Comparison of two "pseudo-bending" raytracers

Fast and accurate ray tracing in heterogeneous media is necessary in many s eismological analyses. In this paper, we present some comparative results o btained from applying two different pseudo-bending raytracers to an actual 3-D velocity model of north-western Italy. This area is characterized by st rong lateral velocity variations and major anomalies, related to the so-cal led Ivrea Body, lead to velocity perturbations greater than 10%. The ray tr acing algorithm implemented in the Simulps12 Code (Thurber [J. Geophys. Res . 88 (1983) 8226]; and Um and Thurber [Bull. Seismol. Sec. Am. 77 (1987) 97 2]) and a ray tracing scheme based on that of Prothero et al. [Bull. Seismo l. Sec. Am. 78 (1988) 1190] were compared, adopting the same velocity inter polation scheme and the same integration parameters. The data set was compo sed of more than 8700 P-phases and 7500 S-phases and the performances of th e two methods were evaluated in terms of travel time difference and ray pat h geometry. The results showed that the scheme based on that of Prothero et at. [Bull. Seismol. Sec. Am. 78 (1988) 1190] nearly always produced smalle r travel time values. In particular, the travel time differences were great er than 0.05 s for 33.2% of the whole S-phases data set whereas, in the cas e of P-phases, these differences corresponded to 18.9%. Considering rays re levant to hypocenters further than 75 km, we obtained travel time differenc es greater than 0.05 s for 75.9% of S-phases and 47.1% of P-phases. Furthermore, travel time differences appeared strongly related to the distr ibution of velocity heterogeneities. Indeed, for all the considered station s, the most significant travel time differences tin some cases greater than 0.15 s) were found to correspond to the azimuthal sector embracing the Ivr ea Body. In the case of incomplete azimuthal coverage, these differences ha d a significant influence on the location procedure. Moreover, the path dif ferences can induce a notable bias in the tomographic procedure. The result s of this study had to be taken into account in its companion paper on the reliability of earthquake location procedures (Spallarossa et al. [Phys. Ea rth Plant. Interiors (2000)]). (C) 2001 Elsevier Science B.V. All rights re served.