[silence]
Thank you, NEHRP organizers, for giving me the opportunity
to present this work on the Santa Cruz Mountains section
of the northern San Andreas Fault.
This work started the final year of my postdoc with Roland Bürgmann
at UC-Berkeley, and the project is my first funded NEHRP grant as
faculty at San Jose State University. So it’s near and dear to my heart.
And, though I am giving the talk, Katherine Guns,
my then-undergraduate intern, completed much of the work
I’m presenting today.
Katherine recently completed her Ph.D. and is now a postdoc at Scripps.
As you will see, she and this project has come a long way.
In this talk, I’m going to summarize our work to estimate how Holocene slip
rates on the Santa Cruz Mountains section of the San Andreas Fault.
I’ll start with why we started the project, how we estimated slip rates,
and discuss the implications of our work.
[rustling sounds]
[silence]
Why we study a fault slip history, for all those engineers out there,
is because slip rate data is a first-order input into the logic tree used for
earthquake probability models. It is a direct input into deformation models
used to estimate the probability of a magnitude 6 or greater earthquake
in the next 30 years on an individual fault.
So this is just a logic tree that shows how geologic data, or slip rate data,
is used into these probability models.
And why do we study the Santa Cruz Mountains section
of the San Andreas Fault? Well, as you can see in this model,
the deformation models, where we provide fault slip to calculate
seismic moment, there are slip rate data for the northern section
of the San Andreas Fault, the central section of the San Andreas Fault,
but the southern section does not have any slip rate constraints.
So, if we focus into the northern San Andreas Fault,
we can see that it’s separated into three main sections.
These sections, from north to south, include the North Coast section,
the Peninsula section, and the Santa Cruz Mountains section.
Along the North Coast section, geologic slip rate estimates by
Grove and Niemi suggest rates of 17 to up to 35 millimeters a year
since 30,000 to 200,000 years ago.
Grove and Niemi also estimate Holocene slip rates of
at least 21 to 25 millimeters a year since 1,000 years ago.
On the Peninsula section of the fault, Holocene slip rates appear to slow
to 13 to 21 millimeters a year, though rates along both sections
overlap with their respective uncertainties at 21 millimeters a year.
In our study, we estimate slip rates for the Santa Cruz Mountains section
of the San Andreas Fault of about 25 millimeters a year.
This rate is estimated over the last 10,000 years of the fault.
Our site is shown by this yellow star.
And now we’re just going to zoom into the star.
And that star takes us to Sanborn County Park in Saratoga, California.
So we’ve talked about why we decided to come here
to determine, or estimate, slip rates. Now I’m going to summarize
how we estimated slip rates of 22 to up to 28 millimeters a year
for the Santa Cruz Mountains section of the fault.
We first started with Lidar data from open topography.
As you can see, the Lidar data shows beheaded and deflected
streams and channels along the San Andreas Fault.
We then walked the fault and mapped different alluvial deposits
based on cross-cutting relationships and surface morphology.
The alluvial fan deposits at Sanborn County Park contain
large sandstone boulders. These sandstone boulders
did not come locally from these hill slopes.
Because these hill slopes are made of Tertiary mudstones.
These sandstone boulders that comprise these alluvial deposits
came from the upper reaches of the Santa Cruz Mountains and
has been transported downstream across the San Andreas Fault.
Along the lower reaches near the mouths of the fault are mudstones.
And so these alluvial fans are a great way to estimate the deposition,
or the age, of the offsets along the fault.
And so, to date the deposition of these fans, we collected samples
from these large sandstone boulders.
When possible, we collected boulders that appear stable.
Like boulders that are imbricated within the
debris flow bar sitting on top of the surface.
The imbrication of these debris flow bars, or these boulders,
suggest that they have not moved since they were deposited.
We also made sure boulders appear smooth, an indicator of fluvial erosion,
and that little to no attrition of the boulder surface has occurred.
Because attrition of the boulder surface would affect the concentration
of beryllium-10 measured in the rock.
This slide shows the location of boulder samples from alluvial fans we interpret
to originate from Todd Creek, Service Road Creek, and Aubry Creek.
