The peaceful hiking trails of the Theler Wetlands at the head of Hood Canal are alive with marsh grasses, blue heron, and bald eagles. In the fall, salmon wash up in the marsh after spawning. The peace and tranquility of this popular preserve and walking trails belie their history of strong shaking, sudden land-level change, and tsunamis that have rocked the marsh in the past. The coastal wetlands of the Salish Sea are rich, diverse habitats, and they are an important focus of coastal ecosystem restoration efforts in our region. However, unknown to many visitors to coastal wetlands, they often preserve in their sediments a rich history of past earthquakes, tsunamis, and strong ground shaking.

Digging down into the sediments of a coastal marsh in the Salish Sea may reveal a layer cake of sediment colors, textures and other characteristics, each providing clues as to the environment in which the sediments were deposited. Paleoseismic study of coastal marshes, where scientists collect, analyze, and interpret sediments from coastal marshes, tells us of their seismic past. Such studies also helps us with the critical work of understanding and preparing for hazards along our populated coastline. Importantly, they tell us that we have been in a long period of quiescence from the most dangerous types of large earthquakes and tsunamis in the Salish Sea. This same message is relayed to us by the many coastal tribes that have made the Salish Sea their home for millennia: their stories describe earthquake shaking and coastal floods that destroyed their communities in the distant past.

What sorts of geologic hazards do coastal wetlands record? Effects of large earthquakes can include the following:

• sudden land-level change. Land can either raise or drop suddenly when slip along a fault deforms the ground surface;
• tsunami deposits. These are formed when an abrupt change in land level on the sea floor generates a tsunami. The tsunamis erode and transport sand that is then left behind as sheets blanketing the marsh;
• strong shaking, which can cause a pressure increase in water-logged sediments below the surface, causing them to erupt onto the surface as sand boils, or cut through marsh sediments as dikes (explained below).

An outcrop in the marsh at the head of Discovery Bay on the Strait of Juan de Fuca reveals at least two distinct sand layers associated with past tsunamis. These sand layers are the light horizontal bands between the darker layers which are the typical material that forms over time in tidal marshes — muddy peat.  Photo by Carrie Garrison-Laney.

Tidal marshes can preserve records in the sediments from all of these events. Abrupt raising or lowering of the vertical position of the marsh in the intertidal zone can cause a lasting environmental change that is recorded not only in the sediments, but also in the marsh plants and microfossils that lived in the marsh before and after the earthquake. Marsh plants prefer specific elevations relative to the tidal range, as do bottom-dwelling phytoplankton and zooplankton. Careful study of plant and plankton fossils in the sediments of a salt marsh can provide insights about land level change during past earthquakes.

Tsunamis rush across shallow tidal flats and erode and transport sediments, depositing the sediment, as well as any organisms that lived in it, in a new location farther inland. Tidal marshes, with their thick layer of vegetation, are good at trapping those sediments, and daily tidal currents are unlikely to erode them away. As time goes on, these tsunami deposits stand out in the marsh sediments as characteristic sand layers that are quite different in appearance from the “business as usual” peaty marsh deposits. The presence of a tsunami deposit along with evidence of land level change is especially compelling, as it suggests that the earthquake caused the land level change and the tsunami.

In some settings, strong shaking causes sands from deep below the surface to undergo liquefaction and be erupted onto the marsh surface. As the water and sediment flow toward the surface, if sometimes leaves traces. These are called dikes when the trace cuts vertically across horizontal layers. They are called sills when they travel horizontally between layers of sediment. When the liquified sediment finally reaches the surface, it can cause a “sand volcano,” also called a boil. These distinctive features, like tsunami deposits, can sometimes be seen preserved in tidal marsh sediments. Several coastal wetlands around the Salish Sea preserve multiple episodes of earthquake shaking, that represent repeated earthquakes. These distinct earthquake histories can be pieced together with careful observations of cross-cutting relationships of sand dikes.

where in the salish sea can you find coastal wetlands that tell these stories?

Since the late 1980s, when the first observations of Cascadia earthquake evidence on the Pacific Coast were first being studied, tidal marshes around the Salish Sea have been searched for similar evidence of past earthquakes.  The map of wetlands that show evidence for past geologic upheaval is littered with sites in the Salish Sea.  These sites (along with other scientific measurements and observations) have helped geologists build a database of past earthquakes and tsunamis generated by the network of faults that underlie and abut the Salish Sea.

Knowing the past seismic activity in an area helps to understand how frequently a fault may experience earthquakes, and how big past earthquakes have been.  This is essential information to prepare for what a future earthquake and tsunami might be like.

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