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A. C. Johnston

Publications and source records attributed to A. C. Johnston.

13 recordsLinked to original sources
USGS2003

Estimating locations and magnitudes of earthquakes in eastern North America from Modified Mercalli intensities

We use 28 calibration events (3.7 < or = M < or = 7.3) from Texas to the Grand Banks, Newfoundland, to develop a Modified Mercalli intensity (MMI) model and associated site corrections for estimating source parameters of historical earthquakes in eastern North America. The model, MMI = 1.41 + 1.68 XM - 0.00345X Delta - 2.08log (Delta), where Delta is the distance in kilometers from the epicenter and M is moment magnitude, provides unbiased estimates of M and its uncertainty, and, if site corrections are used, of source location. The model can be used for the analysis of historical earthquakes with only a few MMI assignments. We use this model, MMI site corrections, and Bakun and Wentworth's (1997 technique to estimate M and the epicenter for three important historical earthquakes. The intensity magnitude M1 is 6.1 for the 18 November 1755 earthquake near Cape Ann, Massachusetts; 6.0 for the 5 January 1843 earthquake near Marked Tree, Arkansas; and 6.0 for the 31 October 1895 earthquake. The 1895 event probably occurred in southern Illinois, about 100 km north of the site of significant ground failure effects near Charleston, Missouri.

Bulletin of the Seismological Society of America↗
USGS2002

Modified Mercalli Intensities (MMI) for some earthquakes in eastern North America (ENA) and empirical MMI site corrections for towns in ENA

Modified Mercalli Intensity (MMI) assignments for earthquakes in eastern North America (ENA) were used by Bakun et al. (submitted) to develop a model for eastern North America for estimating the location and moment magnitude M of earthquakes from MMI observations. MMI assignments for most of the earthquakes considered by Bakun et al. (submitted) are published. MMI assignments for 6 other earthquakes used by Bakun et al. (submitted) are listed in this report: November 18, 1755 near Cape Ann, Massachusetts; January 5, 1843 near Marked Tree, Arkansas; October 31, 1895 in southern Illinois; November 18, 1929 on the Grand Banks, Newfoundland; September 26, 1990 in southeast Missouri; and May 4, 1991 near Risco, Missouri. MMI empirical site corrections developed and used by Bakun et al. (submitted) are also listed in this report.

Arkansas;Illinois;Massachusetts;Missouri;Newfoundl↗
USGS2002

Modified Mercalli intensities (MMI) for large earthquakes near New Madrid, Missouri, in 1811-1812 and near Charleston, South Carolina, in 1886

Large historical earthquakes occurred in the eastern United States on December 16, 1811 near New Madrid, MO, on January 23, 1812 near New Madrid, MO, on February 7, 1812 near New Madrid, MO, and on September 1, 1886 near Charleston, SC. Modified Mercalli Intensity (MMI) assignments for these earthquakes were used by Bakun et al. (submitted) to estimate the location and moment magnitude M of these earthquakes from MMI observations. The MMI assignments used by Bakun et al. (submitted) are listed in this report.

Missouri, South Carolina↗
USGS1996

The enigma of the New Madrid earthquakes of 1811-1812

Continental North America's greatest earthquake sequence struck on the western frontier of the United States. The frontier was not then California but the valley of the continent's greatest river, the Mississippi, and the sequence was the New Madrid earthquakes of the winter of 1811–1812. Their described impacts on the land and the river were so dramatic as to produce widespread modern disbelief. However, geological, geophysical, and historical research, carried out mostly in the past two decades, has verified much in the historical accounts. The sequence included at least six (possibly nine) events of estimated moment magnitude M ≥ 7 and two of M ≃ 8. The faulting was in the intruded crust of a failed intracontinental rift, beneath the saturated alluvium of the river valley, and its violent shaking resulted in massive and extensive liquefaction. The largest earthquakes ruptured at least six (and possibly more than seven) intersecting fault segments, one of which broke the surface as a thrust fault that disrupted the bed of the Mississippi River in at least 2 (and possibly four) places.

Arkansas, Kentucky, Missouri, Tennessee↗
USGS1994

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.

Tennessee↗
USGS1992

Geologic implications of earthquake source parameters in central and eastern North America

The relations between geology and earthquakes remain mostly enigmatic in North America east of the Rocky Mountains. Hypocentral depths and the dips of rupture zones (preferred nodal planes) are among the seismological variables most likely associated with geologic structure in the stable continental region (SCR) of central and eastern North America (CENA). We compiled well-constrained depths and dips for 20 SCR shocks in CENA with magnitudes (M) between 4.9 and 6.8. The appendix lists the values we chose, our reasons for selecting specific values, and the uncertainties associated with each value. The median depth is 10 km, but an aseismic interval from about 15 to 25 km depth separates three midcrustal hypocenters from the others. The median dip of preferred nodal planes is 54°, and the larger earthquakes tend to rupture along shallower dips. These results combined with geologic evidence lead to four inferences about SCR earthquakes with M ≥ 4.9 in CENA. (1) If rupture generally progresses updip, then the tendency for large earthquakes to rupture more shallowly-dipping surfaces than small earthquakes can explain the scarcity of surface ruptures in CENA. (2) The few midcrustal earthquakes might be evidence that mafic or granulite facies rocks are present. (3) CENA seismicity is concentrated in Late Proterozoic and Phanerozoic orogens and extended terranes that rim an older, stable, central craton. Unusually shallow large CENA earthquakes have been observed only in the central craton, where they might cause severe but localized shaking. (4) Earthquakes in the Appalachian orogen tend to be shallower than those of comparable size in the Late Proterozoic rifted continental margin that underlies and borders the northwest side of the orogen. As a result, some Appalachian earthquakes could cause more localized shaking than comparable shocks in the rifted margin.

eastern North America↗
USGS1990

An earthquake strength scale for the media and the public

Let's face it: seismologists do a pretty poor job of communicating the facts about our science to the public. Earthquake magnitude is the classic example. How many of us have struggled to explain the Richter scale? We explain that it is logarithmic, with each unit indicating a factor of 10 increase, but this really represents a factor of 32 increase in intrinsic earthquake size, and in any case we don't use the Richter scale anymore. By then the unfortunate listener is reeling and can be dispatched quietly by mentioning negative magnitudes or saturation. We even wonder why the audience or the reporter has this glazed look when we we finish. A local engineer, E.P Hailey, pointed this problem out to me shortly after the Loma Prieta earthquake. He felt that three problems limited the usefulness of magnitude in describing an earthquake to the public; (1) most people don't understand that it is not a linear scale; (2) of those who do realized the scale is not linear, very few understand the difference of a factor of ten in ground motion and 32 in energy release between points on the scale; and (3) even those who understand the first two points have trouble putting a given magnitude value into terms they can relate to. In summary, Mr. Hailey wondered why seismologists can't come up with an earthquake scale that doesn't confuse everyone and that conveys a sense of true relative size. Here, then, is m attempt to construct such a scale.

Earthquakes & Volcanoes (USGS)↗
USGS1981

Multiple asperity model for earthquake prediction

Large earthquakes often occur as multiple ruptures reflecting strong variations of stress level along faults. Dense instrument networks with which the volcano Kilauea is monitored provided detailed data on changes of seismic velocity, strain accumulation and earthquake occurrence rate before the 1975 Hawaii 7.2-mag earthquake. During the ???4 yr of preparation time the mainshock source volume had separated into crustal volumes of high stress levels embedded in a larger low-stress volume, showing respectively high- and low-stress precursory anomalies. ?? 1981 Nature Publishing Group.

Nature↗