[Contents]
 
Results and Interpretations
     

The application of radar stratigraphic analysis (distinct signature patterns) on the collected data provided the framework to investigate the geometry of the buried tsunami deposits (Jol and Bristow, 2003).

The Tsunami Sand Layer (TSL), as defined by Peterson (Peterson et al., in progress) is identified within the core logs as the anomalous presence of moderately- to well-sorted sand and/or granules layers of >0.25 cm in cumulative thickness that are found in peat and silt/clay deposits (>50 cm thickness). They generally thin with increasing distance landward and sometimes fine upward or are interlayered with silt or detrital organic layers. TSL are established in wetland and floodplain locations by broad, lateral continuity and established near channel forms by longitudinal continuity and by upstream thinning.

Cannon Beach (map)

A near-surface velocity of the 0.07 m/ns was used for all Cannon Beach data.

Larch Street

Larch Street runs along the barrier ridge, parallel to the shore and data was collected from south to north (Figure 8). Return signal penetration reached approximately 9 m subsurface depth using 100 MHz antennae. The profile (Figure 9) images hummocky reflections along with horizontal to sub-horizontal shallow cut and fill reflections (yellow) (2 to 4 m subsurface) in the upper 5 to 6 m of the barrier ridge deposits between 40 and 120 m. A continuous horizontal reflection (orange) at 140 to 260 m (1 to 2 m subsurface) is interpreted as a possible paleosol. The internal stratigraphy of the ridge (red) can be seen in the north (right) end of the profile with dipping on the north end of the ridge.

larch

Figure 8. Starting data collection on Larch Street (looking north).

Figure 9. The profile of Larch Street shows cut and fill features (yellow), a continuous reflection (orange) and the internal stratigraphy of the ridge (red).

Second Street

Second Street (Figure 10) runs across the barrier ridge (perpendicular to shore) and data was collected from west to east. Return signal penetration reached approximately 10 m subsurface depth using 100 MHz antennae. A continuous horizontal reflection (orange) is imaged, likely indicating the ground water surface (Figure 11). Possible cut and fill features (yellow) (2 to 4 m thick) are imaged in the barrier ridge as hummocky reflections followed by horizontal to sub-horizontal reflections (Figure 11).

2nd

Figure 10. Data was collected from west to east on Second Street (looking east).

Figure 11. The Second Street profile shows a continous reflection in orange and possible cut and fill features in yellow.

Spruce Street

Spruce Street (Figure 12) runs along the barrier, parallel to the shore, and data was collected from south to north. Return signal penetration reached up to 9 m subsurface depth using 100 MHz antennae. The Spruce Street data (Figure 13) shows hummocky reflections, along with horizontal to sub-horizontal reflections indicating possible shallow cut and fill structures (2-3.5 m subsurface) between 60 and 140 m along with other cut and fill at 280 to 320 m and 440 to 500 m. A continuous horizontal reflection (1 to 3 m subsurface), possibly a paleosol, is imaged at 420 to 660 m.

spruce

 

end spruce

Figure 12. Data was collected on Spruce Street from south to north. Image on left looks south around middle of line. Image on right looks north at end of line.

Figure 13. Possible cut and fill features, along with a possible paleosol are imaged in the Spruce Street profile.

Spruce Street Marsh

A transect was run from east to west across the marsh, perpendicular to the shore (Figure ). The transect was run using both the 225 and 450 MHz antennae. Return signal penetration reached up to 1.5 m using 225 MHz antennae and to nearly 1 m with 450 MHz antennae. The 225 MHz profile (Figure 15) shows continuous horizontal reflections to a depth of about 50 cm interpreted as sediment deposits along with hummocky reflections followed by horizontal to sub-horizontal reflections, interpreted as possible cut and fill features from 3 to 25 m (yellow, also shown in orange on the 450 MHz profile). Inclined reflectors dip seaward from the 62 m position to the end of the 225 MHz profile at the 72 m position possibly representing scour and fill of the wetland deposits closest to the barrier ridge. The 450 MHz profile (Figure 15) also shows possible cut and fill features (yellow).

marsh

Figure 14. A line was run in the Spruce Street marsh from east to west using both the 225 and 450 MHz antennae.

Figure 15 . Possible cut and fill features (yellow) and sediment deposits are shown in the Spruce Street marsh profiles.

Spruce Street Marsh Liquefaction Site

Data was collected over a liquefaction site at the northeast corner of the Spruce Street marsh with data collected along two transects: one running north to south and one east to west. The transects were run using both the 225 and 450 MHz antennae. Return signal penetration reached to 1.5 m using 225 MHz antennae and to 1 m with 450 MHz antennae. Hummocky reflections followed by horizontal to sub-horizontal reflections are interpreted as possible cut and fill features (Figures 16 and 17). These features are shown below the continuous horizontal reflection indicating sediment deposits.

Figure 16 . These profiles of the Spruce Street marsh liquefation site were run from north to south and show possible cut and fill features (yellow). Sediment deposits are also shown.

Figure 17. The east to west profiles show similar reflections.

