I’m going to give you a quick overview of
what we did in the last couple of years during the pandemic.
Let’s consider two types of creeping fault.
The end member on the left is a creeping fault driven from below.
Seismicity causes creep to propagate to the surface.
The one on the right is a creeping fault that is driven by anti-symmetric shear.
And the seismogenic zone is potentially locked.
And each creep event propagates to a shallow depth or to a greater depth but
eventually loads the seismogenic zone. So this is potentially more interesting
than this kind of creep, which is interesting,
but it only tells you about the processes after seismicity.
Here’s an example of useful data from creepmeter.
In fact, there was an earthquake in San Juan Bautista in 1998,
surrounded by a cluster of creepmeters. And, if you sum three of those
creepmeters and detrend the summed series, you finish up with
this graph on the top, which shows rather clearly that there was a massive
retardation on the creep record preceding that earthquake.
And, of course, this is a process that will occur again and again
near this triple junction. So, in principle, if we could
understand this, we have a way of forecasting moderate seismicity.
On the Hayward Fault, there are five creepmeters.
We’d like greater density, especially the absence of creepmeters on the
Calaveras Fault is a bit of a problem at the moment, but what you can see
is that, near Berkeley, two creepmeters show a concave-up distribution of slip,
which implies acceleration starting around about 2007.
At the same time, there was a deceleration in creep rate at Palisades
in Hayward, and this creepmeter to the south near Fremont has sort of
decadal variation in creep, which could be due to
interaction with the Calaveras Fault.
Unfortunately we don’t have the creepmeters to tell us that.
And the alignment arrays, of course, are much too noisy.
Let’s look at the central creeping zone briefly.
Here are data from Melendy Ranch, XMR, and you can
see that there are large numbers of creep events – these grays things,
and a smaller number of larger ones. In fact, there’s a bimodal distribution
of small ones and big ones. And, if you sum the slip from just the
small ones, you get a large creep rate, from the big ones, a smaller creep rate,
and you can plot it in a graph like this showing what is a proxy
for slip at depth. And I think you can see
fairly clearly that the rapid slip near the surface has resulted in
75% of the slip deficit being removed, but leaving 25%.
But, at deeper depths, somewhere between 3 and 5 kilometers,
we may have a 50% slip deficit.
Here, again, here are creep data that are telling us about seismic hazards.
John Langbein did this for the entire creeping zone.
You can’t see the plots, but let me explain what you
could do with this kind of data.
Middle Mountain data are shown without editing here.
It’s a fairly steady linear creep rate up to the Parkfield earthquake
at about 16 millimeters a year. If you only look at the creep events,
you get a lower creep rate, 6.6 millimeters a year,
and if you detrend those two data, you can see fairly clearly that the
creep rate from creep events is far less contaminated by surface noise.
In other words, one can clean up the data, suppress the noise considerably,
by just looking at creep events. Another way of looking at these data
is to look at the long-term trends. And they’re emphasized by these
black lines, where you can actually see changes of rate influenced
at the time of regional earthquakes. For example, the Loma Prieta
earthquake caused a significant change in rate in the northern creepmeters.
The Parkfield earthquake, of course, influenced most of them.
But we can think of the creeping zone as
a very sensitive strainmeter to regional stress changes.
Now, what we did in the last year or so was to add orthogonal
measurements to strainmeters. So a normal strainmeter just –
a creepmeter measures merely an oblique component.
But it’s sort of ambiguous if one assumes that block-like
motion does not apply. So various geometries were tried,
and the results are shown here. Dilation is plotted vertically,
dextral slip horizontally. And there are seven faults here, and we’ll
look at the extreme results now. The first one is from the Dead Sea.
Second from Salt Creek on the Salton Sea segment of the San Andreas Fault.
And the third one is XMR, which we just looked at.
If you look at the Dead Sea vector, it’s across a normal fault, and you
would expect there to be considerable opening should it move, and it did.
