Seismicity parameters preceding moderate to major earthquakes

Journal Article · Sat Oct 10 04:00:00 EDT 1981 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB086iB10p09325· OSTI ID:5757264

von Seggern, D; Alexander, S S; Baag, C

Seismic events reported in the bulletins of the two large arrays, LASA and NORSAR, were merged with those from the NEIS bulletin for the period 1970-1977. Using a lower cutoff of m/sub b/ = 5.8, 510 'main shocks' within the P range of LASA or NORSAR were selected for this period; and various seismicity trends prior to them were investigated. A search for definite foreshocks, based on a significantly short time delay to the main shock, revealed that the true rate of foreshock occurrence was less than 20%. Foreshocks are almost exclusively associated with shallow (h<100 km) main shocks. Averaging shows that the seismicity level around the main shock increases somewhat for 10 days before main shocks; this feature peaks in the last 3--4 hours prior to the main shocks. Again by averaging, the seismicity about main shocks is shown to tend with time toward the main shock as its origin time is approached, but the average effect is small (approx.10% change).Using a new variable to track the departures from both spatial and temporal randomness, the Poisson-like behavior of deeper seismicity (>100 km) was demonstrated. For shallow events (<100 km) this variable reveals numerous instances of clustering and spatial-temporal seismic gaps, with little tendency toward a uniformity of behavior prior to main shocks. A statistical test of the validity of seismic precursors was performed for approximately 90 main shock regions which had sufficient seismicity. Using a five-variable vector, (interevent time, interevent distance, magnitude, epicentral distance to main shock, and depth difference relative to main shock) for each event in a 'precursory' time window of 500 days before the main shock and for each event in a 'normal' time window of 500 days before that, the null hypothesis of equal vector means between the two groups was tested. At 90% confidence levels, less than 30% of the main shock regions were thus found to exhibit precursory seismicity changes.

• Research Organization: Teledyne Geotech, Alexandria, Virginai 22313
• OSTI ID: 5757264
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 86:B10; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology &amp; Tectonics-- (1980-1989)
EARTHQUAKE MAGNITUDE
EARTHQUAKES
EPICENTERS
FORESHOCKS
MATHEMATICS
PRECURSOR
SEISMIC EVENTS
SEISMICITY
STATISTICS