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John C. Lahr

Publications and source records attributed to John C. Lahr.

31 records · Page 2Linked to original sources
USGS1981

Eastern Gulf of Alaska seismicity: Annual report to the National Oceanic and Atmospheric Administration for April 1, 1980, through March 31, 1981

The objective of this research is to collect and analyze data on earthquake activity in the northeast Gulf of Alaska (NEGOA) and adjacent onshore areas in order to develop a better regional seismotectonic model and more accurately assess the earthquake potential. This information is critical to the establishment of criteria for the safe development of oil and gas. Large (M s > 7) historical earthquakes have occurred in and around the NEGOA, and recent studies suggest that the NEGOA is a likely site for a magnitude 8 or larger earthquake to occur within the next two or three decades. A great earthquake (Ms>8) associated with low-angle oblique underthrusting of the sea floor beneath continental shelf could be accompanied by strong ground shaking throughout much of the eastern Gulf of Alaska, possibly from Cross Sound to Kayak Island (Page, 1975), and could trigger tsunamis, seiches, and submarine slumping, any of which could be hazardous to offshore and coastal structures (Meyers, 1976).

Alaska↗
USGS1979

HYPOELLIPSE: A computer program for determining local earthquake hypocentral parameters, magnitude, and first motion pattern

HYPOELLIPSE is a computer program for determining the hypocenters of local or near regional earthquakes and for each event the ellipsoid which encloses the one standard deviation region. Traveltimes are determined from a horizontally layered crustal structure or from a linear increase of velocity with depth. Arrival times for the first arrival of P-waves and S-waves, and S-minus P interval times can be used in the solutions. Arrival times for crustal refractions such as Pn, even at distances where they do not arrive first, can also be used. Each arrival can be weighted according to the reading clarity, epicentral distance to the station, and the deviation of its residual from the mean. The hypocenter is found using Geiger's method to minimize the root mean square (RMS) of the traveltime residuals. The magnitude of each event is calculated from the maximum amplitude and/or the signal duration. The first motions can be plotted on the printer using an equal area lower focal hemisphere projection. A map showing the location of each epicenter can be plotted directly on the printer or punched card output can be used as input to another program for plotting on a graphic plotter. The azimuth and apparent velocity of a plane wave crossing the array from a distant source can also be determined.

Open-File Report↗
USGS1979

Interim report on the St. Elias, Alaska earthquake of 28 February 1979

On 28 February 1979 an earthquake with surface wave magnitude (Ms) of 7.7 (W. Person, personal communication, 1979) occurred beneath the Chugach and St. Elias mountains of southern Alaska (fig. 1). This is a region of complex tectonics resulting from northwestward convergence between the Pacific and North American plates. To the east, the northwest-trending Fairweather fault accommodates the movement with dextral slip of about 5.5 cm/yr (Plafker, Hudson, and others, 1978); to the west, the Pacific plate underthrusts Alaska at the Aleutian trench, which trends southwestward (Plafker 1969). The USGS has operated a telemetered seismic network in southern Alaska since 1971 and it was greatly expanded along the eastern Gulf of Alaska in September 1974. The current configuration of stations is shown in Figure 9. Technical details of the network are available in published earthquake catalogs (Lahr, Page, and others, 1974; Fogleman, Stephens, and others, 1978). Preliminary analysis of the data from this network covering the time period September 1, 1978 through March 10, 1979, as well as worldwide data for the main shock will be discussed in this paper.

Alaska↗