The work that I’m going to show you today is part of various collaborative
efforts among multiple government agencies,
academic and private institutions, and many colleagues across the USGS,
coastal and marine, and earthquake hazards programs.
Today I will briefly summarize published results of systematic
margin-wide characterization of Cascadia morpho-tectonic variability
and hazard implications that have recently been published in Geosphere
with my colleague Danny Brothers. And then I will present some
preliminary work offshore northern California using multi-scale geophysics
to image and characterize Late Pleistocene and Holocene deformation
along both cross-shore faults in the shelf and along the deformation front.
Cascadia lacks an instrumental record of large megathrust earthquakes,
making estimates of past and future rupture parameters difficult.
However, we can learn from other seismically active subduction zones
where studies suggest that spatial variations in frictional properties
of the megathrust are linked to tectonic and morphological evolution
of the upper plate and rupture behavior along strike.
We were particularly inspired by the work in Sumatra where they noted that
along-strike changes in prism geometry and vergence coincide with the rupture
boundaries of the 2004 and 2005 megathrust earthquakes.
We took a similar approach in Cascadia, focusing on the outer wedge, located
offshore between the deformation front and the outer arc high.
The reason we focused here is because this is the portion of
the marine floor where destructive tsunamis are generated.
Statistical analysis of outer wedge width and steepness reveal four
distinct regions along the Cascadia margin, denoted by the thick
dashed black lines in the map to the right.
Each of these regions have unique patterns of outer wedge geometry,
structural vergence, as denoted in the middle panel,
and mean profile shape, as denoted in the far right panel.
I want to highlight the distinct end members of this analysis here as
Regions 2 offshore Washington, where we have a large area of landward
vergence, denoted in the red, as well as an area of extension at the shelf break.
And also Region 4 offshore southern Oregon and northern California,
where we have numerous cross-shore faults
and a very steep and narrow outer wedge.
According to critical tapered theory, variations in the geometry and
shape of the wedge reflect changes in the strength
of the wedge and the strength of the décollement.
Since we can’t get at the strength of the décollement with a megathrust
[inaudible] using existing data, we decided to map out first-order
material strength boundaries within the wedge in the form of backstops,
denoted on the right in the map with the
bright green lines and the bright red line.
The arrows indicate the dip direction of those boundaries.
Notice that each morpho-tectonic region has a unique backstop
configuration in terms of distance from the trench, lithology, and geometry.
These backstop boundaries are often associated with megasplay faults on
other margins, such as that activated in the 1964 Alaska megathrust earthquake.
We postulate in this manuscript that the morpho-tectonic variability that we see
in Cascadia reflects both geometric and rheological heterogeneity
within the megathrust system that may control megathrust
behavior by modulating pore pressure and effective stress.
The backstop characterization here provides first-order estimation of
strength contrasts within the wedge and highlights potential megasplay
fault structures that we know can amplify tsunami waves
and decrease travel time to the coast.
In addition, this along-strike variability suggests that the earthquake and
tsunami source characteristics likely vary along the margin and need to
be assessed on a regional basis. For example, if you look at our end
members again, perhaps offshore Washington we might expect a scenario
similar to Tohoku, where you have extension in the upper plate and then
slip to the trench further out in the prism. While offshore northern
California, we might expect a scenario similar to that in Kaikoura, where we
have complex rupture of upper plate and the megathrust.
This margin-wide characterization provides the framework for more
detailed work offshore northern California that I will talk about now,
focusing on preliminary results from geophysical imaging to characterize
Late Pleistocene and Holocene deformation.
So here is a 3D perspective view of southern Cascadia looking north
from the Mendocino Triple Junction. The bathymetry is shown
in a rainbow of colors. Along this part of the margin, we have complex
tectonic interaction involving east-west compression along the deformation
front, north-south compression related to the northward-migrating triple
junction, and northwest-southeast- directed transpression associated with
a rotating forearc interacting with the northern extent of
transform motion along the San Andreas Fault system.
As a reminder, this is Region 4, one of the end members in our morpho-
tectonic analysis, that is characterized by very a steep and narrow outer wedge.
This outer wedge is separated from the inner wedge by the outer arc highs
denoted with the white line. The backstops, denoted in black,
and the outer arc high within the wedge bend eastward as they approach the
Mendocino Triple Junction and come onshore in the Humboldt area.
In order to better characterize the Late Pleistocene/Holocene deformation
in this region, we have collected new high-resolution seismic reflection data
shown in the black lines, sediment cores shown in the blue circles,
and we’ve installed two seafloor GPS-A stations straddling
the deformation front offshore Crescent City,
shown by the orange circles. Today I will highlight preliminary
work along both the shelf and the deformation front.
So here is a map view perspective of that same area that we saw
in the last slide. This is showing a preliminary map
of Quaternary faults and folds, denoted in black.
