A Silent Intrusion:
New Insights into Volcano Dynamics
byThomas Andrew Lee
Class of 2019

Submitted For:
Undergraduate Essays in the Natural Sciences

Epitome: Analysis of “noise” data sources from the 2018 Kı̄lauea eruption
yields unexpected results with implications for volcano monitoring.
Word Count: 6,674
April 22, 2019

Contents
1 The Dangers of Volcanoes

1

2 Dynamic Volcanic Processes

3

3 Imaging the Subsurface with Noise7

4 An Unexpected Disappearance

12

5 Shaking Out A Physical Interpretation

14

6 Proposed Model

18

7 Implications for Monitoring

20

List of Figures
1

Geologic Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3

2

Active Fissures and Summit Collapse . . . . . . . . . . . . . . . . . . . . . .

5

3

Collapse of Halema‘uma‘u Crater . . . . . . . . . . . . . . . . . . . . . . . .

7

4

Reflection, Refraction, and Reflectivity Function . . . . . . . . . . . . . . . .

8

5

Station Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

116Time Evolution of Reflectivity Functions at All Stations . . . . . . . . . . .

13

7

Seismic Scattering and Decorrelation . . . . . . . . . . . . . . . . . . . . . .

15

8Proposed Model for the 2018 Kı̄lauea Eruption . . . . . . . . . . . . . . . . .

18

The Dangers of Volcanoes“It was hard to distinguish where the streets had been. Everything was buried under fallen
walls of cobblestone and pink plaster and tiles, including 20,000 bodies....As I look back on
the Martinique experience I know what a crucial point in my life it was....I realized that thekilling of thousands of persons by subterranean machinery totally unknown to geologists...wasworthy of a life work.” – Thomas A. Jaggar, My Experiments With Volcanoes

In the first months of 2018, the Lower Puna district on the island of Hawai‘i was postcardperfect: lush-rainforests, blue-seas lapping against sandy beaches, and a sweet tropicalbreeze. It was truly paradisal, but Madame Pele, the Hawaiian goddess of fire and volcanoes, had other ideas. Lower Puna is home to a flank of Kı̄lauea volcano known as theEast Rift Zone, and by mid-May 2018, it would be scarred by the opening of dozens of newchasms, all spitting molten rock and debris hundreds of feet into the air. By mid-July a tenstory tall spatter cone – a mini-volcano of sorts – had formed over what had once been thesubdivision of Leilani Estates (HVO, 2018a). This lava-spitting cone and the rivers of lava
flowing from it to the sea was a truly frightful and awe-inspiring sight to behold. Luckily,
there was no loss of life. However, by the time Madame Pele decided she had done enoughand returned to quiescence in August, the eruption had claimed 716 homes, and had added
875 acres of land to the island (HVO, 2018b).

Volcanoes are one of the most primal and destructive forces on Earth, and have madea notable impression upon the human consciousness; mention of the word brings to mind
images of fiery destruction and names like Pompeii, Krakatoa, and Mount St. Helens. Unfortunately, for a natural hazard with such significant impact on humanity, there remains
much to learn about the inner workings of volcanoes and how to predict their activity. This1

knowledge is crucial to saving lives by mitigating the hazard imposed upon us by volcanic
eruption. However, up to present day, despite a strong impetus to grow our understandingof this natural phenomenon, as much, if not more, has been learned about the vast expanseof outer space than about the destructive force that lies beneath our feet, threatening ourvery existence.

The disparity in our comprehension of volcanoes relative to other parts of the natural
world is largely the result of two factors: long periods of rest between eruptions relative
to a human lifespan; and the fact that we cannot dig on the massive scale that would be
required to excavate and physically explore a volcano and its plumbing. In other words, ourunderstanding is limited by both the infrequency of eruptions and the indirect observations
that can be made from the surface. This is not to say that trying to understand volcanoes
is a lost cause, but rather that new techniques for examining underground processes areof great interest to both the scientific community and humanity. One of the best ways todiscover these new techniques is by exploring all possible avenues for observation. Although
we are bound to the surface, many different sources of data exist for volcano monitoring,and the development of many of these is in no small part thanks to one of the pioneers ofmodern volcanology, Thomas Jaggar.

