Better characterization of gas-induced earth tremors

The seismological department of the KNMI is responsible for the monitoring of the earth tremors occurring in the North of the Netherlands in and around gas-producing reservoirs. Therefore the KNMI has a sparse network of vertical arrays of geophones around these different reservoirs. In Figure 1, an overview of seismicity in the north of the Netherlands is given. Although the horizontal locations of the quakes are pretty well known, uncertainties in depth are larger. This makes it difficult to connect the earthquake to geological structures and gain insight in the source process. Still, from the epicenters it seems that the quakes are related to existing faults. It is expected that the dense network of LOFAR will contribute significantly to the understanding of these tremors.

Figure 2. Seismicity in North of the Netherlands. Left: Epicentra of earthquakes. Right: uncertainties of earthquake locations. Note: vertical exaggerated (source: KNMI).

A key question is whether the faults as observed in the first few kilometers have a relationship to those in the deeper subsurface. This would particularly be important for the earth tremors seemingly happening along existing faults. In cases where the LOFAR nodes are close to an active fault, LOFAR is expected to contribute significantly. The density of stations in the center of the network is such that the geophones allow improved detection and real-time determination of the direction from which the energy reaches the station. Another issue arising with the characterization of the earthquakes is its strength, especially how it is measured at the station at or near the surface. It is known that the shallow subsurface amplifies the signal from the earthquakes. A key question here is:

• Why and how does the shallow subsurface (< 100m) amplify the signal

These so-called site effects can be studied through the determination of the local shallow shear-wave structure and the difference between the horizontal and vertical amplitude (H/V) spectra. Data from borehole installation at the LOFAR sites will be used to calibrate the site-effect model. This contributes to the understanding of the regional variability of site-specific amplification of seismic signals.