Studies with a small seismic array in east-central Minnesota

Thesis/Dissertation · Mon Jan 01 04:00:00 EST 1979

OSTI ID:5365544

Greenhalgh, S A

The Central Minnesota Seismic Array comprises an FM, telemetred installation of 6 sp vertical seismometers located on a circle of diameter 27 km. The array is capable of detecting teleseisms down to magnitude 4.5 at a distance of 60/sup 0/ and local events down to magnitude 2.1 at a distance of 100 km. Many seismic events, both earthquakes and explosions, have been recorded during the 2 years of array operation but no event has been identified as an earthquake originating within the Minnesota-Lake Superior region. Teleseismic magnitude corrections have been computed for 61 earthquakes. Values are all positive and are largest for the low magnitude events. The author's explanation lies in a lower than average local attenuation for short period P waves. Array bias in event location was tested against 85 earthquakes. Station corrections based on the residuals greatly improve array location capability and agree quantitatively with time delays predicted from earlier upper crustal refraction studies. A long range refraction profile, extending from the Mesabi iron range in northern Minnesota to the seismic array, has also been carried out. A striking feature of the profile is the absence of direct P waves in the distance range 60-120 km. Dispersion relations for the fundamental mode Rayleigh wave, observed on Mesabi mine blast records, have been interpreted to reveal a 1.2 km thick surface layer of shear velocity 2.7 km/s overlying a half space of shear velocity 3.3 km/s. The geologic interpretation of the mode is that of the Animikie Group metasediments overlying the Lower Precambrian gneisses and granites.

• Research Organization: Minnesota Univ., Minneapolis (USA)
• OSTI ID: 5365544
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
ABSORPTION
EARTHQUAKES
EXPLOSIONS
FEDERAL REGION V
GEOLOGIC MODELS
GNEISSES
GRANITES
IGNEOUS ROCKS
MEASURING INSTRUMENTS
METAMORPHIC ROCKS
MINNESOTA
NORTH AMERICA
OPERATION
P WAVES
PARTIAL WAVES
PLUTONIC ROCKS
RAYLEIGH WAVES
REFRACTION
ROCKS
SEISMIC ARRAYS
SEISMIC EVENTS
SEISMIC SURFACE WAVES
SEISMIC WAVES
USA