Seismic tomography is a method to determine (to some level of accuracy) the internal structure (in terms of velocity) of the Earth. Local, regional and global tomography may differ in scale and approach but essentially refer to the same goal to image (part of) the interior of the Earth. The basic information for these seismic tomography studies comes from seismograms (e.g. arrival times, waveform data). Arrival time (or travel time) tomography is the technique to map the arrival time information in seismograms into a velocity model of the interior.

The North-east part of the Netherlands (bounded roughly by 52.4-53.6 N, 6-7.5 E) is characterized by low seismic activity. Since 1987 the KNMI detected and localized 349 earthquakes in this area, ranging in magnitude between -0.1 and 3.3 ML, which were recorded by 32 different station in this area (including different levels in the boreholes, BB and SP surface seismometers, and accelerometers), resulting in 2467 ray paths (Fig. 1). The region has a number of shallow (few km depth) gas fields, which may have a relation with the occurred earthquakes. Although the number of data points is small compared to other studies (less than 3000 ray paths vs. commonly tens of thousands or more) the KNMI has started a tomographic study of this area.

The area is of interest for a tomography study for several reasons: (1) the crustal thickness (Moho depth) is poorly determined in this area, (2) the velocity structure is poorly known, in particular between a few km depth and the Moho, (3) a relation between the (shallow) earthquakes and the gas fields is likely, but yet unknown for most of the earthquakes. Improvement of the velocity structure (1-D and 3-D) and the earthquake hypocenters may help to reveal details about geological structures and seismic active faults. This information, as well as other types of information like source mechanisms, may be useful for a better understanding of the relation between earthquakes and the extraction of gas in this area.