Pitcairn Hotspot Produces Bountiful Earthquake Swarm

The Pitcairn hotspot (Stoffers, et al. Marine Geology, 1990) experienced its first significant earthquake swarm in recorded history during the fall of 2001. During peak swarm activity in November 2001, the residents of Pitcairn Island felt tremors daily. Using 20Hz data from the GSN station PTCN, we have determined that the primary swarm activity began on October 4, 2001 and continued through the first week of January, 2002. Rudimentary particle motion analysis seems to indicate there are two primary sources for the swarm events: one to the southeast of Pitcairn Island and the other to the northeast. Some arrivals appear to come from due east. Pn - Sn times recorded on the island are approximately 5 seconds, giving an approximate epicentral distance of 20km. These observations agree well with the locations published by the NEIC for the two largest events of the swarm (magnitudes 4.4 and 4.6, occurring on Nov. 15 and Nov.23 of 2001), as well as a third event of magnitude 4.7, which occurred on July 24, 2002 well after the main swarm abated. The swarm itself has an unusual quiescent period that lasts at least 5 days in early December. After this period, swarm activity gradually rises to nearly the level it achieved just before the quiet period. Although the swarm had mostly abated by the beginning of January 2002, two large events were observed later in the year: a magnitude 4.7 on July 24, 2002 (PDE from the NEIC); and a magnitude 3.5 on August 9, 2002. Preliminary counting estimates indicate over 10,000 events were recorded at 20Hz by the seismometer at PTCN during the peak months of the swarm. The small events of the swarm can be grouped by their similar characteristics, indicating that there are multiple distinct sources for the small events. There also are intermittent, apparently volcanic sequences interspersed among the earthquakes. These sequences tend to last 30 minutes to 2 hours, are not large enough to be felt on the island, and occur less than once a month. Since all the swarm-related events have sources closer to the island than expected from previous studies of the Pitcairn hotspot, we propose the activity may be related to a new expression of this hotspot.