Four swarms of felt earthquakes centered near Morne Patates . . . have occurred since March (table 1). Activity typically consisted of 10-30 recorded A-type volcanic shocks in a roughly two-hour period, about half of them felt by residents of flank villages. The most recent swarm occurred on 29 October (after 10 days of quiet) when 26 earthquakes were felt between 0000 and 0200. As of 12 November, no new swarms had been recorded.

Table 1. 1986 earthquake swarms at Morne Patates, recorded by three telemetered seismometers on Dominica and 15 others within 100 km of the island.

Three telemetered seismometers on Dominica and 15 others within 100 km recorded the events. Precise depth and magnitude determinations were difficult, but the events were shallow, perhaps at ~3 km depth, and the largest had duration magnitudes of 3-3.5. Instruments were able to detect events of about M 2 or larger. Geologists visited the area in early November and installed four additional seismic instruments in the epicentral area; data will be telemetered to Trinidad. Fumaroles in the crater and on the N flank appeared unchanged and no other surface changes were evident.

Geological Summary. The Morne Plat Pays volcanic complex occupies the southern tip of the island of Dominica and has been active throughout the Holocene. An arcuate caldera that formed about 39,000 years ago as a result of a major explosive eruption and flank collapse is open to Soufrière Bay on the west. This depression cuts the SW side of Morne Plat Pays stratovolcano and extends to the southern tip of Dominica. At least a dozen small post-caldera lava domes were emplaced within and outside this depression, including one submarine dome south of Scotts Head. The latest dated eruptions occurred from the Morne Patates lava dome about 1270 CE, although younger deposits have not yet been dated. The complex is the site of extensive fumarolic activity, and at least ten swarms of small-magnitude earthquakes, none associated with eruptive activity, have occurred since 1765 at Morne Patates.

Information Contacts: J. Shepherd, UWI; C. Antenor-Habezac, Observatoire Volcanologique de la Soufriere, Guadeloupe.