More on Microseisms

The previous post gave me some confidence to suspect that storms in the north Atlantic basin may give rise to much of the microseism activity I see on my seismograph. Earlier this winter I ran across a public US Navy web page that shows the results of computer modeling of significant wave height in the north Atlantic and have been visually comparing those plots to my microseism activity. At least on some qualitative level this new information continues to support the notion that the microseisms I see result from storms at sea. Take a look at the two examples below.

On Jan 14, 2017, the seismograph was quiet so I looked at the significant wave height model to find that it was also very quiet:



Today, Feb 14, 2017 the seismograph is very noisy:


Take a look at the Significant Wave Height now.  The wave heights at the center are in the upper 30 to mid 40 feet values!


I had actually predicted that this would happen on Feb 11, 2017 by looking ahead at the Significant Wave forecast given by the model.  If I can find access to the gridded data or come up with a way to at least roughly re-digitize these charts I will try to see if there is a simple relationship and correlation between the wave height, the distance from the seismograph to the center, etc. and the amplitude of the microseisms.

This suddenly makes the microseisms, which used to be more of an annoyance, now almost as interesting to watch as the earthquakes.

Whence Microseisms?



Normal Seismogram


Noisy Seismogram showing Microseisms








In the twelve years or so that I have been observing earth motions on my home built seismograph I have been puzzled by the occasional appearance of noisiness in the seismometer trace. I have read about these microseisms and tried unsuccessfully to identify a source of them. The literature always identifies storms at sea as the main cause but I discounted that explanation because of my location in Ohio and the fact that I don’t have a professional, broadband seismometer. So I looked at local wind speeds and wind gusts buffeting the house or moving the trees so much that their roots move. I have looked at local barometric pressure and temperature changes even monitoring the temperature inside the enclosure which contains my seismometer. I have wondered whether there is something in my amplifier electronics that might explain these tiny signals. Nothing worked out.

Earlier this month, I noticed that they were back and that they were growing quite strong. Then it occurred to me that Hurricane Matthew was approaching the east coast of Florida at the same time. Maybe the microseisms I see can be attributed to storms at sea!

I began a closer look at this by downloading time series data about the hurricane from the National Hurricane Center ftp site.  I wrote a python script to load the data from the downloaded file and plot the track of Hurricane Matthew and other information I thought might be important such as the maximum sustained wind, central pressure, and the distance of the eye from my home in Millersburg, Ohio which I calculated from the location data.


Note that P0 marks the location of the seismograph in Millersburg, Ohio.

I also downloaded a bundle of specialized microseism analysis tools from the IRIS (Incorporated Research Institutes for Seismology) software site.  This software is designed for advanced study using professional grade, broadband, multi-channel seismographs.  As such, I have most likely not applied it correctly, in fact bypassed important calculations, etc.  Having said that I hope that the “relative” analysis I did shows some direction toward a qualitative relationships between storms at sea and the microseisms I see recorded on my seismograph.  Here is what I found.


This data includes Local, Secondary, and Primary Microseism energy.  I chose to use the Primary band data since they all show similar qualitative response.  Hurricane Matthew dissipated  on Oct 10 and the NHC data file was closed and no more data was added.  More on that later.  I snipped the microseism graph off at Oct 10 and compared to the Hurricane central pressure and distance to the eye:


Actual (non-inverted) data

This is a little hard to visualize in part because one would think that the microseism energy might be inversely related to both the distance and the central pressure.  We can look at the same information taking this into account by plotting the reciprocal of both quantities:


Inverted data (see text)

The last chart is the closest to correlated data that I have seen in twelve years or so of trying to figure this out.  Not perfect due to many known and unknown reasons but something for me now to be aware of when I see this noisiness show up.

The proximity of the storm seems predominant.  The central pressure seems to lead the microseism energy.  And finally, the original microseism graphs that went out to Oct 13 showed the noise staying high long after the NHC downgraded the storm.  Presumably, the ocean does not calm immediately after the storm dissipates.  An unmentioned complication in this study is that Hurricane Nicole was also present and strengthening during this time period further out in the Atlantic.