Geologic Background:
The low-lying Rabaul caldera forms a sheltered
harbor once utilized by New Britain's largest city. The
outer flanks of the asymmetrical pyroclastic shield
volcano are formed by thick pyroclastic-flow deposits.
The 8 x 14 km caldera is widely breached on the east,
where it is flooded by Blanche Bay. Two major Holocene
caldera-forming eruptions took place 3,500 and 1,400
years ago. Three small stratovolcanoes lie outside the
northern and northeastern caldera rims. Post-caldera
eruptions built basaltic-to-dacitic pyroclastic cones on
the caldera floor near the northeast and western caldera
walls.
Historic Activity:
- The first known historical eruption at Rabaul was in
Sept. 1767 from an unidentified cone.
- In May 1791, Tavurvur emitted columns of black
smoke.
- Native accounts describe activity from Sulfur Creek
between 1845 and 1850 that may have caused fatalities
from falling pumice blocks.
- A submarine eruption in Feb. 1878 formed Vulcan
Island and produced large amounts of pumice; Tavurvur
also began eruption at about the same time and continued
for approximately three weeks.
- The largest historical Rabaul eruption, May 29 to
June 2, 1937, caused >500 fatalities, buried Rabaul
town in >5 cm of andesitic ash and pumice, and
generated a tsunami that threw ships onshore. During this
eruption, Tavurvur was active for <1 day, but activity
at Vulcan built the cone from sea level to a height of
243 m.
- Phreatic eruptions occurred at Tavurvur in Feb. and
March 1940, and intermittent explosive eruptions from
June 1941 to April 1942 left a small crater on the floor
of the larger 1937 crater.
- Another eruption during Nov.-Dec. 1943 built a new
small crater at Tavurvur.
Recent Activity:
- August 1983 to July 1985 was a period of seismic and
deformation crisis. Numbers of earthquakes increased from
a background level of a few hundred per month to a
maximum of over 13,000 per month in April 1984. Uplift of
tens of centimeters occurred. The seismicity and
deformation indicated a large intrusion of magma beneath
the caldera and an eruption was expected but did not
occur.
- August 1985 to March 1986 post-crisis subsidence and
weak seismicity.
- April to July 1986 renewed inflation and
seismicity.
- August 1986 to July 1987 irregular deformation and
seismicity declines.
- July 1987 to February 1988 slow subsidence continues
while seismicity remains low.
- August 1988 to May 1989 - limited deformation and
varing levels of seismicity.
- Continued decline in seismicity thru September
1989.
- Increase in seismicity in October thru January
1990.
- Seismicity remains at low levels thru December
1990.
- Brief earthquake swarm in January 1991.
- Brief earthquake swarm in February 1992.
- Broad uplift in May 1992 was accompanied by seismic
activity. There was a marked increase in seismic activity
in July 1992 with 1089 caldera earthquakes compared to
400 to 500 in May and June. Another marked increase in
seismic activity occurred in Dec. 1992 with 1090 caldera
earthquakes compared to 350 to 600 in October and
November.
- In January 1993 seismic activity returned to lower
levels with 352 earthquakes being recorded.
- There was a marked increase in seismic activity in
March 1993; 1685 earthquakes were recorded, compared to
256 in February and 352 in January. This is the highest
monthly total since April 1986 (1769 earthquakes). The
number of earthquakes detected in April 1993 was 1061.
The number of earthquakes detected in May 1993 was
1525.
- Seismic activity declined in June with only 480
earthquakes being detected. Quakes during the last few
months have been located along the north part of the
caldera ring fault. Leveling and dry-tilt measurements
indicate inflation of the caldera's central part.
- Marked increase in seismicity beginning in
mid-October through November and the decreased in
December 1993 and January 1994. Continued uplift of the
caldera's central part.
- Seismic activity began to decline in March 1994 and
continued to decline in April. Increased seismic activity
during May and then decreased in June and July.
Relatively quiet seismicly during August until Aug. 25
when high seismic activity occurred until Aug. 28.
Seismic activity again increased rapidly on Sept.
18.
- Tavurvur began eruption during the morning of
Sept. 19, 1994 followed by an explosion from
Vulcan about an hour later. Within a few hours the
top of the volcanic ash cloud was reported at an altitude
of 15 to 18 km. Over 50,000 people were displaced from
the town of Rabaul and surrounding areas, only 5 deaths.
Heavy damage to buildings in Rabaul town. The eruption at
Vulcan was dacitic in compositon and included activity
from 4 vents including pyroclastic flows. Vulcan's
eruption ended on Oct. 2, but fumarolic activity
continued through end of year. The eruption at Tavurvur
was andesitic in composition and included a lava flow.
Moderate to mild activity at Tavurvur continued until
Dec. 23, 1994. Although the external shape of Tavurvur
appears to be little different, the internal crater
structure was totally changed by the 1994 eruption.
Almost all of the crater features produced by eruptions
in 1878, 1937, 1941, and 1942 were destroyed and replaced
by a single shallow bowl-shaped crater. The low point of
the crater is still on its west side, where the 1994 lava
flow exits. Inside the main crater, slightly off-center
to the southeast and taking up perhaps a third of the
crater floor, is a single cone, which was built up during
the later stages of the 1994 eruption
- The two active cones in Rabaul (Tavurvur and Vulcan)
showed only fumarolic activity in January and early
February 1995 with the level of activity continuing to
decrease during the month. Eruptive activity resumed at
Tavurvur on Feb. 13, 1995 with no precursory activity and
continued until April 16. White vapor emissions continued
from Vulcan. Only low level fumarolic activity in July. A
strong regional tectonic earthquake (Ms 7.8) occurred on
Aug. 16 and was centered about 260 km southeast of
Rabaul. This earthquake and its aftershocks triggered
high-frequency earthquakes in the caldera and ground
deformation. Tavurvur began erupting again on Nov. 28,
1995 creating a parasitic crater. Eruptions continued
through December producing dark ash clouds.
- The Tavurvur eruption which began in Nov. 1995 was
continued into May 1996, waned and then stopped on June
5, but resumed on June 17 and continued into September.
Strong Strombolian eruptions occurred at Tavurvur on Oct.
4-5, but activity was generally low during November and
December. During the October activity a significant
amount of lava was erupted and flowed south covering a
large area of coconut plantation and two houses. Three
lobes of the lava flow moved into the sea, about 1.6 km
from the vent. The volume of lava was estimated at
4,000,000 to 5,000,000 cubic meters, the largest amount
produced at Tavurvur in more than 200 years.
- 1997 began with a strong Strombolian and
lava-producing eruption at Tavurvur on January 9. Strong
Vulcanian explosions on Jan. 21. Subdued activity in
February with some strong explosions the first half of
the month. On March 14 another strong Strombolian and
lava-producing eruption occurred. Additional strong
Strombolian eruptions occurred on April12, June 1, and
July 11-12. Activity continued through October with the
strongest activity on Aug. 17. After 4 months of slow
inflation, a new, though mild, eruptive phase occurred at
Tavurvur in late Dec. and then decreased in Jan.
1998.
- Low level activity through the end on 1998.
- Mild activity through mid-September 1999. New vent
opened on Tavurvur on Sept 17 for eight days with
significant ash emission. Mild activity continued through
end of year.
- Mild activity in early 2000, including reactivation
of the 1995 vent. Increased activity from summit vent in
June. Ashfalls from August to October.
- Mild activity in 2001 at Tavurvur.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 7, No. 8 to V. 14,
No. 12)
- Global Volcanism Network (V. 15, No. 1 to V. 26, No.
10).
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