Azimuthal Dependence of Seismic Scattering in Upper Oceanic Crust at Ocean Drilling Program Site 418A
Type of Culminating Activity
Master of Science in Geophysics
This thesis is an examination of seismic data for the purpose of gaining a better understanding of the small scale structure of the oceanic crust. A more thorough description of the crust, on both a large and small scale, is necessary before an accurate explanation of its creation and evolution is possible. Most of the previous work done in seismic exploration of the oceanic crust has concentrated on describing its large scale structure. In this research, properties of the seismic data affected by the smaller scale structure are analyzed, and the results are geologically interpreted.
The seismic data set examined here was collected from an Oblique Seismic Experiment (OSE) performed at site 418A, located near the Bermuda Rise on 110 Ma crust. Site 418A was chosen for the initial OSE because of the relatively older age of the crust, compared to the 6 Ma site at 504B where a previous OSE was performed. Comparison of the two data sets was intended to provide contrasting descriptions of young and old crust, such as changes with age in seismic velocity by depth and direction. Seismic velocities at the surface of younger crust are relatively slow (2-3 km/s) compared to the velocity at the surface of older crust (4.5 km/s). As oceanic crust ages, it is subjected to many geologic processes including tectonic deformation, sediment deposition, and mineral replacement which close the cracks and voids formed during the creation of the crust. Closure of voids and cracks is generally used to explain the increase in velocity with age of the crust.
Previous analysis performed on the OSE data from site 418A did confirm a regional increase in velocity for the upper layer, but failed to identify any velocity anisotropy. At sites 504B and 417 (7.5 km northwest of site 418), velocity anisotropy was identified with higher velocities along two directions and lower velocities in the two perpendicular directions. However, analysis of the OSE data from site 418A failed to provide similar results. Anisotropy at site 504B was explained by the presence of crustal cracks aligned normal to the slow directions. Waves travelling normal to the cracks are slowed, and a directional dependence can be seen in the travel times. At 418A, these cracks may have closed enough to remove the velocity anisotropy, however the cracks may still exist. This thesis re-examines the data set from site 418A for anisotropy, but instead of concentrating on the travel times, the analysis here examines the scattering properties of the seismic data, as seen mainly in the coda following the principle arrivals. Coda properties have proven to be more sensitive than changes in velocity to the presence of cracks.
Coda is the signal recorded after the main arrivals, identified as random amplitudes decreasing with time. These signals are a summation of seismic waves scattered from crustal heterogeneities and can provide valuable information about the small scale structure and evolution of the crust. Cracks and heterogeneities are altered as the crust ages, and changing the physical properties of these small scale features can affect the scattering characteristics of the entire crust. However, scattering is a random process producing random coda signals and cannot be analyzed as a deterministic process. Only statistical or ensemble properties of the smaller heterogeneities can be described, but this information is still very important for understanding the crustal structure as a whole and its evolution.
At site 418A, measuring the coda energy reveals a definite azimuthal dependence, with an increase in energy toward the directions parallel to crustal spreading. Frequency analysis of the main arrivals (P+S) indicates a decrease in the amplitudes of lower frequencies for directions parallel to spreading direction, corresponding to the increase in coda amplitudes. Coda falloff rates also display a trend with an increase in falloff rates parallel to spreading direction, suggesting that seismic attenuation reaches a maximum in these directions. Cross-correlations between coda signals fails to show any azimuthal dependence, but may prove useful if compared to similar analyses from other sites.
Coda energy is increasing in the directions parallel to crustal spreading, and amplitude spectra suggest the principle arrivals (P+S) are losing energy in these directions. Therefore, scattering is increasing in the directions parallel to crustal spreading. Crustal features that can cause this directional dependence include sheeted dikes, topographic relief, preferentially aligned heterogeneities, or crustal cracks. Groups of sheeted dikes are rarely present in the upper 500 m of ocean crust, and the seismic waves in the OSE do not penetrate the crust below 440 m. Bathymetry and basement relief for the area around site 418A varies by approximately 200 mover 20 km and displays a slope toward the southwest, but no clear topographic trends are present near the borehole. The best geologic explanation for these results is the presence of cracks or heterogeneities, aligned normal to the direction of crustal spreading. Cracks are formed by extensional forces while the crust is still young near the Mid Atlantic Ridge (MAR), and may still be present in older crust. During the evolution of the crust, these cracks are believed to close, but the thinner or filled cracks may still scatter normally incident seismic waves. Heterogeneities may form as linear features along the fault scarps near the MAR and may still be present in the shallow crust at site 418A.
Although seismic velocities do not indicate an anisotropic crust at site 418A, coda energies and spectra do provide evidence for a seismically anisotropic crust. This anisotropic scattering is most likely due to crustal cracks or heterogeneities, aligned normal to the spreading direction, scattering seismic waves and increasing the energy in the coda.
Vincent, Robert John, "Azimuthal Dependence of Seismic Scattering in Upper Oceanic Crust at Ocean Drilling Program Site 418A" (1993). Boise State University Theses and Dissertations. 467.