I like that name, "the sunken market"!
下沉: sink, PP sunken
translation from me
The so-called sunken market,
usually refers to the consumer market of third tier cities, county towns and rural areas,
which (this market) covers a great number of people whose consumption scope is also large,
the amount of retail sales of consumer goods, the scope of internet sales, the volume of e-payment, are all increasing rapidly,
its importance and effect on China's market index grows daily.
To take new energy vehicles as an example,
in 2021 purchases of new energy vehicles became a highlight and a new area of growth,
the sales volume increased year on year by 1.6 times, among which,
the sunken market contributed approximately 47% of the total sales volume
to become the market segment with fastest growth rate and greatest potential.
Can't find an author for the phrase 市场下沉, seems it just comes from the internet.
This phrase comes from the internet,
the meaning of market percolation is: the amplification and deep penetration of the target market ,
(or) in layman's terms,
suppose your original marketing channels barely cover the top tier white-collar internet users group,
after percolating down the market,
your (market) coverage has increased to include the youth of fourth and fifth tier towns.
(if you) continue percolating down,
your client group will increase until it (even) covers the dancers in the local square. (i.e. everyone)
This signifies your target market will be much larger.
So, I would translate 下沉市场 to English as "percolation marketing"