AbstractsEarth & Environmental Science

Seismic analysis of magmatism in the GalápagosArchipelago and East Africa

by Gabrielle Tepp

Institution: University of Rochester
Year: 2016
Keywords: Dike-induced earthquakes; Mantle anisotropy; Seismic tomography; Seismology; Spectral analysis; Volcano
Posted: 02/05/2017
Record ID: 2064462
Full text PDF: http://hdl.handle.net/1802/30881


Magmatism and deformation are consequences of fundamental processes shaping Earth’s ~150 km-thick continental and <125 km-thick oceanic plates. Earthquake seismology encompasses many methods to detect compositional and thermal boundaries from Earth’s surface to the dynamic mantle driving plate tectonics. This work uses three different seismic methods to probe magma migration and storage and tectonism in two intraplate hotspot provinces: the Galápagos and East Africa. First, seismic body-wave tomography is used to image magma within oceanic crust of the largest Galápagos volcano, Sierra Negra. A laterally large, low-velocity region with many smaller, high-magnitude velocity anomalies is imaged at 8-15.5 km depths. No sharp seismic velocity increase is imaged within the resolvable depths, indicating that the thickened crust is at least 16 km deep. The second study involves a spectral analysis of earthquakes induced by the intrusion of thin sheets of magma rising beneath the Afar rift, East Africa. Earthquakes have varying spectral content, some with unusually large amplitude low-frequency content and enhanced surface waves. The analysis showed no clear boundaries between spectral types, suggesting that they are all primarily the result of brittle failure. Deep dike segments (tops > 3 km) induce only high-frequency volcanotectonic earthquakes, while shallower dike segments induce the full range of spectral types. This suggests that low-frequency content is a result of shallow hypocenters, with path and site effects, surface ruptures, and dike fluid interactions all possible secondary causes. In the final study, shear-wave splitting analysis of teleseismic body-wave phases is conducted to evaluate strain and crack fabrics at the base of the continental plate as a consequence of magmatism, mantle flow, and plate stretching in the Western rift, East Africa. On average, fast directions are northeast, consistent with geodynamic models of mantle flow from the African superplume and passive rifting. In the northern study area, splitting directions become complex and rotate northwest. The variational splitting in this region is likely due to mantle flow complexities caused by encounters with deep cratonic roots. Complex flow at craton boundaries may have led to the formation of the magmatic Rungwe Volcanic Province within the largely amagmatic Western rift.