Assessing local impacts of the A . D . 1700 Cascadia earthquake and tsunami using tree ring growth 1 histories : A case study in South Beach , Oregon

We present a spatially focused investigation of the disturbance history of an old-growth Douglass fir stand in South 9 Beach, Oregon for possible growth effects due to tsunami inundation caused by the A.D. 1700 Cascadia subduction zone 10 earthquake. A high-resolution model of the 1700 tsunami run-up heights at South Beach, assuming an “L” sized earthquake, 11 is also presented to better estimate the inundation levels several kilometers inland at the old-growth site. This tsunami model 12 indicates the South Beach fir stand would have been subjected to local inundation depths from 0-10 m. Growth chronologies 13 collected from the fir stand shows several trees experienced significant growth reductions before, during and several years 14 after 1700, consistent with the tsunami inundation estimates. The +/1-3 year timing of the South Beach disturbances are also 15 consistent with disturbances previously observed at a Washington state coastal forest ~220 km to the north. Additional 16 comparison of the South Beach chronologies with regional chronologies across Oregon indicates the South Beach stand growth 17 was significantly and unusually lower in 1700. Moreover, the 1700 South Beach growth reductions were not the largest over 18 the 110-year tree chronology at this location. with other disturbances likely caused by other climate drivers (e.g. drought or 19 windstorms). Our study represents a first step in using tree growth history to ground-truth tsunami inundation models by 20 providing site specific physical evidence. 21 . 22


Introduction 23 31
Here we present a spatially focused investigation of the disturbance history of an old-growth forest in South Beach, 32 Oregon (Figure 1). We also present a new, high resolution model of the 1700 tsunami run-up heights at South Beach 33 to better estimate the inundation levels at the site of the old-growth forest. Our goal is to use tree-growth to ground-34 truth the tsunami impacts and inundation levels as well as for insights into the degree of shaking caused by the 1700 35 magnitude 9.0 Cascadia Subduction Zone earthquake [Satake et al., 2003;Witter et al.., 2011]. 36 37 Interestingly, direct evidence of seismic shaking (liquefaction, landslides, etc) from the 1700 megathrust earthquake 38 is relatively rare along the Oregon coast. This is thought to be due to the high rainfall and water erosion rates in the 39 Pacific Northwest which removes liquefaction evidence in coastal estuaries, and makes landslides in the coast range 40 difficult to identify [Yeats, 2004;LaHusen et al., 2020]. Models of shaking and ground motion along the Oregon 41 coast during the 1700 Cascadia earthquake indicate it should have been violent and widespread [WDNR, 2012], and 42 it seems plausible that evidence of this shaking might be recorded in the ring widths of trees along the coast. 43 44 Very little tree-ring work has been conducted along the Oregon coast; the vast majority of tree-ring research in the 45 Pacific Northwest has entailed climate reconstructions from high-elevation sites in the Cascade Mountains and 46 Olympic Peninsula where competitive effects are low. We sampled a mesic old-growth forest near the Pacific coast 47 . During the 1700 Cascadia earthquake, ground motion and peak ground acceleration (PGA), 65 are modeled to have ranged from ~0.5-1.2 g along the Oregon coast [WDNR, 2012]. Thus ground motion shaking 66 during the 1700 earthquake should have been violent and widespread. 67

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The exact timing of the earthquake was estimated by calculating the travel time for an unexplained tsunami that struck 69 Japan on 26 January 1700 [Satake et al., 2003;Atwater, 2006]. Radiocarbon dating was used to show the earthquake 70 occurred in 1700, where the radiocarbon dates were derived from the remnants of many trees drowned by coincident 71 subsidence, and surviving trees recorded the earthquake's date by anomalous changes in ring width or anatomy of 72 their annual rings [Atwater and Yamaguchi, 1991;Jacoby et al., 1997]. As a result of subsidence, some coastal forests 73 dropped below sea level and were flooded. Boles and root masses of these trees still remain and can be found from 74 northern Oregon to southern Washington. Aligning tree-ring growth patterns of the living trees with those of the 75 flooded, dead trees consistently showed that the last year of growth was 1699, indicating the earthquake occurred 76 between October 1699 and April 1700 [Yamaguchi et al., 1997]. This tree-ring and dating evidence for coastal 77 disturbance is indeed compelling, however the evidence was derived from trees along just 100 km of coastal southern 78 Washington-northern Oregon, or ~5% of the coastline expected to be affected by a Cascadia megathrust earthquake. 79 80 Additionally, a coastal-wide inventory of liquefaction features associated with the 1700 earthquake found no features 81 along the Oregon coast, despite numerous exposures of clean sand deposits that must be susceptible to liquefaction, 82 even at low levels of seismic shaking [Obermeier and Dickenson, 2000]. The locations for these field studies in 83 Oregon were also sites where evidence for great Holocene subduction earthquakes (in the form of crustal subsidence) 84 have been identified [Nelson et al., 1995]. The only liquefaction features identified to date (and thus direct evidence 85 of seismic shaking) were found along the Columbia River 35-50 km east of the coast, and these indicate moderate 86 shaking intensity of 0.2-0.35 g [Obermeier and Dickenson, 2000]. 87 performing hazard assessments, Mean High Water (MHW) or MHHW is assumed over the entire duration of tsunami 117 [Wei, 2017], and using MHHW as the vertical datum usually gives a more conservative estimate of the tsunami 118 impact. In the present study, we prefer to use MHHW, instead of the actual tidal level, as our model reference level 119 due to: 1) the uncertainty of the time of the event, which is based on estimates from Japanese records (Satake et al. 120 1996), and could vary over a window of 1-2 hours; 2) the uncertainty of the earthquake/tsunami source; and 3) the 121 uncertainty in the amount of sea level change, which is > 0.5 m over the past 300 years based on a rate of 1.77 mm 122 annual increase. The impact of these uncertainties on the model could overshadow the difference between MHHW 123 and the actual tidal level, and adds an additional level of uncertainty to the model results. A Manning's coefficient of 124 friction of 0.03 is uniformly applied for both the land and ocean components of the tsunami propagation model. To 125 more realistically estimate the tsunami impact produced by the 1700 event, we removed the two jetties at the entrance 126 of Yaquina Bay from the model DEMs, which leads to greater tsunami inundation levels and impact at South Beach. 127 The tsunami model results discussed hereafter are based on the revised DEMs without the jetties. 128

