Seismic Attenuation

Seismic attenuation describes the energy loss experienced by seismic waves as they propagate. It is controlled by the temperature, composition, melt content, and volatile content of the rocks through which the waves travel. For these reasons, seismic attenuation has the potential to be a valuable source of information about the Earth’s interior, complementing seismic velocity and allowing more definite conclusions to be drawn. The challenge of imaging seismic attenuation in Earth’s mantle is that the wave amplitude requires a more complex interpretation than the wave phase does. In addition to intrinsic attenuation, amplitudes are affected by propagation through gradients in seismic velocity (focusing and scattering), excitation at the earthquake source, and amplification and instrument-calibration errors at the receiver. In order to isolate seismic attenuation, these other factors must be removed.

Here a few of our attenuation studies are described.

Seismic attenuation beneath USArray

We have determined the first continent-scale attenuation model for North America using Rayleigh wave data from the EarthScope USArray. We identify robust features in these maps that were not previously resolvable, including a transition from high attenuation in the west to low attenuation in the central U.S. that aligns with the Rocky Mountain Front, and elevated attenuation along the eastern seaboard. We also identify several areas where we suspect the imaged attenuation values reflect unmodelled focusing effects rather than attenuation. Our results are described in more detail in Bao et al. (GJI, 2016a).

Accounting for focusing effects

Focusing and defocusing effects are the primary obstacle to isolating the signal of attenuation in surface-wave amplitudes. Recent work in this area seeks to better understand and quantify how well and to what extent different theoretical approaches accurately represent focusing effects and how global models of seismic attenuation are impacted by the treatment of focusing effects. We have done this under two different yet complementary sets of conditions: a controlled experiment using synthetic seismograms (Dalton et al., GJI, 2014), and with observations from the real Earth (Bao et al., GJI, 2016b). The results show that while the large-scale attenuation features are robust with respect to treatment of focusing effects, the magnitude of the features is variable. We also find that the amplitudes are best fit when focusing effects are predicted using seismic ray theory at shorter periods and finite-frequency theory at longer periods. Earlier work on this problem showed how global models of seismic velocity could be obtained from seismic amplitudes alone, without any travel-time information, because of focusing effects (Dalton and Ekström, GJI, 2006).

Global seismic attenuation

We developed a global model of seismic attenuation that shows a strong anti-correlation with seismic velocity. See Dalton et al. (JGR, 2008) and Dalton and Ekström (JGR, 2006a).

Seismic Attenuation

Seismic attenuation describes the energy loss experienced by seismic waves as they propagate. It is controlled by the temperature, composition, melt content, and volatile content of the rocks through which the waves travel. For these reasons, seismic attenuation has the potential to be a valuable source of information about the Earth’s interior, complementing seismic velocity and allowing more definite conclusions to be drawn. The challenge of imaging seismic attenuation in Earth’s mantle is that the wave amplitude requires a more complex interpretation than the wave phase does. In addition to intrinsic attenuation, amplitudes are affected by propagation through gradients in seismic velocity (focusing and scattering), excitation at the earthquake source, and amplification and instrument-calibration errors at the receiver. In order to isolate seismic attenuation, these other factors must be removed.

Here a few of our attenuation studies are described.

Seismic attenuation beneath USArray

We have determined the first continent-scale attenuation model for North America using Rayleigh wave data from the EarthScope USArray, w. We identify robust features in these maps that were not previously resolvable, including a transition from high attenuation in the west to low attenuation in the central U.S. that aligns with the Rocky Mountain Front, and elevated attenuation along the eastern seaboard. We also identify several areas where we suspect the imaged attenuation values reflect unmodelled focusing effects rather than attenuation. Our results are described in more detail in Bao et al. (GJI, 2016a).

Accounting for focusing effects

Focusing and defocusing effects are the primary obstacle to isolating the signal of attenuation in surface-wave amplitudes. Recent work in this area seeks to better understand and quantify how well and to what extent different theoretical approaches accurately represent focusing effects and how global models of seismic attenuation are impacted by the treatment of focusing effects. We have done this under two different yet complementary sets of conditions: a controlled experiment using synthetic seismograms (Dalton et al., GJI, 2014), and with observations from the real Earth (Bao et al., GJI, 2016b). The results show that while the large-scale attenuation features are robust with respect to treatment of focusing effects, the magnitude of the features is variable. We also find that the amplitudes are best fit when focusing effects are predicted using seismic ray theory at shorter periods and finite-frequency theory at longer periods. Earlier work on this problem showed how global models of seismic velocity could be obtained from seismic amplitudes alone, without any travel-time information, because of focusing effects (Dalton and Ekström, JGR, 2006b).

