Hello, everyone.
I'm Ninfa Bennington.
I'm a volcano seismologist
at the Hawaiian Volcano Observatory.
And today I'm gonna be talking about the seismicity
that has occurred across the Big Island over the last year.
To begin with, let's talk about the three major regions
where seismicity distributes across the Big Island.
So first, there's Mauna Loa summit
and it's Southwest and Northeast Rift Zones.
Kilauea summit and the associated Eastern Rift Zone.
And then there's the Pahala region here
where we have both onshore and offshore events.
Let's start by talking about Kilauea volcano.
The 2018 eruption completely drained
the lava lake that existed within Halema'uma'u crater.
Following that eruption,
we were left with the building of a water lake
within the crater, shown right here.
Then in December of 2020, we had a new eruption begin
and the rebirth of the lava lake within Halema'uma'u.
As we got into spring of 2021,
the eruption quieted
and you can see here the dramatically cooled lava lake.
However, in September of 2021,
we had the resumption of eruptive activity
and continued building of the lava lake.
And as I talk to you today,
we're seeing a period where we have pauses and restarts
to this ongoing eruption.
So the eruption might start again
and the lava lake continues to build.
Then we have these pauses where the lava lake cools,
and we seem to be alternating between these two behaviors.
Let's start by talking about the seismic activity
that's been occurring at Kilauea summit
since the start of the December 2020 eruption.
First, we're gonna look at the summit earthquakes
that have been occurring there with depth.
So here, each of these open circles represents an earthquake
occurring at Kilauea summit.
And you can see them all across this profile.
So our vertical axis shows the depth
that these earthquakes occur,
and we're going from two kilometers above sea level
all the way down to 10 kilometers below below sea level.
Now the horizontal axis is showing time.
So this red line here shows the onset of that 2020 eruption.
And then as we move across this horizontal axis,
we're progressing in time
towards the time when the eruption quiets
in the spring of 2021.
The big takeaway here is we have the start of the eruption,
and with it, the occurrence of all this seismic activity;
all these earthquakes occurring shallowly
within the summit of Kilauea.
Now, as we progress into the eruption,
we see that seismicity quiets.
You can see here there's a real dramatic drop off
in the number of earthquakes occurring with time.
Now we start to enter the spring of 2021.
And we see the resumption of this seismic activity.
We see more earthquakes occurring than were occurring before
when the eruption was most active.
And we see all this seismicity occurring
and the volcano continues to quiet and to cool.
And then we get into the end of that eruption.
Movies and TV have popularized what seismic data looks like
when it gets recorded.
Here in the left panel, I'm showing the classic seismometer
and it's recording these wiggles here,
which are reflecting the amount of ground shaking going on
in a particular place.
And if that's big ground shaking,
you get high amplitudes to these wiggles,
small ground shaking
and the amplitude of those wiggles diminish.
Folks are kind of used to looking at wiggles
and thinking of them associated with seismologists.
But actually, seismologists like to look
not only at the sort of amplitude of wiggles,
but also look at the frequency of the seismic data
that we pull out.
In the right-hand panel, I'm showing frequency data.
So we're taking some window of these wiggles
and we're looking at them with frequency.
Let me take you through these plots a bit.
This vertical axis here
represents the frequency of seismic waves
from low frequencies, so starting at above zero,
and going all the way to 20 hertz.
The horizontal axis, again, represents time.
So we started at time zero
for some time window we're looking at
and we go all the way to, in this case,
60 seconds or a minute up.
I'm showing three kind of popular cases
for the types of seismic data that we observe
in the frequency domain.
So if nothing's going on, there's no earthquakes occurring,
there's no eruption,
a lot of times what we see is this top panel here.
So we have this bright orange and red color
indicating that our strongest seismic signal
is occurring here below about one hertz,
and this will just continue on and on
if there's nothing happening geologically speaking
at the volcano.
