WEBVTT
Kind: captions
Language: en-US

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With my co-authors,
Chad Trexler and Kevin Furlong,

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today we will discuss the
northern Maacama Fault –

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the young fault propagating
through legacy crustal structures.

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[silence]

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We would like to acknowledge
unpublished data sources

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from Gavin Hayes, Gwen
Erickson, and Adam Woolace.

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And we point out four key published
sources pertinent to this presentation.

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[silence]

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The San Andreas Transform
boundary, north of the latitude

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of San Francisco Bay, consists of three
strike-slip faults – the San Andreas

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Fault proper, the Maacama Fault,
and the Bartlett Springs Fault.

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This presentation focuses on the
northern Maacama Fault within

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the delineated rectangular area.
Note that the Maacama – note that

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the Mendocino Triple Junction
migrated northward past this region

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approximately 3 million years ago.
And subsequent crustal response

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over the ensuing millennia ultimately
resulted in the northward propagation

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of the Maacama Fault
through this region.

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The road map for this talk is that we
will discuss regional geologic history,

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then introduce the active traces
of the northern Maacama Fault

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using a new
Lidar database.

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Then we will discuss paleoseismic
gravity and seismicity investigations

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that inform fault zone
behavior and structure.

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We will place the 2020 Willits
earthquake swarm within the context

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of the fault zone structure, and
finally present a wrap-up summary.

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[silence]

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The map images in this presentation
are constructed from USGS 3D

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elevation program Lidar coverage of
northern California released in 2017.

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The Lidar enables a revised
comprehensive mapping

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of active traces of the
northern Maacama Fault.

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This Lidar-based hillshade map of
the study region shows the

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north-northwest trend in the
topography, reflecting underlying

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structure of the Mesozoic
Franciscan Assemblage bedrock.

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The elevation-based color bands
show that the highest topography

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in the region is in the center of
the image – the Laughlin Range

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with a 1,000-meter
summit elevation.

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The lower – the lower elevation limit is
the Ukiah Valley floor at 170 meters.

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There are two prominent basins in the
field area – the northern Little Lake

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Basin, containing the town of Willits,
and the southern elevation in lower

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Ukiah-Redwood Valley Basin.
These basins are filled with

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Neogene and Quaternary sediment.
Both basins’ structure and

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sedimentation predates inception
of strike-slip tectonics.

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These are not
strike-slip basins.

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Prior to 1 million years ago, drainage
direction through these basins was

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uniformly to the south-southeast –
from the south-southeast-directed flow

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into the Little Lake Basin, through
a narrow channel corridor between

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the two basins, and then south-
southeast through the Ukiah Basin.

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At about 1 million years ago, faulting
instigated a partial reversal of drainage.

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A new divide formed just north of the
Laughlin Range, dividing north-flowing

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streams through Little Lake Valley
from south-flowing streams through

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the Ukiah Basin. The faults active
at 1 million years ago are evident

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through potential field data and
well log records and evident

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on the landscape as fault-line scarps.
The faults ultimately play a role in the

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northern propagation
of the Maacama Fault.

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Since 1 million years ago, the
Maacama Fault has propagated

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north-northwestward
through this region.

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Another derivative product of
the Lidar data is a slope map.

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And we used the slope map to map
active traces of the Maacama Fault,

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shown here
by the red lines.

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We will show these active traces
on subsequent slope maps.

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These active fault traces represent
our mapping and are not fault traces

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from the USGS Quaternary
Fault Database.

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Based on the active fault trace
mapping, we divide the

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Maacama Fault into fault strands.
In the south, the Maacama Fault

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consists of a single strand –
the Ukiah strand.

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Further north, at the Laughlin Range
step-over, the fault steps into two

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strands – the Laughlin Range strand
and the Redwood Valley strand.

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These two strands trend on either
side of the Laughlin Range proper.

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Further to the north-northwest,
these two strands become the

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Willits strand on the west and the
East Willits strand on the east.

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The Willits strand extends
through the town of Willits.

