Cyclic eruptions and sector collapses at Monowai submarine volcano, Kermadec arc: 1998-2007

Repeated multibeam bathymetric surveys at Monowai Cone, a shallow submarine basaltic volcano and part of the Monowai Volcanic Center in the northern Kermadec arc, were conducted in 1998, 2004, and 2007. These surveys document dramatic depth changes at the volcano including negative changes up to - 176 m from two sector collapses and positive changes up to + 138 m from volcanic reconstruction near the summit and debris avalanche deposits downslope of the slide scars. One sector collapse occurred on the SE slope between 1998 and 2004 with a volume of similar to 0.09 km(3), and another occurred on the SW slope between 2004 and 2007 with a volume of similar to 0.04 km(3). The volume of positive depth change due to addition of volcanic material by eruption is of the same order: similar to 0.05 km(3) between 1998 and 2004 and similar to 0.06 km(3) between 2004 and 2007. During these time intervals, monitoring by the Polynesian Seismic Network detected frequent T wave swarms at Monowai, indicative of explosive eruptive activity every few months. An unusual T wave swarm on 24 May 2002 was previously interpreted as the collapse event between the 1998 and 2004 surveys, but no similarly anomalous T waves were detected between 2004 and 2007, probably because the Polynesian Seismic Network stations were acoustically shadowed from the second slide event. We interpret that the sector collapses on Monowai are caused by the unstable loading of fragmental erupted material on the summit and steep upper slopes of the volcano (> 20 degrees). Moreover, there appears to be a cyclic pattern in which recurrent eruptions oversteepen the cone and periodically lead to collapse events that transport volcaniclastic material downslope to the lower apron of the volcano. Volumetric rate calculations suggest that these two processes may be more or less in equilibrium. The repeated collapses at Monowai are relatively modest in volume (involving only 0.1-0.5% of the edifice volume), have occurred much more frequently than is estimated for larger debris avalanches at subaerial volcanoes, and may be characteristic of how persistently active shallow submarine arc volcanoes grow.