WEBVTT
Kind: captions
Language: en-US

00:00:01.328 --> 00:00:03.955
[silence]

00:00:04.007 --> 00:00:06.970
Thank you, NEHRP organizers,
for giving me the opportunity

00:00:06.970 --> 00:00:10.080
to present this work on the
Santa Cruz Mountains section

00:00:10.080 --> 00:00:12.559
of the northern
San Andreas Fault.

00:00:12.559 --> 00:00:16.160
This work started the final year of
my postdoc with Roland Bürgmann

00:00:16.160 --> 00:00:20.189
at UC-Berkeley, and the project is
my first funded NEHRP grant as

00:00:20.189 --> 00:00:24.823
faculty at San Jose State University.
So it’s near and dear to my heart.

00:00:24.823 --> 00:00:27.200
And, though I am giving the talk,
Katherine Guns,

00:00:27.200 --> 00:00:30.700
my then-undergraduate intern, 
completed much of the work

00:00:30.700 --> 00:00:32.640
I’m presenting today.

00:00:32.640 --> 00:00:38.070
Katherine recently completed her
Ph.D. and is now a postdoc at Scripps.

00:00:38.070 --> 00:00:42.780
As you will see, she and this
project has come a long way.

00:00:42.780 --> 00:00:47.609
In this talk, I’m going to summarize
our work to estimate how Holocene slip

00:00:47.609 --> 00:00:50.210
rates on the Santa Cruz Mountains
section of the San Andreas Fault.

00:00:50.210 --> 00:00:54.620
I’ll start with why we started the
project, how we estimated slip rates,

00:00:54.620 --> 00:00:57.810
and discuss the
implications of our work.

00:00:57.810 --> 00:01:01.080
[rustling sounds]

00:01:03.272 --> 00:01:09.045
[silence]

00:01:09.070 --> 00:01:12.540
Why we study a fault slip history,
for all those engineers out there,

00:01:12.540 --> 00:01:19.970
is because slip rate data is a first-order
input into the logic tree used for

00:01:19.970 --> 00:01:26.570
earthquake probability models. It is a
direct input into deformation models

00:01:26.570 --> 00:01:30.640
used to estimate the probability
of a magnitude 6 or greater earthquake

00:01:30.640 --> 00:01:34.880
in the next 30 years
on an individual fault.

00:01:38.600 --> 00:01:43.430
So this is just a logic tree that shows
how geologic data, or slip rate data,

00:01:43.430 --> 00:01:48.713
is used into these
probability models.

00:01:48.713 --> 00:01:51.780
And why do we study the
Santa Cruz Mountains section

00:01:51.780 --> 00:01:56.950
of the San Andreas Fault?
Well, as you can see in this model,

00:01:56.950 --> 00:02:02.310
the deformation models, where we
provide fault slip to calculate

00:02:02.310 --> 00:02:06.931
seismic moment, there are slip rate
data for the northern section

00:02:06.931 --> 00:02:11.440
of the San Andreas Fault, the central
section of the San Andreas Fault,

00:02:11.440 --> 00:02:16.994
but the southern section does not
have any slip rate constraints.

00:02:18.539 --> 00:02:25.483
So, if we focus into the
northern San Andreas Fault,

00:02:25.483 --> 00:02:31.030
we can see that it’s separated
into three main sections.

00:02:31.030 --> 00:02:37.230
These sections, from north to south,
include the North Coast section,

00:02:37.230 --> 00:02:41.544
the Peninsula section, and the
Santa Cruz Mountains section.

00:02:42.593 --> 00:02:47.739
Along the North Coast section,
geologic slip rate estimates by

00:02:47.739 --> 00:02:54.930
Grove and Niemi suggest rates
of 17 to up to 35 millimeters a year

00:02:54.930 --> 00:03:00.099
since 30,000 to
200,000 years ago.

00:03:01.317 --> 00:03:04.574
Grove and Niemi also estimate
Holocene slip rates of

00:03:04.599 --> 00:03:11.450
at least 21 to 25 millimeters
a year since 1,000 years ago.