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In a poorly defined manner,
these two strands merge into

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the single Laytonville strand
to the north-northwest.

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First focusing on the Willits strand,
the Willits strand actively creeps

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through the town of Willits.
Looking down this sidewalk curb,

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you can clearly see the deflection
caused by the actively creeping fault.

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Theodolite surveys across the fault
near the site indicate a creep rate

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of 5.7 millimeters a year based on
a decade of observation.

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The accessibility and the active
creep of the Willits strand prompted

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a paleoseismic investigation
of the strand.

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The chosen site for the trenching
was 3 kilometers along strike to the –

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along strike to the south of Willits,
just south of a prominent

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Late Holocene pressure
ridge along the fault.

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Objectives of the trenching
investigation were to develop

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an earthquake chronology
and to determine a slip rate.

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Hence, the construction of trenches
both parallel to and orthogonal

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to the fault, which is
delineated by the red line.

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For the slip rate estimate,
the strategy was to find

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piercing points of features
correlative across the fault.

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Investigations found evidence both for
discrete ground-displacing earthquakes,

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which occurred 1,170 and 970 years
ago, as well as piercing points

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to measure offset rate.
A channel that crossed the fault was

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abandoned about 630 years ago and
subsequently displaced right-laterally

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4.6 meters, yielding a slip rate of 7.5,
plus or minus 1, millimeters per year.

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This graph of fault offset versus time
shows that the offset channel-derived

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slip rate between 6.4 and
8.6 millimeters a year is greater than

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the theodolite-derived creep rate,
permitting a possible earthquake

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in the last 600 years.
However, the upper bound of the

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creep rate is close in value to the
lower bound of the offset channel rate,

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leaving open the possibility that the
channel was offset entirely by creep.

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This graph of offset versus time also
depicts the geodetically determined

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deep slip rate of 13 millimeters a year
for the Maacama Fault at this latitude.

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That is, the rate across the
entire breadth of the fault zone

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occurring by deep slip
below the locked zone.

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This graph is the same graph
as previous but framed so as to

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depict how slip accrues over time.
For instance, after 1,000 years of

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elapsed time, the Willits strand
would have slipped – the Willits strand

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would have slipped 7.5 meters,
both by creep and earthquakes,

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based on the geologically determined
slip rate. Over the same time period,

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the fault zone as a whole would have
accrued slip of about 13 meters.

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At the latitude of Willits,
where could the remainder

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of the slip be accommodated?
The East Willits strand is a candidate

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to accommodate accrued slip.
The East Willits strand has Holocene

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fault scarps, a pronounced gravity
signature, and active seismicity.

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This Lidar-based slope map shows –
indicated by the red arrows –

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a segment of the Holocene scarps
evident along the East Willits strand.

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On the next image of the
Maacama Fault, we are going to

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shift focus to isostatic
gravity anomalies.

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This map shows recently updated
isostatic gravity anomaly data

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for the northern Maacama based on
Gwen Erickson’s thesis work.

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Gwen established 465 new gravity
stations throughout the northern

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Maacama Fault corridor to
improve existing generalized

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gravity data
for the area.

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The most striking result is the
very strong gravity gradient along

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the east side of the
Little Lake Valley.

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The East Willits strand directly
overlies this steep gravity gradient.

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The active fault traces
are superposed on this map.

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Not only does the East Willits strand
overlie a steep gravity gradient,

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but also an inventory of well logs
on a transect across the fault

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at the steepest gradient shows
that the Franciscan basement rock

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has been displaced down to
the west by at least 80 meters.

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Note that the Redwood Valley strand,
on the southeast margin of the

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Laughlin Range, overlies a steep
gravity gradient in a fashion similar

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to the East Willits strand.
Both of these strands are faults

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that were operative in de-ranging
drainage more than a million years ago.

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These faults, in turn, influenced
the currently active process

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of northward propagation
of the Maacama Fault.

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On the next image,
we will show seismicity

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along strands of the
northern Maacama Fault.

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Epicenters are for seismicity
between 1985 and 2005.

