Hi. I’m John Baldwin. I’ll be presenting the preliminary
findings of a paleoseismic investigation on the North Coast
segment of the San Andreas Fault near Gualala, California.
This is a collaborative research project between Lettis Consultants
International, California Geological Survey, and the U.S. Geological Survey.
The Fish Rock Road site is located southeast of Point Arena
and northwest of Fort Ross near the town of Gualala.
It was originally identified by Koehler and Baldwin as part of
a strip mapping exercise along the San Andreas Fault to identify
potential paleoseismic research sites.
The resultant map is shown here on the left as well as the region
of the study site. On the far right is a schematic geologic map showing
the fault zone defined by a bedrock high on the east, shown in gray,
that impounds Quaternary alluvium being deposited by creeks
that flow from west to east across the fault zone.
The map also shows two trenches, T1 and T2, in black that are part of
the Zachariasen and Baldwin pilot study that were the first trenches to
evaluate the stratigraphy at the site. Those trenches confirm the location
of an older fault located at the base of the bedrock high,
as well as the 1906 rupture located to the west of the bedrock high,
as well as multiple secondary fault strands shown in purple.
As part of the 2021 study, we also developed a ground-based Lidar map,
shown here on the left. We’ll use it to finish the tectonic
geomorphic mapping and geomapping of the site.
What it does show is that it shows the bedrock high.
It shows the impounded alluvium behind the bedrock high in the area.
In the lower right, it shows where we excavated the trenches from the 2021
study, which included six trenches, two of which were part of the pilot
study in 2016 in which we re-excavated portions of those trenches.
The site stratigraphy that was identified consisted of fine- to coarse-grain
alluvium with lesser amounts of colluvium.
We confirmed that the western strand is the 1906 strand and has multiple
potential events along it. And we confirmed the eastern strand
shown in T1 as being discontinuous, and if it does continue to the north,
maybe as a warp. And also recording older events that do not project up
into the younger more modern surface.
We also identified anomalous secondary soft-sediment deformation features,
primarily in T3, that may help with the event chronology.
The upper part of this slide shows a photomosaic developed as part
of the pilot study performed in 2016 by Zachariasen and Baldwin,
which we were to confirm that the site had the proper stratigraphic
and structural relations to collect event timing information.
So you can see we identified a very prominent zone of primary
faulting offsetting bedded alluvial deposits as well as a zone of
shallow secondary faulting. This trench was limited in depth due to
a buried log as well as to a shallow groundwater table west of the fault.
2021 we re-excavated part of Trench T2 to get a longer and closer look.
And the preliminary interpretation of that re-excavation is shown below,
in which we interpret the 1906 trace and several earlier events.
We also excavated a new trench, Trench T3, south of T2,
in which that trench also confirms the presence
of the 1906 rupture as well as multiple earlier events.
In summary, the Fish Rock Road site along the western strand,
we interpret multiple events from the trenches.
And along the eastern strand, we infer a single older event.
The soft-sediment deformation that’s exposed, we intend to evaluate that to
see whether or not it can provide any additional event timing information.
Our next steps, we’re awaiting the C-14 age results, and we plan to
finalize the final technical report. Thank you.
- Hello. This is Chris Bloszies presenting the preliminary findings
of the strip mapping study of the Quién Sabe Fault conducted by
myself and John Baldwin of Lettis Consultants International.
The Quién Sabe Fault is a 25-kilometer-long,
1- to 1.7-kilometer-wide, zone of discontinuous fault strands
located northeast of the San Andreas Fault and
Southern Calaveras Fault junction. Right-lateral strike-slip motion is
inferred for the fault zone with a small component of up-to-the-northeast
vertical, as evidenced by low southwest-facing fault scarps.
The fault’s also associated with a concentrated area of seismicity
and is historically active with two moderate magnitude events
occurring on the fault in the mid-’80s.
Both of these events showed dominant dextral strike-slip motion.
