Seismic tomography : Inversion for P- and S-wave velocities

Abstract: Seismology is undoubtedly the most powerful method for determining the structure of the Earth below the deepest borehole. This thesis describes the, use of seismic signals from both man-made and earthquake sources to model seismic velocities within a volume of the Earth. Based on data from explosions, a two-dimensional model for the P-wave velocities from the Baltic Shield in southern Sweden, across Denmark and into the North GermanBasin is presented. The observed crustal structures show clear signs of extension, estimated to be at least 20% along the profile. However, the emphasis of my work has been on modeling seismic velocities in three dimensions using local earthquake travel times. Two studies have been conducted, one using data from the Long Valley caldera, California. Originally formed in a catastrophic eruption 0.76 My ago, the caldera is today characterized by intense seismic activity. To better understand this volcanic system we have imaged the three-dimensional P- and S-wave velocity structure beneath the caldera. Low seismic velocities are observed below the resurgent dome down to depths of 9 km, which we believe are caused by hydrothermal circulation of fluids rather than by a large magma chamber. The observed lower than normal Vp/Vs ratios (<1.7) are associated with fractured rock containing compressible and low density fluids. Partially molten or hotmaterial will reduce the S- more than the P-wave velocities, thus producing elevated Vp/Vs ratios. The second study exploits the earthquake data catalog from southwest Iceland. Within the study region are several volcanic centers, active and extinct. The seismically most active one is the Hengill volcanic complex, within 30 km of Reykjavík. The large fissure system is characterized by lower than normal velocities and, similar to the Long Valley caldera, low Vp/Vs ratios. We conclude that supercritical magmaticfluids, such as CO2 and H20, circulate in the fissure system and cause the reduced velocities. These findings are important for understanding the nature of the volcanism and volcanic hazard in both study regions.

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