00:03:11.450 --> 00:03:17.060
On the Peninsula section of the fault,
Holocene slip rates appear to slow

00:03:17.060 --> 00:03:25.069
to 13 to 21 millimeters a year,
though rates along both sections

00:03:25.069 --> 00:03:32.500
overlap with their respective
uncertainties at 21 millimeters a year.

00:03:32.500 --> 00:03:36.730
In our study, we estimate slip rates
for the Santa Cruz Mountains section

00:03:36.730 --> 00:03:43.209
of the San Andreas Fault of
about 25 millimeters a year.

00:03:43.209 --> 00:03:48.650
This rate is estimated over the
last 10,000 years of the fault.

00:03:48.650 --> 00:03:52.673
Our site is shown
by this yellow star.

00:03:52.713 --> 00:03:55.953
And now we’re just
going to zoom into the star.

00:03:55.953 --> 00:04:02.790
And that star takes us to Sanborn
County Park in Saratoga, California.

00:04:02.790 --> 00:04:05.588
So we’ve talked about why
we decided to come here

00:04:05.588 --> 00:04:09.169
to determine, or estimate, slip rates.
Now I’m going to summarize

00:04:09.169 --> 00:04:14.529
how we estimated slip rates of
22 to up to 28 millimeters a year

00:04:14.529 --> 00:04:18.579
for the Santa Cruz Mountains
section of the fault.

00:04:18.579 --> 00:04:24.550
We first started with Lidar
data from open topography.

00:04:24.550 --> 00:04:30.360
As you can see, the Lidar data
shows beheaded and deflected

00:04:30.360 --> 00:04:35.930
streams and channels along
the San Andreas Fault.

00:04:35.930 --> 00:04:43.270
We then walked the fault and
mapped different alluvial deposits

00:04:43.270 --> 00:04:48.460
based on cross-cutting relationships
and surface morphology.

00:04:48.460 --> 00:04:52.340
The alluvial fan deposits at
Sanborn County Park contain

00:04:52.340 --> 00:04:56.170
large sandstone boulders.
These sandstone boulders

00:04:56.170 --> 00:04:58.919
did not come locally
from these hill slopes.

00:04:58.919 --> 00:05:02.690
Because these hill slopes
are made of Tertiary mudstones.

00:05:02.690 --> 00:05:06.690
These sandstone boulders that
comprise these alluvial deposits

00:05:06.690 --> 00:05:12.750
came from the upper reaches of
the Santa Cruz Mountains and

00:05:12.750 --> 00:05:19.360
has been transported downstream
across the San Andreas Fault.

00:05:19.360 --> 00:05:25.190
Along the lower reaches near the
mouths of the fault are mudstones.

00:05:25.190 --> 00:05:36.960
And so these alluvial fans are a
great way to estimate the deposition,

00:05:36.960 --> 00:05:43.270
or the age, of the
offsets along the fault.

00:05:43.270 --> 00:05:48.909
And so, to date the deposition of
these fans, we collected samples

00:05:48.909 --> 00:05:52.360
from these large
sandstone boulders.

00:05:52.360 --> 00:05:56.479
When possible, we collected
boulders that appear stable.

00:05:56.479 --> 00:05:58.830
Like boulders that are
imbricated within the

00:05:58.830 --> 00:06:02.720
debris flow bar sitting
on top of the surface.

00:06:02.720 --> 00:06:06.210
The imbrication of these debris
flow bars, or these boulders,

00:06:06.210 --> 00:06:10.720
suggest that they have not
moved since they were deposited.

00:06:12.535 --> 00:06:20.440
We also made sure boulders appear
smooth, an indicator of fluvial erosion,

00:06:20.440 --> 00:06:25.590
and that little to no attrition of the
boulder surface has occurred.

00:06:25.590 --> 00:06:32.490
Because attrition of the boulder
surface would affect the concentration

00:06:32.490 --> 00:06:36.240
of beryllium-10
measured in the rock.