After witnessing the havoc wreaked by eruptions on Martinique in 1902, Thomas Jaggar(Ph.D. in Geology, Harvard, 1897) dedicated his life’s work to better understanding volcanoes. He was the first to install seismometers at the summit of Kı̄lauea in 1912, creating thefirst volcano observatory in the United States. As a testament to his life’s work, the Hawaiian
Volcano Observatory has stood watch over Kı̄lauea and the swinging moods of Madame Pele2

for over a century (Decker et al., 1987). Today, the Hawaiian Volcano Observatory operates
one of the world’s most extensive volcano monitoring networks. It keeps measurements ofground deformation, shaking, and volcanic gas chemistry along with many other quantities(Decker et al., 1987; Babb et al., 2011). I took advantage of the data from this substantial
network during the summer 2018 Kı̄lauea eruption (USGS , 1956) to interpret and link themto physical volcanic processes. In particular, I examined how the ground shaking may beleveraged to gain new insight into subsurface volcanic processes.

Dynamic Volcanic Processes

Figure 1: A cutaway diagram depicting a simplified model of Kı̄lauea and its subsurface
plumbing system fed from a deeper source. Note the location of: A Halema‘uma‘u craterat the summit; B Pu‘u O‘o vent further downrift; and C the area of active eruption on
the East Rift Zone. Modified from Poland et al. (2014).
Before it emerges on the surface as lava, molten rock is called magma (Grotzinger et al.,2010). Volcanoes typically have a shallow area for magma storage fed by a deeper source.
In the case of Hawai‘i’s Kı̄lauea volcano, magma chambers are thought to exist below the3summit at a depth of a few kilometers (Figure 1; Wright and Klein, 2011; Poland et al., 2014).
The locations and sizes of these chambers vary with time as the volcanic system evolves andcycles between periods of eruption and repose. The system is drained during eruptions, andis refilled otherwise (Anderson et al., 2015). Outside of this basic dichotomy, there remain
many gaps in our understanding of these dynamic cycles of deflation and inflation in themagma chambers and the related processes of eruption and recharge.

Even within a single eruption, the progression of observed events across the volcanic systemhint at the complex and deeply interconnected systems hidden beneath the ground. The 2018
Kı̄lauea eruption included activity at Halema‘uma‘u crater (at the summit of the volcano;A in Figure 1), the East Rift Zone (located about thirty kilometers away on the flank of
the volcano; C in Figure 1), and the active Pu‘u O‘o vent (between the East Rift Zone andthe summit; B in Figure 1).

As a precursor to the presence of lava on the surface, Pu‘u O‘o crater experienced significant draining on April 30th, and a sequence of small earthquakes moved east from Pu‘uO‘o crater towards the East Rift Zone between April 30th and May 2nd (HVO, 2018a; Nealet al., 2019). Past precedence suggests such an earthquake sequence can be attributed to
fresh magma moving through the ground and occupying new space, much like a subway
train shaking the ground as it passes (Rubin et al., 1998; Klein et al., 1987). Following these
events, the eruption formally began with lava-spewing chasms opening in the East Rift Zone
on May 3rd (Figure 2; HVO, 2018a).This seqeunce suggests a link between Pu‘u O‘o and theEast Rift Zone at the beginning of the eruption. Following the start of the eruption, a large
Magnitude 6.9 earthquake occurred on May 4th (Masse and Needham, 1989; HVO, 2018a).
4

Figure 2: (a, b) Explosive events that occured as Halema‘uma‘u crater at the summit ofKı̄lauea collapsed over the course of the eruption. (c) A river of lava flows down to the sea
from (d) active vents in the Lower East Rift Zone of Kı̄lauea. Photo Credit: HVO, UnitedStates Geological Survey.
As the eruption evolved, Halema‘uma‘u at the summit became integrated into the system
lower down the flank. Since 2008, a lava lake has been present on the floor of Halema‘uma‘u(Patrick et al., 2013). The level of this lake began to drop significantly beginning on May
6th, indicating movement of the subsurface magma away from the summit system. The
draining of the lava lake was followed by significant explosive collapses of Halema‘uma‘u’s
crater wall (Figure 2). The first of these occurred on May 15th, and created an ash cloud
more than 3000 meters in height (HVO, 2018a). These events are triggered as draining
lava exposes the wall of the crater, steepening the cliff face and creating a rockslide that5

