Deformation of Mauna Loa volcano before, during, and after its 2022 eruption

doi:10.21203/rs.3.rs-4592993/v1

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Deformation of Mauna Loa volcano before, during, and after its 2022 eruption

https://doi.org/10.21203/rs.3.rs-4592993/v1

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Mauna Loa volcano on the Island of Hawaiʻi erupted on 27 November 2022, marking the end of decades-long volcanic unrest since its last eruption in 1984. Here we briefly describe the evolution of the USGS Hawaiian Volcano Observatory’s geodetic monitoring network and show patterns of deformation as measured by Global Navigation Satellite Systems (GNSS), interferometric synthetic aperture radar (InSAR), and borehole tilt in the years leading up the 2022 eruption. We highlight the long-term buildup as well as the imminent pre-eruptive geodetic signals, including subtle changes observed in early 2021 that suggested an eruption might be near, and the significant ramp up of activity in September 2022 that provided strong evidence of likely impending eruptive activity. Of particular importance are the first borehole tilt excursions related to magma movement measured at Mauna Loa’s summit, which began in 2021 and were accompanied by increased rates of seismicity. In addition to describing the evolution of surface displacements, we also model the co-eruption deformation, which can be fit by dike opening that matches the geometry of the surface eruptive fissures.

Geodetic data associated with Mauna Loa’s pre-, co-, and post-eruptive phases provide opportunities for exploring questions related to the volcano’s history and magmatic system. How does the 2022 eruption compare to previous eruptions in 1975 and 1984? What geodetic tools are best suited for tracking volcanic unrest at Mauna Loa? How does caldera faulting relate to eruption timing? What, if any, are the relations between Mauna Loa and neighboring volcano, Kīlauea, which paused in its own eruptive activity as the Mauna Loa eruption waned? The comprehensive and advanced geodetic data from before, during, and after the volcano’s 2022 eruption, unavailable during prior eruptions, offer a means of addressing these questions, which are vital to better understanding and anticipating future eruptive activity and impacts.

Deformation

Volcano monitoring

Eruption

Geodesy

Mauna Loa

Mauna Loa, situated on the Island of Hawaiʻi (Fig. 1), is well-known as the largest volcano on Earth, covering more than half of the island's surface area (exceeding 5,000 km²). Its volume is estimated to be over 80,000 km³ (Lipman and Calvert 2013), and it maintains an impressive record of activity, erupting 34 times since 1843. During the period between the mid-1800s and 1950, eruptions occurred approximately every 3.6 years (Barnard 1995); however, the frequency of eruptions declined after 1950, with only three thereafter, in 1975, 1984, and 2022. Eruptions at Mauna Loa are characterized by initial summit activity that about half the time subsequently migrates into one of the volcano’s two rift zones, with rare eruptions from radial vents on the volcano’s northwest flank (Fig. 1). Significant seismic events along a basal decollement fault also occur (Wyss and Koyanagi 1992).

The recurrent high-effusion-rate lava flows from Mauna Loa pose hazards to human life and property on the volcano's lower flanks and coastal regions, particularly developments like Ocean View Estates, located along the Southwest Rift Zone (SWRZ), and the city of Hilo, which can be impacted by lava flows from the Northeast Rift Zone (NERZ) (Trusdell 1995). For example, a SWRZ lava flow in 1950 traveled from its vent area to populated areas within hours (MacDonald and Hubbard 1951).

Geodetic monitoring efforts were implemented at Mauna Loa in the mid-1960s, with Electronic Distance Measurement (EDM) surveys at the summit (Decker and Wright 1968). These data recorded inflationary episodes prior to the brief eruption of 1975, as well as between that eruption and the substantial summit and NERZ eruption of 1984 (Lockwood 1987; Decker et al. 2008). A robust campaign Global Navigation Satellite Systems (GNSS) network was established in the early 1990s (Miklius et al. 1995, 1997). The first continuous GNSS sites were installed in the late 1990s, with significant expansion of the network in the mid-2000s.

Since the early 1990s, rates of seismicity at Mauna Loa have waxed and waned, sometimes including earthquake swarms with hundreds per day. Rates of deformation at Mauna Loa have also fluctuated over the decades between the 1984 and 2022 eruptions. Mauna Loa’s 2022 eruption began on 27 November at 23:23 HST and initiated at the caldera but within hours migrated into the NERZ. Between 28 November 2022 and 10 December 2022, basaltic lava fed an ʻaʻā flow to the north that ultimately reached within 2.8 km of the major, cross-island Daniel K. Inouye Highway.

Here we present geodetic observations from the U.S. Geological Survey Hawaiian Volcano Observatory’s (HVO) geodetic monitoring network spanning nearly a half century. We begin by describing the geodetic measurements, and then examine the pre-2022 unrest at Mauna Loa’s summit. Next, we present detailed observations of heightened unrest in the ~ 20 months leading up to the 2022 eruption, as well as co- and post-eruptive periods. Finally, we conclude by exploring open questions and important observations related to Mauna Loa’s most recent eruption and post-eruption recovery.

Deformation monitoring at Mauna Loa began in the 1960s (Decker et al. 1966, 1983; Miklius et al. 1995; Decker et al. 2008) with leveling surveys and EDM. These data, supplemented by occasional “dry tilt” (also called single-setup leveling) measurements (Decker et al. 1983; Sako 1987) characterized displacements before, after, and associated with the 1975 and 1984 eruptions, capturing the inflationary period before 1975 and between these two eruptions and complex patterns of post-1984 displacements through the early 2000s (Fig. 2) (Miklius et al. 1995; Decker et al. 2008). Although effective tools for their day, these methods are limited in terms of spatial coverage and temporal frequency, and they were labor- and time-intensive.

