Hi, I’m Chris DuRoss with the USGS in Golden, Colorado.
I’m excited to share new research on a portable optically stimulated
luminescence age map of a paleoseismic exposure.
I’d like to acknowledge my co-authors, Ryan Gold with the USGS in Golden,
Colorado; Harrison Gray with the USGS Luminescence Dating
Laboratory in Denver, Colorado; and Sylvia Nicovich with the
Bureau of Reclamation in Denver, Colorado.
Okay, so, as those who have worked with paleoseismic data know
geochronological data can be messy.
Noisy age data can typically relate to the depositional
and geomorphic setting of the trench or exposure.
Mechanisms, either known or unknown, of sediment transport and deposition.
Age inheritance, such as recycled
charcoal, and post-depositional modification, including burrowing.
These factors lead to a poor understanding of the contextual
uncertainties of samples and the resulting ages,
inconsistent methods of constructing Bayesian models, and over- and
under-constrained models of earthquake timing.
This is illustrated in the conceptual figure here, which plots earthquake
timing uncertainty versus the number of ages.
This is simply meant to illustrate that models with limited ages
do a poor job of refining earthquake timing uncertainties.
For example, at some stages, timing uncertainty may actually
increase with an increase in the number of samples.
So considering these processes and uncertainties, how do we develop
sampling strategies for geochronology that maximize the accuracy and,
critically, the reproducibility of paleoseismic rupture histories.
In this presentation, I’ll argue that portable optically stimulated
luminescence, or portable OSL, can help.
I’ll go through the background of the method, results from a field site,
and end with some practical applications.
Okay, so back to our conceptual diagram.
This is where we want to be, highlighted in yellow, constructing
Bayesian models of earthquake timing based on abundant age control.
So what is portable OSL? Portable OSL is the determination of
bulk luminescence as measured in a portable reader from small samples.
The measurement is made without the usual procedures used for
traditional OSL, for example separating quartz grains
of a uniform grain size, and thus does not yield absolute ages.
However, with constant lithology, dose rate across the exposure,
and some pretreatment, bulk luminescence can reveal
relative age and also show zones of partially bleached sediment.
We applied these methods to a natural exposure of the Wasatch Fault at
the Deep Creek site in central Utah. The figure in the upper right shows
a slope shade map of the site with the Wasatch Fault scarp on a
Holocene alluvial fan surface marked by black arrows. The red arrow marks
a natural exposure of the fault shown here in the photograph.
At the site, several meters of exposed alluvial fan gravel
have been vertically displaced about 1.8 meters by a single
prehistoric surface-rupturing earthquake.
We studied the Wasatch Fault exposure hidden behind the gray trap
here and highlighted by the white box.
The exposure consisted of faulted alluvial fan gravel
with a 30-centimeter-thick paleosol.
The paleosol is faulted and buried by about a 1.5-meter-thick
scarp drive colluvial wedge. The colluvial wedge consists of
moderate- to steeply dipping silt, sand, and gravel interbeds,
shown there with the dashed lines.
Our principle goal was to test whether we could generate an age map for
the exposure using portal OSL. So related questions included,
are we able to resolve spatial trends in the portable OSL data,
how do sedimentary and pedogenic processes affect the results,
and do the data help us understand earthquake timing.
To help make sense of how portable OSL data relate to age, we also extracted
samples for radiocarbon and traditional OSL dating, with the results shown here
in thousands of years before present. In total, 23 radiocarbon ages shown
here by the black pluses, and 11 traditional OSL ages shown by
the white pluses, constrain the timing of sediment deposition
and soil formation at the site. These results suggest that alluvial fan
deposits at the site are as old as about 13,000 to 7,000 years in the footwall
and 8,000 to 5,000 years in the hanging wall.
Paleosol formation occurred between around 4,000 years and
shortly after around 1,000 years. With some exceptions that
I’ll discuss in later slides, colluvial wedge sediments
were deposited mostly in the past 1,000 years.
With then extracted 342 samples for portable OSL, shown here
by the white circles. The samples were arranged in a grid across the site
without regard to the stratigraphic contexts or faults.
Just a few notes about these samples. We collected small 3- to 4-centimeter-
diameter samples using film canisters in darkroom conditions.
The samples were spaced 10 centimeters vertically and
about 15 centimeters horizontally. We sieved the sediment on-site,
and off-site weighed similar-size aliquots before measuring
in the portable OSL reader. We normalized the photon counts
by sample weight because of the extreme range in low to high
OSL results, plot the log of the photon counts.
The portable OSL results are colored here by luminescence.
In this and later slides, the color scheme will be the same,
gradient from yellow to blue, with yellow indicating the
greatest luminescence signal and blue the least.
These results correspond well with the radiocarbon and traditional OSL ages,
thus we used a simple linear regression shown here at the lower left to relate
photon counts from the portable OSL reader to age.
Here’s a surface interpolation based on the portable OSL results.
Again, the color grades from yellow to blue,
showing decreasing OSL signal and thus age.
Here the radiocarbon and traditional ages are also shown and colored in the
same way as the portable OSL map, and you can see the general agreement
across the alluvial fan, paleosol, and colluvial wedge.
In the next series of slides, I’ll focus just on the portable OSL age map
and highlight fine to coarse gradients in age across the exposure.
For example, as expected, the alluvial fan age of both the footwall and
hanging wall decreases vertically, but not horizontally.
We also observe a strong age gradient from the paleosol to the colluvial wedge
and also across the buried free face from the footwall to the hanging wall.
However, we find more nuanced age complexity in the paleosol.
For example, on the right side, adjacent to the fault and buried free face,
the portable OSL map suggests rapid burial of the paleosol.
