Analysis of unrest episodes in the last decade at Mauna Loa volcano through the use of multi-temporal and multiplatform

Mauna Loa is a basaltic shield volcano, located in in the Big Island of the Hawaii archipelago (central Pacific Ocean), which rises over 4 km above sea. It is one of the Earth's most active volcanoes, having erupted 33 times since its first well-documented historical eruption in 1843. Most recent eruption occurred in 1984, from March 24 to April 15. After this event, the volcano entered in a phase of quiescence, interrupted by three main episodes of unrest. The first one occurred in mid-2002, when an inflation phase started just after a brief swarm of deep Long-Period (LP) earthquakes. In late 2004, a dramatic increase in the inflation rate was immediately preceded by a more intense swarm of several thousands LP earthquakes. This Inflation slowed again in 2006, ceased altogether in late 2009, and resumed slowly in late 2010. The latest episode began in April 2014 and continues to date, with a variable rate of inflation behavior. In this work, we investigate the deformation process affecting the Mauna Loa volcano from 2003 to 2014 by exploiting the advanced Interferometric Synthetic Aperture Radar (InSAR) technique referred to as Small BAseline Subset (SBAS) algorithm. This technique relies on the use of a large number of SAR acquisitions and implements an easy combination of small spatial and temporal separation (baseline) interferograms computed from satellite images. This approach allows preserving the characteristics of wide spatial coverage typical of satellite SAR systems since it is able to mitigate the noise effects (referred to as decorrelation phenomena) and, at the same time, to maximize the number of coherent targets. The time series of line-of-sight (LOS) displacements derived from the multi-temporal and multi-platform SAR data were obtained using ENVISAT dataset, acquired by ascending and descending orbits over the 2003 - 2010 time period and COSMO-SkyMed images, taken from both orbital passes in the 2012-2014 time span. All the available SAR data used in this study were obtained through the GEO Geohazards Supersites and Natural Laboratories (GSNL) initiative. For each coherent pixel of the radar images, we compute time-dependent surface displacements as well as the average LOS deformation velocity. We also benefit from the use of the multi-orbit (ascending and descending) ENVISAT and CSK data to discriminate the vertical and east-west components of the displacement field. From the analysis of the space-time deformation trend in the last 10 years, we observe a butterfly - shaped pattern of fringes centered on the volcano summit, which indicates an uplifting trend of about 2 cm from April 2014 to August 2014; this may suggest an inflation of the complex magma reservoir beneath Mauna Loa. Moreover, we compare the latest episode of unrest with those of 2002, 2004 and 2006 to better understand the long-term behavior of Mauna Loa volcano. The final step of this work consist in the inversion of the retrieved DInSAR results, in order to model both deep geological structures and magmatic sources to better understand the dynamics that drive on the volcano process. To characterize the geometry and the evolution of the volcano deformation source, we apply both forward and inverse modeling techniques. The former technique allowed defining roughly the geometrical parameters (shape, position and orientation) of the source, while the latter allowed retrieving its temporal evolution. The obtained results have direct implication in the interpretation of the last unrest episode, providing a useful tool to aid the forecast of possible future eruptions at Mauna Loa. SAR data used in this study were obtained through the GEO Geohazards Supersites and Natural Laboratories (GSNL) initiative.