falls into the lava lake and results in an explosion and ash cloud (Neal et al., 2019). The
explosive collapse events represent an increase in the speed and magnitude of the draining
at the Halema‘uma‘u lava lake, a change that was mirrored by an increase in output from
active vents in the East Rift Zone (Patrick et al., 2019). This draining continued until theend of the eruption in early August, at which point, 885 million cubic meters of material had
been lost from the summit (Figure 3; Neal et al., 2019). Notably, the volume of material lostfrom the summit is remarkably consistent with the volume of lava that erupted from active
vents on the East Rift Zone (Patrick et al., 2019).

The nature and timing of these events strongly suggest a connection between Pu‘u O‘oand the Lower East Rift Zone at the beginning of the eruption, followed by a connectionof the Pu‘u O‘o - Lower East Rift Zone system to other parts of the volcano including
the Halema‘uma‘u system at the summit. However, a large piece of the story, one thatis potentially crucial to better understanding the behavior of volcanic systems, is missing.
Unlike the connection between Pu‘u O‘o and the Lower East Rift Zone that was accompanied
by migrating seismicity, there were no corresponding earthquakes taking place around thetime when the summit and Pu‘u O‘o were connected. How was a connection between these
two systems forged? Without being able to view the subsurface movements of magma, thisis a hard question to answer; however, addressing it could save both lives and infrastructurefrom undetected magmatic intrusions that could result in unwarned-of-eruption. This thesis
seeks to answer this question through systematic analysis of ambient seismic noise data.

6

Figure 3: A comparison of the shape and depth of Halema‘uma‘u crater at the summit ofKı̄lauea (top left) before the 2018 activity and (top right) after the collapse events. (bottom)
An east-west cross-section comparison showing the degree to which the crater was enlarged.
Modified from HVO (2018b).

Imaging the Subsurface with Noise
Much like how x-rays passing through the body can reveal the bones within, seismic waves
passing through the ground can reveal the locations and sizes of large bodies of magmaor the volcanic conduits that feed them. Information about these waves are collected by7

observing how the ground shookin certain locations, at certain
times. From these observations,signals of interest can be ex-

Time

Depth

tracted, and meaningful physical conclusions can be drawn.

The ground beneath us isbuilt up of layers of different materials, one upon an-

a

b

c

Reflectivity

other. When seismic energy encounters the boundary between

Figure 4: (a) A cartoon in depth cross section depicting initial arrival of seismic energy (magenta arrow) at a station(black triangle). (b) This energy is recorded as the initial
peak (magenta) in the reflectivity function whose y-axis istime increasing down, and whose x-axis is reflectivity. (c)The energy reflected from the surface (blue arrow) reaches
a boundary and is reflected (cyan arrow) and refracted (redarrow). This reflected energy coming back up is recorded at
the station, and results in another peak in the reflectivity
function later in time (b).

two layers, there are two options.

The energy can be ei-ther refracted, passing throughthe boundary on a possibly
different trajectory, else it isreflected, bouncing back fromwhence it came to be recordedat the surface (Figure 4; Shearer , 2009). For my purposes, I am concerned with the latterof these two outcomes, reflection. The instances of these reflections can be obtained bycorrelating a seismogram with itself to look for similar peaks separated in time (Figure 4b).

8

The time function with the reflection peak is generated by taking a time series (e.g.,
movement of the ground with time) and comparing it with itself at different time offsets
to find similar waveforms (Bracewell , 1986). Applied to seismology, it is used to identify
the return of a previously recorded seismic wave that has bounced off a reflecting boundary,and is referred to as a reflectivity function (Figure 4b; Claerbout, 1985). By determining
how long it takes for the reflected energy to return, the depth of the reflecting boundary andsubsurface wave speed may be inferred, and strength of reflection is determined by amplitudeof the peaks (Thompson and Cooper , 1972; Yokoi and Margaryan, 2008). This method has
been used for the imaging of various subsurface layers such as the boundary between thecrust and the mantle (e.g., Kennett et al., 2015). In volcano monitoring, it is often used
to detect the boundary between solid rock and magma, and to examine how this boundarychanges over time (e.g., McKee, 2012; De Plaen et al., 2016).