00:06:41.439 --> 00:06:48.210
This slide shows the location of boulder
samples from alluvial fans we interpret

00:06:48.210 --> 00:06:53.319
to originate from Todd Creek,
Service Road Creek, and Aubry Creek.

00:06:53.319 --> 00:06:56.288
These fans are mapped
following the nomenclature of

00:06:56.288 --> 00:07:05.420
[inaudible] 1993 as
Qf3a, Qf3b, and Qf4.

00:07:05.420 --> 00:07:13.500
Qf3a is offset by 320 meters.
This offset is based on re-aligning

00:07:13.500 --> 00:07:17.490
an incised channel into
the fan unit and also re-aligning

00:07:17.490 --> 00:07:20.920
the fan back to
its source channel.

00:07:23.733 --> 00:07:32.990
We did the same for Qf3b and Qf4,
which we will summarize in a little bit.

00:07:32.990 --> 00:07:36.969
From these deposits – all the white
stars show the location of boulders

00:07:36.969 --> 00:07:43.000
we collected to date the individual
surfaces across the fault.

00:07:43.000 --> 00:07:47.400
We dated seven boulders from
each of these landforms and

00:07:47.400 --> 00:07:53.680
two boulders from an older landform
dated to about 20,000 years.

00:07:55.808 --> 00:08:01.219
The results of dating these alluvial
fans indicate the oldest deposit

00:08:01.219 --> 00:08:08.509
within these channels and across
the fault are about 20,000 years old.

00:08:08.509 --> 00:08:14.282
The next-oldest unit is
12,000 years old – Qf3a.

00:08:14.282 --> 00:08:19.169
Qf3b is about
7.7 thousand years old.

00:08:19.169 --> 00:08:25.040
And Qf4 was deposited just
2.4 thousand years ago.

00:08:29.078 --> 00:08:32.800
To estimate the offset of
each deposit, we assigned

00:08:32.800 --> 00:08:37.940
a maximum and minimum distance.
The maximum distance aligns

00:08:37.940 --> 00:08:43.539
the deposit across the fault.
In the case of Qf4, which is

00:08:43.539 --> 00:08:48.850
a debris flow, we aligned the
debris flow bar across the fault,

00:08:48.850 --> 00:08:53.060
along with an incised
channel across the fault.

00:08:54.426 --> 00:09:02.488
In this case, Qf4 is a debris flow bar,
which is offset by 65 meters.

00:09:05.180 --> 00:09:15.651
The minimum offset aligns a channel
that has incised into the debris flow bar

00:09:15.651 --> 00:09:25.990
on the northwest side. So here the
45 meters aligns the southwest channel

00:09:25.990 --> 00:09:34.720
with the northeast channel that
has incised into the debris flow bar.

00:09:37.884 --> 00:09:46.959
For Qf3b, the maximum offset of
190 meters aligns the fan across

00:09:46.959 --> 00:09:56.639
the fault, shown by the black arrows.
A minimum offset of 150 meters

00:09:56.639 --> 00:10:00.000
aligns a channel, shown by
the white arrows, that has

00:10:00.000 --> 00:10:05.320
incised into the fan
deposit across the fault.

00:10:09.178 --> 00:10:14.320
From these data, we can reconstruct
the landscape through time as fans

00:10:14.320 --> 00:10:18.880
are deposited, and subsequently
offset, along the San Andreas Fault

00:10:18.880 --> 00:10:24.810
at Sanborn County Park.
At Time 1, Qf2 is deposited.

00:10:24.810 --> 00:10:28.600
This occurred
about 20,000 years ago.

00:10:29.513 --> 00:10:36.100
And, with time,
Qf2 is displaced along the fault.

00:10:36.100 --> 00:10:43.560
At Time 2, 12,000 years ago,
Qf3a is deposited at the mouths of

00:10:43.560 --> 00:10:47.760
Todd Creek, Service Road Creek,
and Aubry Creek.

00:10:52.214 --> 00:10:59.980
As Qf3a is displaced along the fault,
around 7.7 thousand years ago,

00:10:59.980 --> 00:11:07.389
Qf3b is deposited and subsequently
offset at the mouths of Todd Creek,

00:11:07.389 --> 00:11:10.080
Service Road Creek,
and Aubry Creek.

