Earthquakes far and near

Many local news outlets probably have had stories like the one in today's Lancaster newspaper, which compared the earthquake threat to Haiti (always significant) to that in Lancaster (small, but not zero).  I was asked via email about my own experience with earthquakes. As the article says, the only one I ever personally experienced was in Lancaster in 1984.  Charlie Scharnberger also comments.

Part of the full article is repeated below:

How safe are we?  Though not immune to earthquakes, Lancaster County is relatively secure compared to others areas of the United States.

Sunday News   Feb 07, 2010 00:12 ES

By JON RUTTER, Staff Writer

Shirley Riffle was celebrating her daughter's marriage in Reading when the walls of the reception hall started to flutter. The crystal lighting fixtures began to tinkle.

People looked up from their dinner, Riffle recalled. "We thought 'Oh my God.' "

An earthquake had been the remotest thing from their minds. But Riffle had good cause for alarm.

When she returned to her split-level in Spring Township, Berks County, she saw that half the chimney had toppled and a first-floor window had blown out. Some of her family's possessions, including a valuable Canterbury pottery collection, had been destroyed.

She's still paying off the roughly $30,000 in damage, she said. "We have cracks all over the walls to this day."

Riffle was singularly unlucky that bitterly cold weekend back in 1994. The epicenter of the 4.7 magnitude temblor that also buckled roads and pried open sinkholes happened to be under her house.

Last month's magnitude 7 quake in the Caribbean stirred painful memories for Riffle, who said her trauma was nothing compared to the massive death and destruction suffered by Haitians.

"I'm very lucky," said the mother of four. "I'm over it. I'm past it."

But 16 years is a geologic eyeblink.

Could a damaging earthquake again strike close to home? How secure is this place, anyway?

Even more of a mystery
Pretty secure, scientists say. But not entirely so.

There's simply too little known about the nether regions of the earth to rule out the big one.

Charles Scharnberger, a geophysicist and Millersville University professor emeritus, notes that the Spring Township quake was the strongest ever recorded in the immediate area.

He figures the odds of a local magnitude 6 quake happening in a given year to be one in 1,000.

By comparison, places like Haiti or California, which are perched on the slowly grinding junctions of tectonic plates, are at much greater risk of periodic, calamitous quakes.

The eastern United States sits squarely on the North American plate, which stretches about 2,000 miles seaward to the Mid-Atlantic Ridge.

Plate interiors are relatively placid geological zones, said Rob Sternberg [that's me, above] a Franklin & Marshall College geosciences professor who posts quake information on shakingearth.blogspot.com.

On the other hand, many midplate quakes occur worldwide every year. Most are small. But some, even in the eastern U.S., have been severe.

Foremost in the lore is a trio of 1811-12 quakes in the Mississippi Valley, the strongest and noisiest of which was said to have shot sulfurous fumes into the air, awakened sleepers in Washington, D.C., and opened up fissures that swallowed seven Indians.

Reported one newspaper of the day: "The earth was so convulsed, as to tender it difficult for one to keep their perpendicular position."

An 1886 temblor estimated to have equaled 7.5 on the then-nonexistent Richter Magnitude Scale heavily damaged Charleston, S.C., and clanged bells in Boston, according to Scharnberger.

Seismic survey in Lancaster County

I've been away for a bit: archaeomagnetic sampling and magnetic surveying at the Etruscan site of Poggio Colla in Italy; improving my field geophysics with folks at the Bayerisches Landesamt für Denkmalpflege in Munich; presenting a talk on magnetic properties of legacy sediments at the IAGA symposium in Sopron, Hungary; and along the way also enjoying the Venice Biennale and Budapest.

Anyway, what is shaking around here these days is seismic, but not an earthquake, It's a big truck. Lancaster County is getting "x-rayed" via vibroseis. Funding comes from Pennsylvania's carbon sequestration program.

Here is the full newspaper story, by PJ Reilly, from the Lancaster Intelligencer Journal, Jun 17, 2009:

A statewide project aimed at reducing Pennsylvania's carbon dioxide emissions could provide a first look at what's below the surface of Lancaster County.

"No one's ever drilled it deep," said Jay Parrish, director of the Bureau of Topographic and Geologic Survey for the state Department of Conservation and Natural Resources. "No one's ever done seismic work. All we know is what's at the surface.

"Everybody has seen the surface of Lancaster County for 250 years. Go down 3,000 feet and it's a complete mystery."

If his funding holds up, Parrish hopes to deploy a team to Lancaster County in August to literally pound the ground, north to south and from end to end, and take seismic readings to develop a picture of what lies deep beneath the surface.

