Epicentral Seismogram Filters

Modern seismometers sense the ground motions across a relatively broad frequency range. The particular range that you use depends on what kind of earthquakes that you are trying to detect. The signals from a large earthquake half-way around the world (which we call teleseismic) are generally best observed in a low-frequency band. Those from small, nearby earthquakes are best observed in the higher frequency ranges. Large nearby earthquakes can be detected in either of these bands.

Distant (Teleseismic) Signals

For signals from distant earthquakes, try a range from 0.005 Hz to about 0.05 Hz (200 seconds period to 20 seconds period). When you are focusing on this bandwidth, you should use the long-period channels such as LHZ. These are sampled every one second and are derived from the BHZ channels. With the low sample rate, you can stream signals with a duration of about 1 to 2 hrs (60-120 minutes). For specific earthquake requests, 60-90 minutes usually shows all the interesting seismic waves.

Local Signals

If your interest is in small, nearby earthquakes, you should try a range from 0.5 Hz to about 4 Hz. To see this frequency range, you should focus on the broad-band channels such as BHZ. BH channels are sampled 20-40 times per second, so don't ask for long seismograms when watching these signals; 30 minutes should be ok. If you want to see any details in the signal, you will need less time. If you are requesting seismograms for a specific earthquake, you probably only need a few minutes after the earthquakes.