Seismically Induced Soft‐Sediment Deformation Phenomena During the Volcano‐Tectonic Activity of Campi Flegrei Caldera (Southern Italy) in the Last 15 kyr

Abstract

We report, for the first time, evidence of seismically induced soft‐sediment deformations in the central area of the active Campi Flegrei caldera (southern Italy). We analyzed the marine‐transitional and continental sequences located along the coastal La Starza cliffs and several stratigraphic logs exposed during the excavation of a 1‐km‐long tunnel in the Pozzuoli area. The successions host several soft‐sediment structures including sand dikes and sand volcanoes, which are largely dated within the 4.55‐ to 4.28‐kyr BP interval. The volcano‐sedimentary sequence, deposited within the Campi Flegrei caldera in the last 15 kyr, is schematically formed by the superposition of three layers with different rheological behaviors; from the base progressing upward we recognize (1) a massive tuff, (2) marine‐transitional sands of the La Starza unit, and (3) a dominance of continental volcanoclastics. We envisage that during unrest episodes of the volcano, which included ground deformation and seismic activity, the whole volcano‐sedimentary pile was deformed through brittle mechanisms with the formation of normal faults. However, the intermediate layer, when subject to seismic shaking, behaved locally as a viscous material facilitating liquefaction processes and lateral spreading deformation. Furthermore, new geophysical, stratigraphic, and structural surveys allowed us to model the deformation evolution of this area over the last 15 kyr. The evidence of seismically induced soft‐sediment deformation within the volcano‐sedimentary record suggests that moderate earthquakes could occur during future volcano‐seismic unrests. Consequently, liquefaction and related gravitational mass movements must be considered as a hazard during these unrest and volcanic crises.

Paleoliquefaction Studies and the Evaluation of Seismic Hazard

“…There is a large body of literature on earthquake-induced liquefaction features, including articles about laboratory experiments (e.g., References [90,91]), post-earthquake surveys and studies (e.g., References [6,[26][27][28][29][30][31][32]52,53,58,[92][93][94][95][96]), and paleoliquefaction studies (e.g., References [6,8,9,14,15,[21][22][23][24][25][97][98][99]). In addition, there are reviews on earthquake-induced liquefaction features and criteria for distinguishing them from non-seismic features (e.g., References [2,23,44,51,91,[97][98][99][100]).…”

“…Earthquake-induced liquefaction features can be divided into two categories: (1) features related to deformation extending beyond the layer that liquefies; and (2) features related to deformation within the sedimentary layer that liquefies. Features that extend beyond the liquefied layer include intrusive dikes, sills, and diapirs, and extrusive sand blows or volcanoes (e.g., References [6,[21][22][23][24]26,[28][29][30][31]92,100,108,109]). Features that form within the liquefied layer include disturbed bedding, dish structures, ball-and-pillow structures, load casts and related folds, pseudonodules, convolute bedding and lamination, and folds related to slumping (e.g., References [21][22][23][24][25]32,44,54,91,[97][98][99][100]110,111]).…”

“…Generally, sand blows are thicker and coarser-grained above the vent area and thin and fine away from the vent [6,8]. Several investigators noted that some sand blows also fine upward and are capped by silt or clay (e.g., Reference [22]), including depositional units of compound sand blows resulting from multiple earthquakes in a sequence (e.g., References [6][7][8]26,30,92]. For example, compound sand blows that formed during the 2010-2011 Canterbury (New Zealand) earthquake sequence were composed of multiple fining-upward units capped by silt drapes (Figure 4 [26,30]).…”

“…The sedimentological characteristics of sand blows vary from site to site and region to region and likely depend on a number of factors such as grain-size distribution of the source bed, the flow rate and duration of venting water and entrained sediment, and variations in the flow rate related to ground oscillation and pulsing (e.g., References [5,77,92]). In cases where a large volume of subsurface sediment vented to the surface or where lateral spreading or faulting occurred, the soil horizon buried beneath the blow may dip toward the vent or be displaced downward across the vent ( Figure 5 [22,69]).…”

“…In addition, paleoliquefaction studies were conducted in interplate settings like the Dominican Republic and Puerto Rico in the northeastern Caribbean and the Pacific Northwest in the US, where subduction zones and crustal faults pose a significant seismic hazard (e.g., References [3,[17][18][19][20]). Paleoliquefaction studies have been conducted in a lacustrine setting in eastern Turkey [21] and a volcanic setting in southern Italy [22]. Studies focusing on soft-sediment deformation structures in lacustrine deposits were reported for southern Italy [23], Mexico [24], and Argentina [25].…”