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

Abstract

S U M M A R YA 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 f,, 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 100m 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.

A New Method for Determining Small Earthquake Source Parameters Using Short-Period P Waves

“…6) suggest that they are mainly controlled by the nearreceiver structure. The near-source scatters, on the contrary, would produce energy independent of the earthquake source mechanism, hence cause the AAFs to be mechanism dependent (e.g., Ebel, 1989). That is to say, the near-source scattering is not strong enough to obscure the 0.5-2 Hz P-wave radiation pattern.…”

“…We focus on the direct P arrivals, because the S waves are apparently more complicated due to P-to-SV or SV-to-P conversions (Ebel, 1989). Moreover, rather than fitting amplitude measurements, we intend to match the P waveforms.…”

“…Several studies have emphasized the importance of velocity models in using Pand/or S-wave amplitude data for source mechanism determination (e.g., Saikia and Herrmann, 1985;Ebel, 1989;Ebel and Bonjer, 1990;Sileny et al, 1992). In particular, the details of the near-receiver structure can easily disturb the observed amplitudes.…”

Stress Rotation in the Kachchh Rift Zone, Gujarat, India

“…A local earthquake moment tensor least-squares inversion scheme (EBEL, 1989) is also used to compute the focal mechanisms for some recent BWF and GF events by inverting amplitudes and polarities of 12 direct P's (from vertical) and 8 SH's (from The focal mechanism solutions for the 7th March event, which occurred along the GEDI fault, suggest a pure strike-slip fault (strike 88°, dip 78°, rake 178°), whereas, the focal mechanisms for the 17th February, 19th February and 6th April 2006 events, which occurred along the NWF, suggest a reverse fault with a minor strike-slip component along a preferred south-dipping fault (Fig. 7).…”

P-Coda Amplitude As a Measure of Earthquake Magnitude of Local Microearthquake.

“…The amplitude of P-coda seems to be independent of the direction of focal radiation. Ebel (1989) suggested that vertical P-coda amplitudes appeared to be approximately independent of the earthquake radiation pattern. However, Ebel (1989) used seismograms recorded at only 5 stations in his analysis.…”