Beneath the surface of coastal lowlands: archives of mid- to late-Holocene subduction zone earthquakes

Coastal lowland marshes (Figure 1) are more than just muddy shorelines where your feet get stuck as you try to walk around. Geologically, coastal marshes are low-energy settings where fine-grained sediment is deposited during times of gradually rising sea level. Similar to stratigraphic sequences that a geologist would see in a natural exposure or a road cut, the stratigraphic sequences below wetland marsh surfaces record changing geologic environments and a slice of geologic history in the late Holocene. In this vignette, we will discuss the stratigraphy that is preserved in coastal marsh sequences, and we will address two specific questions. First, what are the processes under which coastal marsh stratigraphic sequences are deposited? Second, how can a conceptual understanding of these processes be used to infer the history of subduction zone earthquakes along coasts that border subduction zones?

Processes under which coastal marsh stratigraphic sequences are deposited
Sequences of fine-grained sediment (clay, silt, fine sand) are preserved in coastal settings that are low energy (protected from the open ocean) and subject to gradually rising sea level. Gradually rising sea level allows for space that can continuously accommodate sediment deposition (hence the term, "accommodation space"). The late Holocene (last 5,000 years) is a period of salt marsh aggradation (build-up) in most temperate coastal settings because this has been a time of gradual rise in relative sea level (van de Plassche, 1991).

Coastal marsh stratigraphic sequences chronicle the depositional environment during sea level rise and are the basis for constructing relative sea level curves, which depict the time-dependent rise of relative sea level (Figure 2). The term "relative" connotes that the rise of the sea is only relative to the land. Rising sea level may be the combined result of increasing ocean volume from melting ice sheets (that causes a rise in the ocean level) and subsiding crust (that may also cause a rise in ocean level), or, relative sea level rise could be the result of increasing ocean volume and uplifting crust (that causes a relative fall in ocean level) in which the ocean level rise outpaces the rise of the land surface. In any case, it is important to appreciate that observed sea level changes are always relative and are specific to the coastal site in question.

Subduction zone coasts
Relative sea level curves for the period of the last 6,000 years along subduction zone coasts (Figure 2) are influenced by two independent processes. One process is the increase in ocean volume (increased ocean mass) that applies a load to Earth and produces a viscoelastic response of the mantle resulting in a predictable, gradual relative sea level change at any specified coastal locality (Clark et al. 1978; Peltier et al., 1978). The second process is strain accumulation and release at subduction zones (the subduction zone earthquake cycle) that results, at most coasts bordering subduction zones, in abrupt subsidence during subduction zone earthquakes (hence termed "co-seismic subsidence") (Atwater, 1987; Atwater and Hemphill-Haley, 1997) and gradual uplift of the land surface as strain accumulates between subduction zone earthquakes (Atwater and Hemphill-Haley, 1997).

In temperate coastal settings such as on the margins of the Cascadia subduction zone (west coast, USA), the stratigraphic record of a subduction zone earthquake is a buried coastal marsh wetland soil (Figure 3) (Atwater, 1987). A subduction zone earthquake abruptly subsides a tidal marsh into the subtidal zone (abrupt relative sea level rise) such that tidal mud is deposited on, and buries, a marsh soil. The coseismic subsidence creates the accommodation space for the deposition of tidal mud. The subduction zone earthquake is followed by another build-up of strain on the subduction zone that causes tectonic uplift and hence a relative sea level fall. Because sea level gradually rises over the course of multiple earthquakes (Figure 2), it is clear that the strain accumulation-induced sea level fall is the smaller signal than the long-term relative sea level rise due to both coseismic subsidence and Earth's response to an increasing mass of water in the ocean basins. The tidal mud that buries the marsh soil eventually aggrades to the level that the mud is exposed during lower tidal levels and then plants colonize the tide flat. With further sea level rise, the tide flat develops into a marsh with a marsh soil.

Repeated subduction zone earthquake cycles will result, in a coastal setting on the Cascadia subduction margin, in an alternation of abrupt relative sea level rise and gradual relative sea level fall (Figure 2). Each earthquake will cause an instance of coseismic subsidence that instigates marsh soil burial (Figures 4 and 5). These buried-soil sequences (Figures 4 and 5) will be preserved in an environment of long-term, gradual sea level rise (Atwater and Hemphill-Haley, 1997; Kelsey et al., 2002). On the Cascadia subduction zone, buried-soil sequences may span more than 6,000 years and represent as many as twelve subduction zone earthquakes (Witter et al., 2003). Repeated sequences of buried tidal marsh soils, representing a succession of subduction zone earthquakes, also have been documented along coasts overlying subduction zones in Chile and Japan (Cisternas et al., 2005; Kelsey et al., 2006).

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