Seismic site characterization of the Wiechert earthquake station in Göttingen

The Wiechert earthquake station in Göttingen is the oldest existing seismograph station in the world, recording earthquakes continuously since 1903. In 2003, the broadband seismometer station GTTG was installed as part of the German Regional Seismic Network (GRSN) in close vicinity of the old instruments.

An analysis of the recordings of regional earthquakes showed a strong amplification of the horizontal signals around 3 Hz. This finding indicates that softer surficial layers overlie the harder seismic bedrock at the site. In order to investigate and quantify this site effect in more detail, different geophysical measurements were performed.

Using 20 three-component sensors, H/V and Rayleigh wave ellipticity curves were obtained for each single sensor. Analyzing 2.5 hours of passive seismic array data, dispersion curves for Love and Rayleigh waves were retrieved.

The Mintrop ball, a steel sphere of about 4 tons, was dropped three times from a height of about 14 m to perform an active seismic experiment. Analyzing the signals as a refraction seismic measurement, a P-wave velocity of about 700 m/s was determined for the uppermost 12 m. Analyzing the array signals of the drops by appropriate methods, dispersion and ellipticity curves were also obtained.

The active and passive dispersion curves are in good agreement. An inversion of the measured curves indicates that a strong velocity contrast at a depth of about 55 m, where the S-wave velocity increases from about 600 to almost 2000 m/s, can explain the amplification peak at 3 Hz.