I have been curious about the source of microseisms on my seismograph since I completed it in 2004. I initially dismissed the explanation that they originated from intense ocean storms but after watching those storms online and comparing to my seismograph traces there is convincing qualitative evidence for that explanation. This past winter I decided to try to collect some data for a closer look. I began collecting data on January 08, 2020 at 12:00 UTC. I have a continuous record of the significant wave height in the Atlantic Basin and a fairly complete record of the microseism record as described below through the end of April and will continue to collect data going forward. This post will describe what I am doing and will introduce the results obtained in January through a short video. I am working on merging datasets and starting a statistical evaluation of the data so I hope some interesting things will come out of it.
An article in Geophysical Research Letters titled ‘Stormquakes’ pointed me to the portal to NOAA’s WaveWatch III gridded ocean surfce model results. Information on the NOAA website showed how to access and use data from the model. I modified the example Python script to access and map the significant wave height data in the North Atlantic Ocean Basin. This runs every six hours as cron job, automatically acquiring, mapping, and filing this information.
I also wrote a small Python script to calculate the standard deviation of 30 minute segments of the seismic trace from the previous 24 hours. I use the trace standard deviation to characterize the amplitude of the microseisms. The chart below shows the trace standard deviation changes, plotted in blue, for the month of January 2020. Several earthquakes occurred which appear as the larger narrow spikes in the data. The shorter spikes are due to foot traffic in the house.
Those spikes are of no interest in the present study. What is really of interest is the envelope of the underside of this graph described by the minimum values. Because I only have the ocean surface information at six hour intervals I evaluate the minimum value of the trace standard deviation on the eleven 30 minute trace segments centered on each of the hours 00, 06, 12, and 18 UTC. These values are plotted as the red line in the graph.
A very near minimum and the maximum value of the red line occur only three days apart, on January 15 and January 18, respectively. Here are the 24 hour helicorder charts for the two days.
And the Significant Wave Height during the same days:
The Geophysics Research Letters paper referenced earlier specifically associate The Grand Banks off the coast of Newfoundland with stormquakes. This particular example seems to support that association. I hope that my long term study will give a little more structure to what we see here. In the meantime here is a short video showing the progression of January’s winter storms across the North Atlantic.
A year or so ago, I upgraded my seismograph software from the original IRIS AmaSeis program to an upgraded version based on Java called jAmaSeis. I ran the new software on a little Asus netbook. The helicorder display that I uploaded to the Live Seismograph page on this site was just a screen shot of the active window every five minutes. The little netbook was at its performance limit and the screenshot made it impossible to do any analysis anyway.
This fall I built up a Linux desktop computer running XUbuntu 14.04LTS and installed jAmaSeis on it. I downloaded from the IRIS website and followed the basic instructions for installation given on the same site. With a little fiddling around, I got the software running. I plugged in the Dataq DI-145 USB Analog to Digital converter…the software could not find it. I fiddled around with it for a long time with no success. I could see the device appear in /dev as ttyACM0 when I plugged it in. After giving up on it a couple of times I finally created a symbolic link from ttyACM0 to ttyS32 where the seismograph software would detect it. I do this manually every time the computer reboots or the ADC is unplugged. I plan to write a udev rule to do this automatically.
To get the helicorder display for the Live Seismograph page I use an open source package of seismology tools for Python called Obspy. jAmaSeis writes the data stream in one hour long segments in sac formatted files arranged in a time based directory structure. The Obspy stream manipulation tools allow one to easily build a continuous 24 hour data stream from the jAmaSeis files and plot it in a moderately flexible way in a Python script. This method yields several improvements over the screen shot. The date and time are unambiguously shown on the vertical axis and the traces alternate through four colors to differentiate the hour in which an event occurs more clearly. The Python script runs as a cron job every five minutes.
The Obspy package also provides tools to parse QuakeML documents which I obtain from a USGS feed inside the same script that plots the data. After parsing each event, I use the obspy.core.util.locations2degrees tool to find the distance from the epicenter to my station. The script then annotates the helicorder display with seismic events selected using magnitudes and distances that might be detected with my seismograph. This selection is arbitrary so there will be some that show on the trace without annotation and others will be annotated when there is nothing showing on the trace.