In the late 1980s, campaign GNSS measurements began in tandem with EDM, leveling and dry tilt surveys (Miklius et al. 1997). Because GNSS measurements provide both horizontal and vertical positions, this tool quickly became the preferred geodetic monitoring method at Hawaiian volcanoes. Beginning in 1996, HVO, in collaboration with institutions including the University of Hawaiʻi and Stanford University, began establishing permanent, continuously monitoring GNSS sites, and by the 2000s campaign and continuous GNSS had completely replaced leveling, dry tilt, and EDM on the volcano (Decker et al. 2008; Tilling et al. 2014). Borehole tiltmeters (installed at depths of 2.5–5 m) were also established during the mid-1990s and early 2000s to provide sensitivity to small but rapid variations that might be associated with magma ascent. Those instruments have proven critical for detecting subtle changes prior to eruptions and changes in eruptive activity at Kīlauea (Decker et al. 2008; Poland et al. 2014, 2021). Elevated unrest at Mauna Loa in 2002, and also in 2004–2005, prompted rapid expansion of the GNSS network, which by the end of 2005 included about 19 permanent sites broadly covering the volcano and intended to track displacements in the summit area, rift zones, and flanks. Since the mid-2000s, the primary geodetic monitoring techniques used to monitor displacements of Mauna Loa have been GNSS (mostly using data from the Global Positioning System (GPS)), interferometric synthetic aperture radar (InSAR), and borehole tiltmeters. Together, these data provide an outstanding combination of temporal and spatial resolution for detecting geodetic changes at Mauna Loa.

At the time of the 2022 eruption, HVO was operating approximately 28 continuous GNSS stations across Mauna Loa (Fig. 1). These stations collect a combination of 15- and 1-second data. An additional ~ 35 benchmarks on Mauna Loa are measured during annual campaigns, which provide denser data in areas with sparse permanent station coverage. Positions shown herein are derived from 15-second data merged into 24-hour daily solutions and processed with Release 6.3 of the GIPSY-OASIS (GNSS-Inferred Positioning System and Orbit Analysis Simulation Software) suite from the Jet Propulsion Laboratory (JPL).

The geodetic network also included 6 borehole tiltmeters (Fig. 1) with measurements collected every minute (Ellis and Johanson 2024a, 2024b). In addition, InSAR data for the Island of Hawaiʻi are plentiful. Hawaiian volcanoes were designated a permanent “Supersite” by the Group on Earth Observations in October 2012, thereby facilitating access to a variety of InSAR datasets from space agencies around the world (Dzurisin et al. 2019; http://geo-gsnl.org). Interferograms spanning various time periods leading up to and during the 2022 eruption provided valuable insights into both discrete events and changes in overall surface deformation of Mauna Loa.

The 2022 Mauna Loa eruption presented the inaugural opportunity for the modern geophysical monitoring network (Fig. 1) to comprehensively measure pre- and co-eruption deformation at the volcano. Collectively, these geodetic data provided the coverage needed to detect, track, and interpret magma accumulation and transport in the decades leading up to the 2022 eruption, immediately prior to the eruption, during the eruption, and in the post-eruptive period.

Geography and eruption patterns

Mauna Loa's summit is dominated by Moku‘āweoweo, a 3 km by 5 km collapse caldera that is elongated in a northeast-southwest direction (Fig. 1) (Trusdell and Zoeller 2017). The full summit region of Mauna Loa, as defined by past eruptions and the volcano’s magmatic system, extends beyond this topographic feature and is bounded by a system of faults to the southeast (Macdonald 1971), making the total summit region ~ 50% larger than the topographic caldera itself.

Rift zones, which contain numerous eruptive vents, extend from Moku‘āweoweo to the east-northeast and to the southwest (Fig. 1). The SWRZ extends well offshore of the south part of the Island of Hawaiʻi and has a bend in its middle reaches, likely a result of westward migration of that part of the rift zone (Lipman 1980). On the NERZ, eruptive activity over at least the past 200 years has been limited to the upper 19 km, which becomes progressively wider and more diffuse in its downrift extent until it is buried by younger lavas from Kīlauea (Trusdell and Zoeller 2017). Roughly half of the eruptions since 1843 migrated into a rift zone. Of these, the distribution is approximately equal between the NERZ and SWRZ (Lockwood and Lipman 1987).

In addition to the summit and rift zones, Mauna Loa eruptions also occur from submarine and subaerial vents that extend radially from the northwest flank (Lockwood and Lipman 1987). These radial vents align with the area of greatest horizontal tension, controlled by intrusions into the two rift zones, suggesting that radial dike eruptions occur during periods of heightened pressure within the magma storage area (Rubin 1990, Poland et al. 2014). Radial vent eruptions may also provide evidence of secondary magma pathways that bypass the summit plumbing system (Wanless et al. 2006). Since 1843, two radial-vent eruptions have occurred: a 300-day-long eruption in 1859 that extended from a vent at 11,000 feet elevation to the northwest coast, and a brief submarine eruption in Kealakekua Bay in 1877 (Normark et al. 1978; Moore and Clague 1987).

Magma plumbing system

The presence of a summit magma source at approximately 3 to 4 km, located slightly to the south of Moku‘āweoweo but within the southern bounding faults of the broader summit region, is a consistent feature of geodetic and seismic studies (e.g., Decker et al. 1983; Lockwood et al. 1987; Okubo 1995; Miklius et al. 1997; Yun et al. 2005; Amelung et al. 2007; Poland et al. 2014; Amelung 2021). This zone of magma storage coincides with an aseismic region beneath the south-southeastern part of the caldera, capped by earthquakes around 4 km in depth (Decker et al. 1983; Okubo 1995). Deformation data collected between 1977 and 1981 suggest an inflating source depth of approximately 4 km leading up to the 1984 eruption (Decker et al. 1983; Lockwood et al. 1987). As the summit deflated due to lava effusion during the 1984 eruption, modeling of tilt and leveling data suggested a source of magma withdrawal at ~ 3.5 km depth beneath the southeast margin of the caldera (Johnson 1995). After this eruption, models using EDM and leveling data indicated an inflating source at 3.7 km depth just east of the caldera (Miklius et al. 1995). The location and depth of this source was very close to those modeled for the source of the 1975–1984 inflation and the 1984 deflation episodes (Lockwood et al. 1987; Miklius et al. 1995).

