Briggs, 1994). However, core logs for each study consistently document relative changes in peat development.

Selected core and cutbank data include UTM coordinates, subsurface depth to the most shallow buried paleomarsh surface, radiocarbon age, total number of burial events, and the maximum depth of paleomarsh (roots or peat) development. UTM coordinates are estimated from original field map locations on U.S.G.S. 7.5 minute topographic maps. Positions are given to the nearest 10 m but actual positions might be less well known in some featureless marshes. The data establish the spatial and temporal variability of buried wetland deposits. Additional information, although not critical for this study, is the elevation of each modern surface (core-top elevation) relative to mean tide level (MTL) where the core is taken. Core-top elevation is not determined for many sites, its position being influenced by local sedimentation rates, regional and local uplift rates, and anthropogenic influences (e.g., diking and tidegates).

The database of core and cutbank data shows a range of values for subsurface paleomarsh depth, number of buried deposits, and basal subsurface paleomarsh depth. Subsurface depth of the most shallow paleomarsh ranges from 0.20 to 3.96 m (0.65 to 12.85 ft.) (Appendix A). The number of buried deposits ranges from 0 to 11. Basal subsurface paleomarsh depth ranges from 0.70 to 10.66 m (2.27 to 34.60 ft.). The greatest number of buried deposits, and the greatest depth of the paleomarsh are observed at Nestucca Bay (Darienzo, 1991). The relation between the number of buried deposits in cores containing at least 2 buried deposits and increasing basal paleomarsh depth is not unexpected in northern Oregon (Figure 9). Statistics from linear regression for this apparent trend are located Appendix F. Although the regression yields an R² (coefficient of determination or correlation coefficient squared) value of 0.49, the Probability value (3.38 x 10^-44) is much less than a (0.05) indicating that statistically a linear trend exists (slope ¹ 0). Increasing depth of marsh development represents greater age and length of record. However, there is a decrease in the number of buried wetland deposits per core moving from northern Oregon bays to southern Oregon bays (decreasing UTM N). To show this a plot of core with episodically buried deposits (>2 buried wetland deposits per 2 m length of core) vs. UTM-N coordinate is shown extending from Siuslaw to sites in northern Oregon in Figure 10. Sites from Siuslaw north are chosen because sites to the south of Siuslaw are complicated by local tectonics of the fold and thrust belt (Briggs, 1994). Using specific sites for examples, Netarts and Nestucca bays record 11 events in 10.66 m (34.60 ft.) of core depth. Umpqua core #337 records a basal paleomarsh depth of 4.70 m (15.25 ft.) and only two buried deposits (Appendix A).

The shallowest subsurface paleomarsh depth of 2.53 m (8.21 ft.) at site #337 yields a radiocarbon date of 2150 ± 80 (RCYBP). This suggests that the wetland deposits for this core did not record episodic subsidence during the last 2,000 years, the period that Netarts and Nestucca record 5-6 subsidence events. Radiocarbon ages generally show young dates (less than 500 RCYBP) for the shallowest paleomarsh

deposits in northern Oregon bays. Radiocarbon age estimates serve as a rough estimate of paleomarsh age, and may not accurately measure the time that burial occurred. This may be due to the lack of available samples deposited at the buried

marsh top (see core logs for Tillamook Bay in Appendix B). Samples that are taken from the buried marsh deposit below the burial contact yield age estimates that are somewhat older than the subsidence event itself.

The database for regional subsidence shows an overall record of multiple burial events in all bays of Oregon. However, the database also shows areas in southern Oregon bays where there is a mixture in recorded episodic and non episodic (< 2 buried wetland deposits in 2 or more meters of core) subsidence. For example, some of sites in the central Oregon bays show continuous wetland submergence for 1,000-3,000 years (Appendix A). The abundance of these sites increases from east to west (decreasing UTM E) in the Siuslaw and Umpqua Rivers, central Coos Bay, and the Coquille River. Whereas the consistent evidence of episodic subsidence implies regional elastic response of episodic subsidence in northern Oregon bays, the central-southern Oregon bays demonstrate evidence of locally-restricted tectonic displacement. To test the potential for regionally-linked paleosubsidence in northern Oregon, one or more events must be analyzed for amounts of subsidence to constrain local vertical movements. Regional strain release from megathrust earthquakes should yield similar amounts of subsidence in adjacent localities (see Results and Discussion).

Tide level indicators are tested to evaluate their discrimination of different tidal settings and associated amounts of paleosubsidence. The tide level indicators tested are plant macrofossils, visual estimate of peat development (lithology), loss on ignition, and identification of fresh, brackish, and marine water conditions from diagnostic diatom species. Included in the analyses are comparisons of twenty modern wetland samples for plant macrofossils (5 samples per setting), visual estimates of peat development, and loss on ignition at each locality. Twelve modern samples (3 per wetland setting) are used for analyses of diatoms. The results of modern wetland samples are then compared to results samples from the most recent buried wetland samples.

