Hello, everyone. My name is Alicia Hotovec-Ellis,
and I’m a research seismologist with the California Volcano Observatory.
A few months ago, Sarah Minson asked me to give a talk on the physics
of swarms, and I figured, sure. This will be a great excuse to
refresh myself on the literature and showcase a little of my own work.
Unfortunately, the format’s a bit short to do both with
any amount of depth of nuance, so I’ll highlight the brief aspect
of my talk’s title to keep in mind as we get rolling.
Naturally, this being part review talk, I have to start with some
quick definitions so we’re all on the same page.
What is a swarm, and how are they different from
main shock/aftershock sequences? Here I’ve two example magnitude
time histories for two real earthquake sequences in Alaska.
Although they are both spatiotemporally clustered,
the behavior of the earthquakes with time is very different.
Main shock/aftershock sequences have the largest event either first,
or very near the start, in the event of a foreshock sequence.
But swarms can have the largest event at any point in the sequence,
with, perhaps, some preference to being in the middle.
And that event tends not to be exceptionally larger than
the next-largest event. This isn’t to say that swarm earthquakes can’t be large.
Magnitude 5s, 6s, potentially even up to magnitude 8 earthquakes
can be part of a swarm sequence. However, when there is a large
regular main shock, it’s followed by aftershocks that themselves
follow Omori’s law. Aftershocks become less frequent
with time in a very predictable way. While I wouldn’t really call swarms
truly random in time, they are certainly less predictable and often occur
in bursts of events. Finally, there’s a difference in the number of small
events compared to large events, usually referred to as the b value
from Gutenberg-Richter. Swarms, on average, tend to have
large b values, meaning that they are depleted in large events
given the number of small events. Of course, these really should be
ranges with some overlap and can be highly spatially heterogeneous.
I’m here because swarms are among the primary modes of seismicity
at volcanoes, across eruptive style and composition,
and whether they’re currently erupting or not.
True, there are some unique sources at volcanoes that don’t involve fault slip,
but when faults do slip at volcanoes, they preferentially tend to do so
in swarms. Same goes for hydrothermally and geothermally
active areas, anthropogenically induced seismicity from both
wastewater injection and hydro-fracturing, at the base of
slipping glaciers, and in the form of low-frequency earthquakes
and tremor during episodes of slow slip. Obviously, this isn’t a comprehensive
list but gives a flavor of the diversity, but also some similarities, in setting.
Similarities that allow us to derive a handful of
common features between them.
First, I’d like to highlight a few properties of the settings themselves.
For one, swarms tend to happen in areas of high heat flux.
For volcanoes in geothermal areas, this is due to the presence of magma.
But, as you go deeper – for example, in a subduction zone –
the hotter things get as well. The implication here is that
the heat lowers the viscosity. And we transition from brittle to
more ductile rheology and behavior. This is also where, in part,
glaciers fit in, as ice can be ductile enough to flow
yet still be brittle enough to fracture. Heat also means hot fluids, which
we’ll get to talk more about in a bit. But those bring in dissolved minerals
that can precipitate and alter the frictional properties of the faults.
Heterogeneities in these properties may also be a factor.
And, of course, having faults available to slip is important as well.
I have host rock here to encompass other bulk properties of the rock that
may be important, like permeability or porosity, in addition to their viscosity,
but also as a nod to induced seismicity, where proximity to crystalline basement
and the old faults there being a good predictor of whether
unintended seismicity will be induced.
Next, I’d like to stress – ha ha – the S in faults.
As we continue to improve the tools available to better observe swarms,
the more frequently we see that swarms involved multiple faults,
sometimes with different orientations and slip directions, not unlike the
interconnected fracture mesh that Dave Hill conceived of back in the ’70s.
In his model, the fracture mesh accommodated mode I opening
of dikes, but these cracks don’t have to be filled with magma.
Which leads us to the next major player. You called it. Fluids.
The geophysical go-to when you need a hand-waving explanation.
In all seriousness, fluids of all kinds are ubiquitous in the subsurface.
