2 Seismic Wave Propagation and Earth models

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2. Seismic Wave Propagation and Earth models Fig. 2.15 Records of Lg, together with other crustal phases, in records of a Kola peninsula mining blast (Ml = 2.4) at the Norwegian array stations ARCES (distance D = 391 km; upper two traces) and NORES (D = 1309 km, bottom traces). cb and sb – P- and S-wave beams (see Chapter 9) of the vertical elements of the array, filtered with 2-8 Hz and 1-4 Hz, respectively (courtesy of J. Schweitzer). Fig. 2.16 Mining induced rock burst south of Saarbrücken, Germany, recorded at station WLF in Luxemburg (D = 80 km, h = 1 km, Ml = 3.7). Note the strong dispersive Rg phase. 18
2.3.4 Mantle surface waves 19 2.3 Surface waves Love and Rayleigh waves travel along great circle paths around the globe. Surface waves from strong earthquakes may travel several times around the Earth. They are termed global surface waves. The first surface wave group arriving at a seismic station at the epicentral distance Δ° will have taken the shorter great circle while the later arrival has traveled the major arc path over 360° - Δ° (Fig. 2.17). Fig. 2.17 Great circle paths for the first three arrivals of global Rayleigh waves. These arrival groups are called R1, R2, R3, R4 etc. for Rayleigh waves and G1, G2, G3, G4 etc. for Love waves, respectively. R3 (or G3) have traveled over 360° + Δ° and R4 over 720°- Δ° etc. Fig. 2.18 gives an example for long-period records of P, SV, SH, R1, R2, G1 and G2 in the vertical (Z) and the two rotated horizontal components (radial R and transverse T). As expected, P appears only on Z and R while S has both SV and SH energy. The Love wave groups G1 and G2 are strongest in T and arrive ahead of R1 in R2, which are visible only on the R and Z components. But Fig. 2.18 is also a good example for inverse (negative) dispersion in the Rayleigh-wave groups. Their periods range from about 60 s to almost 200 s, with the longest periods arriving at the end of the R1 and R2 groups. This is just the period range of inverse dispersion according to Fig. 2.9 for both continental and oceanic mantle Rayleigh waves. This inverse dispersion is not seen in records of classical WWSSN longperiod seismographs with a peak magnification around 15 s because the corresponding periods are filtered out by the system response of these seismographs. Further, one should note in Fig. 2.18 that these surface waves originate from an earthquake in the Tonga trench subduction zone at a depth of h = 230 km. This seems to contradict the above statement, that no or only weak surface waves can be observed from deep earthquakes. However, there is no contradiction. As discussed above, the depth of penetration (and thus constructive interference) of surface waves increases with their wavelength. For the periods considered in Fig. 2.18 Λ ranges between about 230 and 880 km, i.e., it is comparable or larger than the source depth. Therefore, we still can expect significant surface wave energy in that period range for the largest amplitudes in Fig. 2.18. However, no periods below 50 s, as recorded in classical narrow-band long-period records, are recognizable in these surface-wave trains.
Page 1 and 2: CHAPTER 2 Seismic Wave Propagation
Page 3 and 4: 3 2.2 Elastic moduli and body waves
Page 11 and 12: 11 2.3 Surface waves S waves are al
Page 13 and 14: 2.3.2 Dispersion and polarization 1
Page 15 and 16: 15 2.3 Surface waves Strictly speak
Page 17: 17 2.3 Surface waves (Z-E) plane, w
Page 21 and 22: 2.4 Normal modes 21 2.4 Normal mode
Page 23 and 24: 23 2.4 Normal modes First observati
Page 25 and 26: 2.5 Seismic rays, travel times, amp
Page 43 and 44: 2.6 Seismic phases and travel times
Page 53 and 54: Overlay to Fig. 2.47 2.6 Seismic ph
Page 57 and 58: 57 2.7 Global Earth models on verti
Page 59 and 60: 59 2.7 Global Earth models As shown
Page 61 and 62: 61 2.7 Global Earth models The mode
Page 63 and 64: 2.8 Synthetic seismograms and wavef
Page 69 and 70:
2.8 Synthetic seismograms and wavef
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2 Seismic Wave Propagation and Earth models

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