From "Historical Unrest at Large Calderas of the World, Vol 1. USGS Bulletin 1855 (Newhall, CG and Dzurisin, D 1988)

Topographic and bathymetric map of Rabaul Caldera (fig 5.7.1 of Newhall and Dzurisin)
Geologic map of Rabaul Caldera (fig 5.7.2 of Newhall and Dzurisin)
Rabaul Caldera has been the source of many voluminous explosive eruptions in the past few hundred thousand years. Reliable dates are available for two such eruptions within the past 3,500 years. A caldera-modifying eruption about 3,500 yr b .P. produced primarily rhyolite but may have been triggered by an injection of basalt (Walker and others, 1981). A large eruption of dacitic magma roughly 1,400 yr B.P. may have resulted in further caldera collapse, perhaps of an inner caldera inferred from bathymetry and from recent seismicity. Postcaldera volcanics at Tavurvur, Sulphur Creek, and Rabalanakaia range from basalt to dacite; those a t Vulcan and Davapia Rocks are dacite. The vent for the 3,500 yr B.P. eruption may have been in the north or northeast part of the present caldera (J. Mori, 198 8); the vent for the 1,400 yr B.P. eruption may have been near the center of the caldera (Walker and others, 1981). Both the 3,500 and 1,400 yr B.P. eruptions " began with small scale explosions... produced small-volume pyroclastic deposits... (and then) activity built up rather quickly to a climax" (Walker and others, 1981). "Rather quickly" in this context means only that there is no geologic evidence for a time gap between the small-scale and larger scale deposits, although Walker speculates that the gaps may have been as short as a few hours. Consideration of repose periods between large explosive eruptions at Rabaul suggests that another might occur in the geologically near future (McKee and others, 1985).

Historical Activity

1767?, 1791: The eruption of 1767 was probably at Tavurvur, though possibly also at Rabalanakaia; the eruption of 1791 was at Tavurvur.

1850: The eruption of 1850, at Sulphur Creek, was preceded by an unspecified period of strong earthquakes and several meters of uplift between Sulphur Creek and Matupit Island (McKee, 1981; McKee and others, 1988, citing Brown, 1878)

1910, 1916, 1919-23: A sever earthquake was felt at Rabaul on 24 February 1910, resulting in sulfur odor and compass deflection "of four points northeast to north" (Fisher, 1939, citing a German Admiralty document). Another strong earthquake near Rabaul Harbour on 1 January

1916 (M=7.9, shallow, Ripper, 1979) caused the causeway connecting Matupit Island and the Gazelle Peninsula to subside 2 m. Other large tectonic earthquakes occurred nearby in 1919 (M=8.1, shallow),

1923 (M=7.2, shallow) and 1933 (Fisher, 1944). On 7 May 1919, the submerged Matupit causeway was elevated by almost 1 m, accompanied by great fumarolic activity at Matupit and Tavurvur. The causeway continued to rise: "...By 1924 it was once again possible to drive a motor car across to Matupit Island and this condition persisted until the 1937 eruption." (Fisher, 1939). Uplift of the bay floor also occurred: Fr. Mayrhofer, quoted in Arculus and Johnson (1981), wrote that "In recent years, measurements have shown that the seabed between Vulcan Island and Kaia (on the west foot of Tavurvur) rose perceptibly every year and I often thought that if this goes on, one fine day Rabaul, the proud capital and port, will be cut off from the sea." General uplift was accompanied by at least local subsidence; Murry (1973) indicated that "between four and five years ago... a casuarina tree which had been well above high water mark was particularly noticeable as being about 12 inches below water level, and the shoreline seemed to have tilted."

1937: "Tremors" began on 26 or 27 May 1937, and a strong earthquake of local origin (MMI=7) occurred at 1320 hr on 28 May. The intensity of this earthquake was strongest in Karavia, on the west shore of Blanche Bay and immediately west of Vulcan Island; on German missionary's home collapsed, injuring the occupant. After the strong earthquake, no additional earthquakes were felt at Rabaul until the early morning of 29 May, but more or less continuous tremor was felt at Vulcan. Earthquakes became stronger at 5:50 a.m. on 29 May, and from then until the eruption began, they were felt throughout the caldera. During lulls between discrete shocks, a "gentle vibration was maintained"--perhaps volcanic tremor. "For much of the morning it was the larger earthquakes. Beginning about 8 a.m. on 29 May, Vulcan Island was uplifted by about 2 m and a reef at the southern end of Matupit Island (that had "never" before been exposed) was uplifted by at least 2 m. A reef on the west side of Vulcan Island began to rise at 1:10 p.m., disappeared at 1:15 p.m., and reappeared at 1:20 p.m. "During the afternoon many of the natives from the adjacent village engaged in looking for stranded fish on these exposed reefs, and about twenty of them are reported to have lost their lives when the eruption occurred" (Fisher, 1939). At 12:45 p.m. on 29 May, narrow, northeast-southwest trending cracks up to 15 m long developed on Matupit Island. Also noted in the hours before the eruption were deep rumbling noises and a marked and produced 3x10^8 m3 of dacitic tephra. Pyroclastic flows caused approximately 500 deaths. The eruption from Tavurvur was smaller and exclusively phreatic, but it produced mud rains that caused considerable damage. Two new vents opened along the east caldera boundary, just north and south of Tavurvur; these emitted only a small volume of debris but a large volume of CO2, killing animals in the vicinity for several months thereafter. Earthquakes ceased when the eruption began, but uplift continued to expose new land during the eruption. Post-eruption subsidence occurred along the shore between Sulphur Creek and Matiput Island.