The installation of a robust continuous GNSS network and availability of InSAR data by the mid-2000s provided new opportunities to image magma sources beneath Mauna Loa with geodetic techniques, especially as a new phase of inflation began in 2002 following 8 years of little significant deformation (Miklius and Cervelli 2003). These datasets made clear that not only was there a spheroidal source beneath the southeast margin of the caldera between ~ 3.6 and 4.7 km depth, but there was also a tabular structure with a length of about 8 km that extends across the summit region and spans depths of about 4–8 km (Miklius et al. 2004; Yun et al. 2005; Amelung et al. 2007). All deformation data collected since the mid-2000s show varying rates of magma accumulation and withdrawal over time associated with both these sources (Amelung et al. 2007; Poland et al. 2014; Thelen et al. 2017; Varugu and Amelung 2021), and it is likely that the tabular source has always been present but could not be resolved using the limited datasets available before the mid-2000s.

Flank motion and decollement activity

In addition to helping identify magma storage regions, GNSS data on Mauna Loa indicate movement of its southeast flank. Campaign GNSS stations established in the 1990s were the first to suggest that Mauna Loa’s southeast flank surface was moving at a rate of more than 4 cm per year towards the southeast along a sub-horizontal fault underlying the volcano's flank (Miklius et al. 1995; Miklius et al. 1997; Poland et al. 2014). Movement of the southeast flank of Mauna Loa has also been implied by analyses of low-angle thrust faulting in the Kaoiki seismic zone (Endo 1985; Bryan and Johnson 1991; Wyss et al. 1992; Jackson et al. 1992; Varugu and Amelung 2021) and by possible feedback between flank motion of dike intrusion (Walter and Amelung 2004, 2006). These studies suggest that Mauna Loa's southeast flank slips seaward on a basal decollement at ~ 12 km depth (Endo 1985; Thurber et al. 1989; Wyss et al. 1992; Miklius et al. 1995). Significant seismic activity has also been recorded on Mauna Loa’s lower west flank, suggesting decollement activity on that side of the volcano as well (Endo 1985; Wyss and Koyanagi 1992).

Since its eruption in 1984, Mauna Loa has experienced repeated episodes of magma accumulation separated by periods of relative quiescence. Immediately following the 1984 eruption, the shallow magma storage area for Mauna Loa started refilling. Inflation slowed and stopped altogether in the mid-1990s (Decker et al. 2008; Poland et al. 2014; Thelen et al. 2017), with subtle and persistent deflation occurring until 2002, when a new episode of inflation began at about the same time as deep long-period seismicity beneath Mauna Loa and a high-volume lava effusion event at neighboring Kīlauea (Miklius and Cervelli 2003; Okubo and Wolfe 2008; Thelen et al. 2017). Between May and December 2002, summit continuous GNSS stations like sites MOKP and MLSP (Fig. 1) measured roughly 1.5 to 3.5 cm of uplift. Inflation continued at a low rate throughout 2003 (Figs. 2 and 3b).

Another distinct inflation period began in 2004 (Miklius and Cervelli 2003; Amelung et al. 2007; Poland et al. 2014). Surface deformation measured between mid-2004 and late 2005 at summit GNSS sites show ~ 10 cm of uplift. Baseline lengths between sites such as TOUO and ELEP on the volcano's lower flanks show around 20 cm of extension, indicating a rapidly inflating deeper source of magma accumulation. This particular period of inflation was also accompanied by a dramatic increase in deep long-period seismicity approximately 45 kilometers beneath the surface that started coincident with the inflation but slowed dramatically on 26 December 2004 (Figs. 2 and 3a, b)—the time of the great Sumatra earthquake (Okubo and Wolfe 2008). It was this episode of inflation that was the first to suggest, based on modelling using both InSAR (Amelung et al. 2007) and GNSS (Poland et al. 2014) data, the presence of a tabular magma storage zone beneath Mauna Loa’s summit in addition to a spheroidal source beneath the southeast part of the caldera. Kīlauea also experienced rapid inflation during this period due to a surge in magma supply from the mantle (Poland et al. 2012) that Gonnermann et al. (2012) interpreted to have also impacted Mauna Loa, implying a stress connection, if not a direct physical connection (which is not favored by geochemical evidence) between the two volcanoes. Deformation rates slowed considerably by the end of 2005, but inflation, along with elevated shallow seismicity, persisted until 2009 (Figs. 2 and 3a, b).

After five years of effectively no deformation, Mauna Loa began to inflate and shallow seismicity rates increased in 2014 (Figs. 2 and 3a, b). The pattern of ground deformation was similar to that of the 2004–2005 episode and has likewise been modeled by coexisting spheroidal and tabular sources. The locus of inflation moved to the southwest by a few kilometers in late 2015 before returning to the main summit area in 2016—a change in pattern that very likely reflects the underlying magma storage conditions but that is still not well understood (Thelen et al. 2017; Varugu and Amelung 2021). Overall, between 2014 and 2020 the tabular body is modeled to have widened by ~ 2.5 m, and steady decollement slip occurred beneath the volcano’s eastern flank (Varugu and Amelung 2021). The Moku‘āweoweo and southern caldera areas were also the most seismically active regions during the 2014–2020 unrest, consisting of mostly small earthquakes (magnitude less than 2.5) at depths of 2–4 kilometers (Thelen et al. 2017). The rate of inflation waxed and waned over time, with the highest rates in 2014–2015 and 2019–2020 (Figs. 2 and 3a, b).

The rate of summit inflation increased significantly in late 2020 to early 2021, lasting until mid-March 2021 (Fig. 4b), shortly after a M3.2 earthquake that occurred less than 500 m below the surface of the south part of Moku‘āweoweo on 6 March. Interferograms spanning the earthquake identified localized deformation near the earthquake’s epicenter, indicating about 10 centimeters of downward motion of the caldera floor relative to the rim and suggesting motion along one of the caldera-bounding faults (Fig. 4d).

After this earthquake, there was a distinct uptick in daily seismicity (up to 120 events per day) beneath the summit of Mauna Loa—a pattern that persisted, with fluctuations, through April 2021 (Fig. 4a). For a period of about two weeks during late March and early April, inflation of Mauna Loa was interrupted by contraction of almost 1.5 cm along the MLSP-MOKP baseline (Fig. 4b). Corresponding to the GNSS-measured deflation, the MOK tiltmeter measured a ~ 5 microradian decrease in the southeast direction (Fig. 4c), consistent with deflation of the summit (Fig. 4b). This anomaly is the first volcano-related tilt signal measured by the Mauna Loa borehole network since its installation beginning in 1999. Although five microradians over the span of weeks may be interpreted as instrument drift in some cases, in this particular instance the tilt signal agrees very well with GNSS observations, diverges significantly from tilt trends before and after the event, and is far larger than normal diurnal signals or other environmental artifacts. The tilt signal was interpreted as evidence that activity beneath Mauna Loa’s summit may be shallowing.

