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.
There was a moderate, Magnitude 5.0, earthquake in southern Ontario, Canada this morning. It registered as a small but nicely formed signal on my seismograph in northeastern Ohio, showing the P, S, and surface arrivals. The orientation of my seismograph makes it less sensitive to earthquakes north and south of my location than it is to earthquakes east or west of us. By my calculation this earthquake was about 456 miles northeast of my location.
USGS Event page is here: http://earthquake.usgs.gov/earthquakes/eventpage/usb000gxna#summary
The USGS reduced the magnitude estimate to 4.4 after this post was written.