A Magnitude 4.0 earthquake occurred in southern Michigan at 12:23 EDT this afternoon generating a very clean signal on my seismograph at Millersburg in northeastern Ohio. The analysis I did using jAmaseis estimated the distance to the epicenter to be 353 km. The Google Earth ruler measured 348.5 km. The signal overlaid on the jAmaseis travel time curves is shown below.
A Major earthquake, Magnitude 7.3, occurred of the coast of Nicaragua at 03:51:35 2014 October 14 UTC. The image above shows the trace from my seismograph in the Event mode of the latest version of jAmaSeis. The distance is computed by dragging the trace around to get the best fit to the travel time curves. I usually cheat a little by selecting the portion of the trace starting at the actual time of the event.
I have added a new page under Seismology/Lehman Seismograph section in response to occasional questions about the construction details of my Lehman seismograph. I have included some rough dimensions and general layout but in my experience following them exactly is not necessary. A quick scan of the web based collection of Lehman projects shows an amazing array of designs which apparently work. The Lehman concept is very forgiving, at least within broad constraints.
I have tried to answer questions I have already received and will add additional material if new questions arise.
I received an email notification this afternoon that a Magnitude 3.5 earthquake occurred east southeast of Nelsonville, Ohio just before 1:00 pm local time. My heligraph has been very noisy so I couldn’t really see any signal even though the epicenter was only 79 miles south of Millersburg where I am located. When I zoomed in on the trace though I could see an excursion at about the right time. Just to be sure I checked a nearby helicorder located at Kent State University Branch campus in New Philadelphia, one of the OhioSeis network stations. It showed the first arrival at about 20 seconds past 18:00 UTC, the same as mine. There were 373 felt reports by mid afternoon mostly in southeastern Ohio..
Postscript (April 27, 2014) :
In an email exchange with another amateur seismologist, he had included a link to additional information on using the AmaSeis software. As I read some of the entries I found an article on analyzing local events so I applied it to this event in Nelsonville, only 79 miles south of my seismograph. One of the characteristics of local earthquakes is that they have a higher frequency content than more distant teleseismic events. These can be extracted by using a high pass filter on the signal. Following the lead of the article, I applied a 1.2 second high-pass filter twice to the Nelsonville signal with the results shown below. Impressive!
Several strong earthquakes registered on my seismograph in northeastern Ohio during the past 24 hours or so. Because our internet was down during that time, the “Live Seismograph” was not working. For readers who watch these things, I thought I would do a brief summary of the activity.
May 23-24, 2013 Helicorder Display
The Magnitude 8.3 earthquake, which occurred at 05:44:49 UTC, in the Sea of Okhotsk dominates the helicorder display. Ordinarily the great depth of the focus, 601 km, would generate relatively small surface waves but the large magnitude cause those phase to still dominate my display. Note that the travel time curves below automatically scale the trace in a way that the amplitude cannot be compared between them.
M8.3 – Sea of Okhotsk
A second, Magnitude 6.8 earthquake occurred in the same vicinity about 9 hours later (14:56:31 UTC) also at a depth of more than 600 km:
M6.8 – Sea of Okhotsk
The first earthquake of May 24th that registered on my seismograph was a Magnitude 5.9 tremblor in a remote area of Northern California (03:47:08 UTC). This earthquake occurred near to the surface at a depth of 11 km with the result that the body phases are minimized relative to the surface phases.:
M5.7 – 11km WNW of Greenville, California
The final earthquake that I list was actually the first in this collection to occur, a Magnitude 7.4 tremblor, originally described as southwest of the Fiji Islands but later amended to read southwest of Tonga to which it was actually closer, at 23 May 2013 17:19:04 UTC. The focus for this earthquake was also moderately deep at 171 km.
M7.4 – 282km SW of Vaini, Tonga
Again, remember that the scaling of these arrival time plots is not uniform, so the trace amplitudes cannot be compared with one another.