Relative motion; absolute motion কী?

Example 13.7 (The San Andreas fault)

In central California, at $35.5{}^{\circ}$ North and $120.5{}^{\circ}$ West, the rotation poles give the Pacific plate the velocity $(-42.1, +26.4)$ and the North American plate $(-11.9, -14.8)$, in millimetres a year (east, north), in the frame of the whole lithosphere. The Pacific plate relative to North America moves at $(-42.1 + 11.9,\ 26.4 + 14.8) = (-30.2, +41.2)$, that is $\sqrt{30.2^2 + 41.2^2} \approx 51\,\mathrm{mm}$ a year, in a direction $36{}^{\circ}$ west of north (figure below). The San Andreas fault, which runs north-west, carries most of this sliding.