Land surveying and sampling of environments, ranging from colonizing marsh (mudflat edge) to forest edge, are used to analyze tide level indicators. Critical tidal level environments are used to constrain elevation changes for subsidence estimates. These environments include bounded (colonizing, low, and high marsh) and unbounded (forest edge) settings. That is, the mudflat extends to subtidal levels. Likewise, the forest extends to upland elevations. By comparison, colonizing marsh, low marsh, and high marsh are bounded within intertidal settings. The data from surveying, visual estimate of peat development, and loss on ignition are presented in Appendix C.

The forest edge, e.g. lowest growth of contiguous forests, is characterized by the growth of Sitka spruce, Alder, and Willow. Forest edge communities are observed at five localities. Wetland soil samples are gathered from settings where tree roots are exposed. Average forest edge elevations differ from average colonizing marsh elevations by about 2 m (Table 3).

High marsh communities are found at four localities, but not at Elliot Slough, Washington, where no discernible change in high marsh-low marsh plant assemblages was identified. High marsh communities are distinguished on the basis of Potentilla, Juncus, and Deschampsia. in brackish water bays. Average high marsh elevations differ from average colonizing marsh elevations by about 1 m (Table 4).

Low marsh plant communities are not identified at all localities e.g. the Elliot Slough, Washington. Low marsh communities consisting of Salicornia, Distichlis, and Triglochen are identified at Johns River, Washington, Neawanna Creek, Oregon, Kilchis River, Oregon and Schooner Creek, Oregon. Average low marsh elevations

differ from average colonizing marsh elevations by about 0.5 m (Table 5). Results of the wetland survey suggest a variety of species may be present at different

elevations (Appendix C). However, if the average differences are rounded to the nearest 0.5 m, a general trend is observed including a change of elevation of approximately 0.5 m for vertically adjacent communities and about 2.0 m for forest edge communities to colonizing marsh (Table 6). Certain plants such as Potentilla and Deschampsia were only identified at high marsh setting and may support such a paleotidal interpretation. By itself, plant assemblage evidence does not discriminate between high and low marsh settings. For example, Triglochen sp. is found at both

low and high marsh environments at the Kilchis River site making the tidal-level estimate of Triglochen rhizomes problematic. The relevance of these overlaps is further discussed following the buried wetland data (below).

The lithologies or degrees of peat development that are observed at modern settings are presented in Appendix C. To be consistent with previous studies, the visual percentages of peat development in Table 2 are used to describe the wetland samples from the modern setting. For each modern wetland setting a consistent apparent trend of increasing peat development is observed from colonizing marsh to high marsh and in most sites, from colonizing marsh to forest edge (Table 7). One exception is the visual estimates for the Kilchis River transect for Tillamook Bay, Oregon that show a greater degree of peat development at intertidal settings. This is possibly due to limited sampling that took place only above low tide levels. Consequently colonizing marsh settings were sampled closer to low marsh settings than in other study localities. Nevertheless, the five locations generally show increasing peat development with regard to increasing tidal setting from colonizing marsh (tidal flat edge) to high marsh. To statistically show this trend, a linear regression of Loss on Ignition vs. Elevation is completed for five localities below.

Measurement of organic content by loss on ignition (LOI) is described in

methods. Although there is a general trend of increasing organic content from colonizing to high marsh (Appendix C) the forest samples do not continue this trend. Some high marsh samples showed as great an organic content as forest edge samples (Figure 11). This may be due in part to sample size, as small peat samples might have arbitrarily sampled patchy distributions of organic material, e.g.; rhizomes and rootmats. However, the reversing trend of organic content in the forest edge samples might reflect oxidizing conditions in the supratidal soils. Oxidative loss of organics can occur in supratidal soils where gas exchange is facilitated in the vadose zone.

For example, samples from Neawanna Creek (Figure 12) do not discriminate between high marsh and forest edge based on organic content from the LOI technique. For these reasons, it is most appropriate to use LOI analyses of large samples, greater than 50 g dry weight, and of samples from intertidal settings that remain saturated by regular tidal inundation. Small samples have a variability with anomalous root masses that may not be consistent with the overall peat development of a wetland setting. Using linear regression it is clear that a general trend of peat development with increasing elevation from the colonizing marsh to forest edge does exist (Table 8)

Diatom analyses are based on 39 selected diagnostic diatom species of three different salinity divisions. These salinity divisions further constrain tidal elevations in

settings of wetland emergence from brackish colonizing marsh to freshwater high marsh-forest settings. Raw identification counts (Appendix D) and relative abundance counts (Appendix E) are tabulated for 12 modern wetland samples for each site.

Percentages of polyhalobous (marine), mesohalobous (brackish), and oligohalobous (freshwater) diatoms are calculated to estimate the tidal influence on a sample (Table 9) For modern marsh samples the raw counts are averaged, then combined to yield one value each for the three diatoms assemblages. The data show apparent differences in the percentages of marine and fresh water species from high and low marsh settings. However, the data do not discriminate between colonizing marsh and low marsh, nor do they show differences between the high marsh and the forest edge setting.

