Lancaster earthquake - just a phase?

We're still doing an extended tutorial on the Lancaster earthquake.

Go down the page there, and click on Phase (Arrival Time) Data.

This shows the phases recorded at different seismic station. We know already that there are three physically different types of seismic waves - P, S, and L. But phase not only indicates which type of wave, but what path it has traveled.

You can find a listing of seismic phases here.

Note in particular the phases for the Lancaster quake:

• Pg - at short distances, an upgoing P wave from a source in the upper crust
• Pn - a P wave refracted along the crust-mantle boundary
  
• Sg - at short distances, an upgoing S wave from a source in the upper crust
  
• Sn - an S wave refracted along the crust-mantle boundary

from http://www.isc.ac.uk/doc/analysis/2003p13/index.html#crustal

I think the preceding e you find for some phases indicates an "emergent" phase, one that is not too sharp, but builds up more gradually. Confirmation, anyone?

Lancaster quake - how fast do seismic waves travel?

Seismic waves travel pretty fast. There are three physically different types of waves - P, S, and L. The velocity depends on which type of wave and what part of the earth you are in - crust, mantle, core. But my rule of thumb for velocities of seismic waves that travel some distance from the epicenter is about 8 km/second (or 8000 meters/second) for a P wave, and 5 km/second for an S wave. Compare this with common velocities such as driving your car at 60 mph, which is about 100 km/hour, or 2 km/minute, or 0.03 km/second (all back of the envelope calculations).

So look at the NEIC report for the Lancaster quake. Now go to the link for Theoretical P-Wave Travel Times. The figures and table tell you how long it will take the P-waves from this quake to travel different distances. Note that these are essentially spherical wavefronts, since the waves travel out at constant velocities in all directions.

click above for larger image

Now this was a small quake, so the waves won't be large enough to be detectable more than a few hundred kilometers away, but this shows how many minutes it would theoretically take the waves to travel out to various distances: to New York in less than 1 minute, to Chicago in 2 minutes, Miami in 3 minutes, Denver in 5 minutes, and on to San Francisco in 7 minutes. There is another map on the same page for times around the world, and a table with the same information. We'll have to return to this topic at some point for a larger quake to understand the nature of the shadow zone.

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.

click on above for larger image
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.

click above for larger image
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?