Relative dating of Quaternary deposits based on P-wave velocities in weathered granitic clasts

A new quantitative, reproducible method for determining relative ages of unconsolidated Quaternary deposits containing granitic clasts has been developed and tested. The technique makes use of a microseismic timer to determine the compressional wave velocity (clast-sound velocity) in each clast of a group chosen from a single deposit. From these data a group mean velocity is determined that is proportional to the age of the deposit: the youngest deposits having the highest velocities. The clast-sound velocity method was used to study Quaternary deposits in the San Gabriel Valley and San Gabriel Mountains, California, using a previously proposed four-part age classification for the deposits. The clast-sound velocity group means for the four age groups were found to be statistically separable at better than 99% confidence. A velocity/age correlation curve was determined for these deposits using two radiocarbon dates and one paleomagnetic determination. This curve suggests that the clast-sound velocity method may be used to determine ages of deposits up to one million years old when calibrated with sufficient radiometric dates and may also be used as a tool for correlating undated deposits.     

Analysis of input data and seismic hazard assessment for the low seismicity area ‘Belgium,The Netherlands and NW Germany’

From the earthquake catalogue of the region of interest 2–8°E, 49–52°N, only events with intensity > 3 are considered. The fore/aftershocks and the induced seismicity are removed. The completeness and other properties of the remaining data set are investigated. The seismic data, as well as the geological, tectonic, and other geophysical data of the area are used for the determination of the seismotectonic zones. For each zone, the cumulative intensity-frequency relation (taking into account the completeness of the catalogue), the attenuation depending on the direction, the upper bound of intensity and the average depth are calculated. When the seismic activity is changing within a zone, this zone is divided into subzones with an almost constant activity and a b value equal to the value of the whole zone. When necessary for each subzone, a different attenuation coefficient can be used, to take into account more regional effects. Then the seismic hazard is assessed with the modified McGuire program. Annual probability versus intensity for several sites and hazard intensity and probability maps are calculated. Finally, an error discussion of the whole procedure for one test site is given.