159
boulders. Maybe, this graveyard cleaning was done to scare us (very successful!).
The other student swore he would never return, but my scientific pursuits required
me to conduct more research there a few years later. So, like any smart professor…
I sent my students! After hearing my story, they wisely found a different way to the
marsh site and successfully resampled without much trouble or encounters with
graveyards or officers of the law.
The last giant megathrust earthquake on the west coast of the contiguous
U.S. occurred in 1700. Recurrence intervals derived from the geologic record show
that these events reoccur on timescales as short as 200 years and as long as
1000 years. So, we are either past due by ~100 years or still have ~700 years to go.
If you are ever in coastal Washington, Oregon, California (Alaska, Japan, Chile,
Indonesia, etc.) and you feel the Earth move beneath your feet, move inland and
upland to quickly seek out a safe, stable elevation.
How do we use these geologic records to get at past earthquake characteristics?
Well this involves the shells of single-celled marine organisms called foraminifera
(“forams” for short). There are thousands of varieties of these animals, living in different marine environments. The salt marsh forms are agglutinated (see the Kemp
and Horton chapter), which means they use organic cement and sand grains to make
their protective shell. They range in size from the width of a hair to a small bread
crumb and are quite particular about where they live. For example, one foram species only likes to live at the upper landward edge of a salt marsh around the tree line,
whereas another foram species only lives at lower elevation on tidal flats. The distribution of each species is dominantly based on the duration of tidal exposure (time
without being bathed in salt-water) they can withstand. Because of this detailed
zonation of species across a marsh surface, sedimentation associated with earthquakes record changes in the foram community, also known as the assemblage.
Higher salt-marsh species underlying lower salt-marsh species reflect the change
from higher marsh pre-earthquake to lower marsh post-earthquake. This is caused
by earthquake-induced subsidence. And because these elevation zones are robust
and quantifiable, we can actually use the change in elevation (pre-earthquake to
post-earthquake) to estimate the amount of subsidence the land experienced during
past earthquakes. For example, the site we worked on in Oregon experienced about
2 feet (0.6 m) of subsidence from the AD 1700 earthquake. Why would we want to
know how much past earthquakes made the land move vertically? Well, it helps
earthquake scientists gain a better understanding of how the Earth works and details
of the earthquake deformation cycle. It also provides an estimate of potential earthquake magnitude. If we can reconstruct several earthquake events over a large geographic area we can gain valuable information on earthquake recurrence intervals.
Further, these data help to refine hazard assessment, planning, and mitigation –
which in turn can help inform evacuation of people from an earthquake/tsunami area.
By the way, the officer never did come check on us, and we never did see
him again.
10 Earthquake-Driven Coastal Change: Ghost Forests, Graveyards…
boulders. Maybe, this graveyard cleaning was done to scare us (very successful!).
The other student swore he would never return, but my scientific pursuits required
me to conduct more research there a few years later. So, like any smart professor…
I sent my students! After hearing my story, they wisely found a different way to the
marsh site and successfully resampled without much trouble or encounters with
graveyards or officers of the law.
The last giant megathrust earthquake on the west coast of the contiguous
U.S. occurred in 1700. Recurrence intervals derived from the geologic record show
that these events reoccur on timescales as short as 200 years and as long as
1000 years. So, we are either past due by ~100 years or still have ~700 years to go.
If you are ever in coastal Washington, Oregon, California (Alaska, Japan, Chile,
Indonesia, etc.) and you feel the Earth move beneath your feet, move inland and
upland to quickly seek out a safe, stable elevation.
How do we use these geologic records to get at past earthquake characteristics?
Well this involves the shells of single-celled marine organisms called foraminifera
(“forams” for short). There are thousands of varieties of these animals, living in different marine environments. The salt marsh forms are agglutinated (see the Kemp
and Horton chapter), which means they use organic cement and sand grains to make
their protective shell. They range in size from the width of a hair to a small bread
crumb and are quite particular about where they live. For example, one foram species only likes to live at the upper landward edge of a salt marsh around the tree line,
whereas another foram species only lives at lower elevation on tidal flats. The distribution of each species is dominantly based on the duration of tidal exposure (time
without being bathed in salt-water) they can withstand. Because of this detailed
zonation of species across a marsh surface, sedimentation associated with earthquakes record changes in the foram community, also known as the assemblage.
Higher salt-marsh species underlying lower salt-marsh species reflect the change
from higher marsh pre-earthquake to lower marsh post-earthquake. This is caused
by earthquake-induced subsidence. And because these elevation zones are robust
and quantifiable, we can actually use the change in elevation (pre-earthquake to
post-earthquake) to estimate the amount of subsidence the land experienced during
past earthquakes. For example, the site we worked on in Oregon experienced about
2 feet (0.6 m) of subsidence from the AD 1700 earthquake. Why would we want to
know how much past earthquakes made the land move vertically? Well, it helps
earthquake scientists gain a better understanding of how the Earth works and details
of the earthquake deformation cycle. It also provides an estimate of potential earthquake magnitude. If we can reconstruct several earthquake events over a large geographic area we can gain valuable information on earthquake recurrence intervals.
Further, these data help to refine hazard assessment, planning, and mitigation –
which in turn can help inform evacuation of people from an earthquake/tsunami area.
By the way, the officer never did come check on us, and we never did see
him again.
10 Earthquake-Driven Coastal Change: Ghost Forests, Graveyards…