HR: 0800h
AN: T51B-1339    [Abstracts]
TI: Installation of a High Resolution Potassium Magnetometer in the Coast of Oaxaca
AU: * Hrvoic, I
EM: ivan@gemsys.ca
AF: GEM Advanced Magnetometers, 14 - 52 West Beaver Creek Road, Richmond Hill, ON L4B 1L9 Canada
AU: Cifuentes-Nava, G
EM: gercifue@igeofcu.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica Ciudad Universitaria, Mexico, D.F 04510 Mexico
AU: Cabral-Cano, E
EM: ecabral@igeofcu.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica Ciudad Universitaria, Mexico, D.F 04510 Mexico
AU: Hernandez-Quintero, E
EM: estebanh@igeofcu.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica Ciudad Universitaria, Mexico, D.F 04510 Mexico
AU: Wilson, M
EM: mike.wilson@gemsys.ca
AF: GEM Advanced Magnetometers, 14 - 52 West Beaver Creek Road, Richmond Hill, ON L4B 1L9 Canada
AU: Hollyer, G
EM: greg.hollyer@gemsys.ca
AF: GEM Advanced Magnetometers, 14 - 52 West Beaver Creek Road, Richmond Hill, ON L4B 1L9 Canada
AB: This paper describes the state-of-the-art in (electro) magnetic measurements for earthquake research. We also analyze dipolar magnetic fields and gradients of magnetic moments generated by earthquakes with emphasis on their strongly local character. The magnetic moments of two measured precursors are calculated as well as the maximum distances at which those earthquakes can be detected with both present methods and a new proposed method (i.e. short base Potassium gradiometer). Based on the theory of piezomagnetism and / or piezokinetics, magnetics offers a possibility of detection of precursors to earthquakes due to gradual pressure build-up. Three typical limiting factors include sensitivity, long-term stability and a need to eliminate environmental noise (diurnals, man made noise). Early monitoring systems with sensitivities in the nT range and long base differential measurement produced in a few cases, startling precursors that could, however, be neither confirmed nor repeated. Some of the more recent work has employed induction coils with an improved sensitivity (25pT) but limited long term features (bandwidth down to 0.01Hz) and the results have been somewhat better. When detected, corresponding anomalies varied from few nT to few tens of pT (close to instrument's background noise). Piezomagnetic anomalies vary substantially with the earthquake intensity, composition of rocks that come under pressure, geometry of pressure etc. Assuming that they are of dipolar character (3), their fields vary with the cube of distance (i.e. their detectability will be limited to a proximity to epicenters - or better, to hypocenters). More systematic results can only be obtained if the measurements can be done with substantially increased sensitivity; long-term stability; and by taking into consideration the very local character of dipolar magnetic field, large time variations of magnetic field (diurnals) and noise. Man-made (anthropogenic) noise is another formidable barrier that must be overcome. In addition to theory, we present the initial results of the continuous operation of a high resolution Potassium magnetometer in the Trapiche, San Francisco Cosoaltepec, Oaxaca, for the study of processes of accumulation of efforts in the Oaxaca coast. This is a region of great importance for the study of superficial subduction. The high rate of convergence, the relative proximity of the coast to the trench and its consequent high gradients of strains, as well as frequent seismic events, make this an ideal zone for these studies. The recent development of high sensitivity and high resolution Potassium magnetometers allow the detection of resulting piezomagnetics effects during processes of accumulation of strains. This requires monitoring the variations of the magnetic field that can help to understand better the process of accumulation and relaxation of these in a zone of subduction. The Geophysical Institute of the UNAM in collaboration with GEM Systems (Canada) has initiated one experiment of medium term duration in the coastal region of Oaxaca to operate a continuously recording Potassium Super gradiometer. This instrument consists of 3 sensors oriented in an almost orthogonal way with separation of approximately 100 metres and connected to a magnetic high resolution processor of 0.001 pT @ 20Hz resolution, Sensitivity is greater by several orders of magnitude compared with a conventional magnetometric surveys.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 1594 Instruments and techniques
DE: 8194 Instruments and techniques
SC: Tectonophysics [T]
MN: Fall Meeting 2005