1938-43: SO2 and HCl emissions at Tavurvur began to increase in late 1938 and increased further in 1940. Small steam explosions occurred in a mud pool at the bottom of Tavurvur crater on 2, 12, and 16 March 1940, and a strong tectonic earthquake occurred near Rabaul on 12 September 1940. In December 1940, temperatures of Tavurvur fumaroles rose from 100 c at a rate of 10 c/day for two weeks, then rose an additional 22-28 c during the morning before a large earthquake (M=7.0) on 14 January 1941 (13 January GMT), centered approximately 30 km west of the caldera (Ripper, 1979). Tilting at a rate of 1 sec (about 5 microradians) per day, upward to the west-southwest, was observed at Rabaul Volcano Observatory (RVO) for one month before the earthquake (fisher, 1944); the tilt direction was toward the inferred epicenter of the earthquake, not toward the center of the caldera. Fisher (1976) states that seismicity before the 1941 eruption was "tectonic" with certain volcanic connections. Temperatures of Tavurvur fumaroles continued to rise after the 14 January earthquake, reaching 400 c by 3 June 1941. during the time of this remarkable temperature increase, amounts of acid gases (HCl and SO2) also increased (Fisher, 1976). No change was noted in fumaroles along the Matupit Harbour (Greet Harbour) waterfront, 2 km away. A dominantly magmatic (dacitic) eruption began at Tavurvur on 6 June 1941 with small steam explosions. Ground deformation before the eruption was conspicuously minor. Tide gauges indicated 7.5cm of subsidence of tavurvur relative to Rabaul town during the 2 years preceding the eruption ; there is no report of short-term uplift before the eruption. The tiltmeter at RVO and another located 1 km north of Tavurvur were sensitive to 1 arcsecond (5 microradians) and showed no tilt before the eruption. No immediate seismic precursor to this eruption was noted a RVO, using a seismometer with only 150x magnification. Minimal ground deformation and seismicity and the occurrence of a major tectonic(?) earthquake 6 months before the eruption suggest the possibility that the earthquake "short-circuited" the normal precursory buildup by easing the passage of magma to tavurvur. Earthquakes were frequent between June 1941 and March 1942, then absent until another swarm in October-December 1943.

1967: On 14 August 1967, earthquakes of magnitude 6.2, 5.7, and 6.4 occurred beneath St. George's Channel, 35 km east of Rabaul. They shook Rabaul town and especially Kokopo town on the coast southeast of Rabaul. The quakes, with a maximum MMI of 7-8, were of tectonic origin and caused no immediate change in the Rabaul volcanoes (Heming, 1969; Denham, 1971). Unlike the 1941 earthquake, there was no noticeable effect on tiltmeters before or after these earthquakes.

1971-present (published in 1988): Seismic swarms and broad uplift of the caldera floor resumed in late-1971. Two M=8.0 earthquakes occurred in the Solomon Sea in July 1971, 130 and 230 km from Rabaul. Seismic activity at Rabaul began to increase in November 1971. Prior to November 1971, approximately 60 earthquakes/month were recorded at Rabaul (Everingham, 1975). That number increased to 220 earthquakes/month from November 1971 to August 1980, then to 320 earthquakes/month from August 1980 to August 1983. A dramatic increase in seismic activity in late 1983 may have been linked to a major tectonic earthquake (M=7.6, 200 km east of Rabaul) in March 1983. Activity generally declined from November 1984 to December 1985 (pre-1971 levels), increased slightly from early to mid-1986, and returned to pre-crisis levels through late 1986 and early 1987. Epicentral distributions of earthquakes from September 1983 through July 1985 define a striking pattern along a steeply outward-dipping ring-fracture system of the inner, 1,400 yr B.P. caldera (Mckee and others, 1984; Mori and Mckee, 1987).The largest local earthquakes from November 1971 to May 1984 occurred during swarms in September-October 1980 (M=5.2), January-March 1982 (M=5.1), and March 1984 (M=5.1). The total seismic energy release for the crisis period (September 1983-July 1985) was 2.5x10^19 ergs (Mori and others, 1986a).

