Lancaster - Historic Seismicity

Ok, let's go back to the NEIC web page for the recent Lancaster earthquake.

Now click on the link down the page on Historic Seismicity.

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from http://neic.usgs.gov/neis/bulletin/neic_bcah_h.html

This is a cool graphic. Previous earthquake epicenters are indicated with the circles. The radius of the circle indicates the earthquake magnitude. And the depth of the epicenter is shown by the color. So four dimensions of data are shown on a two-dimensional figure.

All of the historic quakes shown are small, and all are shallow.

Note the last line in the figure caption. Plate boundaries, that is, the type of boundary between different plates in the plate tectonics model, are indicated. In this area, there are no plate boundaries. Thus, these are examples of intraplate earhquakes, those that occur within a plate, as opposed to the interplate earthquakes that occur at boundaries between different plates.

It will be useful at a later time to compare this earthquake to one at a plate boundary, in an area of much higher seismicity.

How intense is that? Lancaster earthquake intensities

Notice now that the Lancaster quake NEIC report now lists it as magnitude 3.4 . Re-evaluation of the data?

An earthquake magnitude is an objective measure of the energy released. It is based on seismograph readings, and is a single number that characterizes the size of the earthquake. The first methods for determining magnitudes were derived by seismologist Charles Richter, so although different algorithms are now used, these are still often called Richter magnitudes.

Intensities are another way of indicating the nature of seismic ground motion in a particular area, and the response of people and structures to that ground motion. It is somewhat subjective; intensities vary from being highest near the epicenter, to decreasing values at greater distances. The Modified Mercalli Intensity Scale is the one in common usage today.

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from http://neic.usgs.gov/neis/bulletin/neic_bcah.html

Notice the intensities for the Lancaster quske as shown above are all Roman numerals II and III - compare these to the description above for the Modified Mercalli Scale.

Update on Dillsburg swarm

Jeri Jones, a geologist in York County, PA, has been compiling a list of all felt earthquakes reported to him by residents of the Dillsburg, PA, area. As of January 1, there are 206 earthquakes on the list! Please contact me if you want a copy of the list.

NEIC post on Lancaster earthquake

The U.S. Geological Survey's Earthquake Hazards Program (aka National Earthquake Information Center) web pages have great information on earthquakes. Let's look at the information provided for the recent Lancaster earthquake.

First, the basics, to be found here, or copied below:

(click above for a larger image)

Obviously, this page gives the basic hypocenter or focus, epicenter, and magnitude information on the quake: where, when, how deep, how big, what cities was it close to (where damage might have occurred), and indicators of uncertainty and reliability. Meanings of the terms are given here; note the different types of magnitudes, perhaps the source for a future blog.

Good vibrations (Beach Boys style)

If we want to start the year with good vibrations, what better way than with the Beach Boys:



This 1966 song was a #1 hit for the Beach Boys, and is one of the The Rock and Roll Hall of Fame's 500 Songs that Shaped Rock and Roll.

Perhaps the Southern California Beach Boys hit was influenced by the 1966 Parkfield magnitude 6.0 earthquake earlier that summer?

Shaking earth causes vibrating butt

The Sunday report in the Lancaster Newspapers on the earthquake began its article as follows:

[Name withheld by me] of Ridge Road, Mount Joy Township, was in the bathtub when, he said, "I felt it in my butt. It vibrated."

I'm not kidding - who could make this up?

I know animals are reputed to sense impending earthquakes. I wonder if this is a corresponding human trait? Or was that guy just playing around in the bathtub at midnight on a Saturday?

And how could a reporter and an editor possibly let an article start that way? I guarantee, you would never find that particular wording about an earthquake here in the SF Bay area where I'm spending the week.

Hoping the New Year brings you all good vibrations!

Another view of the Lancaster earthquake

This one is from the Millersville seismograph. You can look up Millersville's current and archived seismograms here.

click above for larger image

Seismologists, what say ye about this event?