With my co-authors, Chad Trexler and Kevin Furlong,
today we will discuss the northern Maacama Fault –
the young fault propagating through legacy crustal structures.
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We would like to acknowledge unpublished data sources
from Gavin Hayes, Gwen Erickson, and Adam Woolace.
And we point out four key published sources pertinent to this presentation.
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The San Andreas Transform boundary, north of the latitude
of San Francisco Bay, consists of three strike-slip faults – the San Andreas
Fault proper, the Maacama Fault, and the Bartlett Springs Fault.
This presentation focuses on the northern Maacama Fault within
the delineated rectangular area. Note that the Maacama – note that
the Mendocino Triple Junction migrated northward past this region
approximately 3 million years ago. And subsequent crustal response
over the ensuing millennia ultimately resulted in the northward propagation
of the Maacama Fault through this region.
The road map for this talk is that we will discuss regional geologic history,
then introduce the active traces of the northern Maacama Fault
using a new Lidar database.
Then we will discuss paleoseismic gravity and seismicity investigations
that inform fault zone behavior and structure.
We will place the 2020 Willits earthquake swarm within the context
of the fault zone structure, and finally present a wrap-up summary.
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The map images in this presentation are constructed from USGS 3D
elevation program Lidar coverage of northern California released in 2017.
The Lidar enables a revised comprehensive mapping
of active traces of the northern Maacama Fault.
This Lidar-based hillshade map of the study region shows the
north-northwest trend in the topography, reflecting underlying
structure of the Mesozoic Franciscan Assemblage bedrock.
The elevation-based color bands show that the highest topography
in the region is in the center of the image – the Laughlin Range
with a 1,000-meter summit elevation.
The lower – the lower elevation limit is the Ukiah Valley floor at 170 meters.
There are two prominent basins in the field area – the northern Little Lake
Basin, containing the town of Willits, and the southern elevation in lower
Ukiah-Redwood Valley Basin. These basins are filled with
Neogene and Quaternary sediment. Both basins’ structure and
sedimentation predates inception of strike-slip tectonics.
These are not strike-slip basins.
Prior to 1 million years ago, drainage direction through these basins was
uniformly to the south-southeast – from the south-southeast-directed flow
into the Little Lake Basin, through a narrow channel corridor between
the two basins, and then south- southeast through the Ukiah Basin.
At about 1 million years ago, faulting instigated a partial reversal of drainage.
A new divide formed just north of the Laughlin Range, dividing north-flowing
streams through Little Lake Valley from south-flowing streams through
the Ukiah Basin. The faults active at 1 million years ago are evident
through potential field data and well log records and evident
on the landscape as fault-line scarps. The faults ultimately play a role in the
northern propagation of the Maacama Fault.
Since 1 million years ago, the Maacama Fault has propagated
north-northwestward through this region.
Another derivative product of the Lidar data is a slope map.
And we used the slope map to map active traces of the Maacama Fault,
shown here by the red lines.
We will show these active traces on subsequent slope maps.
These active fault traces represent our mapping and are not fault traces
from the USGS Quaternary Fault Database.
Based on the active fault trace mapping, we divide the
Maacama Fault into fault strands. In the south, the Maacama Fault
consists of a single strand – the Ukiah strand.
Further north, at the Laughlin Range step-over, the fault steps into two
strands – the Laughlin Range strand and the Redwood Valley strand.
These two strands trend on either side of the Laughlin Range proper.
Further to the north-northwest, these two strands become the
Willits strand on the west and the East Willits strand on the east.
The Willits strand extends through the town of Willits.
In a poorly defined manner, these two strands merge into
the single Laytonville strand to the north-northwest.
First focusing on the Willits strand, the Willits strand actively creeps
through the town of Willits. Looking down this sidewalk curb,
you can clearly see the deflection caused by the actively creeping fault.
Theodolite surveys across the fault near the site indicate a creep rate
of 5.7 millimeters a year based on a decade of observation.
The accessibility and the active creep of the Willits strand prompted
a paleoseismic investigation of the strand.
The chosen site for the trenching was 3 kilometers along strike to the –
along strike to the south of Willits, just south of a prominent
Late Holocene pressure ridge along the fault.
Objectives of the trenching investigation were to develop
an earthquake chronology and to determine a slip rate.
Hence, the construction of trenches both parallel to and orthogonal
to the fault, which is delineated by the red line.
For the slip rate estimate, the strategy was to find
piercing points of features correlative across the fault.
Investigations found evidence both for discrete ground-displacing earthquakes,
which occurred 1,170 and 970 years ago, as well as piercing points
to measure offset rate. A channel that crossed the fault was
abandoned about 630 years ago and subsequently displaced right-laterally
4.6 meters, yielding a slip rate of 7.5, plus or minus 1, millimeters per year.
This graph of fault offset versus time shows that the offset channel-derived
slip rate between 6.4 and 8.6 millimeters a year is greater than
the theodolite-derived creep rate, permitting a possible earthquake
in the last 600 years. However, the upper bound of the
creep rate is close in value to the lower bound of the offset channel rate,
leaving open the possibility that the channel was offset entirely by creep.
This graph of offset versus time also depicts the geodetically determined
deep slip rate of 13 millimeters a year for the Maacama Fault at this latitude.
That is, the rate across the entire breadth of the fault zone
occurring by deep slip below the locked zone.
