Local seismicity increased in the sea NW of Nii-jima on 21-22 September. Residents measured no significant changes in temperatures of the island's hot springs during this period. Because epicenters were close to the island, more than 20 events were felt, the largest of magnitude [3.5 (JMA)]. Earthquake swarms are common around Nii-jima; the most recent was in August-September 1983 [when the largest event was M 4.2].

Geological Summary. The elongated island of Niijima, SSW of Oshima, is 11 km long and only 2.5 km wide. It is comprised of eight low rhyolitic lava domes that are clustered in two groups at the northern and southern ends of the island, separated by a low, flat isthmus. The flat-topped domes give the island the appearance of two large plateaus bounded by steep cliffs. The Mukaiyama complex at the southern end of the island and Achiyama lava dome at the northern end were formed during Niijima's only historical eruptions in the 9th century CE. Shikineyama and Zinaito domes form small islands immediately to the SW and west, respectively, during earlier stages of volcanism. Earthquake swarms occurred during the 20th century.

Information Contacts: JMA, Tokyo.