In comparison, at a more fault-distal position,
the map suggests continued soil formation at the top of the paleosol
and possibly after the rupture may have occurred.
Within the wedge, we note a prominent area of poorly bleached sediment
close to the fault and buried free face. Sediment in this part of the wedge
was likely not fully exposed to sunlight during transport, either as
the result of block formation or a short transport path.
Elsewhere within the wedge, the age decreases laterally
from the fault and vertically toward the surface, as expected.
I just wanted to quickly show that, with these data, we can quantify
horizontal and vertical trends in the portable OSL data.
In all cases shown here, we’re plotting the model
portable OSL ages or, that is, the OSL photon counts converted to
age using our linear regression versus distance.
The left column highlights horizontal trends as portable OSL age is plotted
against distance from the fault. So 1 meter on the X axis indicates
a meter outboard of the fault. The data are shown in the upper figure,
and in the lower figure, grouped into 0.5-meter distance bins.
Horizontal trends in age are especially apparent in the lower figure.
For example, the blue and orange arrows show that the colluvium age decreases
laterally with distance from the fault, whereas, red and black arrows mark
more laterally constant paleosol and alluvial fan ages.
In the right column, distance above or below the paleosol is plotted
against age to highlight vertical trends in the data. In these figures, the paleosol is
marked by the shaded area between the solid and dashed and dotted lines.
In these figures, 1 meter on the Y axis is a meter above
the top of the paleosol and negative 1 meter is a meter below it.
In the lower panel, the portable OSL results
are grouped into 0.1- to 0.3-meter distance bins.
In this case, vertical trends in portable OSL are apparent.
For example, the red and black arrows show that the alluvial fan and paleosol
ages decrease consistently toward the ground surface.
In contrast, the blue arrow shows that the colluvium age
is mostly constant vertically.
Finally, we can use these results to explore how the spatial age
variability relates to earthquake timing.
Here we compare two earthquake horizons.
The first is the stratigraphic event horizon, or stratigraphic position
at which sediments are interpreted as pre-dating or post-dating the
earthquake, shown here by the white arrows.
We also define a chronologic event horizon, or position at which sediments
are older or younger than the earthquake.
Here we show three chronologic event horizons based on Bayesian model
variations with the mean earthquake times converted to portable OSL age
contours and shown on the age map. We find that the portable OSL contours
at around 500 to 700 years, which are shaded gray and labeled
as the apparent chronologic event horizon, have the greatest
spatial overlap with the stratigraphic event horizon.
However, notably, the stratigraphic and chronologic event horizons
diverge close to the fault. This suggests that there are sedimentary
facies within the fault-proximal colluvial wedge that were not mappable
from sedimentary texture alone. Although these sediments are
interpreted as younger than the earthquake based on the
stratigraphic event horizon, the chronologic event horizon
suggests a pre-earthquake sediment age.
This shows that portable OSL can provide important chronological
context for interpreting radiocarbon and traditional OSL ages as well as
Bayesian models of earthquake timing and that these timing interpretations,
based on stratigraphy alone, can be misleading.
Okay, at this point, I’d like to shift gears and talk about practical
applications of this method. If you recall, in portable OSL,
we were measuring luminescence from bulk sediment samples.
Because we don’t correct for organics, grain size, or class lithology,
it’s ideal to try to keep variations in these factors to a minimum.
So ideal site conditions would include low variability in sediment texture,
minimal post-depositional modification, consistent dose rate across the
exposure or information on how dose rate changes spatially,
and sediments that are generally suitable for luminescence dating.
Data density is also an important aspect of this.
For the Deep Creek data set, we generated a 10-centimeter
cell raster, however lower-resolution rasters
are also suitable for resolving the spatial age relations.
The figure here shows variations in raster density from 10 centimeters
in the upper left to 100 centimeters in the lower right.
We find that key details in the portable OSL data, such as the strong
age gradient from the alluvial fan, the colluvial wedge sediments, and the
area of poorly bleached sediment in the fault-proximal part of the colluvial
wedge are still resolvable with the 20- to 47-centimeter cell rasters shown here.
Thus, even in a lower density field application,
this method could be useful for interpreting paleoseismic exposures.
The last aspect to consider is sampling strategy.
Although our sampling density is quite high for the Deep Creek site,
we found the lower-density sampling strategies also reproduce the primary
spatial relations observed in the full density data set.
This figure shows portable OSL rasters generated using
subsets of the original data. Points in black were included
in the raster calculation, and those in white were excluded.
So, for example, in the upper left-hand corner, the raster
was generated using every other point, or 50% of the data.
For the Deep Creek site, rasters generated using about 40 to 50%
of the data reasonably reproduce the spatial age relations
apparent in the original results.
In general, sample transects oriented normal to the
stratigraphic contexts are preferable.
Again, this highlights that portable OSL, even with a low-density sampling
strategy of, say, 100 or so samples can be used in a rapid field application.
In conclusion, portable OSL is an additional tool to consider
in paleoseismic investigations. At the Deep Creek site, the portable
OSL age map yields insight into how spatiotemporal age variability
relates to sedimentary and pedogenic processes.
Our work suggests that stratigraphic and chronologic event horizons may diverge,
which has implications for paleoseismic sampling strategies
and Bayesian models of earthquake timing.
In summary, portable OSL, even in a moderate-density field application,
has the potential to provide insight into pre- and post-earthquake
depositional processes and improve the accuracy
of paleoseismic rupture histories. And so finally, a huge thanks to
Harrison Gray and Sylvia Nicovich who spent many hours sampling.
And I’m excited to announce that this research was just released in Geology.
Thank you.