### Analysis of the Receiver Function Obtained from RUKSA Records

[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).

 Figure 2
[9]  Figure 2 presents Q components of all processed waveforms from 12 events (see the Table 1). The microseismic noise level can be estimated in the time interval from - 5 s to - 1 s before a first arrival (the time t = 0). Differences between records of the stations C1, C3, and C5 are small, except for systematically higher amplitudes at the station C5 as compared to C1 and C3 in the interval from 0 s to 1 s. Waves that experienced conversion, refraction, and scattering in the upper few kilometers under the station are observed precisely in this time interval. The higher C5 amplitudes are well seen on the average C5 trace in Figure 2. The signal-to-noise ratio on average and even some individual traces is large enough to enable further analysis and formulate the inversion problem for the velocity structure determination.

[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.