As referred to in chap. 2, the determination of the
arrival-times (starting time of the first swing) both of the primary-
and secondary-wave is essential for the evaluation of the source
parameters. Now we want to present the individual steps of a simple
analysis of a seismogram as we do it in our seismic-team.
We evaluate the seismogram recorded from the Ascension-earthquake
(island off Western Africa) on 18. 02. 1996.
||Calculation of the distance between the
origin of the earthquake and our station: the epicentre-distance.
||Determination of the time at which the earthquake
broke out : the origin time
||Fixing the strength of an earthquake : the
(surface-) magnitude (position on the Richter-scale)
||Fixing of the geographical coordinates (latitude
and longitude) of the epicentre : the "localisation" of the
Before we start discussing the routine procedure of evaluating a seismogram
let us take a general look at the recording units of a seismographic
Fig. 48 The three components of a seismographic station
A complete seismographic station observes the waves
of an earthquake in all the three spatial dimensions. This is usually
done by two horizontal seismographs, one of which swings from north
to south, while the other one swings from east to west. The third
dimension is caught by a vertical seismograph, also called up-down
or z-seismograph. - A complete seismogram of an earthquake therefore
always consists of three components: a North-South-, an East-West-
and an Up-Down-Seismogram.
Fig. 49 P- and S-waves arrive simultaneously in
all the three components (3d-seismogram)