The study is devoted to a 2D problem for interface cracks under harmonic external loading. The system of boundary integral equations for displacements and tractions is derived from the dynamic Somigliana identity. The distributions of the displacements and tractions at the bonding interface and the surface of the cracks are analysed for the normally incident tension-compression wave. The dynamic stress intensity factors are also computed for different values of the frequency of the incident wave and the distance between cracks. The effect of distance between cracks on the solution of the problem is studied.