Leveling from September 1973 to July 1983 showed about 1.0 m of uplift at the southern tip of Matupit Island, 1 km northwest of an uplift center inferred from patterns of ground tilt to be beneath the mouth of Greet Harbour. the average uplift rate from August 1983 through July 1985 was about 7 cm/month, compared to 0.8 cm/month for the preceding decade. Uplift extended out to the caldera boundaries but was most pronounced within 2-3 km of the inferred deformation center.

Modeling of leveling data indicates that the focal depth of the Matupit source has been steady at about 1.8 km (McKee and others, 1988). This suggests that the magma body responsible for the deformation inflated but did not rise significantly (McKee and others, 1985, 1988). The estimated volume of deformation from 1971 to 1984 was about 50 million m3 (0.05 km3) for the Matupit (Greet Harbour) source (Mckee and others, 1984). The relationship between gravity and uplift was -216 microgal/m, in contrast to the normal free-air gradient of -309 microgal/m. thus an increase in subsurface mass has been inferred, presumably a magmatic intrusion (McKee and others, 1988).

Marine geophysical studies (Greene and others, 1986) suggest a N-S fault zone at the crest of a N-S ridge of uplift. North-south elongation of uplift is consistent with the N-S elongation of the ring of recent seismicity and the inferred boundary of the 1,400 yr B.P. collapse but not wholly consistent with geodetically inferred centers of deformation in Greet Harbour and Karavia Bay. A faulted anticline at the southern shore of Matupit Island is probably related to the offshore structures. The folded and faulted rocks on Matupit Island are overlain by sea-rafted pumice deposits containing shells and coral, perhaps suggesting resurgence. structures defined by the marine geophysical surveys probably predate the current unrest and may provide a new structural context for interpretation of recent changes.

CO2 emissions occurred in an old crater on the south flank of Tavurvur in June and July 1981, killing a number of birds. Minor new fumarolic activity 1983 killed vegetation in a small area 2 km northwest of Tavurvur (Mori and others, 1986).


The remarkable episode of unrest at Rabaul Caldera that began in 1971 is perhaps the most threatening example in this compilation (Historic Unrest at Large Calderas of the World, USGS Bulletin 1855). From 1971 until mid-1984 the intensity of seismic swarms generally increased with time, the maximum earthquake magnitudes stayed roughly constant or increased, and the rate of seismic energy release increased. Volcanologists at the Rabaul Volcano Observatory were justifiably concerned about the possibility of an eruption in the near future; the fact that none has yet occurred is a testimony to the complexity of large magma reservoirs and evidence that some reservoirs can accommodate a substantial intrusion of magma without erupting.

Another fact that has been graphically illustrated at Rabaul since 1971 is that some large magma reservoirs are subject to relatively minor external disturbances--in this case, shaking by regional tectonic earthquakes or regional tectonic strain changes. The susceptibility of the Rabaul volcanic system to minor strains is underscored by a strong negative correlation between diurnal ocean tides and volcanic earthquakes at Rabaul (Lowenstein, 1986). Low tides decrease load and seemingly trigger volcanic earthquakes. The difference in pressure from high to low tide is about 0.1 bars, or 0.025 bars of shear stress on the steeply dipping fault outlined by Mori and others (1986a, 1988), so in some cases very small stress changes can induce failure.

Caldera-forming eruptions only 3,500 and 1,400 years ago, simultaneous eruptions in 1878 and 1938-43 at Vulcan and Tavurvur (on opposite sides of the caldera ring fracture), and the annular pattern of seismicity since 1971 all raise concern over the possibility of another catastrophic eruption in the foreseeable future. Large silicic systems like Rabaul pose a difficult challenge -- frequent unrest at such systems is likely to cause many false alarms, but the sizes and consequences of possible eruptions are so great that each episode of unrest must be taken seriously, often at considerable social cost.

Lastly, volcanologists and public officials in Rabaul have prepared a clear and well-considered plan that defines four levels of volcanic unrest and corresponding alerts to be given to the general public. Stage 1 is a low level of unrest in which an eruption is possible within years to months. Because no immediate eruption is expected, no public announcements or public response is made. Stage 2 is significantly elevated activity, with tens to hundreds of earthquakes per day and measurable ground deformation; an eruption is judged to be likely within months to weeks. Quiet, low-key precautionary measures are taken. Stage 3 is an alarmingly high level of unrest, with several hundreds to thousands of earthquakes per day and marked ground deformation. In Stage 3, an eruption is expected within weeks to days, public announcements are given, and people are readied for evacuation. Stage 4 is critical, with thousands of earthquakes per day and rapid swelling of the ground, indicating that an eruption might begin within days to hours. At Stage 4, an evacuation is ordered. Stage 2 was invoked during a crisis period from October 1983 to November 1984; fortunately, unrest declined and as of early 1988, Stages 3 and 4 have not been required.