Committees

 

The Natural History Committee

Natural History encompasses the study of plants, animals, climate, and geology that are native to an area. It is important to study this history so that one can understand what is natural in the ecosystem and how to preserve that ecosystem. When large scale changes are made, such as building without consideration of the ecosystem, we create soil erosion, flooding, and extinction of wildlife, insects and plants. Also, when animals, plants, and insects are brought into a location not native to them, the balance in nature is shifted.

The Natural History Committee hopes to be able to discuss various topics in Natural History and to encourage people to assist in the preservation of the ecosystem.

Spring is Here!

Spring is here and we are starting to think about our gardens. As we do, why not consider some native plants this year?

Native plants are "at home" in the soil where they originated and are rarely attacked by insects. They have adapted to our culture and can withstand the growing conditions found here. Never remove them from the wild, however. It depletes the native population and many do not survive transplanting.

The Pennypack Ecological Restoration Trust at 2955 Edgehill Rd has 2 native plant sales a year. The Spring sale one is on Friday, May 2nd. and Saturday May 3rd. Another sale occurs in September. The Fall is a great time to put in native plants. In the Spring you are rewarded with hale and hearty plants. Plants are also on sale Monday through Friday during regular business. Knowledgeable people are available to answer your questions.

Many retail nurseries and mail order catalogs also offer native plants. Make sure they are propagated at the nursery and not taken from the wild.

Excerpts from a 2007 article by June Crippen, Natural History Committee.

What Earthquake?

The cause of earthquakes was stated correctly in 1760 by British engineer John Michell, one of the first fathers of seismology, in a memoir where he wrote that earthquakes and the waves of energy that they make are caused by "shifting masses of rock miles below the surface."

For many years the San Andreas Fault in California has made headlines. Awhile back there were questions on whether California would fall into the Pacific Ocean. Early in the 19th Century the New Madrid Fault caused significant damage in the central part of the Country. It was strong enough to ring church bells in Boston, MA. Many do not realize that earthquakes also happen on the East Coast and can be felt in Southeastern Pennsylvania.

Southeastern Pennsylvania is the State's most seismically active region. It is not known to have experienced an earthquake with a magnitude greater than a 4.7, but the historical record only goes back about 200 years. There is no obvious reason to conclude that an earthquake of a magnitude between 5 and 6 could not occur. An earthquake higher than magnitude 6 is less likely but cannot be ruled out.

The majority of earthquakes occur along the boundaries between tectonic plates. The tectonic plates are the large, thin, relatively rigid plates that move relative to one another on the outer surface of the Earth. Eastern North America, including Pennsylvania, is about 2000 miles away from the nearest plate boundary - the Mid-Atlantic Range. The Eastern States, however, do experience a moderate level of earthquake activity, including occasional earthquakes with a magnitude greater than 6. The Seismic activity in the East may be related to what happened about 200 million years ago. At that time, the supercontinent of Pangaera broke up and the Atlantic Ocean began to form. This event called rifting by geologists produced many faults, and some of these faults may be experiencing reactivation by present day stress, which is squeezing eastern North America in a roughly east-west direction.

The following map of Pennsylvania shows the probability of earthquakes in various parts of the State.

The contours represent earthquake ground motions that have a probability of being experienced in 50 years. The numbers are related to the speed of the earth movements. As may be seen, the numbers decrease as you go toward the mountains. Range 8-14 fall roughly into the same earthquake intensity range.

Many earthquakes occurred outside the State but were felt in Pennsylvania. A strong quake on December 18, 1737 toppled chimneys in New York City and was felt in Boston, MA, Philadelphia, PA, and New Castle, Delaware. Other shocks that occurred outside the State were felt in 1758, 1783, and 1791. In 1800, two earthquakes (March 17 and November 29) were reported as "severe" at Philadelphia. On November 11 and 14, 1840, earthquakes at Philadelphia were accompanied by a great and unusual swell on the Delaware River.

Dishes were thrown from tables (intensity 5) at Allentown by a strong shock on May 31, 1884. Towns from Hartford, Connecticut to West Chester, PA reported fallen bricks and cracked plaster from an earthquake centered near New York City on August 10, 1884.

On December 27, 1961, the northeast portion and suburbs of Philadelphia noticed a disturbance that shook buildings, rattled dishes, and disturbed other objects. Police and newspaper offices received numerous calls from alarmed citizens.


The above map of the local area shows where earthquakes have occurred in our region. Here is a listing of the earthquakes that have occurred in Bucks, Montgomery, and Philadelphia Counties.

Some of the local earthquake activity has occurred on what is called the Huntington Valley Fault. To a Geologist it is actually called the Huntington Valley Shear Zone because the action of the plates is quite different.

Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock (see diagram below).

Normal faults are usually experience tensional or compressional stress while a shear zone undergoes shear stress.

Pennsylvanians will probably continue to feel small earthquakes generated on local faults. A large local earthquake of a magnitude greater than 6, though unlikely, is not impossible. It is always wise to take certain precautions such as emergency planning. This would include having on hand a flashlight, battery-powered radio, water and food supply, and a first aid kit.

The source for the material for this article came from the USGS Website, as well as from the Commonwealth of Pennsylvania, Department of Conservation and Natural Resources, Bureau of Topographic and Geologic Survey Website.

Excerpts from a 2007 article by Bob Crippen, Natural History Committee.