Hello. This is Sam Johnson of the USGS Pacific Coastal
and Marine Science Center.
I’ll be talking about the northern San Andreas Fault from a coastal
and marine perspective. The San Andreas Fault forms –
is the master fault in the boundary between the Pacific Plate and this
North American microplate that extends for about 450 kilometers
from the San Juan Bautista area on the southeast up to the south flank of
Cape Mendocino. About 40% of the fault is offshore in the ocean.
That occurs in three different areas – offshore of San Francisco, north of
Point Reyes here, and between Point Arena and Point Delgada.
The entire fault ruptured in a magnitude 7.9 earthquake in 1906,
after which there has been continuing intensive investigations of onshore
parts of the fault. In contrast, detailed studies of the offshore areas
has only occurred in about the last 20 years offshore of San Francisco
in the early 2000s. And, in these two northern areas, in the last decade.
In my limited time today, I’ll be talking about these northern two areas.
I’ll be – I’ll briefly discuss our mapping, the importance of fault strike
on geomorphology, the significance for earthquake hazards and
hazard assessments, and how the fault ends to the north.
All of the data and interpretations have been published – the map and
data sets for the California Seafloor Mapping Program and these two journal
articles authored by me and my colleague, Jeff Beeson.
Just want to review a few basics of strike-slip faulting.
Where slip along this structure is parallel to the plate boundary,
that should result in pure strike-slip faulting.
Where the strike of the fault has a more westerly trend, that should
result in oblique contraction or transpression, higher elevations,
pop-ups, a combination of strike-slip and reverse faulting.
Where the fault trends more northerly, that should result in transtensional
deformation and lower elevations, perhaps basin formation.
That’s validated on this plot, which shows geography on the X axis –
this is San Juan Bautista to the southeast up to Point Delgada
on the northwest – against the strike of the NSAF on the Y axis.
360 degrees, or pure north, on the bottom of the plot.
West-northwest strike of 300 degrees on the top of the plot.
The curvy line is the fault strike. It’s measured at 2-kilometer intervals,
smoothed over 5 intervals to produce a rolling average.
The on-land measurements we used are from the Jennings compilation.
The offshore measurements are from our work.
The green line is the vector of plate motion.
So anything above the line should result in transpression,
or anything below the line should be transtensional areas.
And the offshore areas of the fault are shown by the blue shading and
the red parts of the curvy line. And it’s consistent with the lower
elevations occurring in transtensional areas and upland areas in
transpressional areas. Also this dip here – this marine area
coincides with this transtensional bend that you see right here.
So we’re going to begin by looking at the Tomales Point to Salt Point area.
This rectangle here is expanded on the right and rotated.
This plot shows – this map shows our high-resolution bathymetry in orange.
Our track lines – these were collected at 1-kilometer line spacing.
In most of the area, 500-meter line spacing inside the water body
of Bodega Bay. It’s distinct from the town
of Bodega Bay, which is up a little bit farther to the north.
The map on the right is an onshore/offshore geology map.
The blue line is the shoreline. The pink shows granitic basement
of the Salinian Block. The green shows metasedimentary
basement of the Franciscan terrane. And that’s separated by the northern
San Andreas Fault, which extends through here.
Okay, this is the Bodega Bay area.
Again, the elevated rim on the western margin of the bay.
Blue line is the shoreline.
Three different seismic reflection profiles.
These are vertically exaggerated 12-1/2 to 1.
Red lines are the faults. Orange lines – orange and yellow
are inferred Pleistocene and Holocene deposits.
The mapping reveals that the fault in Bodega Bay
is a distributed shear zone with shallow deformed valley fill.
We recognize two main strands. Strand 2 – the onshore continuation
is where fissuring and rupturing were identified in the Lawson report for the –
for the 1906 earthquake. This map – this inset is shown
in the block over on the lower right where we show
significant offset on another strand of the fault labeled Fault 1.
Here’s a close-up, and you can see clearly 75 meters of right-lateral
offset on the back edge of this sandbar,
which extends across the width of Bodega Bay.
We interpret the bar as an early Holocene analog of the modern
sand spit and shoreface that now forms the natural boundary of –
southern boundary of Bodega Harbor. The blue line shows sea level at minus
18 meters in here – the depth at which we think this sandbar probably formed.
On the seismic profile, it shows this sandbar –
the seismic profile here shows the sand spit and shoreface here.
Another probable shoreface a little bit farther to the north, and then it extends
up to the modern shoreface coming off of the modern sand spit.
The depth at which we think this sand spit formed suggests an age of
7,500 to 8,700 years before present, based on matching a sea level curve.
That slip – amount of slip and that timing suggests a slip rate of
8.6 to 10 millimeters per year, which is significantly less than
the 24 millimeters a year from the NSAF based on GPS
and paleoseismology in other areas.
So the slip must be divided between two different strands.
[silence]
This – apart from its local relevance, this emphasizes the importance
of fault zone mapping for paleoseismology work.
By not mapping an entire fault zone, one could miss the complete
event history or part of the slip rate by working on
one strand in a multi-strand zone.
