EMPIRICAL PATH AND STATION CORRECTIONS FOR SURFACE-WAVE MAGNITUDE, M S, USING A GLOBAL NETWORK

Abstract

One of the most commonly used methods of estimating the size of shallow earthquakes is the surface-wave magnitude scale, $M s $. It has been known since the inception of the scale that the $M s $ of a seismic disturbance recorded at different stations can vary due to path propagation and station terms, but it has been difficult to quantify these variations on a global basis because, until recently, there has been no attempt to record $M s $ in a uniform way on a worldwide network. Here we use $M s $ measurements within the Reviewed Event Bulletin (REB) of the International Data Centre (IDC) which is produced to help monitor compliance with the Comprehensive Nuclear Test Ban Treaty (CTBT). The IDC collects waveforms from the as yet incomplete seismological network of the International Monitoring System (IMS) and $M s $ is then measured using an automatic algorithm, providing a unique set of surface-wave observations. In this paper we show that the $M s $ values reported in the REB show distinct geographic variations. We present preliminary attempts to model these observations in terms of a set of station corrections and a 2-D model which can be used to predict path corrections for $M s $. The model shows a striking correlation with the known tectonic regions of the Earth, suggesting that this type of data set may provide a valuable tool for investigating shallow Earth structure. In this paper we show that the $M s $ values reported in the REB show distinct geographic variations. We present preliminary attempts to model these observations in terms of a set of station corrections and a 2-D model which can be used to predict path corrections for $M s $. The model shows a striking correlation with the known tectonic regions of the Earth, suggesting that this type of data set may provide a valuable tool for investigating shallow Earth structure. After modelling, the residuals show some systematic patterns which cannot be explained using the model basis we have used. These residuals are presumably due either to source radiation patterns or to complicated propagation effects not allowed for in the model, such as refraction at ocean-continent boundaries. From the CTBT monitoring viewpoint, $M s $ station and path corrections are valuable because they should improve the effectiveness of event screening using $m b : M s $. However, we recommend that implementation of such corrections should wait until the full seismological network of the IMS is in place, and until a fuller understanding of path effects on $M s $ is available. In particular, it is vital that path propagation effects do not distort station corrections, and that earthquake radiation patterns should not influence corrections which may be applied to surface-waves from suspected explosions.