The IRIS DMC archives time series (waveform) data from stations around the world.
Active vs Passive Source
Passive Source data is collected continuously, and usually monitors natural seismic activity. Often, known events are used to identify segments of data to fetch.
Active Source data usually involves the creation of seismic events using explosives or mechanical devices like air guns. Discrete data sets may be collected and organized around these events.
See: MDA Networks
Assembled data primarily comes from temporary deployments and in formats other than SEED. Data comes from various sources including IRIS PASSCAL (mostly active source), the USGS, and others.
See: MDA Assembled