NH 31 A 1336 Earthquake Magnetic Precursors In

NH 31 A 1336: Earthquake Magnetic Precursors In Oaxaca, Mexico with High Resolution Potassium Magnetometer Ivan Hrvoic**, Mike Wilson**, Francisco Lopez Garibay** Gerardo Cifuentes-Nava*, Enrique Cabral-Cano*, Esteban Hernández-Quintero*, * Instituto de Geofísica, UNAM, MEXICO * * GEM Systems, CANADA Site location ABSTRACT This paper describes the state-of-the-art in (electro) magnetic measurements for earthquake research. Existing methods have met with limited success due to limited sensitivity and long-term stability of instruments; imperfect elimination of environmental noise; and in the case of induction coils, to limited low frequency features and the skin effect for their bandwidth of measurements. 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). Due to the extreme sensitivity of the Potassium Super. Gradiometer, the new method is at least one order of magnitude more sensitive than presently used induction coils. Super. Grad features and installations are described. INTRODUCTION Magnetic field precursor for the M 5. 0 earthquake at Kawazu, 1978. From Kiyoo Mogi: Earthquake Prediction, Academic Press, Japan, 1985. Precursor for Loma Prieta (M 7. 1) Earthquake. B=5 n. T anomaly at 17 km distance to hypocenter and 1. 7 x 1011 Am 2 magnetic moment Magnetics has played a significant role in Earthquake studies for several decades. Based on theory of piezomagnetism, piezoelectricity and / or electrokinetics, it 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, and ionospheric and man-made noise). Early monitoring systems with sensitivities in the n. T range and long base differential measurement produced only a few cases with startling precursors that could, however, be neither confirmed nor repeated. Some of the more recent work has employed induction coils with an improved sensitivity (25 p. T) but limited long term features (bandwidth down to 0. 01 Hz) and the results have been somewhat better. When detected, corresponding anomalies varied from few n. T to few tens of p. T (close to instrument’s background noise). 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 , 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; longterm 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. Detectability of earthquakes by magnetic and gradient methods. Super. Grad Instrumentation. Includes 3 supersensitive sensors and acquisition console. Both magnetometers, and to a lesser extent, induction coils need to work in differential mode to reach the best sensitivity free of diurnals and man made noise. Reference instruments that measure only temporal variations of the magnetic field are usually placed away from active zones, (long base), resulting often in imperfect elimination of diurnals and man-made noise. RESULTS We present the initial results of the continuous operation of a high resolution Potassium magnetometer in the Trapiche, San Francisco Cozoaltepec, Oaxaca, for the study of processes of accumulation of strains in the Oaxaca coast in the Mexican Pacific. This is a region of great importance for the study of the processes of subduction. The high rate of convergence, the relative proximity of the coast to the trench and its consequent high gradients of strains that can be detected, as well as frequent seismic events, make this an ideal zone for this type of studies. The recent development of high sensitivity and high resolution Potassium magnetometers allow the detection of resulting piezomagnetic and electrokinetic effects during processes of accumulation of strains in purely differential mode (short base) that suppresses environmental and man-made noise more efficiently. Recently a study of the past records reveals precursors to moderate earthquakes (4. 2 and 4. 8) in the vicinity of the instrument (20 – 35 km. distance to hypocenters). The search and analysis of data continues, and we will report case histories with some details. Magnetic Precursor 33 minutes before Earthquake, Magnitude 4. 8, epicenter located 6 km West and 21 Km Deep from Super Grad Site in Oaxaca Mexico