Although GNSS and tilt measurements clearly confirmed contraction at the summit of Mauna Loa in the weeks after the M3.2 earthquake, InSAR did not detect significant changes (Fig. 4e). The bulk of this deformation signal was in the horizontal direction, along the satellite azimuth, and so was effectively “hidden” by the viewing geometry. This case highlights the importance of ground-based measurements in tracking deformation at Mauna Loa—subtle signals that mostly involve horizontal motion may be difficult to detect in interferograms.

For nearly 18 months starting in April 2021, deformation data showed only minor inflation with a few fluctuations (Fig. 4b and Fig. 5b). Then, abruptly in September 2022, unrest at Mauna Loa escalated with surges in daily earthquake counts (Fig. 5a) and rapid uplift recorded by GNSS (Fig. 5b) and at tiltmeter MOK (Fig. 5c). This further increase in seismicity and deformation rates heightened the anticipation among HVO staff of the possibility of an eruption, prompting increased frequency of community updates and public engagement (Hon et al. 2023).

Initial Eruption Onset

Around 22:20 Hawaii Standard Time (HST; UTC-10) on 27 November 2022, rapid geophysical changes were recorded at Mauna Loa's summit. Tilt and seismic swarm alarms alerted HVO staff to escalated activity that suggested an imminent eruption. Immediate precursory seismicity consisted of an hour-long, concentrated sequence of small, shallow (less than a few km below the surface), high‐frequency earthquakes beneath the caldera and south of the summit. Seismic activity intensified about 36 minutes later, and seismic tremor was detected at summit stations (Maher et al. 2023).

Between roughly 22:50 and 23:30 HST, the MOK tiltmeter recorded inflationary tilt with a maximum of ~ 114 microradians downward towards azimuth 033 degrees (Fig. 6). The eruption commenced at approximately 23:23 HST and was marked by the appearance of fissures within Moku‘āweoweo, with visible glow captured in webcam imagery.

During the initial phase of the eruption, fissures opened across Moku‘āweoweo, resulting in the inundation of the caldera floor with fresh lava (Maher et al. 2023; Trusdell et al. 2024, Volume 86, Bulletin of Volcanology Special Issue on “Mauna Loa 2022”). Almost immediately after lava emerged at the surface, the MOK tiltmeter started to measure rapid deflationary tilt. By approximately 04:00 HST on 28 November 2022, the tilt at MOK had decreased by ~ 96 microradians downward towards azimuth 232 degrees and then decayed to a steady, slight deflationary trend (Fig. 6).

Around roughly 01:30 HST on 28 November 2022, SLC tiltmeter, positioned southwest of Moku‘āweoweo in the area where the summit and SWRZ systems converge (Fig. 1), began showing subtle tilt to the east-southeast. By about 02:30 HST, the tilt direction began to change, becoming stronger in the east-northeast direction, and just before 05:00 HST transitioned to strong tilt to the northwest. Around 05:55 HST and 06:45 HST, the instrumental scale limits were reached on the north and east components, respectively, and the instrument no longer gave useful readings. Before going off-scale, SLC ultimately measured a maximum ~ 542 microradians downward toward an azimuth of 308 degrees (Fig. 6).

At the time this deformation was observed at the SLC tiltmeter, there was concern that eruptive activity might be starting to migrate toward the SWRZ, putting residents and places like Ocean View Estates and the village of Miloliʻi at risk of being impacted by lava flows. No significant deformation was ever recorded at tiltmeter BLB located even farther to the southwest of SLC, implying that magma was not moving down the SWRZ (Fig. 1). By about 06:30 HST on 28 November, lava fountaining in Mokuʻāweoweo ceased, and the eruptive activity clearly shifted northeast along the crest of the NERZ, where multiple fissures emerged, designated by numbers 1–4 in order of their appearance. The concern over an eruption occurring along the SWRZ had definitively passed. The tilt at SLC was apparently associated with the summit eruption and reflected the fact that the summit magma storage system extends to the southwest of Moku‘āweoweo.

Evolution of deformation during ongoing eruption

From the onset of the eruption and throughout, the summit GNSS stations measured significant deflation. From 27 November to 10 December, the summit subsided by nearly 40 cm, decaying with distance to ~ 20 cm at sites ~ 5–10 km down the flank (Fig. 7a). At stations located at even lower elevations on the north and south flanks of Mauna Loa, GNSS sites measured ~ 2 to 6 cm of subsidence (Fig. 7a).

InSAR spanning the eruption onset and the first several days of activity provided additional insights into the complexity of surface deformation. The line-of-sight displacement patterns in interferograms showed overall subsidence of the summit area by more than 50 cm as magma flowed out of the main magma reservoir beneath Moku‘āweoweo to feed the eruption in the NERZ. There were also areas of uplift and spreading where the ground ruptured and magma reached the surface, including within Moku‘āweoweo, just southwest of the caldera, and northeast of the caldera into the NERZ (Fig. 8). These data also confirm that magma did not enter the SWRZ and remained in the southwest part of the summit region.

Island-wide deformation in response to eruption

Another phenomenon observed by HVO's geodetic network throughout the 2022 Mauna Loa eruption was island-wide deformation, detectable 50 to 80 km away from the summit of Mauna Loa. GNSS sites on Kīlauea, including in the south caldera, south flank, and even upper East Rift Zone areas began moving northwestward, towards Mauna Loa, starting around 28 November 2022, with a total displacement over the course of the eruption on the order of 1–2 cm (Fig. 9). The one continuously operating GNSS site, designated PMAU, on Hualālai volcano, also moved east by about 1 cm during this time period, consistent with Mauna Loa deflation (Fig. 9).

