Sourcemap: a graphical representation to enhance the low frequency source radiation

Abstract

SUMMARY Data from dense seismic arrays allow nowadays to map directly the radiation of the seismic source in space. However, the main features observed on these maps are usually linked to the propagation, its attenuation and some site effects. Only few parameters like the magnitude or fault extension may be inferred through the shape and the size of the epicentral area. In this work, we propose a new strategy able to produce fast qualitative maps, ‘Sourcemaps’, that highlight directly some basic characteristics of the seismic source. They are based on a parameter which is robust and fast to compute with a minimal processing: the displacement response spectrum (DRS) computed at low frequency. In order to highlight the source effects, we correct the DRS by the geometrical spreading. We have applied this strategy to some earthquakes occurred in Japan in the last 25 yr using K-NET/KiK-net data. In these examples, we show that the source parameters such as the focal mechanism (nodal planes) and the directivity may be clearly seen on the Sourcemaps. Some site effects at large scale are still visible. The strategy proposed in this work is completely automatic without the need of a visual inspection of the input data. Thus it provides a robust assessment of key parameters of the source. We suggest a systematic computation of these ‘Sourcemaps’ after each event inside a dense network.