Amplification behaviour of compressional waves in unconsolidated sediments

Similar to horizontal earthquake motions, vertical motions are ampli!ed depedent on the local site conditions which can be critical for the safety of certain structures. Production of natural gas in Groningen, the Netherlands, results in reservoir compaction causing low magnitude, shallow earthquakes which are recorded with a borehole seismic network. These recordings form an excellent data set to understand how shallow unconsolidated subsurface geology in"uences the ampli!cation behaviour of compressional waves (P-waves). First, we present borehole and single-station techniques (ampli!cation factors, empirical transfer functions (ETF) and V/H spectral ratio implementations) to quantify vertical ampli!cation. We show that vertical-wave incidence is a reasonable assumption. All techniques are capable of emphasising the sites with strong ampli!cation of vertical ground motion during an earthquake. Subsequently, we compare ETF with single-station methods with the aim to develop proxies for vertical site-response using spectral ratios. In a second step, we link vertical site-response with shallow subsurface conditions, like the P-wave velocity and peat content. To better understand the ampli!cation mechanisms, we analytically simulate P-wave propagation. In the simulations, we compute synthetic transfer functions using realistic subsurface conditions and make a comparison with the ETF. The simulations support the hypothesis that thin layers of shallow gas, originating from the Holocene peat, result in wave ampli!cation. We observe strong vertical site-response in particular in the eastern part of Groningen, with industrial facilities and pipeline infrastructure in the region. Here, if high vertical ampli!cations are persistent at large earthquake magnitudes, appreciable levels of vertical loading may be expected. This study demonstrates that vertical motions should be assessed separately from horizontal motions, given that the ampli!cation behaviour of P-waves is affected by distinctive mechanisms.