Elastic modeling of the 2022 eruption suggests that the far-field displacements can be explained by the same sources of deformation associated with Mauna Loa's shallow magmatic system without invoking a deeper source or other more complex mechanisms. To demonstrate this, we inverted GNSS data from sites on the Island of Hawaiʻi spanning 28 November 2022 to 5 January 2023 and developed a simple model that includes a discretized tabular body and a Mogi source that have the same geometry as those used to model pre-eruptive inflation (Johanson et al. 2017). We fit an exponential curve to GNSS time series, as well as steps associated with fissure opening in the first three days of the eruption. The amplitude of the exponential is used in the inversion as our data vectors (Fig. 9). The tabular body is 14 km long and spans a depth below the surface of 7.2 km to 1.2 km, and it is discretized into 1x1 km segments. The Mogi source is located 3.8 km below the surface (Fig. 9).

The bulk of the deflation during this time period is attributed to the Mogi source, with 37 million m³ of volume loss, while closing of the tabular body amounts to a volume loss of ~ 16 million m³. The model reasonably reproduces the displacements measured across the island, indicating that the magnitude of the source changes is sufficient to explain the broadness of the signal even though the sources were contained entirely within the Mauna Loa edifice. These results demonstrate that transfer of crustal stress can occur across the island due to activity at a single volcano with deflation at Mauna Loa leading to extensional strain across Kīlauea. Such far-reaching effects are probably not uncommon—the deformation field due to activity at one volcano can extend to the other and was especially well expressed during the 2022 eruption of Mauna Loa (Fig. 9).

Epoch-by-epoch GNSS solutions are among many new data types collected in 2022 that were not available during prior Mauna Loa eruptions. These data provide insights into sub-minute deformation processes and reveal the kinematics of the opening phase of the eruption, its migration into the NERZ, and focusing of activity at Fissure 3, which by the second day of the eruption was the only active vent.

We processed 15-second data using the software Track (Herring et al. 2020) in an epoch-by-epoch scheme. Rather than use a single reference station, we created a network of station pairs using Delaunay triangulation and ran separate instances of Track on each station pair (Grapenthin et al. 2014). We then performed a network stabilization by minimizing the movement of five stations outside the summit the region (YEEP, KNNE, PIIK, BLBP, and KHKU) during each epoch. The resulting 15-second time series for each station were then averaged into 15-minute samples (Fig. 10). Sample uncertainties were taken to be the standard deviation of the 15-second positions that make up each 15-minute average.

To model the GNSS time series, we use the Extended Network Inversion Filter (ENIF) of McGuire and Segall (2003), which is a modification of the Network Inversion Filter (NIF) (Segall and Matthews 1997)—a Kalman filter modeling framework for geodetic data. The ENIF adds the ability to use the input data to estimate model hyper-parameters and includes a positivity constraint. Both approaches have typically been used for detecting small changes in model source strengths over time, such as in slow earthquakes (Bekaert et al. 2016; Murray and Segall 2005). Here we use the ENIF to model opening and closing of a tabular body beneath the summit, as well as the migration of magma into the NERZ.

Deformation during the first two days of the eruption is well-fit by opening and closing of a discretized tabular body (dike). Data fits did not improve by including an additional point source beneath the caldera, as is typical in pre-eruption models spanning longer time periods (e.g., Miklius et al. 2004; Amelung et al. 2007; Poland et al. 2014; Varugu and Amelung 2021); neither did the presence of a sub-caldera point source change the dike opening and closing patterns in the model. To minimize model elements, the point source was therefore not included.

The tabular body consists of three vertical segments with varying strike, each extending from the surface to 4 km depth and traced from the surface expression of mapped fissures on Mauna Loa (Fig. 1 and Fig. S1 in Online Resource 1). The three tabular body segments are discretized into roughly 0.5 km x 0.5 km elements and combined such that they are treated as one source with spatial smoothing across all three segments.

The NIF and ENIF include a number of hyper-parameters that are intended to define the temporal evolution of the model. These include temporal smoothing, benchmark wobble, uncertainty scaling, spatial smoothing, and rate-positivity constraints. Because the uncertainties for the data samples are derived from the scatter of the 15-s time series, we do not use the uncertainty scaling parameter and instead fix it to be 1. We also keep the rate positivity flexible, as we expect both opening and closing during the model time period. Benchmark wobble is unlikely to be a strong noise source during the 2 days that these data span, so this value is fixed at 3 mm/√year. We vary the spatial smoothing of the tabular body manually while using the ENIF to determine the most likely temporal smoothing, then examine the data misfit versus model roughness to choose a preferred spatial smoothing factor and corresponding temporal smoothing.

A challenge of using this type of model for volcanic processes is that the deformation rates vary considerably over time. Therefore, time-evolution parameters that capture the onset of the eruption would tend to fit noise once the deformation rates diminish. We thus preferred hyperparameter values that produced smooth time evolution towards the end of the activity (Fig. 11); however, as a consequence, the initial high rates of deformation are not immediately captured by the model (Fig. 10).

The timing of the modeled opening is broadly consistent with geological and seismological observations of the first few hours of the eruption (animation of model results in Online Resource 2). The first two hours of the model are dominated by dike opening under Moku‘āweoweo (Fig. 11a), which was where eruptive activity commenced according to webcam imagery, commencing just before the modeled peak opening rate. Once lava began erupting at the surface, the opening rate quickly declined and turned to closing in a similar location. About 2 hours after the eruption onset, the portion of the model under Moku‘āweoweo consistently shows closing, indicating deflation of the tabular body at 1–4 km depth. Simultaneously, opening is indicated to the northwest and southeast of Moku‘āweoweo, which agrees with tremor locations that suggest multiple fissuring sources along the length of the caldera (Fig. 11b) (Maher et al. 2023). By about 8–9 hours after the eruption onset, modeled opening is entirely beneath the NERZ, which coincides with the eruptive activity focusing on Fissures 3 and 4 (Fig. 8 and Fig. 11c). Opening rates in this area wane 15–20 hours after eruption onset as the eruptive activity focuses and stabilizes at Fissure 3 (Fig. S2 in Online Resource 1).

While appreciable volume accumulation and loss happen in discrete sections of the model (Fig. S3 in Online Resource 1), the cumulative volume change throughout the first 48 hours of the eruption is modest at -10 million m³. This indicates a balance between closing in the central portion of the tabular body and opening in the NERZ. It also implies that during the eruption’s opening phase, lava was sourced from magma stored at shallow levels, rather any source outside the model space, such as a deeper sub-caldera reservoir. This is consistent with geochemical analyses that suggest that magma was stored ~ 1.5 km depth before being erupted (Lynn et al. 2024, in review)—a conclusion consistent with tilt and GNSS data that argue for a shallow deformation source in the two months before the eruption.