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The tsunami inundation model presented here indicates the "L" earthquake, with the co-seismic subsidence taken into 130 account, would produce a tsunami that could inundate South Beach to tsunami water levels up to 17 m (Figure 2a), 131 and inundation depths up to 16 m (Figure 3a,b). The height of the water level at the western section of Mike Miller 132 Park is generally between 12-15 m, and reduces to between 9-12 m on the eastern side. It is important to note that 133 "tsunami water level" is a term used to describe the elevation reached by seawater measured relative to a stated datum 134 (MHHW herein). Whereas "inundation depth" refers to the local water depth, or height of the tsunami above the 135 ground after taking into account the co-seismic subsidence at a specific location, as shown in Figure 2c  is located on topography that can be up to 10 m higher elevation than most of the westward terrain. Nevertheless, it 143 would seem these tsunami current velocities would be high enough to cause significant damage to the South Beach 144 trees, through the large mass and momentum of this volume of sea water, that would be observable in the tree growth. 145 146 Lastly, it is worth noting that the "L" earthquake tsunami model presented here also involves the activation of splay 147 faults in the overriding plate above the subduction zone. Motion on these splay faults introduce a larger co-seismic 148 subsidence along the coastline, and therefore represent a more extreme inundation scenario for the A.D. 1700 event 149 than previous models. Based on the turbidites records reported by Goldfinger et al. (2011), the "L" and larger 150 earthquake scenarios occurred four times in the past 10,000 years, and thus is referred as a 2,500-year event, although 151 the general earthquake size class and associated time interval for an "L" event is estimated to be 800 years by Witter Thus the exact timing of tree-ring disturbances due to an earthquake and the resulting ground motion, coastal land 205 subsidence and tsunami inundation can vary within a few years around the event date. This is because tree growth 206 can be affected by several climatological/meteorological factors, including droughts, cold/heat stress, fires, and 207 windstorms and even insect infestations. However, comparison of coastal growth rings with other regional sites can 208 be used to control for these climate/weather disturbance impacts. Thus, despite this temporal variability, Jacoby et al 209 In an attempt to further quantify the widespread effects of the A.D. 1700 earthquake, we obtained tree-ring records 217 from a stand of old-growth Douglas fir trees (Pseudotsuga menziesii) whose ages pre-date 1700 (Figure 4a,b). The 218 stand is located in an Oregon State Park in South Beach, Oregon, roughly 600 m east of Highway 101 (Figure 3a,b). 219 Old growth trees of 300+ years of age are rare along the Oregon coast, thus this stand of trees within the inundation 220 zone presented a unique opportunity to search for direct physical evidence of the impact of a Cascadia Subduction 221 zone earthquake and tsunami inundation in a populated area where tsunami models indicate significant inundation 222 levels and run-up heights. 223