The mapping is based on integration of new high-resolution multi-channel
sparker and CHIRP imaging, combined with legacy crustal-scale
data and a new 30-meter bathymetric DEM as a base map.
It is now published and publicly available on ScienceBase.
On the next slide, I will focus in on the
cross-shore faults along the shelf in the area of the black box.
Here I’m showing a Google Earth image in the background looking
to the west and offshore where our sparker seismic data has beautifully
captured Neogene and younger strata to sub-seafloor depths of greater than
500 meters that are locally folded and faulted along a series of
cross-shore fault zones. I’ve highlighted two sub-seafloor horizons here.
The blue marking the 500,000-year-old Hookton nonconformity and the
orange marking the transgressive surface that records erosion and
sea level rise following the last glacial maximum
that occurred approximately 21,000 years ago.
We used straddle geometry on CHIRP profiles to identify and
map the regional transgressive surface using a strain marker
for characterizing for offshore fault deformation.
The stars here denote structures that are offset and/or deformed
this transgressive surface. And I know that surface is
difficult to see here in this view, so we’ll zoom in here.
Here I’m zooming in on the transgressive surface of erosion on that
same profile and showing the modern seafloor in black for context.
We see two modes of deformation here. One, long-wavelength features that
likely reflect active folding. And punctuated vertical offset ranging
from 5 to 10 meters coincident with fault traces of the Table Bluff,
Mad River, Bald Mountain/Big Lagoon, and Lost Man Fault zones
imaged in MCS and CHIRP seismic data.
Now, the transgressive surface ages at depths between 50 and 100 meters
below sea level along this seismic profile are estimated to be between
11,000 and 15,000 years old, based on sea level curves,
which result in a maximum vertical deformation rate along these –
on these structures between 0.3 and 0.9 millimeters per year.
It should be noted that these rates are less than those estimated on land
and potentially reflect a change from compressional to transpressional
tectonics across the shoreline as these faults either die off
and/or bend to the north along the shelf break.
Now I’m going to take a closer look at deformation in the outer wedge along
the deformation front in the area of the black box.
Here I am showing a sparker profile that crosses the deformation front
along the red line denoted in the map on the left.
It crosses the deformation front, the dynamic backstop boundary,
and the outer arc high. Thinning of the most recent sediment
package onto the anticlines, denoted by the red arrow,
suggest active uplift along these structures and the underlying thrust
faults, including the most seaward thrust fault along the deformation front.
In the next slide, I will zoom into this portion of the line to take a better look.
So here, again, we’re looking at our sparker seismic data
at the deformation front. You see the flat-line strata
above the incoming oceanic plate on the left and the onset of
deformation along the interpreted faults in black.
The thinning of the most recent sediment package onto this first
anticline suggests activity along the underlying thrust fault.
Now, we ask, could this represent geologic evidence potentially
of trench-breaching rupture? That would depend on whether the
deformation we see was the result of aseismic creep or coseismic movement.
Wouldn’t it be nice to take a closer look at this region in much more
detail and get a better look at the style of deformation?
Well, thanks to our colleagues at MBARI, we were able to take
a much closer look at this part of the deformation front at 1 meter –
using 1-meter resolution AUV bathymetry
and CHIRP sub-bottom imaging.
So here is a map view of that part of the deformation front
in our 30-meter bathymetric compilation.
Now, in case you can’t see it, the deformation front
is marked by the blue arrow.
Here is the same view with 1-meter AUV bathymetry.
You can now clearly see the deformation front.
You may also see the beautiful landslide head scarps and runout
along the deformation front – the giant hill discussed earlier.
We talk about their significance.
But I am more interested in linear features – these spark-like
lineations denoted by the red arrows.
Here I’m showing a preliminary geologic interpretation based on
the AUV imagery and associated CHIRP sub-bottom imagery.
The white line in the map shows the location of the
CHIRP profile shown below. What we see here is that those
lineaments in the bathymetry reflect scarps that formed by
both compressional and extensional structures
imaged in that CHIRP imagery.
And, since I know you can’t really see anything at this scale, if I zoom in,
you can clearly see vertical deformation along thrust faults
within the first anticline and normal faults in the second anticline.
And the apparent constant throw suggests single-event
offset of between 2 to 4 meters occurred in the final anticline.
So now, zooming back out, with the sparker data to
provide context, we suggest that the combination of thinning
of recent strata onto the anticline and brittle faulting within the anticline
suggest coseismic slip occurred along the underlying thrust fault,
supporting the suggestion that slip to the trench has occurred in this region.
We look forward to combining the detailed geologic observations
in the previous slide with GPS-A seafloor measurements in the coming
years in the same location to get a better, more comprehensive picture
of shallow fault behavior in southern Cascadia, which is inferred
to be locked based on the latest modeling by Lindsey et al.
using onshore GPS data [inaudible].
So stay tuned for more observations and more results coming soon. Thank you.