The results of the chi-squared test of homogeneity are located in (Table 10). Note that a different degree of freedom is applied to the Elliot Slough site as there are only three observed wetland settings. Again, the null hypothesis is

H₀: For each salinity level the proportions of diatoms in the forest edge, colonizing, low, marsh high tidal settings are the same. If the null hypothesis is rejected the accepted hypothesis is

H₁: For at least one salinity level the proportion of diatoms in the forest edge, colonizing, low, marsh high tidal settings are not the same.

For all five localities, the null hypothesis was rejected so that it can be stated that the proportions of diatoms are different at least one of the three salinity levels (fresh, brackish, and marine) in the modern wetland settings.

Results of diatom relative abundance counts show that forest edge samples contain the fewest number of diatom valves per field (Table 11). Although there is some variation in the setting where diatom abundances are the greatest, there is an apparent decrease in the number of diatoms at forest settings, relative to wetland settings.

Core and cutbank logs (Appendix B) show the visual estimates of peat development (lithology) and identified plant macrofossils (tree roots and rooted mud). Lithologic and plant macrofossil descriptions for buried wetland deposits in Grays Harbor, Necanicum estuary, and Siletz Bay observed in this study are consistent with previous studies (Atwater, 1992; Darienzo, 1991). Stratigraphic records presented in Appendix B show deposits in cores and cutbanks from four bays in Washington and Oregon that confirm abrupt subsidence. The stratigraphic age of the youngest subsidence event at these areas has been constrained through archival records, radiocarbon, and AMS dating at about 300 RCYBP (Present~1950 AD). The results of age dating the shallowest buried wetland deposit in Tillamook Bay are discussed below.

Southern Tillamook Bay (sample FP) contained a Sitka spruce cone found at a buried wetland below the most recent buried contact. The cone yielded an AMS age of 250 ± 40 RCYBP (Beta-89165). For a sample such as this where the dated material is relatively young a C¹²/C¹³ ratio of -24.0 ⁰/₀₀ is calculated. If this estimate is converted to years AD (RCYBP ~1950 AD) then the cone yields a date of 1700 AD ± 40 yr. This sample agrees with accurate estimates from precision tree-ring dates (Jacoby and others, 1995) and archival records (Satake and others, 1996). Wood fragments from buried deposits at Tillamook River (sample TR) and Bay City (sample TFU) are dated by bulk radiocarbon dating. The age determinations are 1210 ± 60 RCYBP (Beta-97668) and 500 ± 40 RCBYP (Beta-97669), respectively. The bulk radiocarbon age estimates from Bay City and southern Tillamook Bay illustrate the problem with samples collected from the middle or basal portion of the buried wetland deposit. However the estimate from Bay City does agree with similarly sampled estimates from Netarts Bay (370 ± 60 RCYBP) and the Necanicum Estuary (480 ± 60 RCYBP) supporting regional subsidence.

Plant macrofossils (tree roots) were identified only at one study locality for the 300 yr buried contact. Tree roots are identified at cutbanks from Elliot Slough in Grays Harbor, Washington (Appendix B). Wood fragments are observed in cores and cutbanks at other study area localities. However, these fragments are not associated with in situ roots that would establish forest soil elevations. Tree roots in buried wetland deposits support an estimate of 2.0 m or more of paleosubsidence when buried by mud containing only descending roots (rooted mud) or no marsh roots (tidal flat). Other methods are required to estimate amounts of subsidence for buried wetland settings with no tree roots or rooted mud.

The relative degrees of peat development (lithologies) observed in cores and cutbanks are presented in Appendices B, C and G. Consistent abrupt breaks in lithology across the burial contacts document changes in tidal level setting (Table 12). Eight sites subsided to rooted mud (colonizing marsh) settings. Pre-subsidence settings (modern marsh) are not discriminated by relative peat development (peaty mud to peat).

The buried sample results show abrupt changes in LOI values across buried wetland contacts (Table 13). Substantial changes in organic content reflect changes in

wetland setting across buried wetland contacts in seven sites. However, two cores show across-contact changes of only 10%. For example, Schooner Creek shows a difference in organic content of only about 6%. As mentioned earlier, oxidation in

upland settings may decrease the organic content (see Appendix B). Loss on ignition in seven of the nine sites confirms visual estimates of peat development (lithology). Notably, the below contact LOI values from sites in Tillamook Bay and Johns River document a larger organic content than high marsh and forest edge modern wetland settings. This may be due to the compaction that deposits high in organic content undergo (Franklin and others, 1973). However, the LOI analysis do not independently establish maximum amounts of paleosubsidence.

The analyses of diatoms from buried wetland samples are described below. Breaks in the freshwater diatom species support lithologic evidence of abrupt changes in tidal level setting (Table 14). For example, freshwater diatoms increase upcore across the contact in all of the nine sites.