The 2022 Mauna Loa eruption ended suddenly on 10 December 2022. Unlike numerous rift zone eruptions of Kīlauea (Montgomery-Brown et al. 2010; Lundgren et al. 2013), where summit inflation coincides with the end of lava effusion, tilt and GNSS data indicate that summit inflation at Mauna Loa started as early as 7 December 2022, preceding the end of lava effusion by several days and possibly providing an indication of the impending end of the eruption (Fig. 7). GNSS data captured a swift and substantial inflationary response within the first week or two following the cessation of eruptive activity, with summit stations showing the most pronounced uplift compared to flank stations. From mid-December 2022 to the end of December 2023, summit sites such as MOKP (Fig. 12), MLSP, and MLES documented uplift ranging from approximately 12 to 36 centimeters. Correspondingly, south flank sites, including PAT2 and PHAN, recorded uplift ranging from approximately 14 to 7 centimeters during the same timeframe. The MOK tiltmeter also recorded inflationary tilt until at least the end of October 2023. Inflation rates waned over time, and by early 2024, significant deformation of Mauna Loa had ceased (Fig. 12). During the time period of post-eruption inflation, the deformation pattern once again resembled that of the pre-eruption period, indicative of both a spheroidal and a tabular source beneath Mauna Loa’s summit region.

Comparing previous 1975 and 1984 eruptions to the 2022 Mauna Loa eruption

Varying types of geodetic measurements span each of the 1975, 1984 (Miklius et al. 1995), and 2022 eruptions. These data together provide a basis for comparison of the eruptions and decades of inter-eruption deformation at Mauna Loa. Observations of unrest prior to the 2022 eruption also provide a basis for reinterpreting the more limited past observations, providing insights into the timing and style of magma accumulation and eruption.

In 1975, EDM measurements showed ~ 60 cm of extension across the caldera, likely caused by emplacement of the dike that fed the July 1975 eruption, but only minor extension between 1970 and 1974 (Fig. 2) (Decker et al. 1983). Because the 1975 eruption was short-lived, no co-eruption-only deformation measurements are available. The 1975 eruption lasted for less than 24 hours, remained contained in the summit caldera, and had an estimated total bulk volume of ~ 30 million m³ (Lockwood and Lipman 1987; Bernard 1995). Following the eruption, extension across the caldera persisted, with about 20 cm accumulating during 1975–1976 and rates of 2–5 cm per year measured into the early 1980s (Decker et al. 1983; Miklius et al. 1995).

Accelerating rates of extension on summit-crossing EDM line lengths (Fig. 2) coupled with increased seismicity prompted HVO scientists in 1983 to make a general forecast of potential future eruptive activity within 2 years if the unrest persisted (Decker et al. 1983)—a forecast that proved remarkably prescient when the volcano erupted in March–April 1984. As in 2022, Mauna Loa’s 1984 eruption initiated at the summit and then moved to the NERZ. The 1984 eruption lasted 12 days and emitted ~ 220 million m³ of lava (Lockwood and Lipman 1987; Barnard 1995). In comparison, the 2022 eruption lasted 22 days with an eruptive volume of ~ 145 million m³ (Dietterich et al., in prep). After the 1984 eruption, EDM lines spanning Mauna Loa’s summit continued to extend, with more than 20 cm accumulating by October 1989 (Miklius et al. 1995). The most rapid displacements occurred within the first year and a half after the eruption (Fig. 2).

For all three eruptions, reinflation was rapid immediately after the end of lava effusion but gradually waned over the following year (Fig. 2). This behavior may suggest that the pressure difference between a deep magma source and the shallow magma storage reservoir tapped by the eruptions drives post-eruption magma ascent (Dvorak and Dzurisin 1993; Poland et al. 2014).

Of potentially more interest is variability in pre-eruptive deformation. There were only sparse, small changes measured in the years prior to the 1975 summit eruption, whereas the 1984 and 2022 eruptions, which included vents on the NERZ, were preceded by years of extension across the summit (Fig. 2). In both cases, HVO scientists correctly anticipated the eruptions based on deformation and seismicity, providing opportunities to spread awareness among the community and partners. There is thus a tantalizing possibility that small and short-lived summit eruptions may not be preceded by significant geodetic and seismic unrest compared to larger and longer-lived eruptions that include rift-zone activity—a hypothesis that can only be tested following future eruptions.

Finally, deformation prior to, during, and immediately following the 1975 and 1984 eruptions was interpreted as due to a single source of pressure change beneath the southeast part of the caldera (Decker et al. 1983; Lockwood et al. 1987; Miklius et al. 1995). Data were sparse prior to the use of GNSS, and no additional complexity in model sources could be justified. In the 1990s, campaign GNSS data suggested the possibility that flank motion was occurring due to some sort of decollement or intra-volcano fault slip. And in the mid-2000s, it became clear, from both GNSS and InSAR, that magma accumulation occurred in both a source beneath the southeast part of the caldera and in a tabular body that extended along the length and to the south of Moku‘āweoweo (Miklius et al. 2004; Yun et al. 2005; Amelung et al. 2007). Both flank motion and magma accumulation/withdrawal in the tabular body beneath the summit were probably also occurring before and after the 1975 and 1984 eruptions but could not be isolated, owing to the limited nature of available geodetic data. Future work could assess whether EDM, tilt, and leveling data from before the 1990s can accommodate those sources in addition to the long-term magma storage area beneath the southeast caldera.

Best techniques for tracking the pre- and co-eruptive deformation at Mauna Loa

The 2022 eruption and its build up offer a prime opportunity to assess the best methods for recognizing various stages of pre-eruptive unrest and to evaluate HVO’s deformation-monitoring strategy for the largest volcano on Earth. In the decades leading up to the eruption, both GNSS and InSAR provided excellent temporal and spatial resolution of surface motion at Mauna Loa and were effective complements to one another. For instance, changes in inflation rate in the early to mid 2000s were well captured by GNSS time series, and subtle changes in the geometry of inflation in the mid-2010s were similarly well characterized by InSAR (Miklius and Cervelli 2003; Varugu and Amelung 2021). InSAR data are also particularly useful for identifying localized displacements, like those associated with the 6 March 2021 M3.2 earthquake, which were not captured by the GNSS network (Fig. 4). Conversely, GNSS data are outstanding for detecting small changes that might not be visible in InSAR data because of uncertainty due to imaging geometry, atmospheric artifacts, and other factors. The joint use of these two methods thus provided a powerful means of assessing spatio-temporal changes in deformation style across a range of scales leading up to the Mauna Loa eruption.

