The Vertical Component of Low Magnitude Earthquakes, Why Do We Care?

Up to now, almost all of the ground motion modeling and hazard assessment for induced seismicity in Groningen, the Netherlands, has been focused on the horizontal components of earthquake waves. Including the vertical component in site response studies is hardly being done for low magnitude earthquakes. Since the top part of the soils in the Netherlands is practically always unconsolidated, the elastic waves generated by deeper (~3000m) seated earthquakes will be subject to transformation when arriving in these layers. Recordings over a range of depth levels in a borehole show most of the amplification in the upper 50 meters of the sedimentary cover. We observe not only a strong amplification from shear waves on the horizontal components, but also from longitudinal waves on the vertical component. Furthermore, the seismic velocities show a large lateral heterogeneity. A better understanding of the vertical component of low magnitude earthquakes (to date, max M=3.6 in Groningen) aims to support the design of re-enforcement measures for buildings in areas affected by induced seismicity. This study focuses on longitudinal wave amplification in a sedimentary basin setting.Generally, the vertical component of ground motion is less than the horizontal, but the longitudinal waves are concentrated in a high frequency band which can cause damage to buildings with vertical periods in this range. Furthermore, interference between the longitudinal and shear waves might contribute to extra damage on structures. In Groningen, a dense borehole network is continuously recording seismic activity. From 19 seismic events with a minimum of magnitude two, transfer functions are retrieved between geophones at 50m depth and accelerometers at the surface, for 70 borehole sites. Peak frequencies and amplitudes derived from the transfer functions, do show significant variability across the region. Highest longitudinal wave amplification is measured in the center of the field, which is also the epicenter of most seismic activity. We investigate if the variations in amplification can be linked to the local geology. Additionally, the possibility and consequences of interference between the shear and longitudinal waves are presented.