Hi Mary. The sensor portion of a seismograph is usually placed just a couple of feet below the ground surface. They are located in areas where there isn’t any traffic in the form of vehicles or people. At one time there was one in the Mission at San Juan Bautista, but they had to remove it as traffic in the area was getting to heavy and it was recording just about everything that cause the ground to vibrate. The exception to this is in the Parkfield area where some of the sensors are placed deep in the earth so they can record the smallest of the small quakes.

The seismograph that was located on Fremont’s Peak not to far from where I live was moved to a ranch near San Juan Canyon Road for the same reason. The increase in traffic was getting to be too much.

Traditionally the activities of an earthquake observatory involved a lot of daily routine. The photographic paper film or magnetic tapes had to be changed everyday, so the previous days earthquakes could be analyzed. With the invention of computer technologies advanced seismological observatories record seismic signals to computers as a regular series of discrete numerical data. Much of the daily drudgery formerly required by the seismologist in traditional observatories has gone because of digital recordings.

Digitally recorded earthquakes give an arithmetic sample of the continuous signal. The seismologist scans the recorded ground motion on a graphics screen and makes selections as they would on drum or photographic recordings. With digital recordings of earthquakes the seismologist can flag the arrivals of P and S waves according to preprogrammed rules. In addition a check can be done for the accompanying time code and print or record the arrival time and amplitude of each selected onset.

After recording the arrival time of every main wave, the seismologist identifies each selected wave according to its type and path and assigns to it a standard symbol. The entire process is much like that carried out by a cryptologist attemting to break a secret code.

The final step is to record the arrival times, amplitudes, periods, and identify these onsets and transmit them to a catalogue or directly to regional or international seismological centers. This is the reason I take the solution shown on the earthquake map with a grain of salt. It is an automatic solution and as such it may not be accurate. The main reason for the automatic solution though is for the major quakes. The shake map that is generated gives the Office of Emergency Services (OES) an idea as to where most of the damage is going to be. This gives them an almost instant location as to where their services are going to be required. Take Care…Don in creepy town

• Seismographs