Paleoseismic information for the fault is poorly constrained.
Net slip estimates of 0.22 to 0.67 millimeters a year
by Bryant 1985 are based on estimates of vertical displacement.
UCERF incorporates this uncertainty conservatively by estimating
a 1-millimeter-a-year slip rate and assumes a fault creep rate
of 0.1 millimeters a year. The mapping of the Quién Sabe Fault
by Bryant in the mid-’80s to date represents the most comprehensive
evaluation of the fault-related geomorphology and activity.
Even so, the FER is limited based on coarse-resolution topographic mapping
and largely inferred geochronologic estimates.
More recent mapping efforts have either adopted the Bryant depiction
or locate fault strands based on bedrock relationships.
Thus the understanding of the location and connectivity of
individual strands within the fault zone and the overall
paleoseismic history for the fault not being updated in the last 35 years.
Our study utilizes a newly acquired Lidar data set to revisit the Quién Sabe
Fault and produce two maps, one showing the tectonic
geomorphology and another depicting the surficial geology
within a 155-square-kilometer mapping area around the fault zone.
Together, these maps represent the key parameters used
to produce an updated map of the Quién Sabe Fault.
The methodology employed for this study is similar to other nearby
studies such as those for the Central Calaveras Fault.
The following slides show some of our preliminary interpretations
and initial findings to date. We start at the southernmost portion
of the original Bryant trace mapped through the Santa Anita Valley based on
aligned troughs and scarps developed in an order alluvial surface.
Our field observations show little to no discernable fault scarp,
and the older alluvial surface is relatively unbroken.
It may be mid- to late Pleistocene in age or older.
To the north and southern Santa Ana Valley, the fault is located
along the eastern valley margin where a series of southwest-facing
scarps are developed in small alluvial fan deposits of varying age.
Here, as shown in this photo, deposits are interpreted to be relatively young,
maybe latest Pleistocene or even early Holocene, based on our observations
of limited argillic soil development or yellowish soil color and well-defined
geomorphic expression in Lidar hillshades.
In the northern Santa Ana Valley, the Quién Sabe Fault is best expressed
as two to three semi-parallel anastomosing fault strands.
Here at the Samarkand Ranch, the younger western strand is expressed
as a 5-meter-high scarp developed in young alluvial fan deposits,
whereas the older eastern strand is defined primarily by bedrock features,
a series of sidehole benches, aligned troughs, notches, and saddle.
The deposits displaced by the western strand are inferred to be of similar age
that is displaced on the previous slide in the southern Santa Ana Valley,
supporting an interpretation of increasing vertical displacement
as the fault goes north.
Our last example is in the southern Hollister Valley, where the Quién Sabe
Fault is best expressed as a single strand consistent with a prominent
southwest-facing scarp. Here, where Lone Pine Road intersects
the primary fault strand, left-stepping en echelon cracks were observed in
the road surface, consistent with right-lateral strike-slip displacement.
It is possible that these are the result of aseismic creep along the fault.
Cursory reconnaissance of other similar fault-normal roads
have thus far yielded no other such examples.
Our study is ongoing, but we can reach several preliminary conclusions.
First, in the southern portion of our study area, fault strands
are interpreted to be, at the very least, Holocene-inactive.
In the Santa Ana Valley to the north, the Quién Sabe Fault may be
Holocene-active, and slip is documented to increase to the north.
To the north in the southern Hollister Valley, the Quién Sabe Fault
is best expressed and may be creeping.
North of Pacheco Creek, there is very limited evidence for recent faulting.
Our remaining tasks include finalizing our field evaluation and submitting
the final technical report, which will include GIS files for inclusion in the
Quaternary Fault and Fold Database. Thank you.
- Today I’d like to tell you about some preliminary results from a high-
resolution seismic study examining the Maacama swarm that occurred in 2020.
