Berkeley Seismological Laboratory
Fall 2009 Seminar Series
Tuesdays, 4pm in 265 McCone Hall
David Gubbins, visiting Miller Professor, EPS Berkeley.
The Earth's core cools as a result of mantle convection: the heat loss through the core-mantle boundary (CMB) differs from place to place, perhaps reaching the point where heat is pumped into the core. These lateral variations in heat flux are transmitted through the core to the lower boundary of the inner core (ICB). At the inner core boundary the adiabatic gradient is weakened by the lower value of g and the convective flux is enhanced by the spherical geometry and by narrow downwellings in the fluid core. It is quite possible that the inner core is melting in some places and freezing in others, while growing on average by a net solidification. It is therefore worth exploring the possible consequences of melting. First, release of heavy material from the ICB could produce a dense bottom layer in the fluid core that could explain the anomalous low acoustic velocities that have been inferred from PKP and Pdiff travel times in the lowermost 150 km of the liquid core. Furthermore, if core convection and the inner core are locked to the mantle through thermal interaction and gravity respectively, it provides a simple and strong explanation of seismic anomalies at the top of the inner core, with low Q, low Vs in regions of rapid deposition of mush (freezing) and high Q, high Vs in regions of melting where compressed and consolidated material is being exposed by melting. There remains a problem in locating the CMB features relative to those of the ICB. In geodynamo models fluid downwellings are almost vertical, placing regions of solidification directly beneath cold mantle, but downwelling in the real Earth could spiral, as seen in some non-magnetic simulations and experiments, making correlation difficult.
Deformation and Texture Development in Lower Mantle Mineral Phases: Implications for Deep Earth Anisotropy and Dynamics
Understanding behavior in the Earth’s deep interior requires a multi-disciplinary approach that combines the fields of seismology, geodynamics, and mineral physics. Seismologists observe anisotropic wave propagation in many regions of the Earth. One cause of observed seismic anisotropy is deformation induced texturing or preferred orientation of crystallites within the polycrystalline rocks that make up the Earth’s interior. If deformation behavior and its relationship to texture development are well understood for the appropriate mineral phases at the proper pressure and temperature conditions, then this knowledge can be combined with geodynamic modeling to predict texture development and resulting anisotropy for deformation scenarios expected to exist in the Earth’s interior. This information can be compared to observed anisotropies to evaluate the deformation state in various regions of the deep Earth. This approach has been quite successful for the upper mantle where seismic coverage is good, natural samples are readily available, and deformation mechanisms of the constituent minerals are relatively well known under a range of conditions. A few attempts have been made to model anisotropy development in deeper regions of the Earth such as the lower mantle and inner core. However, these studies are significantly hampered by the fact that little is known about the deformation mechanisms of these mineral phases. For the lower mantle and inner core, the situation is considerable more difficult than the upper mantle. Natural samples from these depths are not available and the extreme pressure and temperature conditions existing in these regions are challenging to achieve in the laboratory. Mineral phases believed to exist at these conditions are often highly unstable or unquenchable to ambient conditions and thus must be studied in-situ while they are under high pressure and/or temperature. In order to interpret anisotropy and the dynamic state of the deep Earth it is critical to under the deformation behavior of these minerals. This talk will discuss deformation experiments, active deformation modes, and implications for seismic anisotropy in the major mineral phases of the lower mantle and D” region. Focus will be on (Mg, Fe)SiO3 perovskite, two phase aggregates of (Mg, Fe)SiO3 perovskite and (Mg, Fe)O magnesiowüstite, and (Mg, Fe)SiO3 post-perovskite
Stable and transient motion on Kilauea's south flank from InSAR Persistent Scatterers
Kilauea volcano on the Big Island of Hawaii is home to a large variety of deformation sources. Some are volcanic: such as the observed inflation/deflation near the summit caldera, near the active vent Pu'u O'o, and along the east rift. Other deformation sources are related to the seaward sliding of the volcano's south flank. Sliding in this area is accommodated mostly by steady slip on a decollement at the interface between oceanic sediments and the volcanic edifice at 7-9 km depth. Additionally, several slow earthquakes (SEQs) have been observed by a continuous GPS network. In this complex location, with many different deformation sources in close proximity to each other, we seek to explicitly construct a time series of ground motions that captures the evolution of each time-dependent deformation source.