Cannon Beach Summary

The GPR profiles document erosion of the sand ridge and back-barrier marsh where overtopping paleotsunami surged landward. Core sampling completed in the area indicate that the 1700 AD tsunami did not overtop the barrier ridge but three tsunamis which came before the 1700 AD tsunami did overtop the barrier (Peterson et al., in progress). Thin cut and fill sequences are located locally in the overtopped barrier ridge as established by shore-parallel GPR profiles. Scour is indicated in the wetlands at the landwards edge of the barrier ridge imaged in a shore-normal GPR profile. The evidence indicates the 1700 AD tsunami entered the back-barrier wetlands after passing through the Ecola Creek channel.

Seaside (map)

A near-surface velocity of 0.07 m/ns was used for all Seaside data. The GPR data corresponds with the core logs, shown in Table 2.

12th Avenue Bridge

The 12th Avenue site is approximately 75 m north of the 12th Avenue bridge along the Necanicum River (Figure 18). Two transects were run, parallel (12THR, south to north) and perpendicular (12THC, east to west) to the channel margin (and therefore to the assumed 1964 tsunami flow) on the west side of the river. The lines were run with both the 450 and 900 MHz antennae. Return signal penetration reached up to 1.0 m using 450 MHz antennae and to 0.5 m with 900 MHz antennae. 12THC (Figure 19) shows sub-horizontal reflections dipping eastward near the surface while 12THR (Figure 20) shows southward dipping reflections indicating tsunami deposits.

east12
 
west12

Figure 18. Data was collected on the west bank of the Necanicum River (river seen in background of left image) north of the 12th Avenue bridge. Lines were run east to west (A in right image and seen in left image) and south to north (B in right image).

Figure 19. The east to west transects (12THC) show eastward dipping reflections along the Necanicum River. The first image shows 450 MHz data and the second shows 900 MHz data.

Figure 20. The south to north transects (12THR) show southward dipping reflections. The first image shows 450 MHz data and the second shows 900 MHz data.

Pine Street (Horning)

The Pine Street site is situated on the Neawanna Creek channel margin and is the site of observed tsunami flow (1964) (Figure 21). Transects were run both perpendicular (HORNC, south to north) and parallel (HORNR, west to east) to the channel margin (and therefore perpendicular and parallel to the observed tsunami flow). Transects were run using both 450 and 900 MHz antennae. Return signal penetration reached up to 1 m using 450 MHz antennae and to 0.4 m with 900 MHz antennae. Nothing significant was imaged at this site. The profiles consist mostly of continuous horizontal reflections with some sub-horizontal reflections (Figure 22).

westhorn

Figure 21. HORNC (line A) was run south to north, perpendicular to the Neawanna Creek channel margin and HORNR (line B) was run west to east, parallel to the channel margin.

Figure 22. The four profiles show mostly continuous horizontal reflections. 1) HORN-C450, 2) HORN-C900, 3) HORN-R450, 4) HORN-R900.

North Roosevelt Drive (Plum)

The North Roosevelt Drive site is on the west side of the Neawanna Creek (Figure 23). Transects were run parallel (PLUMR, north to south) and perpendicular (PLUMC, west to east) to the observed tsunami flow over the top of the point bar (1964). The transects were run with both the 450 and 900 MHz antennae. Return signal penetration reached up to 0.7 m using 450 MHz antennae and to 0.3 m with 900 MHz antennae. Possible cut and fill features are shown as hummocky reflections followed by horizontal to sub-horizontal reflections. Several dipping reflections are imaged in the profiles, dipping east in PLUMC and south in PLUMR. The profiles are shown in Figures 24 and 25.

plum east   plum north

Figure 23. PLUMC was run west to east as shown on left while the other line, PLUMR was run north to south, shown in the right figure looking north.

Figure 24. Possible cut and fill features and several eastward dipping reflection patterns are visible in the profile of PLUM-C450.

Figure 25. The other profiles for North Roosevelt Drive show similar reflections. 1) PLUM-C900, 2) PLUM-R450, 3) PLUM-R900.

Avenue K (pour-over site)

The Avenue K site is along the Neawanna Creek and is the location of an assumed paleotsunami pour-over flow (1700 AD) (Figure ). Previous coring had established a tsunami sand sheet fan with sediment 10 to 20 cm thick in this area (Jol and Peterson, 2006). Transects were run parallel (KAVER, west to east) and perpendicular (KAVEC, north to south) to the assumed pour-over flow from Avenue K. The transects were run using the 225 MHz antennae. Return signal penetration reached up to 1.5 m. Imaged layers were horizontal from 0 to 0.5 m depth. Possible cut and fill features are imaged around 1 m depth, shown as hummocky reflections followed by horizontal to sub-horizontal reflections.

kave

Figure 26. One line (KAVEC) was run north to south at this site, perpendicular to the assumed Avenue K pour-over flow (looking south).

kave creek

Figure 27. A small creek running through the area approximately 33 m into KAVEC.

Figure 28. The profile of Avenue K, running north to south, shows some possible cut and fill features are shown in yellow.

Figure 29. Possible cut and fill features can also be seen in the profile running west to east.

     
[Previous Section]
[Contents]