This is Shostakovich’s string quartet no. 7 in F# minor, which I heard in a live performance by the Linden Tay quartet.
It sets a very tense mood right at the start of the first Allegretto movement, full of suspense. This is driven by a repeated three note motif which is heard on all the instruments. The movement ends unexpectedly on a major chord.
The Lento movement is more subdued and even more eerie – it’s fairly chromatic and uses some effective glissandos at the start. The melody is passed around the quartet, with the remaining instruments playing a quiet accompanying line. Then there are some sudden loud, fast passages which sound very scratchy – probably achieved by the bow being pressed hard onto the string. The instruments play the same line slightly out of phase, which sounds as though they are chasing each other.
The Allegro movement sounds quite hesitant at the start – a quiet solo line with just occasional accompanying chords. Then we hear a reference to the opening motif from the first movement, followed by a pizzicato section which is quiet but with some quite strong accents. It ends like the first movement with a major chord, which I found an interesting choice but I quite like the contrast from the overall mood of the piece.