We estimate the age for the individual stars located at the lower part of the red giant branch from the LAMOST DR2 K giant sample .
Taking into account the selection effects and the volume completeness , the age–metallicity map for the stars located between 0.3 and 1.5 kpc from the Sun is obtained .
A significant substructure ( denoted as the narrow stripe ) located from ( age , [ Fe/H ] ) \sim ( 5 , 0.4 ) to ( 10 Gyr , -0.4 dex ) in the age–metallicity map is clearly identified .
Moreover , the narrow stripe  stars are found the dominate contributors to several velocity substructures , including the well-known Hercules stream .
The substantially large difference between the observed guiding-center radii and the birth radii inferred from the age–metallicity relation is evident that the narrow stripe  stars have been radially migrated from about \R \sim 4 kpc to the solar neighborhood .
This implies that the Hercules stream may not be owe to the resonance associated with the bar , but may be the kinematic imprint of the inner disk and later moved out due to radial migration .
We estimate that the traveling speed of the radial migration are roughly 1.1 \pm 0.1 kpc Gyr ^ { -1 } , equivalent with about 1.1 \pm 0.1 km s ^ { -1 } .
This is in agreement with the median v _ { R } Â of 2.6 ^ { +1.8 } _ { -1.9 } km s ^ { -1 } of the narrow stripe .
We also obtain that about one third stars in the solar neighborhood are radially migrated from around 4 kpc .
Finally , we find that the radial migration does not lead to additional disk thickening according to the distribution of z _ { max } .