This graph is the same graph as previous but framed so as to
depict how slip accrues over time. For instance, after 1,000 years of
elapsed time, the Willits strand would have slipped – the Willits strand
would have slipped 7.5 meters, both by creep and earthquakes,
based on the geologically determined slip rate. Over the same time period,
the fault zone as a whole would have accrued slip of about 13 meters.
At the latitude of Willits, where could the remainder
of the slip be accommodated? The East Willits strand is a candidate
to accommodate accrued slip. The East Willits strand has Holocene
fault scarps, a pronounced gravity signature, and active seismicity.
This Lidar-based slope map shows – indicated by the red arrows –
a segment of the Holocene scarps evident along the East Willits strand.
On the next image of the Maacama Fault, we are going to
shift focus to isostatic gravity anomalies.
This map shows recently updated isostatic gravity anomaly data
for the northern Maacama based on Gwen Erickson’s thesis work.
Gwen established 465 new gravity stations throughout the northern
Maacama Fault corridor to improve existing generalized
gravity data for the area.
The most striking result is the very strong gravity gradient along
the east side of the Little Lake Valley.
The East Willits strand directly overlies this steep gravity gradient.
The active fault traces are superposed on this map.
Not only does the East Willits strand overlie a steep gravity gradient,
but also an inventory of well logs on a transect across the fault
at the steepest gradient shows that the Franciscan basement rock
has been displaced down to the west by at least 80 meters.
Note that the Redwood Valley strand, on the southeast margin of the
Laughlin Range, overlies a steep gravity gradient in a fashion similar
to the East Willits strand. Both of these strands are faults
that were operative in de-ranging drainage more than a million years ago.
These faults, in turn, influenced the currently active process
of northward propagation of the Maacama Fault.
On the next image, we will show seismicity
along strands of the northern Maacama Fault.
Epicenters are for seismicity between 1985 and 2005.
Epicenters are overlain by the active traces of the Maacama Fault.
In the southern part of the map, the seismicity overlies, or is adjacent to,
the Ukiah Fault strand. Moving north to the Laughlin Range
step-over, the seismicity becomes displaced to the east of the fault traces.
North of the step-over, the seismicity largely follows the Redwood Valley
strand and the East Willits strand. In contrast, seismicity is notably
absent underlying the Willits strand. The yellow lines delineate the
four seismicity cross-sections in the next image.
On these four cross-sections, note that seismicity is not recorded
below approximately 9 to 10 kilometers’ depth, which is
the base of the seismogenic zone. On the bottom cross-section,
the seismicity at depth mostly underlies the surface trace of the Ukiah strand.
Moving northward, on the Laughlin Range strand just north of the
step-over, seismicity at depth is removed to the east of
the surface trace. Even further north,
at the latitude of Willits and north of Willits, seismicity at depth is
distinctly to the east of the surface trace of the creeping Willits strand.
These sections invite a structure interpretation shown on the next image.
Structural interpretation of fault geometry at depth, shown here,
shows that the single fault strand and Ukiah is steeply dipping.
Moving north, at the step-over, seismicity rises to the surface on
the Laughlin Range strand, but there is an indication of seismicity
below the Redwood Valley strand. At the latitude of the Little Lake Basin,
we infer from seismicity, in combination with surface creep,
that the Willits Fault strand must dip to the east to the base
of the seismogenic zone, whereas the seismically active
East Willits strand is a steeply dipping fault.
Here we add focal mechanisms for selected events.
Most of these earthquakes are strike-slip – the blue beachballs –
but note the thrust mechanisms – the red beachballs – for earthquakes
located between cross-sections C-C-prime and B-B-prime.
The thrust – the thrust fault earthquakes are at approximately 9 kilometers’
depth on the east flank of the Laughlin Range.
Interestingly, if we plot epicenters for seismicity of the recent 2020 Willits
earthquake swarm on the same map, the swarm epicenters are located
on the eastern flank of the Laughlin Range within the
epicenter region of the historic thrust focal mechanisms.
In a summary of fault strands and fault behavior, the Ukiah strand
is the only obvious strand noted on the Lidar at the latitude of Ukiah.
And slip rate estimates from trenching by Sickler and others permit all the
deep slip to be resolved at the surface on this strand, but the slip rate
estimates are not well-constrained. The Ukiah strand splits into two strands
at the Laughlin Range step-over. Thrust faults at the base of the seismic
zone at this latitude – that is, beneath and just north of the
step-over – may in part be responsible for the elevation of the Laughlin Range.
The Laughlin Range is the prominent topographic high along the fault zone.
North of the step-over, fault scarps are apparent on all the fault strands,
but the Laughlin Range fault scarps are especially notable, in that the scarps
are present and apparent in rugged, [inaudible] topography where frequent
rejuvenation of scarps would be necessary for preservation.
The Willits strand both creeps and has ground-rupturing earthquakes.
In contrast, strand behavior is not well-known for the East Willits strand.
It could be all creep or creep plus ground-rupturing earthquakes.
The East Willits strand would be a timely target for paleoseismic
investigation to better assess the overall seismogenic slip budget at this latitude.
We leave you with a concluding thought, that the legacy of structures
developed in the prior convergent margin setting provides
the crustal structure through which the Maacama Fault zone
must subsequently propagate northward.
Thank you.
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