A recent extensive seismic wide-angle experiment revealed a new image of crustal and uppermantle structure across Northern Honshu Arc, Japan. The western part of the arc recorded the crustal deformation by the Miocene back arc spreading of the Sea of Japan. The crust is composed of highly deformed Tertiary sedimentary layers, a relatively low velocity (5.75--5.9 km/s) crystalline basement and a 15-km thick lower crust with a velocity of 6.6--7.0 km/s. Clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading. The crust attains the maximum thickness (32--35 km) east of the backbone range for which the magmatic intrusion/underplating since 10--15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crust, probably remaining a stable block since the time of the backarc spreading. ¿ 2001 American Geophysical Union