Radiophysical techniques of short-term earthquake precursors and their congruence. The case of L´Aquila earthquake of 06 April 2009

We studied six different parameters derived from three different radiophysical techniques used to monitor area over the central Italy around the time of L´Aquila M6.3 earthquake of 6 April 2009. Namely VLF monitoring, ground based ionospheric sounding and GPS vertical TEC. The parameters derived from these data are: variations of amplitude of VLF signal on different propagation passes, critical frequency foF2 variations, cross-correlation coefficient for different pairs of ground based ionosondes, vertical GPS TEC time series, local variability index for the network of GPS receivers, and GPS TEC map. High congruence of techniques in time and space is found.