The Earth is not a dry place. Whether it be water, supercritical gas,
or a multiphase combination, fluids are a great way to lower the effective stress
on a fault and promote it to slip or to change the local stress state.
One of the features of swarms linked to the intrusion of high-pressure fluids
is that they migrate with time. In some cases, like this one,
the rate of that movement is consistent with the diffusion of fluid
through permeable host rock. Indeed, if we watch the propagation
of these pulses of seismicity, it’s hard not to imagine fluids or
something else moving up from depth, hopping from fault to fault.
And, in many cases, the most logical culprit is hydrothermal fluids.
Other times, such as for magmatic intrusions, seismicity follows closely
the tip of a propagating dike, which certainly isn’t a diffusive
process, nor are pulses of slow slip. These possibilities are all examples
of a transient perturbation to the system – something, whether it
be fluids, magma, slow slip, or a combination of any of them –
alters the state of the system. It can be highly localized,
such as a pulse of fluids traveling directly through a fault mesh,
or creep along a fault. Or it can be the elastic response
to a nearby intrusion at depth. In fact, the evacuation of mass can
also trigger swarms, such as during the summit eruption at Kilauea in 2018
and the partial draining of the magma chamber there.
The perturbation doesn’t even need to be local.
Areas prone to swarms and tremor are also often dynamically triggered by
large remote earthquakes or are modulated by moon or Earth tides,
suggesting that they are areas near failure already,
requiring very little additional stimulus to become active.
All this to say there are many factors that are likely shared between many
swarms in general, however identifying the unique factors contributing to a
specific swarm can still be a challenge. By way of example, I’ll briefly
show you the highlights from a series of swarms at depth
under Mammoth Mountain.
Mammoth Mountain sits atop the southwest rim of Long Valley
Caldera – home to skiing and some excitement in the late 1980s
due to a vigorous shallow earthquake swarm thought to be
due to the intrusion of a magmatic dike.
Several years later, degassing of carbon dioxide killed patches
of trees near Horseshoe Lake. The gas probably escaped because
the intrusion of the aforementioned dike breached a cap on a reservoir
of exsolved CO2. Flux of CO2 and other hydrothermal fluids are the likely
cause of shallow swarms in the area. But Mammoth also has a decent amount
of significantly deeper activity. Here I show earthquakes with time –
gray dots for shallow and black dots – scaled to magnitude –
for the deeper events. Don’t let the amount of ink here fool you.
There are about 100 times more earthquakes above the dashed line
than below it, but they aren’t the focus of this study.
The dashed line represents the approximate brittle-ductile transition
below which earthquakes less frequently occur.
And, yet, we still have plenty here, just episodically.
Another detail is that, at these depths, there are two different flavors
of earthquakes. On the left, the classic brittle failure – what we call
volcano tectonic earthquake that looks pretty much normal, though deep.
Then, on the right, an earthquake in nearly the same location
but with waveforms enriched in lower frequencies,
called deep long period, or DLP, earthquakes.
These are often thought to be due directly to the movement of magma
at depth, but their exact mechanism is still unknown.
Going back to the depth-time plot, I colored DLPs as open red circles,
and we can begin to see some interesting patterns in the occurrence
of these two types of earthquakes, especially if we zoom in.
Certainly there are differences in the long-term behavior of the earthquakes,
but the temporal proximity suggests a relationship between them.
Using high-precision relocation techniques, we can also clean up
the catalog and begin to see more clearly the spatial relationship
between the two as well. On the left, I’ve got a cross-sectional
view of the original locations with depth. Same color scheme as before.
Middle is the updated relocations. And right is the map view.
I’ll set this to rotate so that you can appreciate the shape of the volume.
Basically, we ended up with a northeast-southwest-trending,
near-vertical plane, where the DLPs that I was
able to relocate preferentially tend to occur in the center between two arms
surrounding an otherwise aseismic volume.
Of course, there are also some interesting behaviors that pop out
when we show the occurrence of these earthquakes with time.
I’ve got a cross-sectional view here, which will march forward in 30-minute
increments. DLPs are denoted as open circles. Otherwise, they’ll be filled.