In order for the reflectivity function to exist, a source of energy is needed to generate
the reflected waves. For example, in oil exploration, anthropogenic energy sources such as
controlled explosions are used (Dobrin and Savit, 1960). However, such sources provide onlya snapshot of the subsurface in the moments directly following the energy release. For a
dynamic environment such as an erupting volcano, it is infeasible to set off controlled blasts
often enough to monitor the evolution of the system. Consequently, it is necessary to findanother source of seismic energy for continuous imaging of volcanic systems.

A source of energy for this type of study needs to occur almost all of the time in order toallow detection of rapid changes. One natural source of seismic energy fitting this criterion
is ocean waves impacting the shoreline and being converted to seismic energy. This seismic
9energy travels underground for hundreds of miles, and when recorded by seismometers, arereferred to as microseisms, a type of seismic noise. Noise is typically defined as anything notrelated to earthquake signals, which are the focus of classical seismology. While it may seem
odd to use “noise”, the main benefit of using oceanic microseisms is that it is continuous andoccurs naturally 24 hours a day, seven days a week without human involvement (Stutzmann
et al., 2009; Ardhuin et al., 2011). These oceanic microseisms can be leveraged as the energy
source for calculation of reflectivity functions that allow for easy monitoring of changes inthe subsurface with a high temporal resolution (e.g., Draganov et al., 2009),and consequently
make Hawai‘i an ideal location for this type of study.

Seismic energy is recorded by seismometers, of which the Hawaiian Volcano Observatory
has a substantial network (Decker et al., 1987). The seismometers used in this study belong
to four stations strung along the flank of Kı̄lauea between the activity on the East Rift Zone
and Halema‘uma‘u at the summit (USGS , 1956) (Figure 5). These stations can be used
to get an idea of the structure directly beneath them. In this case, the areas beneath theseven stations are of particular interest because they likely host the subsurface structure thatevolved during the eruption.

10

Figure 5: A map of the four stations from the HVO seismic network that are considered inthis study (yellow triangles) on the island of Hawai‘i (area shown as white rectangle in inset
map of Hawai‘i). The circles mark: A Halem‘uma‘u at the summit, B Pu‘u O‘o ventmidway down the flank, and the C Lower East Rift Zone where active fissures and flows
were located in the 2018 eruption. Imagery Credit: Google Earth, 2018.

11

An Unexpected Disappearance
At the onset of this project, my intent was to use ambient seismic noise to monitor theevolution of the reflectivity function in the subsurface between the East Rift Zone andHalema‘uma‘u at the summit of Kı̄lauea over the course of the summer 2018 eruptive activity. That is to say, I wanted to seismically map the subsurface plumbing system. To thisend, I retrieved data from seven Hawaiian Volcano Observatory seismic network (USGS ,1956) (Figure 5), and wrote computer programs to compute the reflectivity functions on
an hourly basis for each of the seven stations from the beginning of January 2018 to theend of the eruption in August 2018. The computations took about two weeks to performusing Harvard’s Odyssey super computer. From there, I began my analysis of the reflecting
boundaries between subsurface layers and their evolution.

Through the beginning of 2018, the data showed little to no variation in the subsurface
system, meaning that the layout and structure of the subsurface layers were stable andunchanging (Figure 6). This is to be expected, as the precursory signs of the impending
eruption did not appear until mid-April 2018, and no surface expression was observed until
May 3rd (HVO, 2018a). Though some shifting in the subsurface structure was observed
immediately preceding the eruption and continuing for a few days, a much more significant
and puzzling change began on May 5th.

In my analysis, all signs of reflecting boundaries at Halema‘uma‘u and the summit disappeared on May 5th. Over the next eight days, the reflecting boundaries from the summit
down the flank of Kı̄lauea towards the East Rift Zone also began to disappear one-by-one. By

12May 13th, the reflecting boundaries disappeared at all of the stations (Figure 6). Certainly
something odd was happening. More curiously, the reflecting boundaries returned to all
stations by the end of May. While the original goal of the study was to analyze the evolutionof the subsurface system through examination of the movement of reflecting boundaries, the

JOKA

STCD

PAUD

τ (s)

τ (s)

UWE

τ (s)

τ (s)

disappearance of reflectors offered a much more interesting problem.

