Estimation of Lateral Variations of the Mohorovičić Discontinuity Using 2D Modeling of Receiver Functions
Advisor Dr. Kim Bak Olsen
We have used 2D finite difference modeling to examine the efficacy of estimating a relatively abrupt offset (>~35o) in Moho depth with a slant-stacking technique (Zhu and Kanamori 2000) from radial 0-2Hz P-wave receiver functions (RFs) at a single or a few closely-spaced stations. In particular, we have examined the advantage of considering separate RF stacks (including realistic levels of noise) from opposite directions along a line intersecting a Moho offset. If the Moho depth is estimated using the slant-stacking technique with RFs at station X for events with back azimuth baz1 to be d1 km and for events from baz1+180o to be d2 km, the observations are consistent with station X being located within a ~15 km-wide zone across which the Moho depth changes from d1 (toward baz1) to d2 (toward baz1+180o). If, in addition, the Moho depth is estimated with RFs for events from both baz1 and baz1+180o combined to be d12, the observations are consistent with station X being located above Moho depths between d1 and [d1+d2]/2 if d12~d1, and above Moho depths between [d1+d2]/2 and d2 if d12~d2. Other indications of an abrupt Moho depth change, particularly the case for events incident from the direction where the crust is shallower, includes a split Ps arrival, with delay times to the P-wave corresponding to the two different thicknesses of Moho, resulting in decreased P/Ps amplitude ratios.
In an attempt to validate the patterns obtained from our 2D models we have analyzed observed RFs from two different geographical areas. The first area of validation is located in southern Norway, where a 5+ km Moho offset about 100 km east of the city of Bergen has been proposed from a previous study of RFs (Svenningsen et al. 2007). The available radial RFs with back azimuths along the survey profile at 3 stations show characteristics similar to those for a Moho offset (thicker crust toward WSW), including a thinner crust estimated for events incident from ENE as compared to those from WSW. This result is in general agreement with Svenningsen et al., plus an additional constraint of the Moho step to a 25 km stretch between two of stations and a slightly larger offset of up to 10 km. The second test is carried out for a station located on the western border of the Caspian Sea in Azerbaijan. For this station, our 2D model with a ~10 km step from a shallower Moho to the east and deeper Moho to the west generates, in general, P-Ps delay times, Ps/P amplitude ratios, and characteristic shape of the Ps pulse in agreement with the observed RFs for events incident along an E-W profile. This model is in agreement with estimates of crustal thickness estimated from seismic data (Gök et al. 2011).
Thus, our study proposes, with some support from data, a list of characteristics in radial P-wave RFs that may indicate an underlying Moho offset. Future studies should attempt to further validate the proposed indicators of abrupt Moho topography.