Good afternoon, and thank you for inviting me to present in this session.
My presentation today is a bit fragmented to the extent that I will
give a brief overview of where we stand with our update of
the real-time double-difference system – a project that has been
funded by NEHRP. And then, in the second half,
I will focus on the recent activity on the Maacama Fault near Willits.
We have recently completed a new update of the double-difference base
catalog for northern California for events between 1984 and 2019.
The catalog includes precise locations for close to 900,000 earthquakes.
So these locations are based on analysis of 4 billion phase delay times
formed from arrival time picks and about 5 billion delay times
measured from cross-correlation of P and S waves.
An initial look at the results indicate similar or better resolution
compared to the previous catalogs. The figure on the right side show –
for the region where the San Andreas Fault and Calaveras Fault intersect,
a comparison between the double- difference location and the network
locations for both the new, at the top, and the old catalog at the bottom.
Essentially, what we see is an increase in event density
as the earthquakes since 2011 reactivate pre-existing structures.
The complete catalog is shown in the middle as blue dots –
events between 1984 and 2019 – and then, in red, as the real-time
locations that are continuously being computed.
You can see the active areas in the Geysers and [inaudible] popping up,
and then the red arrow points to the Maacama Fault that I will
show in a little bit.
Just a reminder that the high-precision double-difference locations are being
computed and made available in near real time for over a decade now.
So the way the real-time solutions are computed is a little bit different
from the way we compute base catalogs.
So the new base catalog offers the opportunity to evaluate the
performance of the real-time system. We looked at 250,000 real-time
locations and compared them to the corresponding locations the new –
in the new base catalog. You can see on the figure
on the left that the differences are typically less than 100 meter,
especially along well-monitored faults.
Once we update the real-time system with the new base catalog,
then the differences will be even smaller.
Differences between the real-time locations and the NCSN network
locations are quite larger in most regions.
Moving to the second part of the talk, I am showing on the left a figure
with the new base catalog for the Maacama Fault in blue
and the real-time double-difference locations since the beginning
of 2020, which includes the Willits sequence in red.
Since it – since I will be using real-time double-difference locations
in all upcoming slides, I just wanted to make sure that we are looking at
a high-resolution picture of this area. So I am showing on the right side
of the slide the difference between the real-time double-difference
locations and the new base catalog locations just like in the previous slide.
So, again, these differences are – differences are typically smaller
than 100 meter in the area we are interested in.
So those – the next couple of slides show seismicity map and – map view
and cross-sections color-coded by time for the sequence near Willits.
So you can see that the first sequence actually started in 2019 –
in April 2019, shown in blue.
And the second sequence started in August of 2020, shown mostly in red.
This is obviously a very interesting sequence, which I have to admit,
I just started looking at in preparation for this talk.
The first sequence started with a magnitude 4.3 at about 8 kilometers’
depth and then went straight up through the crust to reach the surface.
And the second sequence started with a magnitude 4 at about
7 kilometers’ depth and, similar to the 2019 sequence,
went also straight to the surface. The 2020 sequence included the
second magnitude 4 event at about 8 kilometers’ depth.
The seismicity in the time period between the two sequences –
mostly that green color – mostly occurred in the
6- to 8-kilometer depth range with no significant large events.
This slide is the same as the previous slide except that I show a close-up of
the Willits sequence, again with the 2019 in blue and the 2020 in red.
In a fault – in a fault-perpendicular cross-section, as shown in the
middle panel, we see that the cluster does not image a narrow fault
but rather a near-vertical tube, or pipe, of seismicity.
This slide shows the seismicity until the start of the second sequence.
So the second sequence is not imaged here.
The interesting thing to notice is that the location of the second
sequence was already active before the first sequence.
Even included some larger events at about 5 kilometers’ depth.
So those events are shown in gray.
There also seems to be a seismogenic connection
between the two sequences at the base of the brittle crust.
This slide shows the temporal evolution of the second sequence in 2020.
It started at the base of the – of the seismogenic zone
just like the 2019 sequence. The 2020 sequence shows
a pipe similar to the 2019 sequence that has more events with
a somewhat more diffused distribution.
Again, it is interesting to note that the 2020 sequence started
close to the nucleation location of the 2019 sequence and then veered off
to the northwest, suggesting that they are somehow triggered
by a common driving force.
The sequence is still ongoing. And I created a special website
that tracks the activity near Willits in real time as part of
the DDRT system operation. You can go to the DDRT website
and click on the link under “earthquake of special interest,”
and then you find the same figure that I showed you.
Now those pipes – those pipe-like structures resemble very much
pipe structures that we observed in subduction zones in Java, Sumatra,
and on the Marianas, and which we all interpreted as being fluid-driven.
So shown on the right lower panels are examples for Java and then Sumatra.
So, if I make similar plots for the Willits sequences, and plot the timing
of the events as a function of depth, as shown on the left side of the –
of the slide, we observe bursts of seismicity that migrate upward
at high speeds. I had a very quick look at the ascent rates, and they are in the
kilometers per hour range, so similar to ascent rates we observed for fluids
that come off the subduction interface after large thrust events.
So that’s definitely something that we should have a closer look at.
But these initial results suggest that somehow fluids got released
by breaking some kind of seal by a larger event.
So, in summary, we have – we’re close to having
a new double-difference base catalog for California.
We need to do some tests and evaluate the location robustness.
Then preliminary evaluation of the real-time double-difference locations
that we produced over the last 10 or so years indicate resolution
better than 100 meter in well-monitored regions.
So moving forward, that system is only going to become better
in terms of location precision.
The Maacama Fault near Willits experienced two earthquake
sequences in 2019 and 2020. The 2021 is still ongoing.
I would say the double sequence is still ongoing.
Preliminary spatiotemporal analysis suggests that they are fluid-driven,
similar to observations we made in subduction zones.
There are three repeating event sequences that occurred near
the recent Willits activity. That’s in the catalog of
repeating earthquakes we have. But it is not clear yet whether
any of the new events are repeating events.
So we will check on that.
Ongoing work in terms of the real-time operation for the current project is we
will be able to release a new double- difference base catalog sometime soon.
We will update and improve the RT-DD system for northern California
over the next couple of months.
So they will be at the same location online.
And then we will be working on implementing the repeating
earthquake database that we have produced over the several –
over the last couple of years and implement that into the
RT-DD system for real-time tracking of recurring events.
Thank you, and I will see you soon.