The effect of crustal scattering on observed high-frequency earthquake seismograms

A comparison of high-frequency (1–100 Hz) local (out to 40 km) earthquake seismograms at stations of the Southern Black Forest Digital Seismic Network is made in an attempt to isolate the effects of near-receiver structure and of crustal scattering on the waveforms of the direct body-waves and on the coda waves. Angle of incidence and azimuth of approach analyses from lower-crustal earthquakes show that near-surface weathering layers with low body-wave velocities and with laterally varying thicknesses exist under all of the stations, even those at hard rock sites of the Black Forest. The similarity of direct P-wave pulse shapes at each site for events of different sizes, the time-domain form of the so-called fmax phenomenon, is attributed to the response from this weathering zone. The amplitude of the P-wave coda of an event is found to be independent of the earthquake radiation pattern and to depend only on the absolute size of the event and on epicentral distance to the receiver. Furthermore, the high-frequency P-wave coda is found to be generated, at least in part, from random crustal scattering, including that within 100 m of the earthquake focus. Direct S waveforms are shown to be more complicated than their P-wave counterparts, probably due to the same near-receiver and crustal scattering phenomena. However, in the case of S waves scattered energy can arrive before as well as at and after the direct S arrival, making the identification of the direct S difficult, especially for nodal arrivals.