[6] The procedure used for the construction and analysis of the receiver functions from P wave records has been repeatedly described in literature. In this work, we follow mainly the scheme described in [Kind et al., 1995]. The processing of P wave seismograms was performed with the use of the Seismic Handler software package [Stammler, 1993] and included the following operations.
[7] (1) The observed vertical ( Z ) and two horizontal (NS and EW) components of a P wave were rotated to the ray coordinate system ( L, Q, and T ). The L axis lies in the ray plane and is directed from the source along the main motion in the P wave. The Q axis lies in the same plane and is perpendicular to the L axis. The horizontal projection of the Q axis is positive in the direction from the source. Together with the L and Q axes, the T axis forms a right-hand orthogonal triple. Displacements along the Q axis depend very weakly on the main motion in the incident P wave, determined mostly by the earthquake source, and are mainly composed of multiple and converted waves arising beneath the station.
[8] (2) For applying the stacking procedure to a signal in order to increase signal-to-noise ratio, records of P waves from different earthquakes must be reduced to a standard source of the impulsive type. To do this, the L component of the signal, considered in this case as a source, is transformed into the standard form with the help of a deconvolution filter. The same filter is then applied to the components Q and T. It is the component Q that is called the receiver function. After this transformation, the noise can be suppressed by stacking Q components of all (or some) events, which yields the averaged receiver function Q obs(t).
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Figure 2 |
[10] The inferred receiver functions display two main peaks at delay times of 0.27 s and 5.1 s. These peaks were initially identified as P to S conversions at an upper crust boundary and at the Moho. Particle motion analysis showed that oscillations at delay times longer than 10 s are associated with arrivals of scattered surface waves having a characteristic elliptic polarization rather than with multiple reflections of body waves. For this reason, we chose the time interval ( - 2 s, 8 s) for the receiver function used below for reconstructing the velocity structure of the medium.
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