Towards the identification of later seismic phases

Abstract

SUMMARY The location of seismic events can be improved if accurate picks can be assigned for later seismic phases, which requires both the detection of an arrival and the recognition of its character. Such phase identifications are particularly valuable if they can be provided in real-time as the seismic disturbance passes across a broad-band seismic recording station. A simple but promising scheme for characterizing arrivals can be constructed by analysing the energy content of the seismic trace as a function of time. Such an approach can be used to detect arrivals by using a method comparing the short-term average energy to a long-term average, with averaging windows that are adaptive to the local frequency of the seismic disturbance. The phase detector can be tuned to different classes of arrivals by utilizing three-component records. By comparing the energy on the vertical component of motion to that in the horizontal plane, it is possible to start to separate P and S arrivals. Phase assignments can be refined by the use of adaptive filtering and by including polarization information. With an estimate of the azimuth of propagation it is possible to use approximate projection methods which attempt to compensate for the influence of the free surface, since the surface corrections are not a strong function of slowness for teleseismic arrivals. By this means, an instantaneous estimate can be made of the relative contributions of P, SVand SH arrivals which can be very helpful in determining the phase assignment for a particular arrival.