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Influence of Model Perturbation on Regional Ground Motions – a Numerical Experiment

Description: 

We are examining the attenuation characteristics for high-frequency regional Pn, Pg, Sn, Sg and Lg waves using numerical seismograms for generic 1D/2D/3D velocity models that extend into the mantle by several tens of kilometers. In this study, we use 1D FK/reflectivity, SPECFEM2D and SW4 codes to compare synthetic explosion seismograms at receivers every 50 km interval over a range of 100 to 1400 km. 1D FK/Reflectivity seismograms are used to establish the reliability of the upper limit of the frequency below which all regional seismograms computed using SW4 and SPECFEM2D are consistent in amplitude and the phase complexity. To the 3D and 2D models, we introduced velocity perturbation in the shallow crust, mid-crust and crust-mantle boundaries separately to generate extended high-frequency coda waves, thus enabling us to understand the various velocity perturbation effects on Pn, Pg, Sn and Sg and the Lg waves. We examined fine laminated structures in 1D model and in the same depth ranges to simulate high-frequency regional coda waves. We used these seismograms to validate for a fast computational method to accomplish the same objective. The upper frequency limit is dependent on the model discretization which is controlled by the available computational resource. Scattering was introduced by perturbing the geophysical parameters in various parts of the 3D model using criteria described in Frankel and Clayton (1986). von-Karman correlation function was used to specify the extent of the scatterers (Goff and Jordan, 1988). This provided an insight on how the regional body and Lg waves are influenced by the velocity perturbation in random locations. The SW4 model was optimally designed for large distances ensuring that seismograms do not suffer from boundary reflections. SPECFEM2D was used to generate 2D seismograms for similar models, were corrected for the point-source seismograms and compared to the equivalent 1D synthetic explosion seismograms.

Session: Exploiting Explosion Sources: Advancements in Seismic Source Physics

Type: Oral

Date: 4/19/2023

Presentation Time: 05:30 PM (local time)

Presenting Author: Chandan K. Saikia

Student Presenter: No

Invited Presentation: 

Authors