- Hey everybody.
Welcome to this brave new virtual world we live in.
Well, thanks for tuning in
and for your interest in this presentation.
And as a reward, I'm gonna be telling you today
about the largest gravity changes ever recorded,
at least to our knowledge.
Now, these were gravity changes that were associated
with the onset and the early evolution
of the 2018 eruptive activity at Kīlauea,
the lower East Rift Zone eruption, and the summit collapse.
And before I launch into this,
I'd like to make sure I acknowledge my co-authors,
that's Daniele Carbone of INGV and Matt Patrick
of the Hawaiian Volcano Observatory.
The two of them really made this work possible.
So, let's talk about these huge gravity changes.
As most of you probably know, Kīlauea had been erupting
from two areas prior to the 2018 eruption.
The first of these areas was Pu'u 'Ō'ō
on the East Rift Zone.
That eruption started in 1983
and went more or less continuously for 35 years.
And then, starting in 2008, a summit eruption was ongoing
and this was manifested by lava lake activity
just on the inner edge of Halema'uma'u crater.
In 2018, of course, we had the extrusion
of over a cubic kilometer of lava
from the lower East Rift Zone.
Now, this was fed by a dike that propagated down
from Pu'u 'Ō'ō and eventually connected back up
to the summit.
In the early hours of this dike propagation,
Pu'u 'Ō'ō collapsed, and you can see here
it was characterized by billowing plumes
and at the summit, the lava lake withdrew.
That took a matter of a few days and eventually,
the summit collapsed to form a new caldera.
So this was all associated with this dike intrusion down
to the lower East Rift Zone.
Gravity monitoring was done by two continuous instruments.
The first was located at the summit,
in fact, just above the summit eruptive vent.
This was station HOVL, right on the eastern rim
of Halema'uma'u crater.
The second was located right on the north rim
of Pu'u 'Ō'ō crater station PUOC.
And these two gravimeters had been running for several years
at the time of the 2018 collapse.
Now, the installations looked something like this.
We had power supplied by solar and batteries,
telemetry as well.
And the instrument was located
in this little protective hut, this little doghouse
with a tile roof that was meant to protect it
from the elements and from any ballistics.
If you open up that doghouse,
you can see that inside is a LaCoste & Romberg.
In both cases, we used g-meters that were fitted
with the Aliod Electronic Feedback System.
And so two-hertz gravity data and tilt data were streaming
from the gravimeter into a data logger.
This system remained in place at the summit as is,
but at Pu'u 'Ō'ō, we eventually replaced the dog box
with a Pelican case with a concrete floor
and slightly more advanced electronics
that were more robust against failure.
Well, let's start by talking about gravity change
at the summit.
What you're looking at here in this movie
is a time-lapse animation of the thermal camera imagery
of the summit lava lake.
You can see it slowly receding through the first few days
of May as the lower East Rift Zone dike
tapped the summit magma system.
And on the bottom is the gravity change
that accompanied the withdraw of the lava lake.
Now, over this few-day period there was over a milligal
of gravity change.
This is the second largest gravity change ever recorded.
Unfortunately during this time,
the summit went from looking something like this,
where Halema'uma'u was located in the center
of the caldera, the gravimeter was located
right on the eastern rim here to looking like this.
This was the ending view after several months of collapse.
The gravimeter had been located right in here,
and unfortunately in mid May of 2018, it was consumed
by the widening of Halema'uma'u as the collapse took place.
After withdrawal of a lava lake,
but before collapse of the summit,
aerial photos were used to construct a structure from motion
DEM of the interior of the summit vent.
And that's what you're seeing here on the left.
There's also a plan view of the system
with the location of the gravimeter noted by that dot
that says HOVL and a cross-section
that shows what the vent looked like
on a north-to-south profile.
And we modeled this vent
by breaking it into parallel pipettes;
these five-by-five meter cells
and we could simulate the level of lava over time
by matching the level of lava
as viewed in that thermal video.