00:11:11.748 --> 00:11:22.370
At Time 4, Qf4 is emplaced at around
2.4 thousand years ago as smaller

00:11:22.370 --> 00:11:30.215
debris flow bars at the mouths of
Service Road Creek and Aubry Creek.

00:11:30.215 --> 00:11:38.149
And movement along this fault of
about 55 meters, since 2.4 thousand

00:11:38.149 --> 00:11:42.450
years ago, forms the
landscape that we see today.

00:11:42.450 --> 00:11:48.050
And this is the cartoon of modern – the
modern-day configuration of the area.

00:11:48.050 --> 00:11:52.361
And this is the mapped configuration
with the Lidar showing how the

00:11:52.361 --> 00:11:57.470
landscape looks today based on
the emplacement of subsequent

00:11:57.470 --> 00:12:01.200
offset and incision
into these fans.

00:12:03.542 --> 00:12:06.595
[silence]

00:12:06.620 --> 00:12:15.030
When we combine all of the offsets
based on displacement of Qf3a, Qf3b,

00:12:15.030 --> 00:12:22.209
and Qf4, we get slip rates of 24.8,
plus or minus 3, millimeters a year

00:12:22.209 --> 00:12:27.000
for the Santa Cruz Mountains
section of the fault.

00:12:27.000 --> 00:12:30.870
Slip rates were calculated using
the Styron slip rate calculator,

00:12:30.870 --> 00:12:34.860
which applies Monte Carlo methods
over tens of thousands of iterations

00:12:34.860 --> 00:12:39.079
to explore the probability space
of each set of offset and ages

00:12:39.079 --> 00:12:43.769
to calculate an overall slip rate.
The input parameters include the

00:12:43.769 --> 00:12:47.670
final weighted mean of each
deposit and the offset distances

00:12:47.670 --> 00:12:51.880
as minimum and
maximum set values.

00:12:51.880 --> 00:12:57.459
A linear slip rate is estimated over
the three different time intervals.

00:12:57.459 --> 00:12:59.117
So that’s how we did it.

00:12:59.117 --> 00:13:04.259
And the implications of this work
suggest that the San Andreas Fault

00:13:04.259 --> 00:13:09.699
in the Santa Cruz Mountains section
appears to be faster than those

00:13:09.699 --> 00:13:17.480
previously estimated for the section
based on a rate at the Peninsula section.

00:13:17.480 --> 00:13:22.300
This higher, faster rate on the fault
suggests temporal and spatial

00:13:22.300 --> 00:13:28.769
consistency in San Andreas Fault slip,
potentially from the North Coast section

00:13:28.769 --> 00:13:33.899
to the Santa Cruz Mountains section
over Holocene time scales

00:13:33.899 --> 00:13:36.709
for the northern
San Andreas Fault.

00:13:36.709 --> 00:13:42.860
And, if you’re interested in coming
to this area and I can’t be with you,

00:13:42.860 --> 00:13:47.269
I just wanted to let you know that
there is a Streetcar 2 Subduction

00:13:47.269 --> 00:13:51.820
field trip based on
Google Earth for the site.

00:13:51.820 --> 00:13:55.570
All you have to do is Google
Streetcar 2 Subduction.

00:13:55.570 --> 00:13:59.569
Once you go to Streetcar 2 Subduction,
you can – you have to do this

00:13:59.569 --> 00:14:05.380
on a Chrome browser.
Don’t do it on Safari.

00:14:05.380 --> 00:14:12.533
You can then scroll down, and you will
see a number of different field trips.

00:14:12.533 --> 00:14:14.370
They’re all great.

00:14:14.370 --> 00:14:17.420
But this one in particular is
called the San Andreas Fault

00:14:17.420 --> 00:14:22.820
at Sanborn County Park.
And you can go in and see

00:14:22.820 --> 00:14:28.438
all the offsets and displaced
landforms that I just summarized.