I've been learning about seismic discontinuities, mainly the 5 defined ones - Conorod discontinuity, Mohorovicic discontinuity, Repetti discontinuity, Gutenberg discontinuity, and Lehmann discontinuity. But I cannot find a discontinuity along the lithosphere-asthenosphere boundary. As far as I know, the asthenosphere is 1% molten rock, which means seismic waves travel relatively slower through it. Then why isn't there a discontinuity between the lithosphere and asthenosphere?
Short answer: There are actually two different Gutenberg discontinuities, and one of them corresponds to one way of defining the Lithosphere-asthenosphere boundary, or LAB.
(The abstract of this article in Science refers to the LAB version of the Gutenberg discontinuity as G.)
The boundary between the lithosphere and asthenosphere can be defined in several ways: mechanically, thermally, rheologically, compositionally, and finally seismically. All are approximations for something humans will never be able to access directly.
Wikipedia has this to say about the seismic definition:
The seismic LAB (i.e. measured using seismological observations) is defined by the observation that there exists seismically fast lithosphere (or a lithospheric lid) above a low-velocity zone (LVZ).
The seismic LVZ was first recognized by Beno Gutenberg, whose name is sometimes used to refer to the base of the seismic LAB beneath oceanic lithosphere. The Gutenberg discontinuity coincides with the expected LAB depth in many studies and has also been found to become deeper under older crust, thus supporting the suggestion that the discontinuity is closely interrelated to the LAB.