Seismic waveform modeling of boundary interaction phases is used to determine the discontinuity structure of the crust in the Subandean and foreland-basin regions overlying the zone of flat subduction beneath east-central Peru. The data analyzed are from intermediate-depth earthquakes (110 to 155 km) recorded on an array of three-component short-period (1 Hz) digital seismographs deployed in the epicentral region. Full use is made of both P-to-S and S-to-P converted phases in the modeling. Results from the determination of crustal structure in the Subandean and foreland basin region of east-central Peru confirm the presence of vast depositional basins comprised of low velocity sediments up to at least 8 km thick which flank the Andean orogen to the east and correlate with a substantially thickened crust atop the Brazilian shield. Crustal thickness in the foreland basin varies from about 35 km or less where sedimentary cover is minimal to 44 km in regions of maximum sedimentary deposition. There is some evidence that the crust thins slightly on the western side of the foreland basin (in the gap between basin and Subandean fold and thrust belt), but it rapidly thickens to 45--50 km beneath the Subandes proper, and to more than 50 km in the southern part of the Subandean belt. The results are consistent with, but do not require, a thick-skinned model of foreland crustal deformation similar to that found for the block faulted terrane in Argentina above the zone of flat subduction there. At least some basin formation appears to be due to block faulting, where faults may penetrate into the mantle.