Distance Downrift from UWE (km)Distance Downrift from UWE (km)Figure 6: The reflectivity functions at all four stations, UWE (blue), PAUD (red), STCD
(yellow), and JOKA (purple). Peaks in the reflectivity functions show reflecting boundaries
in the subsurface. Note the disappearance of these peaks and the resultant flattening of thereflectivity functions at one station after another over time.

13

Shaking Out A Physical Interpretation
Disappearance of reflective boundaries in the ambient seismic noise reflectivity functions
is usually a result of two possible scenarios. Either the seismic waves are not detected due toan instrumental problem, or they are not actually reflected due to some physical change inthe subsurface. The movement of the decorrelation signal downrift to encompass all sevens
stations was a pattern unlikely to have resulted from instrumental problems. Moreover,comparison of the timing of the decorrelation to the ground observations of the eruption,shows that progression of the decorrelation, which started on May 5th, was preceded bya Magnitude 6.9 earthquake on May 4th (Masse and Needham, 1989; HVO, 2018a). The
decorrelation at all stations by May 13th was also followed by the first of the collapse eventsat the summit on May 15th (HVO, 2018a). This remarkable series of events provides a verystrong case for a physical interpretation of the signal involving volcanic-processes, and all
but rules out instrumental error.

If not due to instrumental error, the disappearance of reflectors must come from decorrelation (Figure 7). Decorrelation occurs when seismic energy is scattered such that a reflection
cannot be detected, and therefore the reflecting boundaries effectively disappear from thereflectivity function. This is analogous to how light scattered by fog obscures objects you
would otherwise be able to see. On a clear night with no fog, you can see an object suchas a car some distance away using a flashlight. This is a result of light bouncing off of the
car and then traveling to your eyes with relatively little loss in energy. However, in foggy
conditions, the light from the flashlight interacts with water droplets and is reflected in all
directions. Only some of these weak reflections return to your eye, and these perceived as

14

“blurry” fog. While some of the light may travel to the car and bounce off of it, by the time
it reaches your eyes, it will be so weak that the car can barely be seen, if at all.

Figure 7: (a) A cartoon depiction of a situation where seismic energy is reflecting back toa station (left) allowing for imaging of a subsurface reflector using the reflectivity function(right), and (b) a decorrelation situation where an intrusion of magma (red segments) scattersthe seismic energy (left) such that energy is reflected back to the station nearly continuously
at lower amplitudes, and the reflector is not clearly visible (right). The reflectivity functions
show the initial instance of energy (purple; Figure 4) and the reflected energies (peaks withvarious shades of blue).

The presence of a large earthquake before the appearance of the decorrelation signal,and the beginning of the explosive collapses at Halema‘uma‘u after the migration of thedecorrelation from the summit into the East Rift Zone is significant. The earthquake may
have had a large effect on the subsurface, potentially triggering a change in the plumbing
15

system. The collapses mark the beginning of summit involvement in the eruption. Together,
these suggest that this decorrelation signal which moved downrift was related to the creationof a stronger connection from the summit to the active system erupting further downrift
along the flank of the volcano. Interpreting this decorrelation signal as the creation of a newconnection from Halema‘uma‘u and the summit to Pu‘u O‘o and the East Rift Zone makes
sense for several reasons. First, any new connection would involve injection of liquid magma
into otherwise solid ground. This type of influx has been shown to scatter seismic waves sothat they are not reflected back properly, which in turn causes decorrelation similar to thatobserved in our study (e.g., Obermann et al., 2015) (Figure 7). Moreover, several previous
studies of the Kı̄lauea summit system have come to the concluded that such an intrusion ofmagma into the region directly east of the summit could be triggered by a large earthquake
such as the one that occurred on May 4th (e.g., Judson et al., 2018). There also exists
compelling evidence from ground observations for a shift in the eruption from a Pu‘u O‘oand East Rift Zone connection to a Halema‘uma‘u, Pu‘u O‘o, and East Rift Zone connection.
Together, the data forms a convincing argument that the decorrelation signal was the resultof an intrusion of magma into the area east of the summit, between Halema‘uma‘u and Pu‘uO‘o.
Normally, such an intrusion of magma is accompanied by earthquakes generated by therock fracturing under increased pressure (e.g., Lockwood et al., 1999; Hurst et al., 2018).
Indeed, this is the case for the intrusion which occurred at the beginning of the 2018 activity
between Pu‘u O‘o and the East Rift Zone from April 30th to May 1st. This preceded theopening of the first active vents (HVO, 2018a; Masse and Needham, 1989). However, there isno increase or migration of earthquakes corresponding to the migration of the decorrelation,16

or any possible intrusion from May 5th to May 13th between Halema‘uma‘u at the summit
and the East Rift Zone (Masse and Needham, 1989; USGS , 1956).

