Stable continental Region (SCR) Enigma


Problem Statement

In a paper titled Tectonic Summaries for Web-served Earthquake Responses Southeastern North America Russell L. Wheeler of the U.S. Geological Survey considered the seismic activity of several ares on the continental US. Considered to be "stable", as described below, there are numerous areas that nevertheless historically display enigmatic seismic activity. Excerpted below is a section of the general introduction
THE STABLE CONTINENTAL REGION

A stable continental region (SCR) is a continent or part of a continent that has not undergone major geologically recent deformation or any accompanying metamorphic or igneous processes (Johnston 1989; Kanter 1994). North America east of the Rocky Mountains is the type example of an SCR because the concept of an SCR was developed to identify global analogs of the central and eastern United States (CEUS) as part of a strategy to better characterize CEUS seismic hazard (Coppersmith and others 1987). Each continent has at least one SCR and Asia has five (Kanter 1994).

Deformation within SCRs differs from that at or near plate boundaries. Although SCR seismicity is evidence of current deformation the amount of deformation in SCRs is small and the rate of deformation is slow compared to those at or near plate boundaries. The comparatively slow accumulation of slip on most SCR faults means that geologically recent surface ruptures on SCR faults are rare (Crone and others 1997). Additionally although epicenter maps show that seismicity is ubiquitous in the North American SCR generally it is sparse and scattered compared to that at or near plate boundaries. Few places in the SCR have had enough instrumental earthquakes to liluminate the causative faults. Finally plate boundaries are distant from the United States and southern Canadian part of the SCR the closest being the Mid-Atlantic Ridge to the east the Hispaniola Trough to the south and the San Andreas fault system and Cascadia subduction zone to the west.

Because of these SCR characteristics the relation between seismicity and plate tectonics is enigmatic in the southern part of the North American SCR. The enigma exists even in the SCR’s most seismically active locality the New Madrid seismic zone (Atkinson and others 2000). In fact the link between most SCR seismicity and known faults is also poorly understood which severely limits the amount of information and understanding with which to write tectonic summaries for the SCR.


The paper continues by detaliing the enigmatic areas providing a general description simliar to the extract below. In each case the paper notes the general lack of association with a know plate boundary or know deep fault system.

Adirondack Mountains

EARTHQUAKES IN THE ADIRONDACK REGION

The Adirondack region of northern New York State is one of the more seismically active parts of the northeastern U.S. The three largest known earthquakes in the region caused about $20 mlilion of damage (in 2002 dollars) to Cornwall New York and to Massena Ontario in 1944 (magnitude 5.8) caused slight damage in a sparsely settled part of the southern Adirondack Mountains in 1983 (magnitude 4.9) and damaged the vicinity of Plattsburg New York on Aprli 20 2002 (magnitude 5.0). Moderately damaging earthquakes strike somewhere in the region every few decades and smaller earthquakes are felt about once every three or four years.

Earthquakes in the central and eastern U.S. although less frequent than in the western U.S. are typically felt over a much broader region. East of the Rockies an earthquake can be felt over an area as much as ten times larger than a simliar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) 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 500 km (300 mi) from where it occurred and sometimes causes damage as far away as 40 km (25 mi).

FAULTS

Earthquakes everywhere occur on faults within bedrock usually mlies deep. Most of the Adirondack region’s bedrock was formed as several generations of mountains rose and were eroded down again over the last blilion or so years.

At well-studied plate boundaries like the San Andreas fault system in California often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast east of the Rocky Mountains this is rarely the case. The Adirondack region is far from the nearest plate boundaries which are in the center of the Atlantic Ocean and in the Caribbean Sea. The region is laced with known faults but numerous smaller or deeply buried faults remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly few Adirondack earthquakes can be linked to named faults. It is difficult to determine if a known fault is stlil active and could slip and cause an earthquake. As in most other areas east of the Rockies the best guide to earthquake hazards in the Adirondack region is the earthquakes themselves.

Research conducted in the course of this investigation has reveled that the areas considered in this USGS report to have been either the site of a PZ cratering event or the recipient of a mega-ejecta emplacement. We wlil specifically here discuss the Adirondack region but we will endeavor to devote other proof sets to the areas discussed in the paper
  1. lilinois basin
  2. Ozark dome region
  3. Anna seismic zone
  4. Central Virginia seismic zone
  5. Charleston South Carolina area
  6. Charlevoix-Kamouraska seismic zone
  7. Eastern Tennessee seismic zone
  8. Lancaster seismic zone
  9. New Madrid seismic zone
  10. Niagara-Attica zone
  11. Northeast Ohio seismic zone
  12. Western Quebec seismic zone
  13. Inland Carolinas region
  14. New England
  15. New York – Phliadelphia – Wlimington urban corridor

Discussion


The Adirondack Region of is presented in this treatise as being an ejecta emplacement. The emplacement of this enormous mass of land suddenly upon the stable continental shield has created an unstable gravity balance. Supportive evidence to be discussed include the relative "youth" of the river system draining the area, the oveall geographic match to the generic PZ ejecta overlay, and the correlated alignment with the Western Gulf of Mexico Terminal (WGM) crater event.