By Laurie Brenner – Updated Mar 24, 2022

When most people think of U.S. earthquakes, California dominates the mental image. Yet, Pennsylvania hides a legacy of ancient tectonic activity, most notably the Ramapo fault and seismic zone that continue to generate tremors. According to the U.S. Geological Survey (USGS), the Ramapo fault runs from New York through New Jersey into southeastern Pennsylvania, producing a steady stream of micro‑quakes.

Earthquake Magnitude Scale

To gauge the potential damage of a quake, Michigan Tech University outlines the following ranges:

• ≤2.5 – Generally imperceptible and causes no damage.
• 2.5–5.4 – Felt by many, but typically only minor structural impact.
• 5.5–6.0 – Can produce slight damage depending on building construction.
• 6.1–6.9 – Potentially significant damage in densely populated areas.
• 7.0–7.9 – Major earthquakes that often lead to severe damage.
• ≥8.0 – Capable of destroying entire communities near the epicenter.

East Coast Fault Line Earthquakes

Most East Coast quakes register below magnitude 4.0. However, the 1884 event that reached magnitude 5.2 near the Ramapo fault destroyed chimneys in New York and was felt as far south as Virginia and as far north as Maine. Since then, the region has experienced several 2.0–4.5 events with negligible damage, but the prevalence of historic brick and masonry buildings means that a future mid‑range quake could still pose a risk.

Longest Fault Lines in the U.S.

At over 800 miles, the San Andreas fault is the longest in the country, responsible for the 1906 San Francisco earthquake. The Cascadia subduction zone, spanning 680 miles from Oregon to British Columbia, is second in length and could unleash a magnitude 9.0‑plus quake that would devastate the Pacific Northwest and trigger a tsunami within 20–30 minutes.

The New Madrid Seismic Zone—extending from southeastern Missouri to western Tennessee—produced a 7.5‑magnitude quake in 1811, followed by more than 2,000 aftershocks over five months. In Pennsylvania, the Ramapo seismic zone and an active fault line in Erie produced 5.2‑magnitude quakes in 1998, underscoring the state’s vulnerability.

The Ring of Fire

Roughly 90 % of global earthquake activity—and 75 % of volcanoes—occurs along the Pacific Ring of Fire. The newly discovered Banda Detachment in eastern Indonesia represents the world’s largest exposed fault, spanning over 23,000 square miles and extending 4 miles below sea level.

Types of Earthquake Faults

Faults form where tectonic plates meet. Three primary geological boundary types—divergent, transform, and convergent—give rise to distinct fault mechanisms:

• Strike‑slip faults (common on transform boundaries) involve horizontal plate motion.
• Normal faults (typical of divergent boundaries) occur when one block drops below another.
• Thrust (reverse) faults (characteristic of convergent boundaries) push one block upward over another.

Many seismic zones, such as the San Andreas fault, combine these styles; the San Andreas is approximately 95 % strike‑slip with a 5 % thrust component.

For more detailed data, consult the USGS Earthquake Hazards Program.