Tilt signals attributed to magmatic processes were not observed at Mauna Loa prior to 2021, despite over 20 years of data collection at some sites. Subtle deflation measured by the MOK tiltmeter following the March 2021 earthquake, confirmed by GNSS observations, was the first measured tilt excursion thought to be related to shallow magmatism. MOK tilt showed no additional significant changes until mid-September 2022, about two months before the eruption onset, when it measured an inflationary trend at the summit (Fig. 5c). In retrospect, these tilt excursions provided an indication of shallow magmatic activity, strongly suggesting that an eruption might be likely. The deflationary signal in March 2021 (Fig. 4a, b) might have been a sign that magma had reached shallow levels, while the September 2022 inflationary tilt at MOK, which tracked inflation measured by GNSS (Fig. 5a, b), demonstrated a heightened potential for eruption.

In the hour prior to lava onset and during the first few hours of the eruption, tilt data were critical to assessing the likelihood and course of eruption. When the MOK tiltmeter first detected a large excursion on the order of 50–80 microradians over ~ 20 minutes on the night of 27 November (Fig. 6), observatory staff confidently estimated that an eruption was imminent within minutes to hours. When lava finally reached the surface, tilt at SLC (Fig. 6), coupled with the lack of significant deformation at tiltmeter BLB, provided a strong indication that, while present in the southwest part of the caldera, magma was not intruding into the SWRZ. These minute-by-minute insights into ground deformation patterns were vital for assessing hazards to vulnerable, downrift communities.

The quintessential lesson of the 2022 Mauna Loa eruption is that a diverse approach to deformation monitoring is critical. GNSS and InSAR provided key insights into small changes leading up to the eruption. Even though the borehole tiltmeter network showed no significant signals for decades, the instruments were invaluable for not only detecting when an eruption might be looming, but for also providing timely insights into how the eruption might evolve in the critical first hours of activity. Moving forward, it is imperative to keep both the tilt network robust and operational and to maintain GNSS and InSAR capabilities to track changes to surface deformation over multiple scales of time and space in advance of the next Mauna Loa eruption.

Implications of March 2021 caldera faulting event

The 6 March 2021 M3.2 earthquake was the first observed event known to have ruptured one of the bounding faults of Moku‘āweoweo, although such a process probably would not have been recognized prior to the availability of InSAR data in the late 1990s and early 2000s. Interferograms spanning the earthquake clearly document localized downdropping of a portion of the southeast caldera floor relative to the rim (Fig. 4d), but the isolated nature of the deformation prevented its detection by summit-area GNSS stations (Fig. 4b). Caldera extension, which had been occurring at an accelerated rate since late January 2021, slowed following the earthquake, and the summit experienced a period of rapid contraction about two weeks after the earthquake, with the caldera-crossing GNSS line length shortening by about 1.5 cm between 24 March and 9 April (Fig. 4b). Summit deformation then remained stagnant for months, and earthquake activity was subdued compared to earlier in 2021. These observations raise the possibility of a relation between the shallow M3.2 earthquake and Mauna Loa’s magmatic system.

Similar sequences of inflation and faulting have been noted at other volcanoes. Perhaps most well-known is the “trapdoor fault” at Sierra Negra, Galápagos. Here, both thrust and normal motion occur along a sinuous intracaldera fault in response to both pre-eruptive pressurization (Amelung et al. 2000; Jónsson et al. 2005; Chadwick et al. 2006; Jónsson 2009) and co-eruptive depressurization (Shreve and Delgado 2023). Models of the faulting suggest that it can relieve pressurization of the sub-caldera magma body (Jónsson 2009), possibly postponing eruption and resulting in extraordinary amounts of uplift that could not be sustained without eruption at other basaltic volcanoes (Chadwick et al. 2006). Rupture of caldera faults in M4 + events has also been proposed as a response to summit pressurization at Kīlauea (Wauthier et al. 2013).

It is therefore tempting to interpret the 6 March 2021, caldera faulting event at Moku‘āweoweo as a consequence of the accumulated pressurization of the sub-caldera magma system, which stretched the summit region to the point of inelastic, if localized, failure. If this was the case, the fault rupture may have had the effect of postponing an eruption of Mauna Loa—a particularly appealing inference given the summit contraction that occurred in the month following the earthquake. Correlation is, of course, not causality, and we remain speculative in the absence of additional evidence. We encourage future analysis of this event, and also monitoring of caldera faulting, which may only be identifiable using InSAR data. If the process is a repeatable one, as it is at Sierra Negra, localized failure of Moku‘āweoweo’s walls or other fault systems in the summit region of Mauna Loa may provide an indication of the pressurized state and eruptive potential of the volcano.

Interactions between Kīlauea and Mauna Loa

Mauna Loa’s 2022 eruption provides yet another opportunity to explore the often ambiguous and frequently questioned relation between that volcano and its neighbor, Kīlauea—an issue that has persisted for well over 150 years (e.g., Dana 1850; Jaggar 1917). While geochemical evidence rules out a direct connection between the volcanoes (e.g., Wright 1971; Rhodes et al. 1989; Frey and Rhodes 1993), coincidences in deformation and eruptive activity (including both correlations and anti-correlations) raise the possibility of some sort of stress connection, possibly via an asthenospheric melt layer (e.g., Miklius and Cervelli 2003; Gonnermann et al. 2012; Poland et al. 2014; Dzurisin and Poland 2019).

The eruption at Mauna Loa started during an ongoing Kīlauea eruption in Halema`uma`u Crater that began on 29 September 2021. As Mauna Loa’s eruption progressed, Kīlauea’s effusive activity, which was already occurring at a very low level, gradually waned and ultimately ceased on 9 December; the eruption at Mauna Loa stopped the following day. A few weeks later, on 5 January 2023, Kīlauea resumed erupting. The timing of these changes is intriguing. Did the eruption of Mauna Loa impact the activity of Kīlauea in late 2022?

