New procedures to decompose geomagnetic field variations and application to volcanic activitiy

Abstract

SUMMARY We report the development of numerical procedures for extracting long‐term geomagnetic field variations caused by volcanic activity from an observed geomagnetic field by using statistical methods. The newly developed procedures are to estimate the trend from the observed data, as well as variations of non‐volcanic origin such as periodic components, components related to external geomagnetic variations and observational noise. We also aim at referring to data obtained at a remote standard geomagnetic observatory rather than using a temporarily installed reference site for reasons of data quality. Two different approaches—a Bayesian statistical method and a Kalman filter method—are applied to decompose the geomagnetic field data into four components for comparison. The number of filter coefficients and the degree of condition realizations are optimized on the basis of minimization of the information criteria. The two procedures were evaluated by using a synthetic data set. Generally, the results of both methods are equally sufficient. Subtle differences are seen at the first, several data points due to arbitrarily selected initial values in the case of the Kalman filter method and at the smaller residual for the Bayesian statistical method. The largest differences are in computation time and memory size. The Kalman filter method runs a thousand times faster on a testing workstation and requires less memory than the Bayesian method. The Kalman filter method was applied to the total intensity data at Kuchi‐erabu‐jima volcano. The result suggests that the procedure works reasonably well.

The characteristics of geoelectric fields at Kakioka, Kanoya, and Memambetsu inferred from voltage measurements during 2000 to 2011

“…An arbitrarily large number is set as an initial value of u(t a ), and an estimate of u(t a ) is excluded from the evaluation of τ 2 . The second procedure we have added to the method of Fujii and Kanda (2008) is used to remove outliers such as spikes in the data. A datum is regarded as an outlier and is treated as a data gap if it satisfies:…”

“…The procedure proposed by Fujii and Kanda (2008) splits E i (t) (i = x, y), a vector component of the electric field, into four parts:…”

“…We make the trend component accommodate these sudden changes at a preset time. The trend component in Fujii and Kanda (2008) is assumed to be smooth in time (t) and to follow:…”

“…A trend-estimating procedure using the Kalman filter technique, as developed by Fujii and Kanda (2008), was adapted to estimate T(t) from E(t). Our primarily modification to this method was to add a robust procedure with which to accommodate the geoelectric data, with its anomalous spikes and sudden trend changes.…”

Localized changes in geomagnetic total intensity values prior to the 1995 Hyogo-ken Nanbu (Kobe) earthquake

“…Recent advances in the techniques available for modeling the geomagnetic field (e.g., Walker and Jackson, 2000) enable data correction with greater accuracy. Moreover, data processing methods have been proposed that take into account the induction effect in the Earth's crust (Fujii and Kanda, 2008). The use of such advanced procedures will produce reliable results regarding localized changes in the geomagnetic field, and possibly lead to a breakthrough in studies on the relation between tectonic activity and geomagnetic variations.…”