  We study the clustering properties of fast Coronal Mass Ejections (CMEs) that
occurred during solar cycles 23 and 24. We apply two methods: the Max spectrum
method can detect the predominant clusters and the de-clustering threshold time
method provides details on the typical clustering properties and time scales.
Our analysis shows that during the different phases of solar cycles 23 and 24,
CMEs with speed $\geq 1000\ km/s$ preferentially occur as isolated events and
in clusters with on average two members. However, clusters with more members
appear particularly during the maximum phases of the solar cycles. Over the
total period and in the maximum phases of solar cycles 23 and 24, about 50% are
isolated events, 18% (12%) occur in clusters with 2 (3) members, and another
20% in larger clusters $\geq 4$, whereas in solar minimum fast CMEs tend to
occur more frequently as isolated events (62%). During different solar cycle
phases, the typical de-clustering time scales of fast CMEs are $\tau_c=28-32\
hrs$, irrespective of the very different occurrence frequencies of CMEs during
solar minimum and maximum. These findings suggest that $\tau_c$ for extreme
events may reflect the characteristic energy build-up time for large flare and
CME-prolific active ARs. Associating statistically the clustering properties of
fast CMEs with the Disturbance storm index Dst at Earth suggests that fast CMEs
occuring in clusters tend to produce larger geomagnetic storms than isolated
fast CMEs. This may be related to CME-CME interaction producing a more complex
and stronger interaction with the Earth magnetosphere.