The diatom assemblages collected from above the buried wetland deposit contacts resemble modern wetland samples from the colonizing and low marsh settings. The below contact settings are interpreted to be from former high marsh or forest soil settings.

The z-test is used to test the difference between two proportions of freshwater diatom from above and below buried wetland contacts. A z-test of each buried wetland

sample is completed against the modern wetland settings to determine if a modern wetland setting has similar freshwater diatom proportions to a buried wetland horizon (peat or mud). Again, the null hypothesis states

H₀: p₁= p₂

If the null hypothesis is rejected (|z|>1.96) then it is accepted that:

H₁: p_1¹ p₂

A summary of the results of the z-test are located below in Table 15-Table 23.

Z-test is completed for freshwater proportions except for the above contact sample (FP) from Tillamook Bay that used marine proportions. No freshwater species are observed for this study in the above contact sample making the z-test of the difference

of proportions test invalid. The z-test of the difference of proportions supports two statements: (1) all above contact (mud) samples rejected the null hypothesis of equal proportion for below contact (peat), high marsh, and forest edge sample proportions and (2) all below contact (peat) samples rejected the null hypothesis of equal proportions for low and colonizing marsh samples. Comparisons where the null hypothesis could not be rejected include (1) above contact samples (mud) against colonizing and low marsh settings, and (2) below contact samples (peat) against high marsh and forest edge settings. This indicates that the proportions are equal, although there are z-tests values where all modern settings were rejected. For example, z-tests that compared below sample TFU against modern wetland samples from the Kilchis rejected the null hypothesis for all settings. However, the proportions of freshwater diatoms for the buried sample are at least as great as the forest edge or high marsh, indicating that the sample was at least from a former forest edge or high marsh.

Diatom abundance data from buried wetland deposits show a substantial increase upcore at only two sites (Table 24). For example, relative abundances at Elliot Slough, WA for above and below the contact are twelve and four valves per field, respectively. However, at other sites (Johns River, WA; Tillamook River, OR; Southern Tillamook Bay) the number of valves above the contact and the below contact samples are even. The total abundance data possibly show submergence from supratidal settings of two site (Elliot Slough and Siletz River) but, do not discriminate paleotidal settings at the other seven sites. Samples collected from below the shallowest burial contact showed a similar variability to those above the contact.

Estimates of the amount of subsidence produced by the most recent buried wetland event are based on multiple paleotidal level indicators calibrated for modern settings at Grays Harbor, Necanicum Estuary, Tillamook Bay, and Siletz Bay. Specifically, the plant macrofossils (rooted mud and tree roots), relative peat development (lithology), and diatom proportions define distinct tidal settings. These changes correspond to subsidence estimates of 0.0 m ± 0.5 m, 1.0 m ± 0.5 m, and 2.0 m ± 0.5 m (Figure 13). A 0.0 m ± 0.5 m estimate would represent transitions where the sediment above the contact cannot be distinguished between a colonizing or low marsh setting and the sediment below the contact is that of a low or high marsh setting. An estimate of 1.0 m ± 0.5 m is given for an above contact sediment representative of a colonizing marsh and the sediment below the contact is that of a high marsh setting. Loss on ignition and diatom assemblages are used to confirm changes in paleotidal level. For estimates of 2.0 m ± 0.5 m the above contact sediment is representative of a colonizing marsh or tidal flat setting and the sediment below the contact is that of a forest edge setting. An assumed error of ± 0.5 m is used for the paleosubsidence estimates. This is the average difference (rounded to the nearest 0.5 m) between (1) colonizing and low marsh, (2) low and high marsh, (3) and high marsh and forest edge settings. Adjacent settings might not be discriminated from one another by tidal level indicators used here, leading to the assumed ± 0.5 m estimate error. Using Figure 13, as summary of tidal level indicators (Table 25) of the four bays is shown below with a detailed description of interpretations that follows.

For Grays Harbor, Washington, separate subsidence estimates are given for the Elliot Slough and Johns River sites. The available tide level criteria are lithology, total organic content, plant macrofossils, and microfossils.

From cutbank observations the lithology above the contact ranges from a rooted mud to a barren mud that compares with a modern wetland setting of a colonizing marsh. No plant macrofossils are identified. Loss on ignition of the sample (ES-ac) yields a value of 8.0%. This value compares with colonizing modern marsh samples from this locality. The identified diatom counts shows a majority of marine diatoms (42%) when compared to fresh (22%) and brackish (36) diatoms. Z-tests against modern wetland samples show equal proportions of freshwater diatoms in the colonizing marsh setting (Z=0.74). Using the evidence from above, the sample was from a colonizing marsh or perhaps a tidal flat.

The lithology below the contact is a peaty mud that compares with modern established wetland setting of a high marsh. Plant macrofossils that are observed include Sitka spruce roots. The loss on ignition of the sample (ES-bc) yields a value of 43.4%. The value is well above modern high marsh or forest edge samples from Elliot Slough. The identified diatom counts shows a majority of freshwater diatoms (66%) in comparison to marine (22%) and brackish (12%) diatoms. Z-tests against modern wetland samples show equal proportions of freshwater diatoms in the forest edge setting (Z=0.09). Evidence, especially that of the spruce roots and Z-test of proportions, suggests that this deposit was from a forest edge or a forest.