Earthquakes will fade from yellow to black with age, and we’ll skip
any frames that don’t have any new offense to show.
The date up at the top can skip several weeks at a time, but, for most of
the swarms, we’ll follow without any breaks.
And I will go through and narrate as it loops through the second time.
All right. So the first major activity starts the deepest, then intermittently
works its way to the – way up the southwest arm.
Activity then moves deeper again and begins to fill out the bottom
of the northeast arm in this slowly expanding and the largest of the
swarms that I was able to relocate. Which then lights up in sort of
these smaller swarms. A few months later, activity jumps
to the top of the northeast arm and migrates down, then jumps over
to the other arm and migrates both up and down simultaneously.
It pops back over to the other arm briefly, and then most of
the longer-lived DLP activity that just kind of flew by
in this view, fills in the rest of the middle.
Another curiosity with the swarms was the presence of earthquakes
that I had called flipped polarities. Here, I have the waveforms for
two earthquakes that occurred closely in both time –
about 10 minutes apart – and space – of order of, perhaps,
50 to 100 meters apart. But I flipped the waveform of the
red earthquake upside-down to show how similar it is to the black one.
This implies that the earthquakes occurred on faults of very similar
orientation but with opposite senses of slip.
While this observation is unusual, it’s certainly not unprecedented.
For example, White and others observe a similar flipping of focal
mechanisms during a dike intrusion in Iceland, which they interpreted
as either fault slipping with opposite slip on either side of the dike tip,
or as slip on either side of a plug of solidified magma within the dike
and the country rock surrounding it.
An alternative possibility is that the orientations of the two faults are similar
but just ever so slightly different. And that their orientation relative to
the regional stress field causes the difference in slip factor.
In this example in Texas from Rutledge and others, they can see the slight
difference in fault orientations from the relocated hypocenters.
In our work, we’re looking at sort of a similar elongated system of faults
but rotated 90 degrees and at 15 to 20 kilometers’ depth.
So we don’t have the resolution to see these small deviations
in orientation if they exist.
Tying it all together, we have the earthquakes and, by proxy,
the fault system they occur on situated in the mid- to lower crust with
the Moho here at the bottom and the surface at the top,
and then the usual brittle-ductile transition closer to the surface.
Hot fluids, whether they be magma itself or exsolved supercritical CO2
and/or water, are sourced from depth. And they work their way through the
system in discrete bursts of activity. During these swarms, we see flipped
mechanisms with two possible explanations.
Either the response to the opening of a tip of an intruding dike
or the opening of a fracture mesh with slightly different orientations.
Both of these possibilities infer mode I opening of fractures
to accommodate the intruded volume of fluids.
Given the amount of bouncing around that the activity does, there is a pretty
good possibility that there must be some amount of aseismic slip –
sorry, aseismic flow going on, either through this central portion
or up the arms in pulses that have already slipped.
Speaking of the central portion, its aseismic nature and the presence of
DLPs suggest that it likely has a different rheology than the outer arms,
which we suspect may be due to the – sorry, due to an area of partial melt.
Additionally, horizontal feature in the seismicity being distinct breaks
in the start or endpoint of a swarm, suggest that there may also be sills
or some other horizontal barriers to vertical flow.
Magma occasionally intrudes more shallowly than this body,
such as it likely did in 1989. Shallow swarms are then driven
by volatiles exsolved from the magma at depth,
which then finally escaped the surface.
One of the primary conclusions from this work is that the speed at which
these swarms migrated highlighted the permeability of this structure,
which acted like something of a geologically high-speed chimney,
connecting the root of the volcanic system to the surface
with implications for how quickly fluids can ascend.
That said, this ties back to my point from prior to this section.
Even with a great set of observations for this set of swarms,
we still aren’t able to definitively pin the blame on these – for these
swarms on intrusions of new basaltic magma or if they were other
magmatically derived fluids. Though, to be fair, much of
the challenge with this work was that everything was so deep.
Shallower swarms certainly have their own challenges as well, though.
And, with that, I’d like to thank you for your attention,
and I look forward to your questions.