6. On the evaluation of a seismogram
As referred to in chap. 2, the determination of the
arrivaltimes (starting time of the first swing) both of the primary
and secondarywave 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 seismicteam.
We evaluate the seismogram recorded from the Ascensionearthquake
(island off Western Africa) on 18. 02. 1996.
(1a) 
Calculation of the distance between the
origin of the earthquake and our station: the epicentredistance.

(1b) 
Determination of the time at which the earthquake
broke out : the origin time 
(2) 
Fixing the strength of an earthquake : the
(surface) magnitude (position on the Richterscale) 
(3) 
Fixing of the geographical coordinates (latitude
and longitude) of the epicentre : the "localisation" of the
earthquake 
Before we start discussing the routine procedure of evaluating a seismogram
let us take a general look at the recording units of a seismographic
station.
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 updown
or zseismograph.  A complete seismogram of an earthquake therefore
always consists of three components: a NorthSouth, an EastWest
and an UpDownSeismogram.
Fig. 49 P and Swaves arrive simultaneously in
all the three components (3dseismogram)

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As referred to in chap. 2, the determination of the
arrivaltimes (starting time of the first swing) both of the primary
and secondarywave 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 seismicteam.
We evaluate the seismogram recorded from the Ascensionearthquake
(island off Western Africa) on 18. 02. 1996
 Calculation of the distance between the origin of the earthquake and our station: the epicentredistance.
 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 Richterscale)
 Fixing of the geographical coordinates (latitude and longitude) of the epicentre : the "localisation" of the earthquake
Before we start discussing the routine procedure of evaluating a seismogram let us take a general look at the recording units of a seismographic station.
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 updown
or zseismograph.  A complete seismogram of an earthquake therefore
always consists of three components: a NorthSouth, an EastWest
and an UpDownSeismogram.
Fig.49: P and Swaves arrive simultaneously in all the three components (3dseismogram)
An diesem Seismogramm ist gut zu erkennen, dass sowohl
die Primärwelle "P" als auch die SekundärWelle "S"
in allen drei Seismogrammen synchron eintreffen. (Man spricht vom
"Einsatz" der P und der SWelle.)