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Geofísica internacional
versión On-line ISSN 2954-436Xversión impresa ISSN 0016-7169
Geofís. Intl vol.44 no.2 Ciudad de México abr./jun. 2005
Articles
Simulation of seismograms in a 2-D viscoelastic Earth by pseudospectral methods
1 Istituto Nazionale di Oceanografía e di Geofísica Sperimentale (OGS), Borgo Grotta Gigante 42c, Sgonico, Trieste, Italy, Fax: 0039 040 327521 Email: jcarcione@ogs.trieste.it
2 Norsk Hydro a.s., 0 & E Research Centre, N-5020 Bergen, Norway; Fax: +47 55 996920 Email: hans.b.helle@hydro.com
3 Departamento de Sismología, Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata Paseo del Bosque sin, B1900WA La Plata, Argentina, Fax: 0054 221 4236591 Email: milton@fcaglp.unlp.edu.ar
Using an improved global pseudospectral modeling algorithm we synthesize seismograms generated by oceanic and continental earthquakes. Attention is given to attenuation, to explicit modeling of boundary conditions at the ocean-bottom interface, simulation of the Rayleigh window and interface-wave propagation. The algorithm is based on Fourier and Chebyshev differential operators and a domain-decomposition technique - one grid for the fluid and another grid for the solid. Wave propagation in the oceanic and continent crusts and mantle is modeled by using a viscoelastic stress-strain relation based on memory variables.
The main physical phenomena associated with an ocean-crust system are modeled, including Scholte waves, leaking Rayleigh waves, dispersive modes, and the Rayleigh-window phenomenon due to a minimum in the reflection coefficient of the ocean bottom, which has not been simulated with direct methods. In particular, we model Rayleigh modes (mainly the M 11 mode), and coupled Rayleigh-Scholte waves, for which the dispersion relation is solved in simple cases. Also, we model the effects of random inhomogeneities in the crust and mantle by using a von Kármán autocovariance probability function, which simulates scattering-Q-effects.
The 2-D modeling code allows general material variability, and a complete and accurate characterization of the seismic response of oceanic and continental earthquakes. A synthetic seismogram for an earthquake in the South Atlantic region is provided.
KEY WORDS: Seismogram; earthquake; attenuation; modeling; Rayleigh waves; Scholte waves; Rayleigh window
El algoritmo de modulación seudoespectral es mejorado y aplicado a la simulación de sismogramas generados por sismos oceánicos y continentales, como atención a la atenuación y a la modelación explícita de condiciones a la frontera en el fondo oceánico y a la simulación de la ventana de Rayleigh y la propagación en interfases. El algoritmo se basa en los operadores diferenciales de Fourier y de Chebyshev con una técnica de decomposición de dominios, una malla para el fluido y otra para el sólido. Para la propagación se usa una relación de esfuerzo-deformación basada en variables de memoria. Entre los fenómenos modelados se incluyen las ondas de Scholte, las ondas evanescentes de Rayleigh y los modos dispersivos, así como la ventana de Rayleigh, un mínimo del coeficiente de reflexión en el fondo oceánico que nunca ha sido simulado con métodos directos. Hemos modelado los modos de Rayleigh (principalmente M 11) y las ondas acopladas Rayleigh-Scholte, resolviendo la relación de dispersión para casos simples. Se modeló también efectos de inhomogeneidades aleatorias en la corteza y manto mediante una función de autocovarianza tipo von Kármán que simula los efectos de dispersión de ondas.
El programa bidimensional permite una variación material general y una caracterización completa y exacta de la respuesta para sismos oceánicos y continentales. Se desarrolla un ejemplo para un sismograma originado en la región del Atlántico Sur.
PALABRAS CLAVE: Sismograma; sismo; atenuación; modelado; ondas de Rayleigh; ondas de Scholte; ventana de Rayleigh
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Received: June 06, 2004; Accepted: November 15, 2004
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