  Magnetic field variations are detected before rupture in the form of `spikes'
of alternating sign. The distinction of these `spikes' from random noise is of
major practical importance, since it is easier to conduct magnetic field
measurements than electric field ones. Applying detrended fluctuation analysis
(DFA), these `spikes' look to be random at short time-lags. On the other hand,
long range correlations prevail at time-lags larger than the average time
interval between consecutive `spikes' with a scaling exponent $\alpha$ around
0.9. In addition, DFA is applied to recent preseismic electric field variations
of long duration (several hours to a couple of days) and reveals a scale
invariant feature with an exponent $\alpha \approx 1$ over all scales available
(around five orders of magnitude).