These fans are mapped following the nomenclature of
[inaudible] 1993 as Qf3a, Qf3b, and Qf4.
Qf3a is offset by 320 meters. This offset is based on re-aligning
an incised channel into the fan unit and also re-aligning
the fan back to its source channel.
We did the same for Qf3b and Qf4, which we will summarize in a little bit.
From these deposits – all the white stars show the location of boulders
we collected to date the individual surfaces across the fault.
We dated seven boulders from each of these landforms and
two boulders from an older landform dated to about 20,000 years.
The results of dating these alluvial fans indicate the oldest deposit
within these channels and across the fault are about 20,000 years old.
The next-oldest unit is 12,000 years old – Qf3a.
Qf3b is about 7.7 thousand years old.
And Qf4 was deposited just 2.4 thousand years ago.
To estimate the offset of each deposit, we assigned
a maximum and minimum distance. The maximum distance aligns
the deposit across the fault. In the case of Qf4, which is
a debris flow, we aligned the debris flow bar across the fault,
along with an incised channel across the fault.
In this case, Qf4 is a debris flow bar, which is offset by 65 meters.
The minimum offset aligns a channel that has incised into the debris flow bar
on the northwest side. So here the 45 meters aligns the southwest channel
with the northeast channel that has incised into the debris flow bar.
For Qf3b, the maximum offset of 190 meters aligns the fan across
the fault, shown by the black arrows. A minimum offset of 150 meters
aligns a channel, shown by the white arrows, that has
incised into the fan deposit across the fault.
From these data, we can reconstruct the landscape through time as fans
are deposited, and subsequently offset, along the San Andreas Fault
at Sanborn County Park. At Time 1, Qf2 is deposited.
This occurred about 20,000 years ago.
And, with time, Qf2 is displaced along the fault.
At Time 2, 12,000 years ago, Qf3a is deposited at the mouths of
Todd Creek, Service Road Creek, and Aubry Creek.
As Qf3a is displaced along the fault, around 7.7 thousand years ago,
Qf3b is deposited and subsequently offset at the mouths of Todd Creek,
Service Road Creek, and Aubry Creek.
At Time 4, Qf4 is emplaced at around 2.4 thousand years ago as smaller
debris flow bars at the mouths of Service Road Creek and Aubry Creek.
And movement along this fault of about 55 meters, since 2.4 thousand
years ago, forms the landscape that we see today.
And this is the cartoon of modern – the modern-day configuration of the area.
And this is the mapped configuration with the Lidar showing how the
landscape looks today based on the emplacement of subsequent
offset and incision into these fans.
[silence]
When we combine all of the offsets based on displacement of Qf3a, Qf3b,
and Qf4, we get slip rates of 24.8, plus or minus 3, millimeters a year
for the Santa Cruz Mountains section of the fault.
Slip rates were calculated using the Styron slip rate calculator,
which applies Monte Carlo methods over tens of thousands of iterations
to explore the probability space of each set of offset and ages
to calculate an overall slip rate. The input parameters include the
final weighted mean of each deposit and the offset distances
as minimum and maximum set values.
A linear slip rate is estimated over the three different time intervals.
So that’s how we did it.
And the implications of this work suggest that the San Andreas Fault
in the Santa Cruz Mountains section appears to be faster than those
previously estimated for the section based on a rate at the Peninsula section.
This higher, faster rate on the fault suggests temporal and spatial
consistency in San Andreas Fault slip, potentially from the North Coast section
to the Santa Cruz Mountains section over Holocene time scales
for the northern San Andreas Fault.
And, if you’re interested in coming to this area and I can’t be with you,
I just wanted to let you know that there is a Streetcar 2 Subduction
field trip based on Google Earth for the site.
All you have to do is Google Streetcar 2 Subduction.
Once you go to Streetcar 2 Subduction, you can – you have to do this
on a Chrome browser. Don’t do it on Safari.
You can then scroll down, and you will see a number of different field trips.
They’re all great.
But this one in particular is called the San Andreas Fault
at Sanborn County Park. And you can go in and see
all the offsets and displaced landforms that I just summarized.