We present measurements of the field-induced change of the microwave surface impedance in optimally doped Bi 2 Sr 2 CaCu 2 O 8-x crystals. Measurements are taken with microwave fields in the (a,b) planes. A moderate (< 0.7 T) dc magnetic field is applied normal to the (a,b) planes. Measurements are taken for temperatures above 70 K. At fixed temperature, with increasing field the surface resistance presents a sharp onset at low fields, a steep rise then a plateau or a peak, followed by a second, slower rise with the field. The surface reactance presents similarly a steep raise at low fields above the onset, then a sudden decrease. We discuss the applicability of commonly established vortex motion models to the observed response, and we find that substantial inconsistencies emerge. Possible influences of the layered nature on the vortex response are investigated.