The paleotidal indicators demonstrate an abrupt transition from a forest edge setting to an overlying colonizing marsh for a vertical change of 2.0 m ± 0.5 m.

The lithology above the contact ranges from a rooted mud to a barren mud that compares with a modern wetland setting of a colonizing marsh or tidal flat. The loss on ignition of the sample (JR-ac) yields a value of 8.9%. This value is comparable to modern colonizing or low marsh values from Johns River. The identified diatom counts shows a majority of marine diatoms (62%) when compared to fresh (8%) and brackish (30) diatoms. Z-tests against modern wetland samples show equal proportions of freshwater diatoms in the colonizing (Z=0.69) and low marsh settings (Z=-1.96) The above contact sample was probably from a colonizing marsh, low marsh or perhaps a tidal flat.

The lithology below the contact ranges from a peaty-mud to a peat that compares with modern wetland settings of a high marsh or forest edge. Plant macrofossils observed include Potentilla rhizomes. Loss on ignition of the sample (JR-bc) yields a value of 56.4%. This value is nearly twice the loss on ignition values for modern high marsh and forest edge samples from Johns River. Identified diatom counts shows a majority of freshwater diatoms (52%) in comparison to marine (16%) and brackish (32%) diatoms. Z-tests against modern wetland samples show equal proportions of freshwater diatoms in the forest edge setting (Z=-1.42). This evidence, especially that of plant macrofossils and diatoms, indicates that this was a high marsh or perhaps a forest edge environment although no tree roots are observed in the cutbanks.

The paleotidal indicators show an abrupt transition from a forest edge or high marsh setting to an overlying low or colonizing marsh setting for a vertical change of 1.0 m ± 0.5 m.

For Necanicum Estuary, Oregon a subsidence estimate is given for the Neawanna Creek site. The available tide level criteria are lithology, total organic content, and microfossils.

From the observations of cores the lithology above the contact ranges from a rooted mud to a slightly peaty mud that compares with modern wetland setting of a colonizing marsh or a low marsh setting. The loss on ignition of the samples (NE-ac and 12^th St.-ac) have values that range from 12%-20%. These ranges are comparable to the values obtained in modern colonizing and low marsh samples. The identified diatom counts shows a majority of marine diatoms (58, 46% for NE-bc and 12^th St.-bc samples, respectively) in comparison to fresh (10, 20%) and brackish (32, 34%) diatoms. Z-tests of NE-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the colonizing (Z=-1.61) or low marsh (Z=-1.24) setting. Similarly, Z-tests of the 12^th-St.-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the colonizing (Z=0.00) and low marsh (Z=0.43) settings. The above contact sample is from a former colonizing marsh or low marsh.

The lithology below the contact ranges from a peaty-mud to a peat that compares with a modern wetland setting of a high marsh or forest. Loss on ignition of the samples (NE-bc and 12^th St.-bc) range from 37% to 40%. These values compare to modern high marsh and forest edge samples. The identified diatom counts shows a majority of freshwater diatoms (68% for both NE-bc and 12^th St.-bc samples) in comparison to marine (12%, 16% for NE-bc and 12^th St.-bc samples, respectively) and brackish (20, 16%) diatoms. Z-tests of the NE-bc sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z =0.34) and forest edge (Z =-0.27) settings. Similarly, z-tests of the 12^th-St. bc sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z =0.34) and forest edge (Z =-0.27) settings. This site lacks plant macrofossil data, however evidence from diatoms supports the interpretation that this was at least high marsh environment.

As the average differences in elevation from colonizing marsh to low marsh was relatively small (0.17 m or 0.55 ft.), and the lithologies document a pre-subsidence high marsh or forest edge setting, an estimate of 1.0 m ± 0.5 m is used for this site.

For Tillamook Bay, Oregon, subsidence estimates are given for the Bay City, Tillamook River, and Southern Tillamook Bay sites. The available tide level criteria are lithology, total organic content, plant macrofossils and microfossils.

From cutbank observations the lithology above the contact is a rooted mud that compares with a modern wetland setting of a colonizing marsh or tidal flat. Loss on ignition of the sample (TFU-ac) has a value 3.6% and is comparable to modern colonizing samples from the Kilchis River. The identified diatom counts document a majority of marine diatoms (50%) when compared to fresh (8%) and brackish (42%) diatoms. Z-tests of the TFU-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the colonizing marsh (Z=-1.69) setting. The above contact sample is a former colonizing marsh.

