Good afternoon.
The topic of my talk today is the magnitude 6.2 offshore
Petrolia earthquake that took place on December 20th, 2021.
The subtitle, slip on the Mendocino Fault Zone,
has a question mark attached to it.
And the purpose of that question mark will be made clear as the talk goes on.
This talk contains contributions from many other people at the
Earthquake Science Center, at the NEIC, and at UC-Berkeley.
And I’m very grateful to everyone who contributed.
To begin, let’s start by looking at the tectonics of the area where
this earthquake took place – the Mendocino Triple Junction.
The Mendocino Triple Junction is the intersection of the Pacific,
North American, and Juan de Fuca Plates.
It’s one of the most seismically active parts of California,
and it has hosted several significant earthquakes in the past.
Some of these earthquakes include the 1992 magnitude 7.2 Petrolia
earthquake, which is shown here – a reverse fault.
And the 1980 magnitude 7.2 Trinidad earthquake, a strike-slip fault.
Both of these earthquakes interestingly produced aftershocks
on structures other than the main shock structure.
An important plate boundary fault in this area is the Mendocino Fault Zone,
a right-lateral oceanic transform fault separating the Pacific and Gorda Plates.
So this area produces quite a few earthquakes and people living in
Humboldt County and Eureka are no strangers to feeling earthquakes.
So, on December 20th, 2021, around midday,
a magnitude 6.2 earthquake took place.
What we knew about the earthquake initially is that
its event location appeared to be offshore, as shown here.
And its focal mechanism appeared to be a strike-slip earthquake.
So, armed with these two pieces of information, it seemed likely
that the event took place on the Mendocino Fault Zone.
It was felt widely throughout the area, and in some cases, the shaking
was strong enough to be disruptive, as shown here in a grocery store
in Ferndale, California.
For the next few slides of the talk, I’m going to go into some of the
geophysical data that was produced during this earthquake and then
move into the implications of what that geophysical data implies
for the source processes of the earthquake.
So first let’s look at ground motion data.
The ground motion coming in from seismic stations throughout northern
California shows shaking intensity of up to MMI 7 near Petrolia.
This is quite strong for an offshore event, suggesting almost immediately
that the event was either closer to shore than was initially reported
or that it had an eastward rupture directivity, perhaps.
As an example, in the town of Ferndale, California, the PGV was up to
25 centimeters per second, and the PGA was about 29% g.
So next we can look at the seismic data from the event.
And shown here are two types of record sections.
On the left is a record section with local stations from David Shelly.
And on the right is a more teleseismic record section from Paul Earle.
Relatively quickly, we were also able to pull
a solution from the GCMT catalog for the event.
Now, these seismic data have some unusual characteristics.
I highlighted them here in green. Some of the unusual features are
an arrival between the P wave and the S wave in the local record section,
and in the case of the GCMT catalog, a parameter called centroid time
minus hypocenter time ended up being 12 seconds in this solution.
12 seconds is unusually long because other events in this area of this
magnitude have centroid time minus hypocenter time more typically
between 1 and 4 seconds. So 12 seconds is quite long.
Another interesting feature is that the location of the GCMT solution
is farther to the east than the initial location of the event.
Another interesting seismological observation is that the aftershocks
fall into two distinct clusters. One of these clusters is aligned
with the Mendocino Fault Zone here, and the other is onshore and is a little
bit deeper – around 25 kilometers deep. These two distinct clusters of
aftershocks became apparent almost immediately, in the first day even,
but the aftershock activity has continued over the few weeks after the earthquake
and remains relatively vigorous for an event in the Mendocino Triple Junction.
So next we will look at some of the geodetic data from this event.
Initially, on the first day, and in the very few days after, it appeared that
there were relatively small, perhaps unable to be determined, offsets.
So, from a Sentinel-1 interferogram, shown here on the left, this is a 12-day
interferogram spanning the event. It is difficult to see any offsets at all.
And, in the case of the rapid – the high-rate GNSS solutions,
it also appears that the offsets are within the level of the noise.
This is not terribly surprising for an event with an offshore location.
However, in the days after the earthquake, as more data came in,
it became clear that there may be some small offsets in the GNSS time series.
So, on the left, I’m showing plots of east and north position at GNSS stations.
This is daily position at GNSS stations in this area.
And, on the right, we’re seeing a plot of offsets produced by Jerry Svarc,
solved from the GNSS time series – the daily time series.