Let’s talk about late Pleistocene paleogeography.
It’s important to realize that, for about 85% of the last 70,000 years,
the prominent strike valley of Tomales Bay is actually
about 15 kilometers longer. It extended beyond Bodega Bay –
the town and the water body.
50 kilometers long and 1 to 2 kilometers wide – straight as an arrow.
It’s actually – and it was non-marine during this period of lower sea level.
It’s very similar to what one sees now in the non-marine valley of the Garcia
and Gualala Rivers, like in this narrow, straight-as-an-arrow section
of the fault trace. And what’s pretty remarkable is the similarity in fault
strike between the two different areas – a clear indication of the importance
of fault strike on geomorphology.
I now want to turn your attention to these two odd polygons offshore
of the Russian River here. Here and here – they’re each
about 5 to 6 kilometers in area. This is what they look like.
Here and here – they consist of chutes and lobes that are being – appear to be
being draped now by recent sediment. They have very low relief, as seen on
this seismic reflection profile. And our hypothesis is that these
represent liquefaction-generated slope failure associated with
the 1906 earthquake. In arriving at this interpretation,
there’s a very important comparison with coseismic 1980 slope failure
offshore of the Klamath River associated with the magnitude 7.2
earthquake. The upper four rows in this table show the similarities between the
two slope failures in terms of the width of the zone, the length of runout,
the water depth, and sediment thickness.
The lower rows shown in yellow font actually point out why slope failures
on the – offshore of the Russian River should be much more common.
Specifically, coarser grain size suggests an easier flow, steeper slopes,
proximity to earthquakes of much greater magnitude, and much –
in much greater proximity to the rupturing fault.
So what this tells me is that these kinds of slope failures should be associated
with every large earthquake event on the northern San Andreas Fault.
[silence]
In our paper, we also describe how onshore relief and uplift rates of marine
terraces increase to the north from the Bodega Bay area, where they’re
about 0.1 millimeters per year, to about 0.5 millimeters per year,
and we associate that with this transtensional bend
in the nearby offshore northern San Andreas Fault.
This is just a couple of Google Earth images showing the higher relief of
the terraces in the north at the mouth of the Russian River compared to
the lower elevations to the south. This is the sand spit of Bodega Bay.
Now we want to jump to the north and show our – some data from
the area between Point Arena and Point Delgada.
We crossed the fault 120 times with line spacing of 1 kilometer.
Here’s a couple of examples of the – from our beautiful data set showing
the fault as a multi-strand feature.
The area is – the fault is farther offshore, receives less sediment,
and therefore multibeam bathymetry actually shows some exciting intra-fault
zone geomorphology, including scarps, shallow depressions,
these intra-fault zone uplifts. This is an area where we think
we’re actually seeing slip transferred from an eastern strand of the fault
to a western strand of the fault. And, in the process, capturing material
and transferring it from the Pacific to the North American Plate.
On this map, it shows a significant 9-degree bend in the northern
San Andreas Fault that’s associated with the maximum
divergence from the plate vector as much as 27 degrees.
Also the maximum distance offshore – 20 kilometers.
And the minimum elevation is minus 200 meters below sea level
for the occurrence of the fault zone.
That bend is also associated with development of an asymmetric
lazy-Z style sedimentary basin 11 kilometers wide, 37 kilometers long.
I want to talk – now I want to talk about how the fault ends.
We know that it goes onshore here at Point Delgada in the Shelter Cove area.
It’s never been conclusively mapped onshore that led these geologists
to suggest, very speculatively, that the fault could actually extend
offshore just below the – just offshore of the King Range.
Our grid of seismic reflections effectively rules out a fault
in this location. There’s a couple of the seismic profiles on which we
don’t see that kind of a fault. We now think that the fault actually,
in a very ad hoc interpretation – one where we have no data,
but we think that the fault could actually occur in the area between
the shore – the shoreline and the northern end of our seismic profiles,
shown in this location here. Again, it’s ad hoc,
but it’s supported by a few lines of evidence.
There’s significant structural relief between the top of the basement
in the King Range in our – on our offshore seismic profiles.
We know that the King Range is actually going up at
very, very high rates – as much as 5 millimeters per year.
Whereas, we see – know from our seismic profiles that the offshore
is going down, so there’s a real contrast there.
The strike of the fault is actually not anomalous, even though it
goes through this big bend, you know, from the maximum
transtensional spot associated with the Noyo Basin.
But this strike was actually similar to the strike of the northern San Andreas
Fault and the Santa Cruz Mountains. And it’s a very viable interpretation
to extend it to the northwest to continue into the Mattole Canyon Fault.
And this just basically shows the Mattole Canyon Fault as
a significantly – juxtaposing different panels on seismic – different panels
with different properties on seismic reflection data.
Okay, so what to take home from this talk.
One, that the offshore northern San Andreas Fault is now
completely mapped. Two, that the fault controls coastal
and marine geomorphology at regional to local scales.
Three, that it’s important to understand and map multi-strand fault zones.
Four, that massive near-shore slope failures are probably a
common and underappreciated earthquake hazard.
Thanks very much for your attention.