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Current Research Projects
Seismic Imaging of the Hawaiian Mantle Plume
Hawaii is the archetype hotspot that is assumed to sit above the archetype mantle plume.
Yet many geophysical observables appear to contradict the notion that its volcanoes are
fed by a deep–seated single hot plume. One of the fundamental problems is that
seismology has not yet been able to provide reliable high–resolution images of the
deep structure beneath the islands and the surrounding seafloor. Our Hawaiian
PLUME experiment is the first long–term deployment of broadband ocean bottom
seismometers around Hawaii that will enable us, for the first time, to obtain regional
tomographic images that are not restricted by the geometry of the Hawaiian Islands.
Seismology
IGPP seismologists are studying earthquake sources both in southern
California and around the world. We recently released a new catalog of
earthquakes in southern California with dramatically improved location
accuracy (Shearer et al., 2005). Using results from a UCSD/Caltech
collaboration to process and analyze millions of seismograms to obtain
more precise timing of seismic arrivals, the new locations reveal a
much more detailed picture of seismicity and fault structure than
previous catalogs.
Data from IGPP's Anza Seismic Network provide
high–quality observations of earthquakes along the San Jacinto Fault.
Recently we used these data to examine the issue of earthquake scaling,
whether larger events are simply enlarged versions of smaller events
or if different physical processes are involved. By stacking P and
S wave spectra, we found that earthquakes at Anza appear to be self
similar, at least over the M = 1 to 3 interval (Prieto et al., 2004).
In global seismology, we have been developing new methods to produce rapid
images of fault rupture for large earthquakes, which could help both in
tsunami warning and rescue efforts. Our results for the devastating 2004
Sumatra–Andaman earthquake show that the rupture spread over the entire
1300–km aftershock zone by propagating northward at about 2.8 km/s for
8 minutes (Ishii et al., 2005), in contrast to some early models that
confined the fault slip to the southern part of the aftershock zone.
- Ishii, M., P.M. Shearer, H. Houston and J.E. Vidale, Extent, duration and
speed of the 2004 Sumatra-Andaman earthquake imaged by the Hi-Net array,
Nature, doi:10.1038/nature03675, 2005.
- Prieto, G., P.M. Shearer, F.L. Vernon and D. Kilb, Earthquake source
scaling and self-similarity estimation from stacking P and S spectra,
J. Geophys. Res., 109, B8, B08310, doi:10.1029/2004JB003084, 2004.
- Shearer, P., E. Hauksson and G. Lin, Southern California
hypocenter relocation with waveform cross-correlation, Part 2:
Results using source-specific station terms and cluster analysis,
Bull. Seismol. Soc. Am., 95, 904-915, doi:10.1785/0120040168, 2005.
Global Bathymetry and Marine Gravity
Bathymetry is
foundational data, providing basic infrastructure for scientific,
economic, educational, managerial, and political work. We have two
initiatives to improve the global bathymetric charts. The primary applied
research uses dense satellite altimeter measurements of ocean surface
height to recover the marine gravity field to an accuracy of about 3
milligals. These gravity data are inverted in combination with sparse
ship soundings to construct global bathymetric charts. Our secondary
applied research is to compile and evaluate the diverse array of global
depth soundings collected over the past 40 years. The global gravity and
topography data are used for the following research activities:
- Understanding
the geologic processes responsible for ocean floor features unexplained
by simple plate tectonics, such as abyssal hills, seamounts, microplates,
and propagating rifts.
- Determining the effects of bathymetry and seafloor roughness on
ocean circulation, mixing, climate, and biological communities, habitats,
and mobility.
- Mapping the marine gravity field to improve inertial navigation and provide homogeneous coverage of continental margins.
- Providing bathymetric maps for numerous other practical applications, including reconnaissance for submarine cable and pipeline routes, improving tide models, and assessing potential territorial claims to the seabed under the United Nations Convention on the Law of the Sea.
IDA Global Seismographic Network
During the past year we completed a major upgrade of the IDA station at Warramunga, Australia to the latest instrumentation and real–time telemetry, and we recently broke ground for a new station at Ambohimpanompo, Madagascar. Maintenance expeditions to the IDA stations in Sulewesi, Indonesia; Borovoye and Kurchatov, Kazakhstan; and Las Juntas, Costa Rica kept the network in top working order.
Recent upgrades of the telemetry links to some of the remotest stations in the IDA network now brings continuous streams of IDA data to IGPP from the stations in the Seychelle Island; Ascension Island; South Georgia Islands and the Azores. Of the 39 IDA stations, all but four return a substantial portion of the data collected over the Internet to computers at SIO. In addition to their scientific value, telemetered data are used to monitor for clandestine nuclear tests and for earthquakes that generate tsunamis. The IDA network is the oldest, continuously operating seismic network in the world – the first station was established in Canberra in 1974.
SIO Develops Marine EM Transmitter Technology
In support of its academic and industrial experiments using marine EM methods, SIO researchers developed and commissioned SUESI – Scripps Undersea EM Source Instrument. In 2004 a 200 amp transmitter (SUESI–200) was built and used for experiments on Hydrate Ridge and the East Pacific Rise (see below). This year (2005) we built a larger transmitter, SUESI–500, capable of 500 amp transmissions on a 200 meter antenna, yielding a source dipole moment of 127 kAm for a square wave transmission. Combined with the SIO seafloor receiver fleet (now up to about 50 instruments), these new transmitters provide SIO with a research tool comparable to the best available in industry.
Mapping Gas Hydrate using Marine EM Methods
In August 2004 we carried out a pilot experiment on Hydrate Ridge, offshore Oregon, to test our abilitiy to map seafloor gas hydrate using marine EM techniques. Seafloor hydrate, which occurs worldwide, is a hazard in offshore drilling, an important factor in submarine slope stability, a significant potential energy source, and has been implicated in climate change. Gas hydrate is more resistive than surrounding sediments and so is amenable to study using electrical methods. With only 3.5 days on station, using the SIO research vessel New Horizon, we deployed and recovered 25 seafloor receivers and operated our newly commissioned deeptowed transmitter for about 24 hours. Excellent data recovery (100%) resulted in an image of seafloor electrical conductivity which helps delineate the areas of highest hydrate concentration.
Imaging the Magmatic and Hydrothermal Systems of the East Pacific Rise with EM
An NSF–funded project provided us the opportunity to take our 40–instrument seafloor EM receiver fleet, along with our recently commissioned deeptowed transmitter, to the East Pacific Rise at 9 degrees north in early 2004. A total of 69 seafloor magnetotelluric (MT) sites and 80 km of EM transmitter tow make this one of the most comprehensive experiments of its kind.
Global Sea level
SIO scientist Walter Munk continues his studies on global sea level. His current interests include:
- Role of tides on climate variations on the time scale of ice ages: It has recently been established that tides are an essential source of pelagic mixing. Mixing plays a role in the transport of heat from equatorial to polar latitudes. A modulation of tidal amplitude caused by the change in obliquity with a time scale of 41,000 years may play a significant role in climate variability.
- Acoustic Shadow Rays: It has been known for over fifty years that long–range sound transmissions in the ocean include some "shadow rays" which are excluded by the usual physics. D. Rudnick and Walter Munk find that reflections from the base of the near–surface mixed layer may be responsible.
Archive of current research projects
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