From cutbank observations the lithology below the contact is a muddy peat that compares with a modern wetland setting of a high marsh. Loss on ignition of the sample (TFU-bc) is 45.7% and is comparable to modern forest edge samples from the Kilchis River (Appendix C). Identified diatom counts shows a majority of freshwater diatoms (72%) in comparison to marine (18%) and brackish (10%) diatoms. Z-tests of the TFU-bc sample against modern wetland samples rejected equal proportions of freshwater diatoms in all samples, although proportions are greater than high marsh (Z=2.16) and forest edge (Z=2.08) settings. Evidence from diatoms and organic content supports the visual estimate that this at least a former high marsh.

The paleotidal indicators indicate an abrupt transition from a high marsh to an overlying colonizing or low marsh for a vertical change of 1.0 m ± 0.5 m.

From trench wall observations the lithology above the contact is a rooted mud that compares with a modern wetland setting from a colonizing marsh. No plant macrofossils are identified. Loss on ignition of the sample (TR-ac) has a value of 12.4% and compares to modern colonizing and low marsh samples from the Kilchis River. The identified diatom counts document a majority of marine (44%) and brackish (40%) diatoms when compared to the fresh (16%) diatoms. Z-tests of the TR-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the colonizing marsh (Z=-0.32) setting. The above contact sample is a former colonizing or low marsh.

The lithology below the contact is a muddy peat that compares with a modern wetland setting from a high marsh. Loss on ignition of the sample (TR-bc) is 55.2%. This value is comparable to modern forest edge samples from the Kilchis River. Identified diatom counts shows a majority of freshwater diatoms (62%) in comparison to marine (14%) and brackish (22%) salinity levels. Z-tests of the TR-bc sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z=0.91) and forest edge (Z=0.82) settings. Plant macrofossil evidence at this site include woody shrub roots. Supporting evidence from diatoms and LOI indicates that this is a former high marsh or forest edge environment.

The paleotidal indicators across the contact indicate an abrupt transition from a high marsh or forest edge to an overlying colonizing or low marsh for a vertical change of at least 1.0 m ± 0.5 m.

From cutbank observations the lithology above the contact is a rooted mud that compares with modern wetland setting from a colonizing marsh. No plant macrofossils are visible. Loss on ignition of the sample (FP-ac) has a value of 14.3%. and compares to Kilchis River modern colonizing and low marsh samples. The identified diatom counts document a majority of marine (54%) and brackish (44%) diatoms when compared to the fresh (0%) diatoms. Z-tests of the FP-ac sample against modern wetland samples show equal proportions of marine diatoms in the colonizing marsh (Z=1.47) setting. The evidence from above the contact indicate the sample is from a former colonizing marsh.

The lithology below the contact is a muddy peat that compares with a modern wetland setting from a high marsh. Loss on ignition of the sample (FP-bc) is 53.6% and is similar to modern forest edge samples from the Kilchis River (Appendix C). Identified diatom counts shows a majority of freshwater diatoms (60%) in comparison to marine (14%) and brackish (26%) diatoms. Z-tests of the FP-bc sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z=0.66) and forest edge (Z=0.58) setting. Evidence from diatoms and organic content indicates that this is a former high marsh or forest edge environment.

The paleotidal indicators across the contact establish an abrupt transition from a high marsh to an overlying colonizing marsh for a vertical change of 1.0 m ± 0.5 m.

The problems with the Tillamook River and Southern Tillamook Bay sites are that no undiked modern marsh analogues exist nearby. Estimates for the site require extrapolating modern marsh samples from the Kilchis River site, on the east side of Tillamook Bay. Furthermore, mudflat-colonizing marsh transition was not surveyed for this modern wetland analog.

For Siletz Bay, Oregon, subsidence estimates are given for Schooner Creek and Siletz River sites. Available tide level criteria include lithology, total organic content, and microfossils.

From core observations the lithology above the contact is a rooted mud that compares with a modern wetland setting from a colonizing marsh. Loss on ignition of the sample has a value of 12.7% and compares to modern colonizing marsh samples from Schooner Creek. Identified diatom counts document a majority of marine (60%) diatoms when compared to the fresh (14%) and brackish (26%) diatoms. Z-tests of the SC-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the colonizing (Z=-1.83) and low marsh (Z=-1.93) settings. Using the evidence from above, the above contact sample is from a former colonizing marsh.

The lithology below the contact is a muddy peat that compares with a modern wetland setting from a high marsh. Loss on ignition of the sample is 18.9%. This value is comparable to low and high marsh samples from the Schooner Creek site. Identified diatom counts shows a majority of freshwater diatoms (64%) in comparison to marine (28%) and brackish (8%) diatoms. Z-tests of the SC-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z=-0.50) and forest edge (Z=-0.76) settings. Evidence from diatoms and lithology indicates that this was at least a high marsh environment as no tree roots are observed in core..

The paleotidal indicators demonstrate an abrupt transition from a high marsh to an overlying colonizing marsh for a vertical change of 1.0 m ± 0.5 m.

