IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS008 Oral Presentation 1998
A method for parameterization of magnetic ULF signals
Dr. James Cutler
IASPEI
Jacob Bortnik, Clark Dunson, Tom Bleier
We analyze long-term (1995-2007) ultra low frequency (ULF, less than 10Hz) tri-axial magnetometer
data measured in California. We process data from fourteen ULF sensor systems installed along active
California zones that are operated by Quakefinder, Stanford University, and UC Berkeley with data
collections starting in 2005, 2004, and 1995 respectively. L-shell values of the sensors range from 1.6 to
1.9. Twelve years of data as measured by two UC Berkeley systems provides the basis for determining
typical signals levels in California through a full solar cycle. More recent data collected by Quakefinder
and Stanford allows us to measure the extent of wide-area signals and characteristics of additional local
signal environments. In this analysis, we develop statistical summaries of measured signal strength
parameterized according to sensor site, coil orientation, frequency range, time of day, season, and
global geomagnetic activity (Kp index). For each parameter, we calculate the mean, median, variance,
and quartiles of the magnetic signal recorded at multiple ULF stations, and use these quantities as
baseline values from which signals are assumed to deviate. With these statistical quantities, we
construct expected signals as a function of the six parameters and subtract them from absolute
measurements to obtain signal residues. Results show that this technique can be effective in reducing
large background variations and thereby increasing the signal to noise ratio (SNR), allowing much lower
amplitude signals of local origin to be detected. Network-wide trends in parameterized statistics are
discussed. We also describe the use of parameterization output to create data quality metrics,
determine sensor health, remove of wide area signals, and indentify local noise signals. We then
examine time periods surrounding earthquake near the sensors, and to improve SNR, we superpose
multiple earthquake events and discuss the results in light of possible seismogenic ULF signal sources.
Keywords: ulf, magnetic, parameterization