Despite inconsistency with what is expected from an intrusion of this sort, the lack ofearthquakes could be reconciled with the magmatic intrusion inferred from the decorrelation.
Rather than rupturing through completely solid rock, the intrusion may have entered an area
already fractured by a previous intrusion. In that case, the magma would have only needed
to re-open existing fractures, thus releasing less energy than fracturing solid rock would.
Such a situation would create fewer and smaller earthquakes. If these earthquakes were too
few and too small to be detected, the intrusion would effectively be silent.

17Proposed Model
StationsabStage 1b: Pu`u O`o collapses and seismicity
migrates into the ERZ with an intrusion.

Stage 1a: Pre-erup�ve subsurface system withvent at Pu‘u O‘o and summit lava lake.

cdStage 1c: Ac�ve vents open up in the ERZ, andlava erupts.

Stage 2a: M6.9 flank earthquake triggered byfirst intrusion occurs.

Intrusion withmicroearthquakesefStage 2c: Conduit connects summit to Pu`u O`oand ERZ, and autocorrela�ons return to preerup�ve form.

Stage 2b: Halema`uma`u drains and second
intrusion causes decorrela�on to propagate
downri�.

Figure 8: A two-stage eruption model where the steps for each stage are presented as cartoon
depth cross sections. The color of the stations indicate how similar the reflectivity functions
are with the pre-eruptive form, with light colors indicating a greater degree of decorrelation.

18

Embracing this possibility, I propose a model for the evolution of the subsurface magmatransport system (Figure 8; Lee et al., 2018). The model divides the development into twostages. The first stage leads up to the May 4th magnitude 6.9 earthquake and involves thearea spanning Pu‘u O‘o to the Lower East Rift Zone. The second stage takes place after the
May 4th magnitude 6.9 event and incorporates the Halema‘uma‘u summit system into thePu‘u O‘o and Lower East Rift Zone system.

The model starts with a pre-eruptive confirguration of Kı̄lauea (Figure 8a). The Pu‘u O‘ovent is active (continuous since 1983, Poland et al., 2014), and a lava lake connected to a
deep magma reservoir exists at Halema‘uma‘u (active since 2008, Figure 1; Patrick et al.,2013). The second step brings an interruption to this state when Pu‘u O‘o collapses, andan intrusion occurs into the area between Pu‘u O‘o and the Lower East Rift Zone. The
intrusion is accompanied by migration of seismicity between April 30th and May 1st (Figure8b). The third step begins when magma reaches the Lower East Rift Zone, new vents open
up, and eruption of lava onto the surface starts. This marks a connected and active Pu‘uO‘o and Lower East Rift Zone system (Figure 8c).

The second stage begins with a magnitude 6.9 flank earthquake occuring on the interface
between the oceanic crust and the island itself (Figure 8d). This earthquake was likely
triggered by the rift-opening motion of the first intrusion between Pu‘u O‘o and the Lower
East Rift Zone (Chen et al., 2019; Neal et al., 2019). The shaking from the earthquake
then triggers an intrusion of deep-seated magma beneath Halema‘uma‘u at the summit into
the previously fractured region to the east (Figure 8e). Due to the fractured nature of therock in the area, only microearthquakes occur and magma intrusion into fractures begins to19

scatter seismic energy. This scattering and microseismicity cause the reflectivity functions todecorrelate, with timing of the decorrelation tracking the magma front. Finally, the intrusion
reaches the Pu‘u O‘o and Lower East Rift Zone system established in the first stage, resulting
in increased lava output in the Lower East Rift Zone and increased rate of collapse at thesummit. This creates a fully connected Halema‘uma‘u, Pu‘u O‘o, and Lower East Rift Zone
system (Figure 8f).