From core observations the lithology above the contact is a rooted mud that compares with a modern wetland setting of a colonizing marsh. Loss on ignition of the sample has a value of 9.4% and compares to modern colonizing marsh samples from Schooner Creek. Identified diatom counts document a majority of brackish (52%) and marine (36%) diatoms when compared to the freshwater (12%) diatoms. Z-tests of the SR-ac sample against modern wetland samples rejected equal proportions of freshwater diatoms of all samples although proportions are smaller than in the colonizing (Z=-2.13) and low marsh (Z=-2.21) settings. Using the evidence from above, the above contact sample represents a former colonizing marsh or perhaps a tidal flat.

The lithology below the contact is a muddy peat that compares with a modern wetland setting of a high marsh. Loss on ignition of the sample is 20.6% and is comparable to Schooner Creek modern low and high marsh samples. Identified diatom counts shows a majority of freshwater diatoms (58%) in comparison to marine (18%) and brackish (24%) diatoms. Z-tests of the SR-ac sample against modern wetland samples show equal proportions of freshwater diatoms in the high marsh (Z=0.25) and forest edge (Z=0.00) settings. Evidence from diatoms and lithology supports the lithologic interpretation that this is a former high marsh environment.

The paleotidal indicators demonstrate an abrupt transition from a high marsh to an overlying colonizing marsh for a vertical change of either 0.0 m ± 0.5 m (see below) or 1.0 m ± 0.5 m. The estimates for this site may be equivocal as the above contact sediment does not suggest a marine water dominance from diatoms, but rather a brackish dominance.

The most recent buried wetland regional database is represented by published and unpublished data collected throughout the central Cascadia margin (Appendix G). The database provides a framework for comparing criteria used for estimating coseismic subsidence from plant macrofossils and lithologic paleotidal indicators. Diatom evidence confirms paleosubsidence interpretations based on plant macrofossils or peat development.

Components of the database are organized into location (UTM), plant macrofossils, lithology, total organic content, diatom evidence, and the distance (km) to the CSZ trench.

A regional trend of decreasing subsidence is found from north to south (decreasing UTM-N) (Figure 14) in several bays, and locally from east to west (decreasing UTM E) in sites along the Columbia River (Figure 15).. Statistics from regression analysis for these plots are located in Appendix F. The calculated R² value is 0.49 and the P-value (5.06 x 10^-14) is much less than the value of a (0.05) statistically showing that a trend exists for Figure 14.

A plot of subsidence versus east-west distance is shown for the Columbia River demonstrating increasing then decreasing paleosubsidence from west to east. The limit of detectable subsidence is found of a distance of about 40 km (24 mi.) east of the coast of the Columbia River and ~129 km from the Cascadia Trench.

If this plot is inverted (subsidence negative values), it resembles a portion of Figure 2. Both of the regional plots of paleosubsidence for the most recent buried wetland event demonstrate large changes over broad distances but, small changes between adjacent sites. For example, paired localities at north and south Willapa Bay, Netarts and Tillamook bays, and at Yaquina and Alsea bays show no differences in estimated subsidence of the 0, 1, and 2 m intervals.

These results indicate that the vertical tectonic displacement is forced by regional strain release from megathrust earthquake rather than from multiple independent ruptures along local faults.

Potential flooding impacts from combined regional subsidence and estimated flooding conditions are described below. Selected sites in Grays Harbor and Necanicum Estuary are chosen to illustrate the difference between the current flooding elevations and flooding elevations which could result from coseismic subsidence. Detailed topographic maps of these sites are available from the city of Aberdeen, Grays Harbor, Washington and the city of Seaside, Necanicum Estuary, Oregon (Spencer Gross, 1991; CH₂M-Hill, 1973). Lines are drawn from the two foot contour topographic maps to show the extent of increased flooding potential from the maximum subsidence estimate. This analysis assumes that estuary basin effects from elastic response are minimal and that the drainage basin area does not change (see limitations in Discussion).

An inundation analysis is completed by plotting the 100 yr flood estimates from flood insurance studies(Army Corps of Engineers, 1971; Soil Conservation Service, 1979) on topographic maps for Aberdeen, Washington and Seaside, Oregon. Flooding maps and profiles for Seaside and Aberdeen are digitized using the program Canvas (Deneba software, 1993). Critical data that are digitized for maps here include stream or river location, bridge location, street location, existing 100-year flood elevation (MSL), and elevations of subsidence estimates added to 100-year flood elevations. The 100-year flood elevation is used here because flood insurance studies use that frequency for determining the degree of probable susceptibility (Army Corps of Engineers, 1971; Soil Conservation Service, 1979; FEMA, 1990). Profiles show the impacts to other areas not shown on the maps completed in this study. These profiles show elevations of (1) 100 yr. and 10 yr. floods, (2) 100 yr. and 10 yr. floods with added subsidence, (3) roads, and (4) dikes (where present).