Global seismic attenuation

We developed a global model of seismic attenuation that shows a strong anti-correlation with seismic velocity. See Dalton et al. (JGR, 2008) and Dalton and Ekström (JGR, 2006a).

Seismic Attenuation

Seismic attenuation describes the energy loss experienced by seismic waves as they propagate. It is controlled by the temperature, composition, melt content, and volatile content of the rocks through which the waves travel. For these reasons, seismic attenuation has the potential to be a valuable source of information about the Earth’s interior, complementing seismic velocity and allowing more definite conclusions to be drawn. The challenge of imaging seismic attenuation in Earth’s mantle is that the wave amplitude requires a more complex interpretation than the wave phase does. In addition to intrinsic attenuation, amplitudes are affected by propagation through gradients in seismic velocity (focusing and scattering), excitation at the earthquake source, and amplification and instrument-calibration errors at the receiver. In order to isolate seismic attenuation, these other factors must be removed.

Here a few of our attenuation studies are described.

Seismic attenuation beneath USArray

We have determined the first continent-scale attenuation model for North America using Rayleigh wave data from the EarthScope USArray, w. We identify robust features in these maps that were not previously resolvable, including a transition from high attenuation in the west to low attenuation in the central U.S. that aligns with the Rocky Mountain Front, and elevated attenuation along the eastern seaboard. We also identify several areas where we suspect the imaged attenuation values reflect unmodelled focusing effects rather than attenuation. Our results are described in more detail in Bao et al. (GJI, 2016a).

Accounting for focusing effects

Focusing and defocusing effects are the primary obstacle to isolating the signal of attenuation in surface-wave amplitudes. Recent work in this area seeks to better understand and quantify how well and to what extent different theoretical approaches accurately represent focusing effects and how global models of seismic attenuation are impacted by the treatment of focusing effects. We have done this under two different yet complementary sets of conditions: a controlled experiment using synthetic seismograms (Dalton et al., GJI, 2014), and with observations from the real Earth (Bao et al., GJI, 2016b). The results show that while the large-scale attenuation features are robust with respect to treatment of focusing effects, the magnitude of the features is variable. We also find that the amplitudes are best fit when focusing effects are predicted using seismic ray theory at shorter periods and finite-frequency theory at longer periods. Earlier work on this problem showed how global models of seismic velocity could be obtained from seismic amplitudes alone, without any travel-time information, because of focusing effects (Dalton and Ekström, JGR, 2006b).

Global seismic attenuation

We developed a global model of seismic attenuation that shows a strong anti-correlation with seismic velocity. See Dalton et al. (JGR, 2008) and Dalton and Ekström (JGR, 2006a).

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Seismic Attenuation

Here a few of our attenuation studies are described.

Seismic attenuation beneath USArray

We have determined the first continent-scale attenuation model for North America using Rayleigh wave data from the EarthScope USArray, w. We identify robust features in these maps that were not previously resolvable, including a transition from high attenuation in the west to low attenuation in the central U.S. that aligns with the Rocky Mountain Front, and elevated attenuation along the eastern seaboard. We also identify several areas where we suspect the imaged attenuation values reflect unmodelled focusing effects rather than attenuation. Our results are described in more detail in Bao et al. (GJI, 2016a).

Accounting for focusing effects

Focusing and defocusing effects are the primary obstacle to isolating the signal of attenuation in surface-wave amplitudes. Recent work in this area seeks to better understand and quantify how well and to what extent different theoretical approaches accurately represent focusing effects and how global models of seismic attenuation are impacted by the treatment of focusing effects. We have done this under two different yet complementary sets of conditions: a controlled experiment using synthetic seismograms (Dalton et al., GJI, 2014), and with observations from the real Earth (Bao et al., GJI, 2016b). The results show that while the large-scale attenuation features are robust with respect to treatment of focusing effects, the magnitude of the features is variable. We also find that the amplitudes are best fit when focusing effects are predicted using seismic ray theory at shorter periods and finite-frequency theory at longer periods. Earlier work on this problem showed how global models of seismic velocity could be obtained from seismic amplitudes alone, without any travel-time information, because of focusing effects (Dalton and Ekström, JGR, 2006b).

Global seismic attenuation