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Epicenters are overlain by the
active traces of the Maacama Fault.

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In the southern part of the map,
the seismicity overlies, or is adjacent to,

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the Ukiah Fault strand.
Moving north to the Laughlin Range

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step-over, the seismicity becomes
displaced to the east of the fault traces.

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North of the step-over, the seismicity
largely follows the Redwood Valley

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strand and the East Willits strand.
In contrast, seismicity is notably

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absent underlying the Willits strand.
The yellow lines delineate the

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four seismicity cross-sections
in the next image.

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On these four cross-sections,
note that seismicity is not recorded

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below approximately 9 to
10 kilometers’ depth, which is

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the base of the seismogenic zone.
On the bottom cross-section,

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the seismicity at depth mostly underlies
the surface trace of the Ukiah strand.

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Moving northward, on the Laughlin
Range strand just north of the

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step-over, seismicity at depth
is removed to the east of

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the surface trace.
Even further north,

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at the latitude of Willits and north
of Willits, seismicity at depth is

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distinctly to the east of the surface
trace of the creeping Willits strand.

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These sections invite a structure
interpretation shown on the next image.

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Structural interpretation of fault
geometry at depth, shown here,

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shows that the single fault strand
and Ukiah is steeply dipping.

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Moving north, at the step-over,
seismicity rises to the surface on

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the Laughlin Range strand,
but there is an indication of seismicity

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below the Redwood Valley strand.
At the latitude of the Little Lake Basin,

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we infer from seismicity,
in combination with surface creep,

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that the Willits Fault strand must
dip to the east to the base

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of the seismogenic zone,
whereas the seismically active

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East Willits strand is
a steeply dipping fault.

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Here we add focal mechanisms
for selected events.

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Most of these earthquakes are
strike-slip – the blue beachballs –

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but note the thrust mechanisms –
the red beachballs – for earthquakes

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located between cross-sections
C-C-prime and B-B-prime.

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The thrust – the thrust fault earthquakes
are at approximately 9 kilometers’

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depth on the east flank
of the Laughlin Range.

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Interestingly, if we plot epicenters for
seismicity of the recent 2020 Willits

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earthquake swarm on the same map,
the swarm epicenters are located

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on the eastern flank of the
Laughlin Range within the

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epicenter region of the historic
thrust focal mechanisms.

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In a summary of fault strands and
fault behavior, the Ukiah strand

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is the only obvious strand noted
on the Lidar at the latitude of Ukiah.

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And slip rate estimates from trenching
by Sickler and others permit all the

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deep slip to be resolved at the surface
on this strand, but the slip rate

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estimates are not well-constrained.
The Ukiah strand splits into two strands

00:12:57.920 --> 00:13:02.000
at the Laughlin Range step-over.
Thrust faults at the base of the seismic

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zone at this latitude – that is,
beneath and just north of the

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step-over – may in part be responsible
for the elevation of the Laughlin Range.

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The Laughlin Range is the prominent
topographic high along the fault zone.

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North of the step-over, fault scarps
are apparent on all the fault strands,

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but the Laughlin Range fault scarps
are especially notable, in that the scarps

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are present and apparent in rugged,
[inaudible] topography where frequent

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rejuvenation of scarps would
be necessary for preservation.

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The Willits strand both creeps and
has ground-rupturing earthquakes.

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In contrast, strand behavior is not
well-known for the East Willits strand.

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It could be all creep or creep plus
ground-rupturing earthquakes.

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The East Willits strand would be
a timely target for paleoseismic

00:13:46.400 --> 00:13:51.360
investigation to better assess the overall
seismogenic slip budget at this latitude.

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We leave you with a concluding
thought, that the legacy of structures

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developed in the prior
convergent margin setting provides

00:14:00.960 --> 00:14:04.480
the crustal structure through
which the Maacama Fault zone

00:14:04.480 --> 00:14:07.256
must subsequently
propagate northward.

00:14:07.280 --> 00:14:08.960
Thank you.

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[silence]