And so by doing this,
we could actually simulate the gravity change
that would occur as the lava lake withdrew.
And since we know the level of the lava over time
and the shape of the vent,
the only thing that we had to solve for
to match the gravity data was the density of the lava lake.
The top plot shows the change in gravity
over the first few days of May.
And on the bottom is the lava level
during the same time period, the lava level being derived
from that series of thermal camera images.
Now, by combining the lava level change with that structure
from motion model of the interior of the summit vent,
we were able to calculate a gravity effect
simply by changing the density of the lava.
And when we do that, the best fitting gravity change
is obtained with a density of about 1,700 kilograms
per cubic meter for the upper 200 meters of the lava lake.
Now, this is a little bit denser than we'd seen before.
During a past episode of lava lake withdraw in 2011,
we calculated the density
of about 1,000 kilograms per cubic meter.
So it's possible that the density
of the lava lake had increased over time.
It may also be that we had a much, much more accurate model
of the vent and so we were able
to get a more realistic density, although still clearly
much less than the density of basalt,
which reflects the gas-charged nature
of the lava in the vent.
Well, let's move now to the Pu'u 'Ō'ō vent.
After Pu'u 'Ō'ō collapsed and once the ash plumes cleared,
there were helicopter overflights and lots of photos taken,
and we were able to do the same thing at Pu'u 'Ō'ō
as we were at the summit.
That is, use the photos to construct a structure
from motion model of the interior of the eruptive vent.
You can see that model here in perspective view on the left,
and in plan view in the lower left.
That also features the location of the PUOC gravimeter.
And then the cross section on the right shows
what this looked like in side view from East to West.
Now, the difference between Pu'u 'Ō'ō and the summit
was that we did not have any information on the lava level
over time at Pu'u 'Ō'ō.
Unfortunately, the camera views were not able
to see into the vent.
So we only know about the time that the lava level started
to drop at Pu'u 'Ō'ō.
We don't know anything about how that drop occurred.
But that's something we were able to invert for
as part of our analysis of gravity change at Pu'u 'Ō'ō.
And the gravity change at Pu'u 'Ō'ō was really impressive.
You might recall a few slides ago,
I showed the summit gravity change.
We had about a milligal of gravity decrease
over several days, the second largest change ever recorded.
Well, at Pu'u 'Ō'ō, over a matter of just a couple of hours,
we had one and a half milligals of gravity decrease,
easily the largest gravity change ever recorded.
Now, as I mentioned we didn't have
any lava-level information, but that was something
that we could invert for along with lava density.
When we do that, we get a lava-level change
that looks something like this,
where about there were 300 meters of lava level drop
over the course of the two hour or so draining
of the lava level at Pu'u 'Ō'ō.
So now, we can also use this to calculate the lava density.
And we get about 1,900 kilograms per cubic meter
and the volume loss over this two hour period
was 11 million cubic meters
or 8 million cubic meters dense rock equivalent.
Now, this density is significant for two reasons.
It's greater than that at the summit.
And so first, that may reflect pre-eruptive degassing
at the summit.
We know that some of the magma that degassed at the summit
then later went down to Pu'u 'Ō'ō where it erupted.
So it stands to reason that we would see a higher density
of the lava lake at Pu'u 'Ō'ō relative to the summit.
And second, the lava level elevation at the summit
is higher than that at Pu'u 'Ō'ō.
And that might also reflect a greater density
of lava at Pu'u 'Ō'ō.
And in fact, this difference of about 200 kilograms
per cubic meter is roughly what you would expect,
given the lava level elevation difference
at the summit and Pu'u 'Ō'ō.
So the density that we're seeing at Pu'u 'Ō'ō
and the summit are very consistent with observations
from degassing and surface elevation.
Now, there's another signal here that I think we need
to spend a little bit of time focusing on.
And that's this little transient right here,
a 200 microgal decrease followed by a 400 microgal increase,
just a few minutes before Pu'u 'Ō'ō collapsed.
