Third Day of the Zenith

The Ramapo Fault runs from northeast to southwest across the northern part of the State of New Jersey, from the New York State border down to the Round Valley Reservoir near Lebanon, NJ.  Along much of its length, the Ramapo Fault separates Mesozoic-age sedimentary rocks to the southeast from much older Precambrian intrusive volcanic rocks to the northwest. 

Near the Round Valley Reservoir, however, the Mesozoic bedrock occurs on both sides of the Ramapo Fault. In this area, the older volcanic rocks are separated from the younger sedimentary rocks by a series of mostly unnamed faults parallel to and northwest of the Ramapo. 

At 10:23 this morning, a magnitude 4.8 earthquake occurred along one of those unnamed, parallel faults, approximately 3 miles below the ground surface. The earthquake was widely felt in New York City and as far south as Washington, DC.  No injuries or significant damage were reported, but the quake still got much press coverage as earthquakes are relatively unusual in this part of the continent and because so many people felt it.

Scientists can often determine the specific fault that is responsible for an earthquake at well-studied plate boundaries like the San Andreas fault system in California. In contrast, this is rarely the case east of the Rocky Mountains. The New York-Philadelphia urban corridor is far from the nearest plate boundary, which is under the center of the Atlantic Ocean. The urban corridor is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the traces of known faults are poorly known at earthquake depths. Accordingly, few, if any, earthquakes in the urban corridor can be linked to a specific fault. 

Much of the bedrock beneath the New York-Philadelphia urban corridor was formed as ancient continents collided to form a supercontinent about 300-500 million years ago, raising the Appalachian Mountains. However, the intrusive volcanic rocks northwest of the Ramapo and unnamed, parallel faults are older than the continental collision and are remnants of the earlier, pre-supercontinent land masses. Most of the rest of the bedrock, such as the Mesozoic sedimentary rocks northwest of the fault system, formed when the supercontinent rifted apart about 200 million years ago to form what are now the northeastern U.S., the Atlantic Ocean, and Europe.

According to the U.S. Geological Survey, people in the New York-Philadelphia urban corridor have felt small earthquakes since colonial times and have suffered damage from infrequent larger ones. New York City suffered earthquake damage in 1737 and 1884. Moderately damaging earthquakes strike somewhere in the urban corridor roughly twice a century, and smaller earthquakes such as today's are felt roughly every 2-3 years.

Although less frequent, earthquakes in the central and eastern U.S. are typically felt over a much broader region than in the western U.S. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 60 miles from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 300 miles from where it occurred, and sometimes causes damage as far away as 25 miles.

And, no, the earthquake has nothing to do with D.E.I., Taylor Swift, or the impending solar eclipse.