The Earth's surface is divided into a small number of tectonic plates that move as units. The cold, upper part of the earth, called the lithosphere, is stiff, enabling the plates to move without significant internal deformation above a deformable, softer layer called the asthenosphere. Thus, it is the physical properties of the lithosphere that control the surface expression of convection within the Earth's interior, enabling plate tectonics. Despite its fundamental role in governing tectonics, the thickness of the lithosphere is difficult to measure. We propose to measure the azimuthal anisotropy of Rayleigh wave propagation within two ocean-bottom seismometer (OBS) arrays in the western Pacific as a means of unambiguously determining the thickness of the old oceanic lithosphere.
Thermal models of seafloor subsidence indicate that the oceanic plates should be ~ 90 - 125 km thick, with temperatures approaching steady state in very old seafloor. In contrast, seismic surface wave studies indicate that velocities continue to increase as a function of age, with the velocity changes occurring at depths greater than the thickness of the best-fitting cooling slab models. The most direct and unambiguous way to determine the thickness of the lithosphere and to resolve this controversy is to map the transition from static structure frozen in the plate to actively deforming fabric in the convecting, deforming asthenosphere. This change should induce a change in anisotropic fabric associated with the alignment of the mineral olivine in a deforming Earth, which we propose to detect by measuring the variation of azimuthal anisotropy of Rayleigh waves as a function of period.
In a relatively small area of the western Pacific in seafloor approximately 155 million years old, there are major changes in the direction of spreading in seafloor of the same age and similar spreading rate. Thus, the fossil component of anisotropy in the lithosphere should change direction dramatically, but the asthenospheric component due to flow beneath the plate should be nearly constant. With a deployment of arrays of OBSs where the spreading directions change, it should be possible to clearly distinguish the fossil component of anisotropy from the dynamically maintained component in the asthenosphere. We will collect continuous seismic records of earthquakes occurring around the world. In addition to measuring the azimuthal anisotropy of Rayleigh waves as a function of period, we will look for lateral heterogeneities in velocity within and in the vicinity of the arrays, measure shear wave splitting, P and S delays, and study the regional propagation of surface waves in the oldest parts of the Pacific.
Broader Impacts. An important component of the proposed activity is education of students and communication with local public schools. Graduate students will be supported at Brown and at CalState Northridge and undergrads will work as assistants. At least four students will participate in each of the two seagoing legs; a good way to introduce oceanography as a field to students. Student participants will be expected to visit local elementary and middle schools before and after the cruise to communicate the excitement of going to sea and to prepare a daily weblog on board to communicate with the classes they have visited. We expect that of the Brown University participants, at least 50% will be women, and we will attempt to recruit underrepresented minorities from the CalState Northridge student body.
In addition to presentations at scientific conferences and publication in professional journals, we will work with our local press officers to prepare press releases to communicate findings to the general public. Data gathered will be archived at the IRIS Data Management Center and made available to seismologists and the general public.
The goal of this project was to resolve a controversy about the thickness of the oceanic plate. The oceanic lithosphere or plate increases in thickness with increasing age of the seafloor because it gradually cools with time, losing heat to the surface. Some studies suggest that the plate reaches a maximum thickness of 80 to 100 km at an age of about 80 Myr, but global seismic studies indicate that it continues to grow in thickness, with changes observed to a depth of about 150 km in seafloor 150 Myr old. We addressed this issue by conducting an ocean-bottom seismograph (OBS) experiment in the oldest part of the western Pacific that has not been affected by subsequent hotspot volcanism. We chose an area south of the Shatsky Rise where there is a magnetic bight, i.e., a 90 degree bend in the magnetic anomalies indicating that the seafloor in that area was formed at two orthogonal spreading centers. We deployed 16 OBSs for a one-year period to record earthquakes from around the world as well as regional earthquakes in the nearby subduction zones. Using records of seismic waves from these earthquakes, we measured anisotropy (seismic velocities different depending on direction of propagation or polarization of the wave), average seismic velocities, and attenuation of seismic waves, all as a function of depth beneath the seafloor. We expected to find the fast anisotropic direction to be in the direction of seafloor spreading in the lithosphere, but aligned with plate motion underneath the lithosphere in the asthenosphere. In addition we expected seismic velocities to decrease and attenuation to increase below the plate. Although some of the analyses are still in progress, we have completed studies of Pn propagation, short period surface wave propagation using ambient noise sources, and attenuation of P waves. We found that both Pn and short-period Rayleigh surface wave propagation were fastest approximately in the direction of seafloor spreading, although the Pn fast direction was skewed approximately 10 degrees from the expected direction on both limbs of the magnetic bight. These observations indicate that the anisotropy in the shallow lithosphere is frozen in and little changed since the seafloor in this area was formed 150 Myr ago. The Pn velocity also varied more rapidly than expected with azimuth of propagation, perhaps indicating the existence of a second, deeper layer with a different anisotropic structure. The P wave study showed, as expected, that attenuation was very low in the cold lithosphere. Attenuation was much higher than expected in the asthenosphere; from 100 to 250 km depth, average attenuation was comparable to that in the mantle wedge above subducting ocean plates. The high attenuation probably requires the presence of melt beneath the plate. Another surprising result was that attenuation in the mantle transition zone deeper than 410 km was very low, indicating that at least this part of the oceanic mantle is very dry. Broader impacts. This research supplied the material for Master's theses for two graduate students and part of the PhD thesis of a third student. In addition, several other graduate students were introduced to marine geophysical research by participation in the OBS deployment and recovery cruses. The data also formed the basis for a group project in a course on Earthquake Seismology. The research itself addresses a fundamental question in geophysics; the nature of the evolution of the oceanic lithosphere.