"It's like a new world no one has ever looked at," he said. "We have no idea what's here."

DCNR is studying Pennsylvania's geology as part of the requirements of House Bill 2200, which was signed into law in October by Gov. Ed Rendell.

The bill orders DCNR to evaluate the state's potential for "geologic carbon sequestration."

According to the agency's Web site, that's a process in which carbon dioxide emissions are captured at coal-fired industrial plants before they are released into the atmosphere for injection and storage deep underground.

Pennsylvania ranks third among all 50 states for its total emissions of carbon dioxide, which is a greenhouse gas linked to global warming.

One percent of all the carbon dioxide emitted into the atmosphere in the world comes from Pennsylvania, Parrish said.

"As part of a broader portfolio of technologies, geologic sequestration appears to be capable of playing an important role in stabilizing carbon dioxide concentrations in the atmosphere," DCNR's Web site states.

Preliminary studies and general knowledge of Pennsylvania's geology suggest it is well suited for geologic carbon sequestration, which requires reservoirs a half-mile or more underground that are sealed by impermeable layers of rock, the Web site states.

Four primary reservoir areas DCNR expects to find are deep saline formations, depleted and producing oil and gas fields, coal beds that can't be mined and organic-rich shales.

Parrish, who was director of Lancaster County Geographic Information Systems department from 1997 to 2001, said his DCNR bureau has been assigned the task of studying the entire state to find such areas and others that might be suitable for storing carbon dioxide.

Scientists already know what lies beneath many areas of Pennsylvania, where coal mining and gas- and oil-well drilling have been going on for years.

But the extreme southeast corner of the state, including Lancaster County, is unexplored territory.

"Theoretically, there's no oil here, and that's why nobody has ever looked," Parrish said.

Discovering what lies beneath the county is important, according to Parrish, not only to see if the potential for carbon sequestration exists here, but also to simply have the subsurface data available.

"What does it mean to the average person?" he said. "It means there's a better understanding of the rocks under them, and that leads to every aspect of life, because you have radon flows, you have earthquakes, you have sinkholes.

"You just don't know what potential is there, and you also don't know, in the future, what may be useful information to have."

On Dec. 27, an earthquake measuring 3.4 on the Richter scale rattled Lancaster County. The epicenter was just west of Manheim Borough.

That's one of the areas Parrish wants to explore. He said scientists suspect there's a fault there, based on formations found in some local quarries.

"What we're looking at are little tiny thrusts at the top, so we have a good indication there's a fault there," he said. "But we don't know, maybe there's a huge faulted thrust underneath."

DCNR has budgeted about $3 million to collect seismic data from 41 of Pennsylvania's 67 counties, including Lancaster.

Parrish said the "project is in a constant state of flux" and "my priorities change from week to week," which could mean there won't be enough money to study Lancaster County.

For now, however, Parrish said the plan is for a seismic team to descend on the county in August.

He said it would take about three weeks to get readings along a north-south line through the heart of the county.

Since most of Pennsylvania's geologic formations run east to west, a north-south survey near the center of Lancaster County will give scientists a "pretty good idea" of what the whole county looks like beneath the surface, Parrish said.

To collect data, a large truck carrying a heavy weight would crawl along back roads at a rate of about 3 miles per day. Highways generate too much noise, which would interfere with the seismic readings, Parrish said.

Periodically along the route, the weight would be pounded on the ground and a series of small seismometers attached to a cable, which would be extended for three miles on both sides of the truck, would track the vibrations.

The data would be plugged into a computer "to give us a nice picture of the underground" up to three miles deep, Parrish said.

Ideally, Parrish would like to take readings on a perfectly straight line down the center of the county.

"Obviously, that's not possible with the roads we have around here," he said.

The line of readings likely will zig-zag along public roads.

Once he knows for certain there's enough money to study Lancaster County, Parrish plans to map out a work route and then contact officials in municipalities along that route to let them know what's happening.

"It's going to be a pretty big production that's going to attract a lot of attention," he said. "And we want people to know ahead of time what's going on."

