Gilroy Garden Seismograph located at: 121° 37’ 44.12” W    37° 00’ 14.1" N    Elevation 326 feet


Drum Recorder

The drum is 36 inches in circumference (around) and just over 48 inches wide… It turns at 1 revolution per hour and the pen advances at 1/10 of an inch per hour.  So the traces are every hour apart and every 2.4 inches is one day; 24 hours. We can record about 18 days of data on one sheet of paper.

  Geotech Model 8700C

Is a long period horizontal seismometer located in a vault away from people and man made noise. It is oriented in the North/South direction, making it most sensitivity to waves coming from the East or West.  It has a 20 second period which gives it the capability of detecting distant… as well as local events.

The wiggles

Most of the time the wiggles we see, are what are call “microseisms”… earth hummm or earth vibrations.

Lots of things can cause microseisms… But mostly wind… wind blowing in the trees… wind blowing across the land… wind blowing across the ocean.

The wind blowing across the ocean causes ocean swells, and we can see these swells as they come ashore on the coast at Santa Cruz & Monterey; the swells actually cause us to tilt. In the winter when there are big storms… the pen movements caused by these swells/tilting can completely obliterate the recordings.

The wiggles that don't make any sense... are probably man made disturbances caused by people working close by the seismometer.

Earthquakes     Local events…                                 5 and under… tight sharp short recording...  we normally feel these as a simple jolt

                           Distant and large events….             6 and larger…  long drawn wiggles… tilting sometimes going on for hours or more after an event.

Rules of thumb
     Our seismometer can sense Richter Scale 2 earthquakes out to about 10 miles

3 earthquakes out to about 30 miles

4 earthquakes out to about 100 miles

5 earthquakes out to about 500 miles

6 earthquakes anywhwhere in north American and

7 or larger earthquakes anywhere in the world.

Scientific seismometers located in quite areas can sense Richter Scale 4 earthquakes anywhere in the world... our device located here in the park with lots of people and ride noise can only sense world wide quakes of 7 or more.

Latest Earthquakes in the World - Past 7 days
California Nevada local earthquakes 
Public Seismic Network... amateur seismic group interested in building seismographs and talking about seismology