So this was a swarm within the seismicity of the Maacama Fault
Zone in northern California. This is work in collaboration with
Rob Skoumal and Jeanne Hardebeck. And, as part of this work, I’ll briefly
discuss a new approach to S-to-P amplitude ratios,
which may help us to characterize the complexity of this swam.
Okay, so takeaway number one. This swarm is incredibly geometrically complex.
We’ve applied high-resolution detection and integrated precise location.
We’ve used 388 routinely cataloged events as waveform templates.
And this has enabled us to detect more than 3,500 events and precisely locate
them using correlation-derived differential times with HypoDD.
So this gives us about nine times as many events, and this enables us
to get a lot clearer view of the faulting geometry of this swarm.
So this is a map view and a 3D perspective view from the northwest
with events color-coded by time over the nearly four-month analysis period.
So we see a great deal of complexity with widespread conjugate faults
striking north-northwest and northeast and perhaps pervasive
en echelon faulting in the northern part of the swarm.
So understanding these structures will help us
to understand the underlying source physics.
Just to give you a little bit better sense of this complexity, I’m going to
play a depth slice animation. This is going to show – start at
the shallow end of the swarm and show a 100-meter-thick zone
moving down through the swarm depth from 6 to 8 kilometers
with events color-coded by time. So here at the shallow end of the swarm,
most events are striking toward the northeast.
Up here in the north, you can start to see what may be more en echelon faults
striking toward the northeast, but there are also faults striking toward
the north and the north-northwest. I’ll quickly play this one more time
so you can get a little bit better sense of the complexity here
at different depth levels.
[silence]
So seeing this complexity in the locations motivated us to consider
what tools we could use to help constrain focal mechanisms.
In the past, we’ve used correlation measurements to constrain relative
polarities, and we’re also using – also measuring amplitude ratios
for correlated event pairs. And we do this separately for P and S.
And we’ve averaged these amplitude ratios to constrain magnitudes of newly
detected events, but we’re actually throwing away a lot of useful
information when we just average these values.
So we realize that we can set up an inverse problem with all of these
amplitude ratios, plus a few traditional S-to-P amplitude ratios, and solve
for the S-to-P amplitude ratios on each individual seismic channel.
So here is an example from one seismic channel.
You can see that some of the variations in S-to-P amplitude ratios
correspond with some of the location structures.
And finally, we can incorporate this information
to get improved constraints on focal mechanisms.
So our combined analysis, including S-to-P amplitude ratios,
can constrain focal mechanisms for a large population of events.
This is still a work-in-progress, but this plot is showing focal
mechanisms for about 1,600 events, which is about four times the number
of events included in the routine catalog.
It’s difficult to show so many mechanisms on one plot, so this plot
is showing – these are dominantly strike-slip mechanisms.
And each symbol here – the cross symbol is showing these two line
directions. So these are showing the two possible strike orientations
with the line length approximating the physical rupture dimension.
And the color scale is redundantly showing the strike direction, but it helps
highlight variations in strike direction for spatially clustered events.
So having mechanisms for so many events is a really powerful complement
to the detailed hypocenter interpretations – hypocenter patterns
to help us make – start to make fault structure interpretations.
Such as this visual interpretation, where we can start to resolve faults
on the scale of tens of meters. So this is a plausible – an example
of a plausible fault interpretation, but you can see the potential
to illuminate faulting complexity at increasingly small scales.
I think the combination of earthquake detection, precise location, and focal
mechanisms is a really powerful – toward our goal of understanding
the faulting physics underlying this and other swarms. Thank you very much.
- Hi. I’m Litong Huang from UC-Santa Cruz.
And I’m glad to present my ongoing study on the migration of
microseismicity on the Calaveras Fault between March to August 2021.
In Bilham’s 2021 SCEC meeting poster, he proposed a propagating pulse of slip
based on the migration of seismicity, surface creep, and radon and CO2
flows increases. I’m trying to increase the
microearthquake catalog in this area during the proposed slip episode
using template matching to provide more details to the previously identified
earthquake migration and the proposed slip event.