Flooding maps for Aberdeen, Washington show lines of potential flooding (Figure 16). Digital topographic maps (Spencer-Gross, 1991) for Aberdeen, Washington contain two foot contour intervals photo-interpreted from 1:1200 scale. An existing 100-year flood elevation of 3.1 m (10.0 ft.) (NGVD-1929) is digitized as a layer for the comparison of added subsidence estimates. A subsidence estimate of 2.0 m (6.5 ft-Elliot Slough) is added to the existing 100 yr flood elevation of 3.1 m.(10.0 ft) for a total flooding elevation of 5.1 m (16.6 ft.). The map shows the portion of Wishkah St. (Highway 12) that would serve as the main road artery for aid, that would be cutoff from seasonal flooding. The additional area that is inundated after coseismic subsidence may not appear to be a significant hazard when compared to the 100 yr. (present floodline). However, the present 10 year flood of 2.7 m (8.8 ft. MSL) post-coseismic subsidence of 2.0 m (6.6 ft.) could reach an elevation of 4.7 m (15.4 ft). The 10 yr. flood frequency is most appropriate for 50-100 years of post seismic rebound and vertical strain accumulation. This makes the damage from a 10 yr flood post-subsidence-pre-uplift greater than that of a 100 yr flood pre-subsidence. This is particularly evident in a profile of south Aberdeen, a particularly low-lying area (Figure 17). Here flood dikes have been constructed to prevent inundation by a 100 yr. flood event. However, a 10 yr. flood event with added subsidence would crest the dike to the east, inundating Marion St. with nearly seven feet of water. Notice also that highways 101 and 105 through south Aberdeen are inundated. This profile assumes that dikes are maintained, which might be required after a great subduction zone earthquake. For dikes consisting of fill material, the potential for liquefaction and failure from a great earthquake suggests that post seismic repair and maintenance of dikes is critical.

Another critical aspect of flooding is the management of city drain systems, i.e., tidegate and outfall tunnels that serve to remove runoff within the city. A map is constructed to indicate the outfall tunnel elevation relative to (Figure 18). These two outfall tunnels show flooding hazards that could occur as a result of coseismic subsidence. The outfall tunnel to the west at elevation -2.0 m (-6.4 ft.) MSL currently, would be influence by stronger tidal forces that could carry flood tide water up the system perhaps beyond Heron St. Another potential hazard with respect to outfall tunnels is the tidegate associated with them. Consider the remaining three outfall tunnels in the map with elevations -1.2 m (-3.9 ft.), -0.4 m (-1.4 ft.), and -1.2 m (-4.00 ft.) from west to east. With estimated subsidence of 2.0 m (6.6 ft.), the increased tidal forcing would restrict city runoff through the tidegates causing a back up resulting standing water within low-lying areas of Aberdeen throughout the winter. Topographic maps of Seaside, Oregon were generated by CH₂M-Hill (1973) using 2-foot contour intervals. Estimates of coseismic subsidence applied to existing 100-year flood elevation illustrate the potential for flooding. A flooding elevation of 3.9 m (12.5 ft.) MSL is digitized from a flood insurance study of Seaside, Oregon for Neawanna Creek (Soil Conservation Service, 1979). A subsidence estimate of 1.0 m (3.3 ft.) is added to the 100 yr flood estimate of 12.5 ft. for a total flooding elevation of 15.7 ft .

The flooding map shows the additional inundation expected after coseismic subsidence from a great Cascadia earthquake (Figure 19). Specifically, this map shows the hospital essentially cutoff from Seaside as the access Wahanna Road is submerged by six or more feet of water during a 100 year flood. Although Highway 101 for this area is at an elevation greater than that of the flood >5.1 m (>16.5 ft.), the access bridge on Broadway Drive is effectively inundated by six feet of water. Again, at this location the difference in 100 yr flood elevation of 3.9 m (12.5 ft MSL) and the 10 yr. elevation of 3.5 m (11.5 ft.) is quite small (~0.3 m or 1 ft.) so that the impact of a 10 yr. flood with added subsidence (~14.8 ft) is well above the current 100 yr and 500 yr. (13.0 ft) flood elevations. .A profile of south Seaside along the Necanicum River shows a slightly increased flood elevation for the 10 yr and 100 yr floods (Figure 20). For this location, the 100 yr. and 10 yr. flood elevations are 4.4 m (14.4 ft.) and 4.0 m (13.1 ft.), respectively (Soil Conservation Service, 1979). As in Aberdeen, the 10 yr. flood elevation with added subsidence posses a significant impact. In the profile of   Seaside, this moderate event inundates Highway 101. Unlike the Highway 101 elevation near Broadway Drive, here it is comparatively lower (~4.9 m or 16 ft.). With a combined 100 yr. flood event and estimated subsidence this locality is inundated by nearly two feet of flood water.

While there are no flood dikes in Seaside and the only tide gate at Mill Creek has been removed, high tides have had an effect on storm drains (Chris Davies, Engineer, City of Seaside, pers. communication). This has been observed as high tides effects run off within the drain network backing up the drainage system for a half an hour. The effect of coseismic subsidence could have a similar impact to storm drains and outfall tunnels becoming routinely backed up during rainy winter seasons.

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