Surface loading effects complicate the derivation of internal pressure source characteristics from volcano deformation signals

An increasing number of crustal deformation studies relate volcano deformation to internal pressure sources (e.g., magma chambers). However, surface loads such as lava flows provide an additional source of deformation. The initial elastic response due to a load on the surface of the Earth is followed by a visco-elastic response of the ductile crust below the uppermost elastic layer. Thus, a deformation signal recorded in the vicinity of a volcano is often composed of at least two contributors: an internal pressure source (the magma chamber) and a surface load (a composition of previously erupted lava flows) - at the extreme the volcano edifice itself. A test case for a circular lava flow on top of a deflating magma chamber shows that the crust will adjust to the load towards final relaxed response. During this relaxation process gradual subsidence occurs that may mistakenly be interpreted as due to pressure decrease in a magma chamber since the deformation pattern of both processes are very similar. This poses a problem when characteristics of a magma chamber are to be derived. We suggest that additional factors posed by the surface load must be considered and constrained by careful measurements and interpretations of observations and eruptions histories to correct the recorded data for composed signal sources. We conclude that neglecting the surface loads (elastic and visco-elastic responses) may lead to incorrect estimates of magma chamber depths (i.e., too shallow) in magma chamber studies. We conducted a case study in which Green's functions are utilized to estimate the final relaxed response due to recent lava flows of the Icelandic volcano Mt. Hekla. We compare the model results to InSAR data and to the deformation pattern induced by pressure changes in a hypothetical shallow magma chamber. The magma chamber is modeled as a point source of pressure utilizing the so-called Mogi model. The final relaxed response fits the deformation due to the point source very well. In fact, the modeling results show almost identical deformation signals in the vertical displacement field directly under the load. In the far field, however, the response due to the Mogi model is up to 16 % below the final relaxed response. Very significant differences show up the horizontal displacement fields with the horizontal displacement in response to the pressure change in the magma chamber being and order of magnitude higher than the final relaxed response. Thus, to identify whether elastic chamber pressure response or visco-elastic deformation due to surface loads is recorded in the data we suggest to carefully look at the horizontal displacement in addition to vertical displacement.