OVERVIEW

The first long term scientific deployment of the portable broadband seismic array will be part of MASE (Middle America Subduction Experiment) that is being run by the California Institute of Technology in collaboration with Universidad Nacional Autónoma de México (UNAM). Last year we proposed the first long term experiment of the portable array be a San Gabriel/San Andreas experiment. Field deployment funds we sought were not available. Subsequently we were asked to join MASE and tentatively accepted their offer. Scientifically, we will be mapping the subducted slab beneath Mexico and examining slow earthquakes which have been observed at this subduction zone, volcanic earthquakes and the propagation of seismic waves in Mexico City. Technologically, we will have a comparison between the 50 instruments of our wireless array and the 50 instruments of a stand alone array that will be installed by Caltech.

Our orignal goal of constructing a broadband seismic network was to make data collection easy and reliable in remote regions. Although a bit earlier than we might have liked, the invitation to join California Institute of Technology and UNAM to apply the seismic networking technology to a sesmic array study in Mexico is an excellent opportunity. MASE will pay the bulk of field costs. The transect is 500 km long between Acapulo-Mexico City - Tampico. A proposal was submitted to UCMEXUS to facilitate UCLA’s involvement. We continue to develop algorithms to model seismic waves and have recognized caustics forming in LA basin. The Mexico experiment will make an excellent comparison between the LA basin and the Mexico Valley where a devastating earthquake in 1985 toppled high rise buildings.

Figure 1. Map of proposed MASE seismic line. The CENS networked array will be deployed to the central (white) part of line.

APPROACH

We purchased 50 broadband seismometers that will make up the broadband network with the CENS DCC and 802.11 radios. (Two of the purchased seismometers have been returned after testing.). The proposed Mexico network will have stations every 5 km linked by radios so that the data packets are routed along the line to a base station in Mexico City. We are in the process of testing radio networking of the 50 stations from tens of meters to tens of km.

Figure 2. Parabolic antennas for the large scale CENS seismic network. (left) Battery, Quanterra digitizer and CENS DCC (black box) ; (right) Parabolic antenna for > 10 km transmissions.

SYSTEMS / EXPERIMENTS

Experiments on timing and fabrication of the CENS DCC are described in Section 1. In preparation for Mexico we are investigating networking over distance of tens of km. An initial experiment has shown that 802.11T radios transmit at > 2 Mbit/s over 6.27 km using Yagi 6 dB antennas. We are in the process of testing recently purchased 24 dB parabolic antennas and a field trip is planned for a specific test.

ACCOMPLISHMENTS / PARTNERSHIPS

A reconnaissance field trip was made to the field area in April with Caltech researchers involved in the experiment and hosted by researchers at UNAM. Informal agreements were discussed with the plan to draft a memorandum of understanding between UCLA, UNAM ( Prof. Sri Krishna Singh, Director of the Geophysics Institute) and Caltech (Prof. Robert Clayton).

Seismic Wave Propagation and Basin Edge Amplification Study

The Los Angeles Basin Passive Seismic Experiment (LABPSE) provided a small scale pre-study to the near-surface portion of the MASE study. Data from LABPSE was used to develop an amplitude amplification model within the greater Los Angeles region. In addition, we were able to determine differences in scattering and anelastic attenuation. Scattering and ringing within the basin actually increases the time of decay of the wave at the surface. Of particular interest are structural focusing and defocusing effects that do not appear in the standard amplitude model, but from selected azimuths can give localized amplifications several times those predicted (Husker et al., 2004). Such focusing effects are thought to have caused anomalous damage at the time of the Northridge earthquake in regions near basin edges (Gao et al., 1996, Davis et al., 2000, Baher and Davis 2003; Baher et al., 2002). While the effects of geological focusing are approximately described by optical focusing theory (Davis et al., 2000), it has now become apparent that a better description is in terms of the folds and cusps of catastrophe theory (Rial, 1984). The optical focus is an unstable end member of the class of three-dimensional catastrophes (Thom, 1975; Berry et al., 1979; Nye 1985). Catastrophes are more likely to form than focus points in a geological situation because of their stability with respect to the generating structures. The peak amplitudes of the associated diffraction patterns show a characteristic dependence on frequency and spatial decay. The LABPSE study is being written and will be submitted to a scientific journal. Similar phenomena will be investigated during the MASE experiment.

FUTURE GOALS AND OBJECTIVES

Timeline

• Construct all 60 CENS DCC’s. May 2004.
• Field test 12 unit network at 4 Seasons. May – June 2004.
• Field test data long relay link 3 station. April 2004.
• Field test data long relay link 12 stations. June 2004.
• Reconnaissance to Mexico with 3 station link test electromagnetic noise and propagation conditions. Sept 2004.
• Install Network. January 2005.
• Return Network to UCLA. Summer 2006.
• Analyze Data (Husker Ph.D.) 2005-2006.

PEOPLE

FACULTY

Paul Davis
Deborah Estrin

STAFF

Jeremy Elson, Researcher
Monica Kohler, Researcher
Igor Stubailo, Engineer

GRADUATE STUDENTS

Allen Husker

UNDERGRADUATE STUDENTS

John Propst

ASSOCIATES

Robert Clayton, Caltech
Sri Krishna Singh, UNAM.