Application of a hybrid modelling technique to calculate ground motions of Fennoscandian earthquakes

Small to mid-magnitude earthquakes may generate very strong, potentially damaging, ground motion close to their epicenter. Fortunately, these ground motions attenuate quickly and a few tens of kilometers from the epicenter they barely exceed the range of perceptibility. Even if Fennoscandia is one of the most seismically quite regions, about 10-15 earthquakes are felt yearly in Finland.As the number of potentially sensitive infrastructure increases, it becomes important to investigate the potential ground motions generated locally by small earthquakes. Traditionally the concern has been nuclear installations, but industrial and chemical plants may also be source of hazard if equipment is damaged. The expansion of the IT infrastructure, especially the increasing numbers of data centers also results in a set of potentially affected sites.In Fennoscandian it is hard to collect data in the very close vicinity of earthquakes, because seismicity is diffuse and earthquakes do not occur in preferential locations. Only measurements of earthquake swarms by specially deployed temporary stations may offer some insights to the intensity of ground motions close to the epicenter location. However, computer modelling has evolved significantly and realistic modelling of earthquake events is becoming feasible.In this work we present a hybrid modelling approach for estimating ground motions in the vicinity of mid magnitude earthquakes. In a first computer model (3DEC) we simulate the dynamic rupture of the earthquake fault. The properties of the fault are adjusted to the expected earthquake. The stress state on the fault reproduces the expected stresses in the Earth’s crust. A shear failure is initiated in a small patch of the fault by lowering the shear strength. Once this failure is initiated it propagates in the fault area. We record the movements in a very large number of points in the fault area and transfer these movements to a second computer model (COMPSYN) where the ground motions are calculated using kinematic modelling.The method has been benchmarked in predicting ground motions of hypothetical earthquakes in the moment magnitude range of Mw=5.5 (Fülöp et al, 2017). The results show reasonable agreement with ground motion prediction equations from similar tectonic regions as Fennoscandia. However, with modelling we are able to take into account specific features of the Fennoscandian geology, when they differ from these international references.The predicted ground motions in the immediate vicinity of earthquakes sources can be used to estimate the hazard for potentially vulnerable buildings and other civil infrastructure