Seismic recording of small zero frequency displacement frommoderate events

The use of modern broadband seismometers allows the
observation of dynamic and static near-field effects. In the
fortunate case of the great 1994 Bolivia earthquake a 6 mm
coseismic permanent offset was observed at distances of
about 600 km. On the other hand no surface static
displacement from moderate events has been observed
yet. This is mainly due to the intrinsic difficulties in the
instrument removal. In the present paper we analyze
broadband waveforms from a couple of events in southern
Italy, recorded at distance of 50 km, by applying the
technique for instrument removal recently introduced by
Zhu [2003]. We derive stable and reliable measures of very
small coseismic static offset produced by moderate
magnitude earthquakes. Our results, successfully tested
against synthetic prediction, give permanent displacement
of a few tenths of millimeters, one order of magnitude
smaller than usual geodetic resolution.