Arch C. Johnston

The Seismicity of ‘Stable Continental Interiors’

In terms of seismic moment or seismic strain release, the deep ocean basins and the ancient cores—the Stable continental interiors’—of the continents are the deadest places on earth. Averaged over the past 200 years, stable continental crust has generated only ~0.3% of the earth’s annual seismic moment release. This translates into ~600 plate boundary events of M w ≥ 6.0 for every one that occurs in stable continental crust. Moreover, in the last 200 years the largest earthquakes to occur in the stable interiors or at the passive continental margins have been the 1819 Kutch, India, event and the 1812 (Feb 7) New Madrid, North America, event. With estimated seismic moments in the range of M0 = 0.7–2.5×1028 dyne-cm, these are great earthquakes (M w =7.S-8.2) yet their moments are nearly two orders of magnitude less than those of maximum plate margin events. Virtually all of the largest stable continental earthquakes (M w ≥ 7.0) occur in crust that has been stretched and extended in a rifting process that leads to either failed rifts imbedded in the continents or to passive margin formation, whereas the largest plate margin events occur in convergent tectonic settings. These are just a few of a number of the fundamental seismological differences between stable continental interiors and plate boundaries that are explored and quantified in this paper.

The Effect of Large Ice Sheets on Earthquake Genesis

Two continent-scale ice sheets—Antarctica and Greenland—currently exist on earth. The interiors of both continents are virtually aseismic. Is this coincidental or does a causal connection exist between the two observations? An examination of this question is the subject of this paper. It is concluded that with a few reasonable assumptions, ice sheets will indeed inhibit earthquakes by stabilizing potentially seismogenic faults in the underlying brittle crust. This same mechanism may also provide an explanation for the intense late-glacial faulting in Fennoscandia reported elsewhere in this volume.

A seismotectonic model for the 300-kilometer-long eastern Tennessee seismic zone

Ten years of monitoring microearthquakes with a regional seismic network has revealed the presence of a well-defined, linear zone of seismic activity in eastern Tennessee. This zone produced the second highest release of seismic strain energy in the United States east of the Rocky Mountains during the last decade, when normalized by crustal area. The data indicate that seismicity produced by regional, intraplate stresses is now concentrating near the boundary between relatively strong and weak basement crustal blocks.

Refinement of thrust faulting models for the Central New Madrid seismic zone

Abstract We present the results of the joint relocation of events recorded during 1989–1992 by the PANDA network in the central New Madrid seismic zone. The near-surface material in the study area is a gently-dipping layer of poorly consolidated sediments with low P-wave velocity and high V p / V s (estimated values: 1.8 km s −1 and 3). The sediments are underlain by high-velocity Paleozoic rocks. Under the network the difference in sediment thickness is only 0.6 km, but because of the low velocities the location of the events using layered models is affected by errors. Application of the joint hypocentral determination (JHD) technique to a subset of 580 events shows that the single-event locations may be in error by as much as 1 km in depth, depending on where the events are located. Analysis of synthetic data generated for a realistic 3-D velocity model supports the JHD results. The analysis of synthetic data also suggests that a V p / V s ≤ 2.3 is more appropriate for the post-Paleozoic Mississippi embayment sediments. Based on the JHD locations we present a new interpretation of the seismicity, with two en-echelon SW-dipping thrust faults connected by a west-dipping thrust fault. These faults appear associated with the Reelfoot scarp and its northern extension, the Kentucky bend scarp.

Stable continental regions are more vulnerable to earthquakes than once thought

Seismic events at shield areas throughout the world suggest that the stable continental regions (SCR) are much more vulnerable to earthquakes than was once thought. Earthquakes have struck SCRs at a number of locations, including the New Madrid Zone, United States; Tennant Creek, Australia; Ungava, Canada; and Kachchh, Koyna, Latur, and Jabalpur, India. In several developing countries, such as India, the problems caused by SCR earthquakes have become very serious because of high population density and the proliferation of structures not built to withstand earthquake damage. A recent Chapman Conference on SCR earthquakes attracted 90 researchers from 12 countries. Eighty papers were presented. Globally, the stable continental region earthquakes account for about 0.5% of total seismic energy released. The largest SCR earthquakes occurred in the New Madrid zone from 1811 to 1812, when three earthquakes of Mw 7.8-8.1 occurred within just 53 days.

Fault traces Australian quakes

Surface rupture caused by earthquakes is extremely rare in stable continental environments such as the Northern Territory of Australia. However, the Tennant Creek earthquakes of January 22, 1988, rank among the largest known onshore Australian earthquakes, whose surface damage can be seen by these photographs and the cover photograph. The top picture shows the main surface scarp of the Tennant Creek earthquakes. The view is east, and the throw of nearly 1 m (ruler in center of photograph is 30 cm) is south-over-north. Elsewhere on the rupture the scarp took on a more ridgelike morphology, often accompanied by extensive Assuring.