SCSN stations all consist of a seismic sensor and the datalogger (also known as a digitizer). The sensor measures the ground motions, which is then locally digitized, recorded and transmitted in real-time to Caltech, by the datalogger.  

The key function of a seismic network is to record as much ground motion on-scale and with as good resolution as possible. Signals generated by the earth vary hugely, with energy varying over 10 orders of magnitude from the local background noise of the site to the most violent earthquake ground motions. Motions also vary in frequency content from signals associated with oceans tides to high frequency impulsive signals from rock fractures. There are various different types of both sensor and datalogger available to the scientific community, which have different sensitivity and range. The choice of which seismic sensors to deploy depends on what spectrum of ground motion is required.  

In the past, sensors were wholly mechanical devices which recorded directly onto paper. This limited the frequency bandwidth, and the resolution of paper records was not more than 3 orders of magnitude. Seismic stations recorded only small bands of energy, and a particular design would only be capable of recording strong motions, local events or teleseimic waveforms, not all three.  

In the last 20 years, force-feedback mechanisms and improved digitization techniques mean that both sensors and digitizers now have very wide frequency bandwidth and each record over 7 orders of magnitude. This means that the entire region of interest, with frequency coverage between 20Hz to near DC, can be covered using 2 sensors, and a single datalogger. This is the configuration employed at most SCSN stations, as well as in many high quality seismic network stations around the world.  

Figure 1 - Table of SCSN Seismic Sensors

The two sensors are a high gain broadband sensor, such as a Streckeisen STS-2, and a low gain strong motion sensor, such as the Kinemetrics EpiSensor.

The broadband sensor is proportional to velocity over from about 50Hz to 120s (or 360s) [see Figure 1] and is sensitive to the motions from below the background noise of almost every site on earth to moderate earthquakes at moderate distances (clip level is approximately 1cm/s). It also records on-scale almost all teleseismic earthquake ground motion. This is a relatively large, heavy instrument, which requires great care in installation.  

The strong motion sensor typically is proportional to acceleration from about 100Hz to DC, and is designed to record the strongest ground motions expected in the near-source region from the largest earthquakes (its broadband clip level is typically 2g), to small events at close distances, and regional events at moderate distances.  

A summary of how these two sensors combined record the entire range of earth motions is shown here (figure 2.1):

Tables with the typical hardware used within the SCSN are in Table 1 (seismic sensors) and Table 2 (dataloggers).  

Also located in the network are some stations which only have strong motion sensors. These stations are typically associated with boreholes or buildings, or urban sites where the background noise is so large the dynamic range of the broadband sensors would not be efficiently used. Many of these sites also have dataloggers with only 5 orders of magnitude dynamic range.  

Figure 3 - High and Low Gain Broad-Band Seismometer Frequency Range, Sensitivity, and Clip Level

The SCSN also operates some analog stations which are also transmitting data in real-time to Caltech, but are not broadband, with more limited dynamic range. The primary function of these stations is event location and magnitude determination. These stations fill in geographic 'gaps' in the network which are not covered by the digital stations.

Fig 4.a - Median Phase Response vs. Frequency Plot for STS-1/2, CMG3ESP, and CMG40T Instruments

Fig 4.b - Velocity Response vs. Frequency for STS-1/2, CMG3ESP, and CMG40T Instruments.