A vibration actuator for mechanically generating a shear wave, comprising: a housing; a plurality of n rotational vibrators coupled to the housing in a geometric arrangement characterized in that the plurality of n rotational vibrators is configured to enable generation of a vibration vector with desired directional behavior selected from a plurality of vibration vectors (34,36,38) of different directional behaviors, wherein n is a number of at least three, wherein each rotational vibrator (141,142,143) comprises an independently controllable motor having a drive shaft and an eccentric disk coupled to the drive shaft in a plane perpendicular to an axis of the drive shaft; wherein the vibration actuator further comprises: an accelerometer coupled to the housing, wherein the accelerometer is arranged to detect a vibration vector generated by at least two of the plurality of n rotational vibrators and generate an accelerometer output signal based on the detected vibration vector; and a controller operatively coupled to each rotational vibrator (141,142,143) for selectively controlling (i) a first set of two (141,142) of the plurality of n rotational vibrators to rotate respective eccentric disks in a first coordinated manner based at least on the accelerometer output signal to produce a first vibration vector, and (ii) a second set of two (142, 143) of the plurality of n rotational vibrators to rotate respective eccentric disks in a second coordinated manner based at least on the accelerometer output signal to produce a second vibration vector, wherein a directional behavior of the second vibration vector is different from a directional behavior of the first vibration vector. a housing; a plurality of n rotational vibrators coupled to the housing in a geometric arrangement characterized in that the plurality of n rotational vibrators is configured to enable generation of a vibration vector with desired directional behavior selected from a plurality of vibration vectors (34,36,38) of different directional behaviors, wherein n is a number of at least three, wherein each rotational vibrator (141,142,143) comprises an independently controllable motor having a drive shaft and an eccentric disk coupled to the drive shaft in a plane perpendicular to an axis of the drive shaft; wherein the vibration actuator further comprises: an accelerometer coupled to the housing, wherein the accelerometer is arranged to detect a vibration vector generated by at least two of the plurality of n rotational vibrators and generate an accelerometer output signal based on the detected vibration vector; and a controller operatively coupled to each rotational vibrator (141,142,143) for selectively controlling (i) a first set of two (141,142) of the plurality of n rotational vibrators to rotate respective eccentric disks in a first coordinated manner based at least on the accelerometer output signal to produce a first vibration vector, and (ii) a second set of two (142, 143) of the plurality of n rotational vibrators to rotate respective eccentric disks in a second coordinated manner based at least on the accelerometer output signal to produce a second vibration vector, wherein a directional behavior of the second vibration vector is different from a directional behavior of the first vibration vector. an accelerometer coupled to the housing, wherein the accelerometer is arranged to detect a vibration vector generated by at least two of the plurality of n rotational vibrators and generate an accelerometer output signal based on the detected vibration vector; and a controller operatively coupled to each rotational vibrator (141,142,143) for selectively controlling (i) a first set of two (141,142) of the plurality of n rotational vibrators to rotate respective eccentric disks in a first coordinated manner based at least on the accelerometer output signal to produce a first vibration vector, and (ii) a second set of two (142, 143) of the plurality of n rotational vibrators to rotate respective eccentric disks in a second coordinated manner based at least on the accelerometer output signal to produce a second vibration vector, wherein a directional behavior of the second vibration vector is different from a directional behavior of the first vibration vector.