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To ground-truth the model of the A.D. 1700 tsunami, we collected tree cores at breast height from 37 dominant or 225 codominant old-growth trees at the South Beach site using a 32" increment borer. Two cores were collected from 226 each tree, after which cores were mounted, sanded with increasingly fine lapping film, and cross-dated (Phipps 1985). 227 Each core was then measured using a Velmex TA Tree-Ring Measuring device to the nearest 0.001mm (Velmex, Inc. 228 Bloomfield, NY). Cross-dating was then statistically verified using the program COFECHA [Holmes 1983]. A master 229 growth-increment chronology was then developed by detrending each measurement time series using a negative 230 exponential or regression functions to retain as much low-frequency variability as possible as well as a second 231 chronology developed using 50-year 50% frequency-cutoff cubic spline to highlight interdecadal to interannual 232 growth variability. All chronology construction was performed using the program ARSTAN Cook and Krusic 2005]. 233 Figure 3a,b shows the location of the Douglas Fir trees sampled for this study in relation to the modeled tsunami 234 run-up heights for South Beach. Of all the trees sampled, a total of twelves cores from eight trees pre-dated 1700 235 (Figure 4b). 236 237 As noted, the tsunami inundation model presented here (Figure 2a,b) indicates the "L" earthquake would produce a 238 tsunami that could inundate the lowlands of South Beach to inundation depths up to 18 m. However, the Douglas fir 239 old-growth stand that is the subject of this study lies on, and along the western edge, of two parallel north-south 240 striking topographic highs (likely paleo-dune ridge lines). The tsunami model presented here indicates that while 241 many of the trees in this area may have experienced as much as ~10 m of inundation depth, several trees are also on 242 high ground and may have experienced much less, or even zero, inundation. 243 244 Tree-ring data detrended using negative exponential functions did not reveal major stand-wide releases or 245 suppressions around 1700 (data not shown) nor did data detrended using the 50-year spline functions. Detailed 246 examination of the growth-ring samples indicates that although individual cores have below-average growth, and one 247 experiences what could be interpreted as a post-1700 growth release, variability around 1700 is not necessarily 248 exceptional in the longer-term context of the ~310 year history of the dataset (Figure 4a,b) .

Summary 302
We presented a series of tree-ring data from an old-growth Douglas-fir forest in South Beach, Oregon that shows 303 significant growth disturbance at the time of the A.D. 1700 Cascadia subduction zone earthquake. In addition, we 304 presented a new, high resolution model of the 1700 tsunami inundation at South Beach old-growth site. Due to 305 significant variation in the South Beach topography, several areas are predicted to experience water levels up to 17 306 m and a range of inundation depths up to 16 m, however the location of the old-growth stand may be subjected to a 307 range of inundation depths from 0-10 m. To better detect tree growth anomalies near AD 1700, we also compared the 308 South Beach Douglas-fir tree-ring data to two other Douglas-fir data sets from the Oregon Coast Range and western 309 Cascade Mountains, which would have experienced similar climate conditions but not tsunami inundation. When 310 compared to these control sites, South Beach tree growth is significantly lower in 1700, and reaffirms that the South 311 Beach Douglas-fir growth is unusually low for the region. Thus the timing of the observed growth reductions in the 312 South Beach Douglas-fir stand is consistent with these disturbances being associated with the A.D. 1700 Cascadia 313 megathrust earthquake and the resulting tsunami, subsidence and ground motion. Overall, we think our study further 314 supports the view that tree-ring data is a promising tool for providing insights on the spatial distribution of co-seismic 315 impacts from megathrust earthquakes, as well as potential ground-truth information for tsunami inundation models. 316 317

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Upon acceptance of the manuscript, the Mike Miller, Cape Perpetua, and Marys Peak tree-ring data used in this 319 study will be contributed to the NOAA National Centers for Environmental Information International Tree-Ring 320 Databank, https://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring 321

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The tsunami model can be made available with a request to oar.pmel.tsunami-webmaster@noaa.gov followed by a 323 model software training course provided by the NOAA Center for Tsunami Research 324

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RD prepared the manuscript with contributions from all co-authors. BB and RD collected the tree-ring samples, 326 performed growth disturbance analysis, and wrote the manuscript. YW developed the tsunami model code and 327 performed the simulations, and wrote tsunami section of manuscript. SM drafted initial maps used in manuscript. 328

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The authors declare that they have no conflict of interest. 330

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The authors wish to thank the editor and two reviewers. The research in this paper was sponsored by the 333 NOAA/Pacific Marine Environmental Laboratory, PMEL paper contribution number 5184. Yong Wei's work is 334 funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative 335 Agreement NA15OAR4320063, Contribution No. 2020-1084. All data is available from the authors upon request, 336 without undue reservation, to any qualified researcher. Park, South Beach, Oregon (see Figure 1). Vertical axis shows ring growth in cm, time range covers several decades before 456 and after AD 1700. The color of each growth record was relates to alpha-numeric labels of individual trees shown in legend, 457 with location of trees shown in Figure 3b. Designation "A/B" represents two cores from same tree. Black arrow marks AD 458 1700 date, red arrows highlight the AD 1691, 1738 and 1745 large growth reductions that may have been caused by a 459 significant climatological events. (b) Shows detailed growth record of trees in (a) 4 years before and after 1700 (black arrow). 460 461