Unveiling the effects of soil composition on surface motion during
seismic events
Abstract
The degree of damage on buildings due to earthquakes is strongly
dependent on the properties of the subsurface at that specific site. The
shallow geology of the Netherlands consists of an heterogeneous soft
sediment cover, which has a strong effect on seismic wave propagation
characteristics and in particular the amplitude of ground shaking. Where
seismic velocities are lower, seismic wave amplitudes are higher. By
studying local velocity and amplitude variations from seismic waves, we
can obtain constraints on the seismic hazard. Gas extraction in the
Groningen field, in the northern part of the Netherlands, is regularly
causing shallow (3 km), low magnitude (Mw max= 3.6), induced
earthquakes. This region forms an excellent study area due to the
presence of a permanent borehole network and detailed subsurface
knowledge. The earthquake wavefield consists of shear and compressional
waves. Whereas a lot of research has been carried out on the shallow
behaviour of the shear waves, this project includes the characterisation
of the compressional waves in the shallow subsurface. In this way,
ground motions in the vertical direction can be determined in order to
support the re-enforcement design for buildings in the areas affected by
induced seismicity. The Groningen borehole network is continuously
measuring since 2015 and besides earthquakes, it records a wealth of
background signals, which is usually called ‘noise’. This noise contains
low-energetic elastic waves which also resonate within the sedimentary
layers. The local earthquake recordings are used to assess how the
shallow unconsolidated subsurface geology influences the amplification
of compressional waves, amplification can directly be measured because
there are geophones on multiple depth levels. For compressional waves we
observe a strong relationship between locations with high amplitudes and
the presence of peat in the subsurface. Peat is generating biogas,
resulting in a partly gas-saturated soil, hence very low seismic
velocities. The noise resonance and earthquake amplification patterns
are well-matched. Therefore, the learnings from the densely sampled
Groningen region are of interest for other areas in the Netherlands with
risk of seismicity since the noise resonance can be used as a first
proxy to assess wave amplification.