We thank Mitchell et al. (2013) for their Comment. It confirms that there is actually present movement on Pico Island’s southeastern flank, and opens up the opportunity to clarify important points.
Mitchell et al. (2013) point out two eventual inaccuracies in our paper (Hildenbrand et al., 2012a): (1) in their opinion, we considered only one of the several options mentioned in their previous works. This is certainly not the case. In the section “Slump Mechanisms and Propagation” (p. 941), we first state “previous studies have considered that the present configuration of the southeastern flank of Pico could reflect vertical caldera collapse.” A few lines after we state “lateral collapse has been proposed, but is considered as currently inactive,” again citing Mitchell and co-workers. Therefore, the two main options they discussed have been clearly considered by us. (2) Mitchell first identified a submarine debris field, but because it is not visible on the multibeam acquired in 2003, Mitchell et al. (2012) suggested instead that the “traces” visible on the sidescan sonar data are from ancient material buried under a thin sedimentary cover, while the steep submarine slope of the island was created recently from lavas reaching the sea during the delta emplacement. The latter hypothesis is not inconsistent with our interpretation. We, indeed, proposed that the creation of the S3 scarp by recurrent detachment of blocks is “consistent with the presence of a moderate-sized debris field on the southern submarine slope of the ridge.” Partial concealing of the debris field by fine-grained sediments and recent lava flows that cascaded on S3 seems plausible to us.
In their Comment, Mitchell et al. (2013) question the existence of an active slump and discuss the slumping mechanisms proposed by us. However, their arguments are rather disputable:
1) Mitchell et al. question the faults we mark as active, arguing that they did not find evidence for recent activity. We remind Mitchell et al. that the absence of evidence is not evidence of absence. Active faults in the slump area are hard to detect because the steep topography and dense vegetation do not offer easy access to the lava flows cascading over the various scars. It is fairly obvious to us that most of the young volcanic cones developed along the trace of the structures (main failure, S1, and S2) are associated with recurrent movements along the faults over the past few thousand years. The youngest lava flows (Hildenbrand et al., 2012a, their figure 2), especially, have apparently been erupted from lateral fissures at the foot of S2. Some recent flows also show signs of deformation, and clastic breccia deposits are being produced/accumulated at the foot of the scars. Overall, this points to very recent/active flank movements.
2) Mitchell et al. claim that our SAR data do not show evidence for rigid body movement. In Hildenbrand et al. (2012a, their figure 4), the sector west of the main scar shows very limited downward movements (green and yellow circles), whereas the portion east of the scar shows pronounced subsidence (blue points), with overall higher displacements southeast of S2 (dark blue circles). Therefore, the bulk of our data clearly shows differential movement. We did not say that present movement involves rigid body motion; instead, we stated that the main mode of deformation is creeping. Instantaneous measurements are rarely representative of long-term behavior. For instance, short-term differential deformation recorded by GPS and SAR data on active slumps such as the Hilina in Hawaii appear rather complex. On the longer term, block rotation may develop over several kilometers across the island flank, but may appear rather subtle at the foot of the fault headwall where downward motion dominates.
3) We are very pleased to read that one of Mitchell et al.’s co-authors measured deformation of a monument inside the slump area, and now concludes that GPS data acquired over 11 years confirm that there is actually present-day movement. This seems to complement our study, and contrasts with Mitchell et al.’s present Comment. Unfortunately, these data are not shown in the Comment or published elsewhere, so it is impossible to check their significance and examine in detail the possible mechanisms of present-day displacements. Noticeably, the now alleged horizontal displacement is in apparent contradiction with their argument of compaction.
4) In their Comment, and in previous papers, Mitchell et al. (2012, 2013) use a shallow platform located a few hundreds of meters from the island shore to assess potential recent movements. They assign an age of 7 ka to this platform, and claim that no visible fault in the platform reveals that any potential movement has to be very recent. If the abrasion shelf studied by Mitchell et al. (2012) were really 7 ka in age, which is merely speculative at this stage (absence of unambiguous datable markers), it certainly does not record the whole history of an island’s flank, as recently shown by Hildenbrand et al. (2012b) on the neighboring Faial Island. The development of a slump is most likely discontinuous; i.e., it may involve the gradual development of curved fault structures, with alternating phases of activity and quiescence. On Pico, the overall prominent morphology of the main scar, and the various lava successions cut by the several faults point to an incremental process initiated well before 7 ka. We agree that active movement recorded in our study likely reveals a renewed phase of downward motion that started recently. This therefore deserves particular attention, especially as potential sudden detachment of part of the flank cannot be ruled out.
We thank Mitchell et al. for their suggestion of deploying a network of continuously recording seismic and GPS stations, but we already installed a dense network of new tiltmeters, microseismic stations, and GPS stations in 2012.