I’m using all the earthquakes in the NCEDC catalog in 2021 as template
events and relocate them according to Real-Time Double Difference catalog.
And I used the EQcorrscan package to generate templates and detect
matches in this area. After removing repeated detections and earthquakes
which are already in the catalog, I got 44 new detections using templates
in 2021. And here is a example of one of the matches in 2021.
I also got 61 cases that one template matches the other template.
By analyzing the distances between this cross-match pulse, we can see that
most of the matches have distance smaller than 0.5 kilograms, which allows us
to locate the new detect events by the location of their corresponding templates.
On the right side is a map of all the template events
in 2021 and all the new detected events.
And the stars represent the template events,
and circles represents a newly detected event.
We got a similar result to Bilham’s poster that seismicity migrates
from northwest to southeast. However, those migration of these
three clusters are apparent. There’s still not very much detail in the gap area.
That’s an intrinsic problem of our template matching because all
the matches are clustered near the templates.
So one possible way to provide more details in this gap area is to use more
template in this area from past few years, as in the left figure.
There’s many events occurring in the above-mentioned gap area.
I already processed the template event in 2020 and found 11 new matches.
And, in the future, I’m trying to do the same for template events
from 2012 to 2020 and see if we will get more details.
Thank you and welcome to leave any comments or any suggestions.
[silence]
- Today I’d like to give you a brief overview of work that was recently
published in G-cubed describing marine paleoseismic evidence
for seismic and aseismic slip along the Hayward-Rodgers Creek
Fault system in Northern San Pablo Bay.
Distinguishing between seismic and aseismic fault slip in the
geologic record is difficult yet fundamental to estimating the
seismic potential of faults and the likelihood of multi-fault rupture.
Currently, the maximum earthquake magnitude, or M-max, along the
Hayward-Rodgers Creek Fault system is estimated to be between 7 and 7.4.
And this includes a scaling factor for creep.
Now, the presence of creep essentially lowers the M-max for this fault system.
In addition, prior to this study, the extent of creep beneath
San Pablo Bay was largely unknown but suggested by the presence of
repeating earthquakes denoted in the blue circles on the map on the left.
The previous paleoseismic work, combined with the recent work
documenting fault connectivity between the Hayward and Rodgers Creek Faults
suggest that mutli-section fault rupture is possible.
So we integrated chirp sub-bottom imaging with targeted cross-fault coring
and core analyses of sedimentary proxy data to characterize vertical deformation
and slip behavior within an extensional fault bend along the Hayward-Rodgers
Creek Fault system in northern San Pablo Bay, as shown on the map.
We identified and traced four key seismic horizons across the fault,
all younger than approximately 1400 CE.
We developed a stratigraphic age model using detailed
down-core radiocarbon and radioisotope dating combined
with measurements of anthropogenic metal concentrations down-core.
Now, the onset of hydraulic mining within the Sierra Nevada in 1852 left
a clear geochemical and magnetic signature within core samples.
And this key time horizon was used to calculate a local reservoir correction
and reduce the uncertainty in our radiocarbon age calibration and models.
Vertical fault offset of strata younger than the most recent surface rupturing
earthquake on the Hayward Fault in 1868 suggests that near-surface
vertical creep is occurring along the fault in northern
San Pablo Bay at a rate of approximately 0.4 millimeters per year.
In addition, we present evidence of at least one, and possibly two,
coseismic events associated with growth strata above horizons R1 and R2
with median event ages estimated to be 1400 CE and 1800 CE, respectively.
Now, the overlap in timing of E2 and events on adjacent fault sections,
plus the amount of coseismic vertical offset of 0.5 meters, and the off-fault
deformation that we recognize in the chirp data strongly suggest that
multi-fault section rupture occurred around 1400 CE.
