In this electronic supplement, we include the explanation of the computation of the isostatic stress variation due to unloading in the upper crust, as well as five tables that contain macroseismic and seismic phases data used in the present study to revise and relocate the 1951 seismic events and the 1786 historical event, and a table of all hypocentral parameters calculated for the 1951 events.
Following the approach suggested by Boussinesq (1885),
we show the computation of the isostatic stress variation
due to unloading in the upper crust.
Equations for the stress and strain induced in a homogeneous, isotropic, linearly elastic halfspace, with a plane horizontal surface, by a point load F perpendicular to the surface and acting at the surface was first solved by Boussinesq (1885).
Assuming that the Poisson ratio is 0.5, the equations for the principal stresses reduce to simple forms (Fung, 1965). In particular, for most practical analyses of the settlement behavior of soils, it is assumed that the volume of the soil is controlled exclusively by the vertical stress :
In the present study, the force F is due to the removal of the gas mass:
In spherical coordinates, the only nonvanishing stress component is : in which
Table S1. Marcroseismic intensities (Mercalli–Cancani–Sieberg [MCS] scale) for the 15 May 1951 earthquake (mapped in Fig. 2 in the main article). Special cases are reported in the column SC: MS, multiple settlement; SS, small settlement; SB, single building; D, damage to a single building; F, felt.
Table S2. Macroseismic intensities of the 7 April 1786 earthquake (mapped in Figure 2 in the main article).
Table S3. Onset times and associated phases from the International Seismological Summary (ISS) 1951 bulletin (original; ISS, 1951) and modified by this study for the 15 May 1951 seismic event. Station codes are taken from the International Registry of Seismograph Stations (ISC, 2011). Onsets marked by an asterisk (*) have been used by HYPOSAT code (Schweitzer, 2001), and those marked by a § have been used by HYPOINVERSE-2000 code (Klein, 2014).
Table S4. Onset times and associated phases from the ISS 1951 bulletin (original; ISS, 1951) and modified by this study for the 16 May 1951 seismic event. Station codes are taken from the International Registry of Seismograph Stations (ISC, 2011). Onsets marked by an asterisk (*) have been used by HYPOSAT code (Schweitzer, 2001), and those marked by a § have been used by HYPOINVERSE-2000 code (Klein, 2014).
Table S5. Complete parameter set for each 1951 location. For each parameter x, Δx indicates the associated uncertainty.
Boussinesq, M. J. (1885). Application des potentiels à l’étude de l’équilibre et du mouvement des solides élastiques, principalement au calcul des deformations et des pressions que produisent, dans ces solides, des efforts quelconques exercés sur und petite partie de leur surface ou de leur intérieur; memoire suivi de notes étendues sur divers points de physique mathématique et d’analyse, Gauthier-Villars, Paris, France (in French).
Fung, Y. C. (1965). Foundations of Solid Mechanics, Prentice-Hall, London, United Kingdom, 198–202.
International Seismological Centre (ISC) (2011). On-Line Bulletin, International Seismological Centre, Thatcham, United Kingdom, http://www.isc.ac.uk/iscbulletin/ (last accessed March 2015).
Klein, F. W. (2014). User’s guide to HYPOINVERSE-2000, a FORTRAN program to solve for earthquake locations and magnitudes, U.S. Geol. Surv. Open-File Rept. 02/171, revised June 2014, 148 pp.
Schweitzer, J. (2001). HYPOSAT—An enhanced routine to locate seismic events, Pure Appl. Geophys. 158, 277–289.
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