The Gulf of California rift forms a geologically young and active plate boundary that links the San Andreas strike-slip fault system in California to the oceanic spreading system of the East Pacific Rise. Although this is a classical example of a transform-rift plate boundary, the tectonic evolution of the Gulf of California and surrounding regions is complex and poorly understood due to a lack of geological and geophysical data. In 2002, the Network of Autonomously Recording Seismographs (NARS)-Baja network was installed. It consists of 19 broadband seismic stations deployed in the Baja-California and Sonora provinces of Mexico (Figure 1).
= NARS = CICESE
Since NARS-Baja surrounds the Gulf of California rift system, it is ideal for constraining earthquake faulting processes and the crust mantle structure of the region. Moreover, NARS-Baja, in combination with permanent Mexican and U.S. arrays, forms a unique linear array in excess of 4000 km that should lend itself ideally to seismological studies of the North American Pacific plate boundary on a larger scale. NARS-Baja is planned to operate for at least 5 years. To promote involvement from the entire research community, the data collected from the stations will be made available immediately following routine data quality checks.
The need for a broadband seismic network surrounding the Gulf of California is clear from catalogues of the International Seismological Centre (ISC) and the National Earthquake Information Center (NEIC), which contain an unrealistically low number of earthquakes with magnitudes smaller than 4. Owing to a nearly complete station distribution achieved with NARS-Baja around the Gulf of California, an improved detection level should allow accurate earthquake locations and well-constrained focal mechanisms of moderate (M > 3-4) earthquakes to be determined. This will enable us to delineate active faults more accurately and improve our understanding of strain release and tectonic deformation in the region.
In addition, NARS-Baja data will be crucial for studying the crust and upper mantle structure beneath the entire Gulf of California. While global and continent-scale seismological models suggest that the seismic velocity structure in the mantle beneath the Gulf of California is as anomalous as that of the East Pacific Rise, NARS-Baja data will allow us to make models of the crust and mantle with unprecedented resolution. Resulting crustal and mantle models will provide new constraints on the nature of this young plate boundary and its transition from strike-slip faulting along the San Andreas Fault system to ocean spreading along the East Pacific Rise. The Moho depth can be mapped using body wave coda and short-period surface waves, while the lithospheric and asthenospheric structure can be determined using body wave travel times and waveforms and long-period, surface-wave dispersion measurements. The surface-wave data further provide important constraints on anisotropy in the uppermost mantle, and together with shear wave-splitting measurements, mantle flow can be imaged. These are several examples of well-established techniques that can readily be applied, but the NARS-Baja network, especially in conjunction with existing Mexican and Californian seismic arrays, offers ample research opportunities for deep mantle and core studies as well. We hope that our policy of full data availability will encourage many members of our research community to participate in analyzing the NARS-Baja data.
NARS-Baja originated from a desire to combine scientific interest, regional as well as global, with a maximum ease of operation in terms of instrument availabilty, data storage, and manpower. It is a joint effort between Utrecht University (The Netherlands), Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE, Mexico), and the California Institute of Technology, and it consists of 19 broadband seismic stations deployed on either side of the Gulf of California with a 100-150 km station spacing. Every 3 months or so, all stations will be visited by CICESE researchers for maintenance and downloading of the data. The raw data will immediately be sent to Caltech where, after preliminary quality inspection, full seed volumes will be assembled and archived. Both the IRIS Data Management Center in Seattle and the SCEC Data Center at Caltech will store continuous data that will be made available to the community via convenient, Web-based request tools.
The NARS-Baja project has been initiated by the following people: Jeannot Trampert, Hanneke Paulssen (Utrecht University), Jeroen Ritsema, Robert Clayton (Caltech), Raul Castro, Cecilio Rebollar (CICESE).
The project thanks its succes to the efforts of the technicians Arie van Wettum (Utrecht University), and Arturo Perez-Vertti (CICESE).
We are grateful to SCEDC and IRIS DMC for agreeing to archive and distribute our continuous waveform data. Funding for this project was provided by Utrecht University, the Dutch National Science Foundation (grant number NWO-GOA-750.396.01), and the U.S. National Science Foundation (grant number EAR-0111650 of the MARGINS program). Maintenance of CICESE broadband stations is partly funded by CONACYT project 37038-T. We also greatly acknowledge the participations of Oscar Galvez, Antonio Mendoza, Luis Orozco and Luis Inzunza (CICESE).
Example: recordings of the 2003/05/26 Honshu event.