And that energy below one hertz
is simply due to the crashing of ocean waves on the shore,
or the interaction of ocean swells with the ocean floor.
So kind of the ocean hum going on, on our little island.
Now, what if there's an earthquake that occurs?
Well, on top of this ocean noise that I just talked about,
we get this energy arriving
with both low and high frequencies.
And you see here it kind of looks like this triangle shape.
And this is the arrival of an earthquake
at the summit of Kilauea.
So besides those two kind of fingerprints
to activity that's going on at the volcano,
what else might we see?
Well during an eruption if we have fluids moving
within the shallow system at Kilauea,
we often see these low-frequency signals
around five hertz and below,
you see this orange and red color here,
and that represents something called volcanic tremor,
or the movement of shallow magma
and other fluids through the shallow system
within Kilauea summit.
So in this way,
the volcano leaves different sorts of fingerprints
that we can look at and identify
to know what kind of activity is going on at Kilauea summit.
Now that folks have a basic understanding
of what kind of activity
is associated with a particular frequency domain,
I'm gonna start to show you the pictures
in the frequency domain
of the seismic signals we've recorded
since the onset of that 2020 eruption.
First, let's look at this signal in the frequency domain
that we recorded as the 2020 eruption started.
Unlike that figure I showed you in the last side,
you see these high-energy seismic signals arriving
at about five hertz and below.
And if you recall from before,
that was something called volcanic tremor
and it was associated with magma
and other fluids moving through the shallow system
at Kilauea summit.
And that jibes really well with the onset of the eruption
that we see here.
If we see these sorts of low-frequency signals occurring,
we often then look to other data streams
to see if the volcano is erupting or inflating;
any sort of indication
that magma and other fluids are moving around
and there's some sort of volcanic unrest occurring.
Now let's look as we move into the eruption;
we see the persistence
of the most energetic seismic arrivals happening
in this sort of tremor band.
So again, fluid and magma was moving in the system.
And then if you recall,
with the resumption of earthquake activity
in the spring of 2021
we had the start of quieting and cooling
of Halema'uma'u crater and the end of the eruption.
And you can see with that
we lose that volcanic tremor signal,
and our entire picture.
What we see come about is that ocean noise and nothing else.
So we can actually really chart the eruption
and the associated activity
by looking at our seismic signals in the frequency domain.
Now let's talk about volcanic activity that has occurred
since the end of the eruption that began in 2020.
Since that eruption's end
we've had the resumption of seismic activity or earthquakes
occurring at Kilauea's summit.
You can see here,
as we're moving with time away from that eruption end.
And if we look in the frequency domain,
we see that that high-energy seismic signal
at frequencies of five hertz and below have disappeared.
So we no longer have that volcanic tremor signal.
And that makes a lot of sense
because if we look at Halema'uma'u crater,
we can see there that it's obviously cooling
and eruptive activity has ended.
Now of course,
we also have this September of 2021 eruption that occurred.
And we see here this large amount of seismic activity
that occurred right as the onset of the eruption happened.
And then following the onset,
similarly to the 2020 eruption,
we see that seismicity
really started to dissipate dramatically
as the eruption went on.
And if we look in the frequency domain,
we see that we have the appearance of volcanic tremor again
indicating fluids or magma moving shallowly
within Halema'uma'u.
And we can see that similarly in the picture here,
where we have the resumption of building of the lava lake.
And so that's where we're at right now.
We have this ongoing eruption going on at Kilauea.
And as we've had pauses and resumptions
in the lava lake activity,
we've had both the appearance of the volcanic tremor
and then moving towards an absence of the volcanic tremor
and sort of going between those two behaviors.
So that sums up what's been going on at Kilauea
in the last year.
Now let's move on to the Pahala region,
where we've seen a real change
in annual rates of seismicity.
If we look in this left-hand panel,
I'm showing just a week of seismicity occurring in 2013.
In 2013, seismicity rates in Pahala were pretty low.
So here I'm showing, this is a week of seismicity.
