Time lag in reporting does not cause anything to be missed. The data just piles up until it gets processed. it may be that there is insufficient reporting from your area to locate everything, or perhaps they can't get a good azimuthal distribution on the epicenters there.

trend? what kind of trends are you thinking of?

The 30 year window comes from the nature of the data. You do a recurrence analysis and find quakes happen every 100 years +- 15 years on average, so you use a 30 year window.

Roger