Also, based on the recent dynamic rupture modeling work of Harris et al.,
multi-section ruptures are possible, however, extending the area
of significant creep northward along the Hayward Fault reduces
the ability of ruptures to propagate through San Pablo Bay.
Thank you for listening.
- Hello. I’m Jenna Hill with the USGS PCMSC speaking today
about the relationship between submarine landslides and
earthquake recurrence records in southern Cascadia.
I’m showing here a rotated bathymetry map for offshore
southern Cascadia from Cape Blanco to Cape Mendocino.
In Cascadia, the primary offshore record of earthquake recurrence,
which is thought to be about 500 years for full-margin ruptures,
comes from correlation of abyssal turbidites interpreted to be
synchronously deposited at multiple sites along the margin.
Additional mud turbidites have been found at sites in central and southern
Cascadia and used to suggest more frequent shorter-length ruptures,
but these interpretations have been subject of much debate.
As a result, we have been doing extensive work in this region to
examine the sources and pathways of these abyssal turbidites to
better understand both the timing and spatial extent of shaking.
We have noted that there appears to be very little transfer of sediment
from the other shelf and upper slope to the abyssal plane in this region.
So here I’m showing several examples of sediment core CT scans where the
purple is low-density mud and the pinks to yellows are coarser sand and gravels.
In the mid- to upper portions of both Rogue and Trinidad Canyons,
we see mostly mud with no depositional or erosional events.
In contrast, we see evidence of debris flows in the lower slope
and sandy turbidites on the abyssal plane, which suggest
the primary contributions here are from the lower slope.
In between, in the Eel Basin, we see no evidence of deep-sea connected
canyons, where we have a robust Pleistocene turbidite record and
thick Holocene deposits that suggest the basin is a sediment sink.
Since it appears that most of southern Cascadia canyons are not delivering
sediment to the deep sea in the Holocene, we’ve hypothesized
that the abyssal turbidite record is instead derived from disintegration
of the outer wedge. To test this, I began mapping seafloor failure scarps,
which are shown here as the thin black lines along the entire length of the
margin, and a major trend emerged. There appear to be far more failures
along the lower slope than anywhere else.
So I’m showing a few examples of this where you can see extensive
failures in the lower portions of both Rogue and Trinidad Canyons
and a nearly continuous band of slope failures
along the lower slope outside of these canyons as well.
I’m going to zoom in here to this region in this little tiny red box at the bottom
and show some of our AUV and ROV data that was recently acquired
in collaboration with our partners at MBARI.
So the background map here is 1 meter AUV bathymetry from
the toe of the deformation front. We can see a well-defined slope
failure zone with a 6- to 8-meter-high headwall scarp and associated
mass transport deposits at the base of the slope.
The yellow line indicates the location of the AUV chirp sub-bottom profile,
which is shown here on the bottom, that shows multiple buried MTDs
overlain by a few meters of layered strata.
We collected two transects of precisely located ROV vibracores
whose locations are shown by the colored stars.
The cores within the failure zone show three to four well-defined
turbidites atop the larger buried MTDs, while the adjacent core transect shows
much less distinct event deposits just outside of the failure zone.
To me, this is a nice example of how multiple earthquake-triggered
turbidites may be sourced from the same failure zone.
While not every large earthquake produces a large slope failure,
since there are a lot of factors required to precondition slopes for failure,
when these large failures do occur, they can debuttress the slope such that
subsequent earthquakes may shake loose additional material and smaller
failures that are recorded as recurring turbidites.
And I think that’s what we see along much of southern Cascadia.
Now, to take a step back, I’m showing here the full distribution of seafloor
failure scarps for southern Cascadia, represented by a density of features
where the hotter colors, the higher density of scarps.