Six months of south-central Pennsylvania seismicity

The figure and the list below are from the LCSN:

    Date        Time      Lat     Long   Depth Magnitude
year-mo-dy  hh:mm:sec    (N)     (W)    (km)  (Mc)
--------------------------------------------------
Lancaster County, PA
1972-12-08  03:00:32.5  40.150   76.220   4  0.0
1978-07-16  06:39:39.6  39.900   76.220   0  0.0
1984-04-19  04:54:55.1  39.946   76.323   7  2.9
1984-04-23  01:36:02.0  39.946   76.323   4  4.1
1984-04-23  02:46:00.5  39.946   76.323  10  2.5
1994-01-16  03:03:31.3  40.532   76.247   4  2.0
1995-03-11  21:10:16.0  40.100   76.400   5  2.7
1997-11-14  03:44:11.7  40.164   76.276   6  3.0
2000-03-22  20:48:00.0  40.070   76.300   1  1.8
2000-10-05  23:33:59.0  40.080   76.300   7  2.0
2001-07-17  14:41:20.2  39.937   76.340   1  1.8
2008-12-27  05:04:34.5  40.114   76.403   3  3.4

Adams County, PA
1994-05-26  17:21:58.2  39.950   77.187  11  2.8

Dillsburg, York County, PA

1997-06-16  05:43:16.6  40.098   76.967   5  1.9

2008 (21 events)
2008-10-05  22:36:48.1  40.054   76.967   2  2.0
2008-10-06  01:06:13.4  40.120   77.025   0  1.2
2008-10-06  06:15:38.4  40.069   76.992   0  1.1
2008-10-19  08:21:22.7  40.080   77.019   5  1.9
2008-10-19  08:22:07.3  40.078   76.963   5  2.1
2008-10-19  08:26:20.5  40.168   77.038   5  1.2
2008-10-19  08:28:09.0  40.087   76.998   5  1.5
2008-10-19  08:58:31.7  40.092   76.974   3  1.8
2008-10-19  09:06:01.5  40.091   77.018   5  1.0
2008-10-19  09:08:20.7  40.056   76.962   5  0.8
2008-10-19  09:08:44.3  40.092   76.987   5  1.1
2008-10-19  09:17:34.8  40.100   77.009   5  1.7
2008-10-19  09:49:19.1  40.095   77.002   5  1.0
2008-10-19  09:49:51.7  40.075   76.973   5  1.5
2008-10-19  09:50:11.4  40.091   76.989   5  1.6
2008-10-20  00:08:26.6  40.159   77.107   5  1.2
2008-10-20  00:14:03.9  40.075   77.005   5  1.1
2008-10-20  02:16:02.4  40.090   76.984   5  1.5
2008-10-23  15:55:32.3  40.068   76.962   5  1.2
2008-11-07  04:07:03.0  40.096   77.006   1  1.4
2008-12-31  05:34:08.8  40.107   77.003   1  2.1

2009 (3 events)
2009-04-22  13:21:14.1  40.073   77.002   1  1.1
2009-04-23  22:26:04.4  40.085   76.998   1  2.4
2009-04-24  05:36:48.8  40.064   77.026   1  2.9

See any patterns here? Maybe I'll try that next.

My only earthquake, 25 years ago on Easter

I experienced my only earthquake on Easter Sunday, 23 April 1984. I had been in Lancaster, PA, for a year, my 1-year old son was sleeping upstairs, my wife and I were sitting in the kitchen. All of a sudden we heard a low rumble, and then kind of a boom. My wife thought maybe there had been a truck accident, or an explosion in our boiler, both common reactions to eastern earthquakes. Geophysicist that I am, I realized immediately it was an earthquake (perhaps assisted by the fact that a foreshock had occurred during the previous week). This was the best kind of earthquake – exciting, a little cracked plaster, some data for seismologists to play with, but no one hurt, almost no damage. Reports from F&M said that during the showing of the weekend movie (before the days of DVDs), people had panicked and run outdoors. This would make it intensity V on the Modified Mercalli Scale.

The cooperation that event fostered between seismologists at Millersville University and Columbia University was important in the development of the group that led to this blog.

Now my two sons are grown up, and living in the San Francisco Bay area. I hope they don't experience anything worse than a magnitude 4 earthquake.

Lancaster earthquakes - how dense are they?

Or, after another entry on the Lancaster earthquake, you might be asking how dense is Rob?

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

The gray color suggests that there is about 0.1 magnitude 5 earthquakes in that region per year, or only 1 every 10 years (which actually seems a bit high to me). But in the Lancaster area, colored white, the frequency is even less. This actually isn't a very interesting diagram here because of the paucity of large earthquakes. We'll have to look at this type of diagram later for a larger earthquake, presumably in some other part of the world near a plate boundary.

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.

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.

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?

Hometown earthquake!

How ironic! While I am spending the week virtually on top of the Hayward Fault in the East Bay area, a magnitude 3.4 earthquake happens back home in Lancaster.

At least I have F&M's seismogram to look at.

click above for larger image

Our current daily seismogram can be found here, and our archives can be found here.

Does this count as a holiday quake?

More on the hilarious reporting of this event by the Lancaster Newspapers later.