Thirty centimeters of rain in two hours on 14 May dislodged pyroclastic deposits from the upper flanks. Approximately 5-6 x 10⁶ m³ of breccia, volcanic sands, ash, surficial cover, and vegetation flowed down the 40 -60 E flank into the valleys of the Tunggeng and Sat Rivers. The mudflow killed 252 persons, left 152 injured and 120 missing, and flooded 626 hectares of rice fields and 16 villages along the river's banks. It eroded old mudflow deposits and washed away a dike built in 1912 after a similar event had destroyed the city of Lumajang (40 km E of the volcano) in 1901. The devastated area appears to be within the alert zone on VSI's 1973 hazard map.

Fresh nuée ardente deposits on the upper S flank, combined with the onset of the monsoon season, had prompted the VSI to warn local authorities in January of the danger of mudflows S and SE of Semeru. Although a mudflow also moved down the S flank on 14 May, no casualties were reported there. Activity at Semeru was normal during May, with about 80 gas eruptions each day. The lava dome continued to grow at about 100 m³/day.

Geological Summary. Semeru, the highest volcano on Java, and one of its most active, lies at the southern end of a volcanic massif extending north to the Tengger caldera. The steep-sided volcano, also referred to as Mahameru (Great Mountain), rises above coastal plains to the south. Gunung Semeru was constructed south of the overlapping Ajek-ajek and Jambangan calderas. A line of lake-filled maars was constructed along a N-S trend cutting through the summit, and cinder cones and lava domes occupy the eastern and NE flanks. Summit topography is complicated by the shifting of craters from NW to SE. Frequent 19th and 20th century eruptions were dominated by small-to-moderate explosions from the summit crater, with occasional lava flows and larger explosive eruptions accompanied by pyroclastic flows that have reached the lower flanks of the volcano.

Information Contacts: A. Sudradjat and L. Pardyanto, VSI.