There’s a strong clustering of failures along the outer wedge and in areas
around Rogue and Eel Canyons where the seafloor gradient is steeper due to
the plate boundary interactions. Notably, though, there’s a significant
lack of failures across the upper slope despite the fact that this region has
thick, continuous, and relatively rapidly emplaced Quaternary strata,
all of which are hallmarks of a slope preconditioned for failure.
I find this absence of slope failure across such a large portion of the
margin particularly interesting in comparison with the latest M9 scenario
predictions shown here from Wirth et al. in 2021 that indicate this entire region
is likely to experience violent, severe shaking.
The striking absence of failures further up the slope leads to the question
of whether the most intense shaking is actually more restricted to a narrower
zone along the trench, as represented by the submarine landslide distribution,
or whether there are other geomechanical factors at play here.
Although our initial analyses suggest that might not be the case.
So here is my summary. For next steps, we’re working through
our vast array of core transects and geophysical data to characterize the
variability and reduce age uncertainties. We’re also looking to do geotechnical
analyses of slope failure susceptibility and site response
and welcome any potential collaborators on this front.
- Hi. My name is Vicki Langenheim, and I’ll be talking about new
aeromagnetic data in northeastern California.
Why this region? Well, it lies at the intersection of the Cascade arc,
the Basin and Range, the clockwise- rotating Oregon Coast block,
and the Walker Lane.
And one of the questions we want to address is how much
right-lateral displacement occurs along the Walker Lane
Fault Zone and how far it encroaches into this area.
Especially since previous estimates of right-lateral displacement
range from zero to 30 kilometers in this region.
And we also want to address where faults extend beneath cover.
So we’re going to now zoom into our study area.
This is a simplified geologic map. Indicates that aeromagnetic data
should provide a great way to address these questions as the area is blanketed
by geologically young volcanic rocks. Volcanic rocks are very magnetic,
and offsets of these rocks produce characteristic magnetic anomalies.
And, as you can see, there’s a lot of faulting in this area,
so we should see a lot of characteristic anomalies.
I’ll point out that the region also encompasses Medicine Lake Volcano,
which is the largest volcano in the Cascade arc, and the Pit River,
which is the largest tributary to the Sacramento River,
and it’s fed by some of the largest freshwater springs
in the western United States here in Fall River Valley.
It also hosts a number of dams and power houses along the stretch.
So this what the aeromagnetic data looked like before our new data
were flown. They were flown decades ago along flight lines
that were 1- to 2-mile line spacing and at a constant elevation.
This was all before the advent of GPS.
We used these data in our 2016 Geosphere paper to examine the
geometry of the Hat Creek Fault, which is one of the more prominent
normal faults in the region, and to propose little discrete
cumulative right-lateral offset on faults that cross this
prominent northeast-striking magnetic gradient.
Let’s now look at what the new data show, which is a huge improvement
in resolution, much richer data set. And I’m just going to overlay
the faults here. These new data confirm little cumulative displacement
along that northeast-striking gradient as well as little discrete right-lateral
displacement across these east-west- striking bands of magnetic highs.
The likely fault, which is an enigmatic fault with a funny name,
has likely little discrete offset. But filtered versions of the magnetic
data suggest as much as 5 to 10 kilometers of right-lateral
shearing across the fault zone. We could also use the magnetic data
to extend faults south of Medicine Lake Volcano,
to connect to the springs here in Fall River Valley, as well as
extend faults across the Tule Lake and Lower Klamath Basins.
So stay tuned, and new aeromagnetic data are coming online in the coming
months and years in California and Nevada, in large part funded
for assessment of mineral and geothermal resources.
Jon Glen is leading the efforts in central Nevada and in the
Salton Trough, and I am leading the efforts elsewhere.
These data should aid in the characterization of faulting in these
regions in terms of location and offset. And I just point out that there are
even more surveys in the pipeline due to a huge influx of funding.
So potential for incredible data sets for students, particularly in the
Mojave Desert and in the Great Basin. So talk to me or Jonathan Glen for
additional details. And thank you so much for your attention and time.