Gradients from GOCE reveal gravity changes before Pisagua Mw=8.2 and Iquique Mw=7.7 large megathrust earthquakes

ALVAREZ ORLANDO; NACIF SILVINA; SPAGNOTTO, SILVANA; FOLGUERA ANDRES

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2015 vol. 64 p. 1 - 17

0895-9811

Considerable improvements in themeasurement of the Earth gravity field from GOCE satellite mission have providedglobal gravity field models with homogeneous coverage, high precision and goodspatial resolution. In particular, the vertical gravity gradient (Tzz), in comparison to the classic Bouguer anomaly, defines moreaccurately superficial mass heterogeneities. Moreover, the correction of thesesatellite-derived data from the effect of Earth topographic masses by means ofnew techniques taking into account the Earth curvature, improves results inregional analyses. In a recent work we found a correlation between Tzz and slipdistribution for the 2010 Maule Mw=8.8 earthquake. In the present work, wederive the vertical gravity gradient from the last GOCE only model, correctedby the topographic effect and also by the sediments on depocenters of theoffshore region at the Peru-Chile margin, in order to study a spatialrelationship between different lobes of the gravity derived signal and theseismic sources of large megathrust earthquakes. In particular, we analyze thisrelation for the slip models of the 1996 Mw=7.7 Nazca, 2001 Mw=8.4Arequipa, 2007 Mw=8.0 Pisco events and for the slip models of the 2014 Mw=8.2 Pisaguaand Mw=7.7 Iquique earthquakes from Schurr et al. (2014), including the previouslyanalyzed 2010 Mw=8.8 Maule event. Then we find agood correlation between vertical gravity gradients andmain rupture zones, correlation that becomes even stronger as the eventmagnitude increases. Besides this, a gravity fall in the gravity gradient was noticedover the area of the main slip patches at least for the two years before 2014Mw=8.2 Pisagua and Mw=7.7 Iquique earthquakes. Additionally,we found temporal variations of the gravity field after 2010 Mw=8.8 Maule event,related to the main patches of the slip distribution, and coseismic deformation.Therefore, we analyzed vertical gravitygradient field variations as an indirect measure of the variable seismiccoupling finding a potential relationship between Tzz and the seismic b-value. Theserelationships exemplify the strong potential of the satellite only derivedmodels as a predictive tool to determine potential seismic energy released in asubduction segment, determining the potential size of a potential rupture zone,and in particular internal slip distribution that allows inferring coseismicdisplacement field at surface.