Image Captures From Two Miniature
Horizontal Seismometers
David F. Nelson
Brett M. Nordgren
For more information: Brett Nordgren <
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These image files are captured from the outputs of two miniature horizontal seismographs, located approximately two miles from the Pacific Ocean and 19 miles from downtown Los Angeles. The instruments, oriented N/S and E/W are mounted on a concrete pier, which rests on shallow bedrock made up of weathered Miocene marine sediments. They are each approximately 5"H x 4"W x 9"L
The instruments use a completely new principle for detecting ground motion, discovered and refined by Dave Nelson over a period of years. Their high-gain outputs have a sensitivity of approximately 122 V/cm/sec (12,200 V/m/sec) over a frequency range of very roughly 20 seconds to 4 Hz, while the low gain outputs are 2.44 V/cm/sec (244 V/m/sec) over the same bandwidth. Both outputs clip at a level of 10 Volts, which represents 820mm/sec for the high gain channel and 4.1cm/sec for the low gain one.
Unlike the Force-Balance Vertical, these do not incorporate feedback and can never be observatory-class instruments. Their frequency response and waveform delays are not as well characterized or tightly controlled, nor is their sensitivity. However for amateur use they are easier to construct, and are much easier to set up and operate than a Lehman. With an effective seismic mass of only 19 grams, they perform better, use significantly less space and will directly drive a +/-10V A/D input.
As with all horizontal instruments the seismic background is significantly higher than is usually seen with verticals so their useful sensitivity will not be as great, though these seem to be able to display distant quakes quite well. The occasional 2-3 minute noise bursts are mostly the result of human activity in the vicinity of the vault but sometimes are also instrument "pops".
The images below are refreshed every 60 minutes.
E/W Instrument high gain channel
914nm/sec/line WinSDR Y-scale factor = 1 20sec – 12.5sec (0.08Hz) Clipping level 820mm/sec.
Large quakes will be off screen, although the channel clipping level (10V out) is still 820mm/sec. or 36 x this screen height.
N/S Instrument high gain channel
914nm/sec/line WinSDR Y-scale factor = 1 20sec – 12.5sec (0.08Hz) Clipping level 820mm/sec.
