Quantitative Analysis of Early Seismograph Recordings - Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes - page 380

8

Benavente 23-04-1909
DBN, Bosh-Omori

4

0

-4

0 100 200 300 400 500 600 700 800

Time (s)

dbn09b Benavente 23-04-1909

0 50 100 150 200 250 300 350 400

Time (s)

0.01 0.01

P Wave S Wave

0.001 0.001

1e-04 1e-04

1e-05 1e-05

1e-06 1e-06

Uo = 0.70498E-04
fc = 0.255

Uo = 0.28570E-03
fc = 0.136

1e-07 1e-07

0.01 0.1 1 0.01 0.1 1

Frequency (Hz) Frequency (Hz)

Fig. 3 ( a ) Digitized record of the Bosch-Omori seismograph at De Bilt seismic station for
the 23 April 1909 Benavente earthquake, near Lisbon. It is clearly seen how a displacement of
the equilibrium centre of the recording mass occurs during the P and S wave arrivals. Note also the
whole dimension of the record, peak to peak maximum amplitude is just 15 mm. In the horizontal
scale 100s are equivalent to 25 mm. ( b ) Even though mass displacement is noteworthy, after HP
filtering (Butterworth filter at half the free period of the instrument) the P and S wave spectra give
reasonable results