And this may indicate something that occurred prior
to the onset of the great dike intrusion
that fed the big lower East Rift Zone eruption in 2018.
So here's a zoomed view of that gravity transient
that occurred just a few minutes before collapse
of Pu'u 'Ō'ō, and on the bottom is tilt,
measured at a tiltmeter
just on the north flank of Pu'u 'Ō'ō.
And you can see the tilt was starting to accelerate early
in the hours of May 1.
But this gravity transient really showed up quite strongly
before there was really strong ground tilt.
And in fact, it showed up just after the onset
of some interesting seismicity out at Pu'u 'Ō'ō,
an increase in seismic tremor.
This may have been associated with the onset
of a small fissure eruption on the west flank of Pu'u 'Ō'ō.
So here is Pu'u 'Ō'ō cone.
This photo was taken after the collapse,
but at the time it was noted that there was a small fissure
that ran down the west side of the cone.
And this fissure had a very small amount of lava
that had come out of the ground, mostly degassed lava.
This gravity signal, the decrease followed by the increase,
may indicate the onset of this small fissure,
the opening and then filling of this fissure with magma.
So, it may be that we're seeing the opening sequences
of the entire eruption.
A small fissure on the west flank very strongly manifested
in the gravity data from Pu'u 'Ō'ō.
Now with only one gravity station, it's really impossible
to model this, but we can come up with a conceptual model
of what this might mean.
Now the opening, gravity decrease,
could represent the opening of a dry crack,
something that you might see in front of a propagating dike.
So the dry crack opens, the overall density decreases
because you're replacing rock with air
and gravity goes down.
And eventually, magma fills that crack
and we get an increase in gravity.
And in this case at Pu'u 'Ō'ō,
the gravity increase was much greater than the decrease
because the magma filling that crack
would have a much, much higher density
than the agglutinated spatter that makes up the cone.
And so we might explain this gravity transient
as the opening of this dry crack and then the filling
with dense mostly de-gassed magma that had a small eruption
on the west flank of this fissure.
So gravity data provide an interesting perspective
into what could have been the very opening sequence
of the entire 2018 eruption, a small fissure that opened
on the flank of Pu'u 'Ō'ō.
So we just find these results incredibly exciting.
Not only can we use the continuous gravity data
to identify things like the volume of magma that drained
from Pu'u 'Ō'ō and the density of that magma,
we may also be able to detect much more strongly then
other measures, other monitoring data like tilt
or seismicity, the occurrence of very important transients.
So in conclusion, we've seen gravity changes of a milligal,
and 1.5 milligals at the summit and Pu'u 'Ō'ō respectively.
These are the largest gravity changes
that we're aware of ever having been recorded.
Now, using these data and models of the eruptive vents,
we were able to calculate a density of the summit lava lake
of about 1,700 kilograms per cubic meter
while the Pu'u 'Ō'ō lava lake density was slightly higher
at 1,900 kilograms per cubic meter.
This density difference is very interesting.
It's consistent with pre-eruptive degassing
that was occurring at the summit vent
and also the difference in the elevations
of the two lava lakes.
And finally, there's this major gravity transient,
a 200 microgal decrease followed by a 400 microgal increase
all occurring over the course of just a few minutes.
This preceded the onset of the 2018 dike intrusion.
It proceeded the collapse of Pu'u 'Ō'ō.
This may have been the opening event of the entire sequence
and it was indicated more strongly in gravity
than any other data set.
This really is a great argument for a continued investment
in monitoring using continuous gravity, which hopefully,
in the years to come will become cheaper and easier
as new technology is applied.
Well, there you have it,
the story of the largest gravity changes ever recorded.
I hope you found that interesting.
And if you have any questions,
feel free to email me anytime, mpoland@usgs.gov.
Thanks for tuning in and seeing it out to the bitter end
and hope to see you at a future conference sometime soon
when this is all over and we're back to normal.
Take care, everyone.