The offsets are quite small – around 2 to 4 millimeters, typically.
They’re small enough that it’s quite difficult to see them visually in the
time series at all, but they are spatially coherent, and they do make sense for
some of the – for some of the source – estimates of source processes
for this event. So we think that they are
real offsets even though they are only 2 to 4 millimeters.
So, when we look at all of these geophysical observations in total,
some of them, as I mentioned, are unusual for an event of the
initial location – the strong on-land shaking, the second phase arrival,
the parameters of the GCMT solution, the aftershock pattern, and some of the
offsets. These are a little bit unusual. And, as a result of these observations,
several people at the USGS and UC-Berkeley got together
and determined that there was another solution for the origin
of the earthquake that made more sense in light of these observations.
And that solution was two earthquakes.
They are separated by about 11 seconds, and they are separated
by 30 kilometers in spatial distance. One of them – the first was a smaller
event of magnitude 5.7, and the second was larger – about magnitude 6.2.
So the solution that is now presented in the USGS catalog has two events –
5.7 being here offshore and the 6.2 onshore.
The moment tensor for this event is likely dominated by the
moment release of the larger of the two events.
And this new solution has more success at explaining some of those unusual
observations, such as the second phase arrival and the strong on-land shaking.
It also has success in explaining the GNSS offsets.
This is ongoing research. It’s very preliminary.
But we’re able to forward model two point sources. It’s not an inverse.
It’s simply a forward model of two point sources – one located onshore
and one located offshore – and forward models the GNSS displacements.
What we find is quite consistent with the observations in the quadrants
and the overall pattern of the displacement field.
This forward model also allows us to understand why the InSAR
displacements from the observations were so low because the predicted
vertical here in this forward model is only plus or minus 2 millimeters,
which is very difficult to observe using InSAR.
So let’s look at the implications of these events and place the
December 20th events in the context of the Mendocino Triple Junction.
Typical seismicity in this area includes strike-slip earthquakes
on the Mendocino Fault Zone, as you can see here.
And it also includes strike-slip earthquakes within the
Gorda Plate itself. There have been large earthquakes, up to magnitude 7 –
strike-slip earthquakes within the Gorda Plate, and their
orientations are in a northeast or southeast trend.
Some of these earthquakes, based on their aftershock patterns, such as the
2014 earthquake, appear to be on this northeast-trending lineation.
However, there’s also historical precedent for some of these earthquakes
taking place on the southeast trend of this focal mechanism.
And then, within the Triple Junction itself, the seismicity becomes
a little bit more of a diffuse cloud, but there are some identifiable
bands and structures within the Triple Junction.
So the events of December 20th suggest that the first one – first event may have
been on the Mendocino Fault Zone, as shown here – a strike-slip event.
And that the second, with its deeper depth –
about 27 kilometers, is likely within the Gorda slab.
There haven’t been too many of these
strike-slip earthquakes right around this area, though.
And the location and the timing of the two events do suggest
a dynamic triggering relationship between the two of them.
Another interesting feature about this area is that the Mendocino Fault Zone
has some indication of partial creep from repeating earthquakes and from
accounting of moment budget. But it is important to remember
that even partially creeping faults are still capable of magnitude 6
and larger earthquakes. And this event shows the
seismic potential of a fault like the Mendocino Fault Zone.
So what have we learned? We have seen a moderate-sized
earthquake on the Mendocino Fault Zone and a larger inter-slab earthquake.
These events, in general, are consistent with historical seismicity in the area,
but the magnitude 6.2 is of a mechanism and a location
that we have not seen too many of in recent years.
The two events were separated by about 11 seconds and 30 kilometers,
and they produced a vigorous onshore aftershock sequence.
The geodetic displacements are quite small, but they are consistent with the
story of the source processes that we’re deriving from seismic data.
And what is there still to learn?
There are a lot of ongoing research directions generated by this earthquake.
For example, where exactly was the moment released, and how much?
What is the connection between the offshore and onshore faults?
And interestingly, what structure is hosting the onshore events?
These events underscore that the Mendocino Triple Junction is a
complex area with a history of earthquakes on multiple faults.
Modeling using multiple data sets will be needed to understand the
connections between these events, specifically the structures that hosted
them, the aftershocks sequences, and the major plate boundaries.
And lastly, the events raise some interesting questions about seismic
and aseismic slip on faults and about dynamic triggering relationships
between earthquakes in the Triple Junction. Thank you.
[silence]