Investigation of seismic precursors before major earthquakes and of the state of stress on fault planes

Thesis/Dissertation · Fri Jan 01 04:00:00 EST 1982

OSTI ID:5862582

von Seggern, D H

With seismic bulletins from the National Earthquake Information Service, and the LASA and NORSAR arrays for the decade of the 1970's, a search was made for seismic precursors of various types. Using a cutoff of m/sub b/ = 5.8, 510 main shocks were identified to guide the search. The true rate of foreshock occurrence was statistically established to be less than 20%. Foreshocks were almost exclusively associated with shallow main shocks. Very weak trends were found for seismicity level, mean magnitude, and the distance to the main shock of events as the main shock approaches by a method of averaging data over main shock regions. A new variable is proposed to track departures from spatial-temporal randomness in seismic zones - it is the product of interevent time and distance of the events to the main shock. An experiment using LASA recordings of nearly 400 events in the lower Kuril Islands revealed that measured stress drops did not appear to change prior to five main shocks in that region. Unrelated to precursory behavior, it was found that the stress drops increased significantly with magnitude (m/sub b/) or with seismic moment. A numerical one-dimensional fault model was employed to simulate seismic behavior. This model produced seismic events with statistical properties akin to true ones. Altogether the empirical and model studies lead to a theory which emphasizes the random component of stress on fault planes. It is hypothesized that the wavenumber spectral content of the stress field is whitened after main shocks and that subsequent activity reddens this spectral content, thereby eventually making conditions favorable for another main shock.

• OSTI ID: 5862582
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
ASIA
EARTHQUAKES
EASTERN EUROPE
EUROPE
FORECASTING
ISLANDS
KURILE ISLANDS
SEISMIC EVENTS
SEISMIC WAVES
SEISMICITY
USSR