Coincidences in inflation at Mauna Loa and an episode of high-volume effusive activity at Kīlauea in May 2002 led Miklius and Cervelli (2003) to suggest that a pulse of magma introduced into Mauna Loa’s plumbing system increased the pressure in Kīlauea’s shallow magma system. Gonnerman et al. (2012) further explored this possibility, developing a model where stress is transferred via asthenospheric pore pressure at much higher rates than direct flow of melt and that can explain observed instances of coinciding ground deformation and eruptive activity without requiring a direct connection (which is not considered possible based on differences in isotopic compositions of the erupted lavas). This model provides a mechanism by which an eruption at one volcano may hinder eruptions at the other.

Applying this model to the 2022 observations, it is possible that the outpouring of lava from Mauna Loa lowered the pressure in Kīlauea’s plumbing system, causing the already-tenuous eruption to cease. Once the Mauna Loa eruption ended, pressure was able to build once again, allowing lava effusion at Kīlauea to resume after a few-week pause. While speculative, the observations are consistent with past coupled behavior of the two volcanoes and provide additional fodder for exploring the relations between these two adjacent and highly active volcanoes.

The 2022 Mauna Loa eruption provided a rare opportunity to both witness a decades-awaited eruption, as well as to utilize modern geodetic tools to investigate the characteristics and sources of the volcano’s pre-, co-, and post-eruptive deformation. The patterns of deformation before the eruption varied over both space and time, introducing considerable uncertainty into forecasts of when and how the volcano might erupt. Caldera faulting in March 2021 and the detection of signals in borehole tiltmeters in March 2021 and from September to November 2022 provided the clearest sense that an eruption might be imminent. Co-eruptive displacements were complex and large in magnitude, being manifested across the Island of Hawaiʻi, and can be attributed to emplacement of a dike at shallow levels beneath the summit and upper NERZ, as well as volume loss from sub-caldera magma storage areas, reinflation of the volcano began three days before the end of the eruption, which was unexpected given experience at neighboring Kīlauea where reinflation usually coincides with the cessation or pausing of eruptive activity.

Future efforts to track deformation of Mauna Loa can build on the experience gained prior to and during the volcano’s 2022 eruption. The complementary capabilities of continuous GNSS, InSAR, and borehole tilt are vital to assessing displacement patterns, which can vary markedly over space and in time during the years to decades before an eruption. Deformation and seismicity in the caldera area, and especially localized displacements associated with caldera faulting, may offer an important indication of the potential for eruption within months to years. Borehole tilt data were particularly valuable in providing pre-eruptive warning in two ways: first, through the initial detection of tilt signals after decades of unrest began to culminate, and second by indicating the initial propagation of a dike from magma storage areas to the surface in the minutes before the eruption started. These insights provide a blueprint for monitoring future unrest of Mauna Loa and anticipating the volcano’s next eruption, and the available datasets—past, present, and hopefully future—offer myriad opportunities for researching magmatic processes at the largest active volcano on Earth.

Funding

Research and monitoring data collection were supported by the U.S. Geological Survey.

Conflicts of interest/Competing interests

The author declares no competing interests.

Acknowledgments

The monitoring of unrest and response to the 2022 Mauna Loa eruption was a team effort and would not have been successful without the expertise and meticulous work of staff at the U.S. Geological Survey’s Hawaiian Volcano Observatory (HVO), Hawaiʻi County Civil Defense, and Hawaiʻi Volcanoes National Park. We are exceptionally grateful to those who maintained geodetic monitoring networks and collected data over many decades—particularly Asta Miklius, Michael Lisowski, John Dvorak, Maurice Sako, Peter Cervelli, and many others. The geodetic network is operated in collaboration by the USGS and Pacific GPS Facility at the University of Hawaiʻi, where Ben Brooks and James Foster contributed to the development of the GNSS network. We thank HVO’s Field Equipment and Instrumentation Team, led by Kevan Kamibayashi, for their diligent efforts in installing and maintaining equipment in harsh, high-altitude and corrosive environments. Ongoing gratitude is extended to David Okita of Manuiwa Airways (Volcano Helicopters) for safe and timely access to sites and exceptional field support. The authors appreciate the recommendations and insights of reviewers, which benefited and improved this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Availability of data and material

Continuous Global Navigation Satellite Systems (GNSS) data used in this study are available from EarthScope Consortium. The facilities of EarthScope Consortium are supported by the National Science Foundation’s Seismological Facility for the Advancement of Geoscience (SAGE) Award under Cooperative Agreement EAR-1724509 and Geodetic Facility for the Advancement of Geoscience (GAGE) Award under NSF Cooperative Agreement EAR-1724794. Borehole tiltmeter data for sites on Mauna Loa in 2021 and 2022 are available in Ellis and Johanson (2024a) and Ellis and Johanson (2024b), respectively. Interferometric synthetic aperture radar (InSAR) data shown in this report were acquired by the COSMO-SkyMed satellite system and provided by the Italian Space Agency via the Hawaiʻi Supersite as established by the Geohazard Supersites and Natural Laboratories initiative.

Code availability

Not applicable.

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You are reading this latest preprint version

Deformation of Mauna Loa volcano before, during, and after its 2022 eruption

Status:

Version 1

Abstract

Figures

Introduction

Deformation monitoring at Mauna Loa

Mauna Loa’s magmatic and fault systems

Geography and eruption patterns

Magma plumbing system

Flank motion and decollement activity

Build-up to the 2022 Mauna Loa eruption

Co-eruptive observations and processes

Initial Eruption Onset

Evolution of deformation during ongoing eruption

Island-wide deformation in response to eruption

Co-eruptive modeling and implications

Post eruption observations

Discussion

Comparing previous 1975 and 1984 eruptions to the 2022 Mauna Loa eruption

Best techniques for tracking the pre- and co-eruptive deformation at Mauna Loa

Implications of March 2021 caldera faulting event

Interactions between Kīlauea and Mauna Loa

Conclusions

Declarations

Funding

Acknowledgments

Availability of data and material

Code availability

References

Supplementary Files

Status:

Version 1