With full connection across the flank, material would be transported relatively smoothly
in the established conduit, and there would be less magma pushing into the surroundingrocks. Magma will stop re-opening and filling cracks, and those already open will either
drain and close or solidify. Consequently, without the numerous magma-filled cracks, thescattering stops, and the reflectivity functions return to the pre-eruptive form. The timingof this return to pre-eruptive form retreating back up the flank makes it more likely that thedecorrelation is due to scattering, as microearthquakes would likely cease across the flank assoon as the conduit is established.

Implications for Monitoring
Outside the scope of the 2018 eruptive activity at Kı̄lauea, the results of this study still
have wide-reaching implications. Whether the lack of earthquakes accompanying the inferred
intrusion is due to an actual dearth of seismic activity, or an inability to detect said activity
is not the point. The fact is that the typical earthquake-based methods for detecting suchsignificant events did not suggest the notable intrusion of magma which is otherwise inevidence (HVO, 2018a). That a significant subsurface change could be essentially silent
20poses an interesting problem to volcanology. Perhaps more concerning though, is what thissilence means in the realms of monitoring and hazard.

The detection of magmatic movement, especially that preceding eruption, is necessary
for the effective management of volcano hazard and issuance of warnings to the public. For
example, during the 1995 eruption of the Soufrière Hills Volcano on the island of Montserrat,
clear signs of escalation of seismicity, including swarms of earthquakes, led to the city of
Plymouth being successfully evacuated before it was almost completely covered by pyroclastic
flows, saving many lives (Robertson et al., 2000). Around the world, millions of people livein the shadow of volcanoes, including Kı̄lauea in Hawai‘i, Mount St. Helens in the PacificNorthwest, or Campi Flegrei in Naples, just to name a few (Small and Naumann, 2001).
Failure to detect significant subsurface changes in any volcano could put lives at risk, making
the implications of a silent intrusion dire. If this type of silent intrusion were to occur
as an undetected precursor, a major eruption could quietly materialize with potentiallycatastrophic results.

Fortunately, while my results suggest the possibility of a silent magmatic intrusion, thetechniques I utilized also provide a methodology with which to recognize such an event,decorrelation in reflectivity functions computed from ambient seismic noise. Other studies
have already shown that analysis of ambient seismic noise can be used to monitor the inflationof and pressure within magma chambers (e.g., Bennington et al., 2015; Brenguier et al., 2008;
Sánchez-Pastor et al., 2018). The suggestion from my work that these data sources can also
detect otherwise silent intrusions further contributes to the body of work supporting use of
seismic noise data analysis as a volcanic monitoring tool. The computations required for21this type of analysis can be performed quickly enough to be done in near real time andwith as few as one station. Therefore, incorporating such a technique would require little
addition to existing monitoring infrastructure. This low-cost coupled with the the potential
benefits for detection and risk reduction will hopefully serve to grow the momentum behind
this methodology, which if added to the suite tools already used for volcano monitoring,
could help keep populations living on and near volcanoes further from harm’s way. On
a larger scale, the favorable results stemming from leveraging ambient seismic noise will
hopefully serve to encourage the continued exploration of novel data sources in pursuit ofdeeper understanding.

One might argue that this continued growth of the array of tools and body of scientificwork are the greatest testament to Thomas Jaggar and his founding of the Hawaiian Volcano
Observatory. However, I am compelled to argue that his true legacy lies not with thescience itself, but rather with the millions of people whose lives are safer as a result. A
silent intrusion that leads to unwarned-of-eruption is most certainly “subterranean machinery
totally unknown,” and it is easy for me to agree with Dr. Jaggar’s sentiment that thissubterranean machinery is truly worthy of a life’s work. While this study presents interesting
results, it is but a first step, and continued study of this phenomenon and refinement of themodel presented here would help paint a clearer picture of the eruptive processes for not
only the 2018 Kı̄lauea eruption, but also other events in both the past and future.

22References
Anderson, K. R., M. P. Poland, J. H. Johnson, and A. Miklius (2015), Episodic deflation inflation events at Kilauea volcano and implications for the shallow magma system, Hawaiian
Volcanoes: From Source to Surface, 208, 229.
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