It started – the first six months of data didn’t do very much, and then,
in September, it started slipping slowly, 24 millimeters a year.
However, the first six months of data were accompanied by dextral slip.
And only when it started moving rapidly did you get a sinistral
motion across the fault. Second example is from Durmid Hill
on the southern San Andreas. The creepmeter array looks like this.
You have three simultaneous equations to solve.
And when you do that, you finish up with an observation that the fault zone
is closing slowly at about 0.2 millimeters per year,
a very small amount, but it’s closing both in a secular sense
and in creep events – individual creep events.
So here is the creep during the Chiapas event. It closes at 7.9 degrees.
During Ridgecrest, at 5. During a recent creep event,
it’s still ongoing at 7.5. All these angles of transpressional
closure are very similar to that inferred for the entire fault system.
Now we can look at the XMR data. This is from the northern creeping zone.
And you can see a very complicated fault slip vector.
First of all, periods of heavy rain make the fault go completely nuts.
But, in between, during the dry season, you can see that the fault zone opens
quite uniformly at about 12 degrees. Here’s an example of a single
creep event. And, during the inter-event period, it also closes
at about 12 degrees. And the difference in velocities
between these two are about 24,000. So this is a very unexpected result.
We have a sort of explanation for that. If you look at the fault zone,
you can find that it consists of these curious clasts of clay that are obviously
rotating relative to each other and interlinked in some way.
So, during slip, we suppose that these – during a creep event – rapid creep,
we suppose dilation occurs through rotation or wedging of these clasts.
But, during the inter-event period, they rotate back.
And during saturation conditions, when heavy rain occurs,
they completely relax into a tight packing arrangement. We’re not sure.
Now, what about gas? During the Sivrice earthquake
in 2021, there was a huge amount – or, sorry, 2020 the earthquake occurred.
In the first few days of that event, when afterslip was just beginning,
we saw a large amount of gas coming out of the fault zone.
So the natural question is, does that occur on the San Andreas system?
Is it caused by ventilation – the fault moving – or is it actually
causing the fault to move through some kind of lubrication effect?
Well, the answer is that gases are indeed coming out of the fault zone,
but largely because of biogenic processes.
Microbes become active when the fault zone is wet.
But what we did notice was that, after creep events, not during them,
we had bursts of gas coming out. And we suppose that’s caused by
en echelon crack formation, since here is an example of
crack formation – we saw these cracks appear soon after we
witnessed gas coming out. Radon seems to come out of the fault
with carbon dioxide as a kind of carrier. Now, looking at the creep events
in detail, this is the opening velocity stacked for five events.
And the carbon dioxide vented, one-minute samples for five events,
also stacked. And what you can see is that, when the fault opens,
there is a reduction is the concentration of carbon dioxide.
This would be atmospheric levels, and this would be the fault zone levels.
So there is – this reduction means the fault probably did indeed
dilate sufficiently to suck air in from the atmosphere.
The fault sucks during creep events.
This example I’m going to show you is from the Calaveras Fault,
and it shows something much more interesting.
What we’re seeing here is clearly a gas venting before a creep event and
turning off a few weeks afterwards. Again, same thing happened here.
Very large creep event – these are 16 millimeters.
And it switches off afterwards. And there’s, you know,
normal oscillations of gas in between. But these are off-scale, both for radon
and for carbon dioxide. What we think may be going on there is
that fault valving of some sort nucleates gas near microearthquakes that are
occurring in the seismogenic zone, propagating to the surface slowly,
and coming to the surface with creep events. Since they precede creep
events, we infer that maybe they are lubricating the fault in some way.
So gas venting actually brackets slip.
So, in summary – and we found various new novel ways of
looking at creeping faults. And, if they are to be incorporated in
the next century of fault measurements, we need to ensure data continuity.
In other words, right now, they are a bit of an outlier.
They’re maintained almost by accident.
We need more creepmeters, and we need multiple sensors.
Thanks very much for listening.