To accomplish this, we use the Stanford Method for Persistent Scatterers (StaMPS) to identify >25,000 persistent scatterers (PSs) from 28 descending-mode SAR scenes (27 interferograms) acquired between 2003 and 2007 by the European Space Agency's ENVISAT satellite. StaMPS isolates pixels that show consistently good spatial correlation with other PS candidates and then extracts phase change information from all interferograms to produce a time history of deformation at each PS location. Atmospheric effects are mitigated by applying a spatially correlated noise filter which removes signals that are long-wavelength in space, but short-wavelength in time. Our dataset captures the overall seaward motion of the south flank, as well as small-scale deformation features associated with the surface expressions of the Koa'e and Hilina Fault Zones. Evaluation of the time series suggests that small-scale deformation features near the Hilina Fault Zone are unaffected by slow earthquakes, implying that the shallowest parts of the sliding plane are not involved in the SEQs. We construct a 2D model of flank motion that includes slip on the basal decollement and the landslide blocks that partition slip between shallow, deep-seated, and transient sources.
The 2007 Pisco earthquake and the new data generation – Exploring the various stages of a seismic cycle
In the last couple of decades, advances in the analysis techniques and instrumentation have improved significantly our capability to document the different stages of the seismic cycle, namely the co-, post- and inter-seismic phases. To this respect, the Mw8.0 Pisco, Peru, earthquake of August 2007 is exemplary. The coseismic rupture is well constrained for a variety of data including seismological records, Interferometric Synthetic Aperture Radar (InSAR), tsunami waveforms, field observations of coastal uplift, subsidence and run-up data. Postseismic deformation is also well constrained from a dedicated local network of GPS stations which was set up right after the event, and interseismic deformation could be partially quantified thanks to regional GPS campaigns carried one before the earthquakes. In this seminar, I will first focus on the analysis of the coseismic rupture and then present how the various components of the seismic cycle can be combined to develop some model of how slip on the Peru Megathrust proceeds with time. The study sheds some light on the relationship between the seismic behavior (or timing and extent of large earthquakes) and friction properties of the Megathrust.
Slow Slip and Tremor in the Northern Costa Rica Seismogenic Zone
Several episodes of slow slip and tremor are believed to have occurred at the plate boundary of northern Costa Rica between 2000 and 2008. The evidence for these events varies and consists of: 1) correlated fluid flow excursions and seismic tremor recorded on ocean bottom instruments in 2000; 2) offsets in continuous GPS data in September 2003; 3) offsets in GPS data accompanied by seismic tremor in May 2007; and 4) strong prolonged seismic tremor in August 2008. Modeling of the 2000 event suggested that slip occurred at shallow depth, between the surface and ~15 kilometers. The much better constrained slip distribution of the 2007 event consisted of 2 patches, the stronger centered at ~30 km depth, near the down dip transition from stick-slip to stable sliding, and the weaker patch located at ~6 km depth at the up dip edge of the shallow frictional transition. The 2003 event was recorded on too few instruments to be modeled. These results are significant in that they are the first to suggest that slow slip occurs at the up dip transition from stick-slip to stable sliding; locations of slow slip in other environments have been limited to the down dip frictional transition.
Due to the relatively small surface displacements (1-2 cm) associated with Costa Rica slow slip events, the coincident occurrence of seismic tremor is important for their detection and study. Similar to tremor observations in southwest Japan, some Costa Rica tremor consists of swarms of low-frequency earthquakes that occur as repetitive stick-slip motion on the plate interface, contains very low frequency earthquakes, with dominant energy between 20-50 s, and appears to be tidally modulated. The diverse slow slip and tremor behaviors observed in this subduction zone will be discussed, compared to normal (fast) seismicity and contrasted with slow slip in other environments.