Two strong explosions took place at the intra-caldera cone Tavurvur on 30 March; after that, the repose intervals between explosions at Tavurvur lengthened, lasting from several hours to more than 24 hours. Tavurvur discharged several noteworthy explosions on 13-15 April; explosions ceased on 16 April.

During the first half of April, explosions sent ash clouds 1-2 km above the crater, but they were typically spasmodic and relatively mild. Ash predominantly fell to the SE (mainly over Talwatt and occasionally at Kokopo, with smaller amounts in Rabaul on a few days). Accompanying the normally gray ash emissions were weak roaring sounds heard late on the 3rd, low rumbling sounds on the 9th, and lightning seen in and around the billowing ash column on the 11th.

At 1206 on 13 April an impressive explosion occurred. It began with fast-rising, spear-headed jets of dark ash, which fed a billowing ash cloud that rose to about 2 km above the crater. Some ballistic blocks landed in the bay immediately W and NW of Tavurvur. On 14 April, moderate-to-strong explosions started at about 0920, with the most intense activity occurring between 1030 and 1040. Resulting eruption clouds were dark gray and quite dense; fallout was heavy at Tavurvur and immediately downwind (SE). In and around the eruption column, lightning was noted. The activity declined slowly through the day and stopped at about 2320.

Strong explosions resumed at about 1320 on 15 April. During a roughly 1 hour period, several large eruption clouds rose to about 2 km. These ash clouds remained intact as they drifted to the SE. Prolonged moderate ash emission also took place from early to mid-afternoon. During the early hours of 16 April, mild explosive activity took place; it stopped at about 0600. From that time onward activity chiefly consisted of weak white vapor emissions. Following a period of heavy rainfall on the 24th, however, these emissions again became more voluminous, but by the next day they returned to a very low level.

Seismicity in the first half of April, until the 16th, partly consisted of low-frequency earthquakes associated with Tavurvur's explosions. Explosion sizes appeared to correspond to earthquake amplitudes. Six high-frequency earthquakes also occurred (compared to 5 in March and 4 in February). These earthquakes all had epicenters outside the caldera--five to the N-NE and one to the SW.

During April, electronically measured tilt in the interior of the caldera at Matupit Island continued to show a trend of very slow deflation. Other ground deformation measurements failed to show significant trends.

An aerial inspection, on 8 April, revealed that Tavurvur's surface was covered with fresh black ash. Numerous gray blocks had also landed, mainly on the S flank and inside the old crater. The fumarole previously emitting blue-vapor (located about 1/3 of the way down the 1994 lava flow) was inactive. One white-vapor fumarole was noted where the lava had advanced over the crater rim. The crater displayed variably colored sublimate deposits and small erosional gullies. A step-like structural form was seen on the crater's E side, and a smooth, bowl shape was seen on its W side. Inside the crater there were neither visible vents nor a lava mound.

Vulcan continued weak white vapor emissions, coming mainly from the crater of the 1994 cone. Fumaroles at the base of the 1994 crater had been mostly buried by mud leaving only one on the W side of the crater. The upper one of the two pit craters on the N flank of the 1994 cone had caved in. Temperature of hot springs along the N shore were consistent with previous months' readings at ~100°C.

The State of Emergency in Rabaul was lifted on 10 April, making way for the Gazelle Restoration Authority to promote the rehabilitation process.

Geological Summary. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: David Lolok and Ben Talai, RVO