This is generically outlining the Pahala region.
You don't see a ton of events.
This figure in the top panel
is showing weekly counts of seismicity.
This vertical axis shows the weekly counts
and the horizontal axis shows time.
So you can see in 2013,
the amount of earthquakes occurring was pretty low
relative to the amount of earthquakes that you see
as you move later in time.
Let's move to 2017, or even let's look earlier,
you can see that by 2015, relative to earlier years,
we're starting to see an uptick
in the number of earthquakes occurring per week in Pahala.
And I'm showing here, this middle panel,
this shows a week in 2017 of seismicity on the island.
And so you can see here,
I'm just sort of generically outlining the Pahala region,
but again, seismicity is slightly increased
relative to the distribution of events happening
in this region in 2013.
But the real jump happens in sort of mid-2019.
And you can see the number of events
being counted in the Pahala region
really starts to take off to dramatic levels.
So if you see here in this right-hand panel,
this is just a week of seismicity occurring
in Pahala in 2021.
So it's an extensive amount of seismicity.
So what could the swarm
or high number of earthquakes occurring in Pahala mean
for the region?
What could it represent geologically?
There was an interesting study done in 2015
by scientists at the USGS,
where they postulated this very question.
And they looked at not only those earthquakes occurring,
but they also identified tremor,
similar to what I was talking about earlier at Kilauea,
but at this time occurring within the Pahala region.
And this tremor was located deep
about 40 kilometers below Pahala.
And as I talked about earlier,
it's often associated with the movement of fluids and magma.
Now, right above this tremor that they located
were all of those Pahala earthquakes
that I was showing you in the previous slide.
And when you plot those and look at them with depth,
they form a nice linear chain,
you can actually see even better in this figure above,
moving towards Kilauea.
And they hypothesized that this region represents
a mantle-fault zone towards Kilauea
where perhaps these magmas and fluids
move towards the region, and perhaps even act as a source
for deep magma into Kilauea.
Of course, this is currently a hypothesis
and permanent stations that HVO has in this region
are fairly sparse.
If USGS can densify the number of stations or seismometers
within the Pahala region, at least temporarily,
we can then start to image the structure
of the subsurface in this region
in order to better understand
why we're having all of these seismic events.
And so that's a future project happening with the USGS
likely beginning this spring or summer,
where we'll be placing these very small seismometers densely
through the Pahala region,
collecting data for a few months and then using that data
to model what the subsurface in the region looks like
so that we can try to better understand
what's going on at Pahala
and the reason for these swarms of earthquakes.
With respect to island-wide seismicity,
let's finally look at Mauna Loa.
In spring of 2021, it produced seismic swarms.
Those are most easily seen
if we look at this plot of earthquakes per week.
Again, the counts are gonna be on the vertical axis
versus time.
In the spring of 2021,
we have these upticks in weekly seismicity
and that went down following the spring
to these lower rates seen here
as we march in time to the present.
This swarms plotted in two regions:
Here, across Mauna Loa summit
with this northeast/southwest trend,
and then also a second region of events here,
located immediately west of the summit.
And these are two common regions where we see seismicity,
whether in a swarm level
where we have lots of seismicity occurring
or just kind of weekly low levels of seismicity.
Following these swarms, though, we've gone into this period
of very low activity at Mauna Loa with not much going on.
Finally, we move offshore to Lo'ihi Seamount.
This newly named submarine volcano
is located immediately east of the south point
of the Island of Hawaii.
And in the last several weeks,
it's had a swarm of seismic events occurring there.
This culminated in a magnitude 4.9 event in this region.
These events may represent
either the intrusion of magma and fluids
into the volcanic system there or volcanic activity.
At this time,
due to the fact that we're observing these events
from our network of onshore stations,
we can't differentiate between the two.
This concludes our summary of seismic activity
across the Big Island
for this month's Volcano Awareness program.
Thank you very much and have a wonderful day.