Regional earthquakes followed by delayed ground uplifts at Campi Flegrei Caldera, Italy: Arguments for a causal link

Highlights

• Uplift activity occurring at Campi Flegrei is affected by regional earthquakes.

• Seismic waves focus stress in the hydrothermal system and at the roof of the magmatic reservoir.

• Proposed new extended time window for dynamic triggering.

• Proposed new model to explain fluid release and migration between deep and shallow reservoirs.

Abstract

Earthquake-triggered volcanic activity promoted by dynamic and static stresses are considered rare and difficult-to-capture geological processes. Calderas are ideal natural laboratories to investigate earthquake–volcano interactions due to their sensitivity to incoming seismic energy. The Campi Flegrei caldera, Italy, is one of the most monitored volcanic systems worldwide. We compare ground elevation time series at Campi Flegrei with earthquake catalogues showing that uplift events at Campi Flegrei are associated with large regional earthquakes. Such association is supported by (yet non-definitive) binomial tests. Over a 70-year time window we identify 14 uplift events, 12 of them were preceded by an earthquake, and for 8 of them the earthquake-to-uplift timespan ranges from immediate responses to 1.2 yr. Such variability in the response delay may be due to the preparedness of the system with faster responses probably occurring in periods during which the Campi Flegrei system was already in a critical state. To investigate the process that may be responsible for the proposed association we simulate the propagation of elastic waves and show that passing body waves impose high dynamic strains at the roof of the magmatic reservoir of the Campi Flegrei at about 7 km depth. This may promote a short-lived embrittlement of the magma reservoir's carapace otherwise marked by a ductile behaviour. Such failure allows magma and exsolved volatiles to be released from the magmatic reservoir. The fluids, namely exsolved volatiles and/or melts, ascend through a nominally plastic zone above the magmatic reservoir. This mechanism and the associated inherent uncertainties require further investigations but the new concept already implies that geological processes triggered by passing seismic waves may become apparent several months after passage of the seismic waves.

Introduction

Dynamic stresses associated with passing seismic waves generated by large earthquakes may trigger volcanic activity in the near- and in the far-field within days (Hill et al., 1995, Hill et al., 2002; Linde and Sacks, 1998). More recently it was shown that geological processes activated by passing seismic waves may become apparent beyond the commonly accepted time window of few days (Parsons, 2005, Jagla, 2011, Shelly et al., 2011, Watt et al., 2009, Johnson and Bürgmann, 2016). It is suggested that surface waves rather than body waves (Hill et al., 1995, Hill, 2012, Hill et al., 2002; Husen et al., 2004a, Husen et al., 2004b; Manga and Brodsky, 2006) are more efficient in promoting earthquake–volcano interactions. However, proposed models are still difficult to test due to the paucity of recorded triggered events (Prejean and Haney, 2014). As calderas are sensitive to incoming seismic energy (Hill et al., 1995; Husen et al., 2004a, Husen et al., 2004b) they are considered ideal natural laboratories to investigate earthquake–volcano interactions. Some magmatic systems underlying calderas periodically undergo rapid ground uplift phases (Acocella et al., 2015; Chiodini et al., 2012; Hurwitz et al., 2007, Hutnak et al., 2009, Todesco and Berrino, 2005) associated with intense local seismic activity and emission of volcanic gases marked by a strong magmatic component (Bodnar et al., 2007; Chiodini et al., 2012, Chiodini et al., 2003; Hurwitz et al., 2007, Hutnak et al., 2009, Todesco et al., 2014, Todesco and Berrino, 2005). Uplift phases (known at Campi Flegrei as bradyseismic episodes), generally followed by slow ground deflation (Del Gaudio et al., 2010), are thought to be driven by the rise of hydrothermal fluids or magmas (Battaglia et al., 2006, Bodnar et al., 2007, Chiodini et al., 2003; De Natale et al., 2006).

The Campi Flegrei caldera near Naples, Italy, is characterised by frequent bradyseismic episodes that may reach vertical ground displacement rates of about 1 m/yr (Del Gaudio et al., 2010). Most models relate bradyseismic episodes to the pressurization of the shallow hydrothermal system by injection of deep fluids (Battaglia et al., 2006, Bodnar et al., 2007, Chiodini et al., 2012, Hurwitz et al., 2007, Hutnak et al., 2009) while others invoke the intrusion of magma at shallow depths (Amoruso et al., 2014, Macedonio, 2014, Woo and Kilburn, 2010). Numerical modelling (Todesco and Berrino, 2005, Hurwitz et al., 2007, Hutnak et al., 2009, Chiodini et al., 2012) in agreement with measured fumarole gas signatures (Chiodini et al., 2003, Chiodini et al., 2012, Chiodini et al., 2016) suggests that CO₂-rich fluids would be particularly efficient in generating the observed uplifts. Previous studies suggest that these fluids are released by a cooling magmatic body at ca. 6–7 km depth (Bodnar et al., 2007, Zollo et al., 2008) and accumulate beneath a low-permeability layer at ca. 3 km depth from which they are then episodically discharged.

In spite of longstanding scientific efforts, a consistent and predictive model accounting for different geological processes at Campi Flegrei is still lacking. By combining analysis of uplift time series, statistical studies and forward simulations of seismic waves, we show evidence for an association between regional earthquakes and bradyseismic episodes. We then develop a mechanistic explanation for a causal link between regional earthquakes and ground uplift that integrates several of the above-mentioned aspects into a single coherent model. Most notably, our mechanism is compatible with the two scenarios suggesting the release of CO₂-rich brines or the intrusion of magma at shallow depths as the driving force of the bradyseismic episodes.