Earthquake Incident Research Articles

Further evidence of tidal influence on earthquake incidence

The rate of release of earthquake energy over the past 65 yr is compared to the relationship of the longitude of perigee of the Moon to the Earth's longitude of perihelion. Certain perculiar phase and local hour angle relationships, of the Moon to earthquake epicenters, are also presented. Finally, the clustering of epicenters near the longitudes of Neptune and Uranus is examined.

A philosophy of seismic zoning

Seismic zoning involves both economic questions, and questions of public safety. If it is accepted that the latter are more important, little justification for differing structural codes in different parts of New Zealand can be found. Consideration of the economic importance of the varying incidence of smaller earthquakes is hampered by the absence of properly costed engineering studies. Microzoning and the study of foundation conditions are of high importance.

The Parkfield, California, earthquakes of 1966

Abstract The characteristics of the Parkfield, California earthquake sequence of 1966 are presented. Historically, the epicentral region is one of the three most seismic areas along the San Andreas fault in central California. It is characterized, however, by a relatively high incidence of large earthquakes in proportion to smaller shocks, compared to other active zones. The 1966 sequence occurred in an area where measured deformation across the fault for 1959-1965 shows a decrease from about 2 cm/year to the north to zero to the south of the area. Neither micro-earthquake nor normal seismic activity prior to the sequence gave indication of its coming. Seismicity before the sequence was confined to the north of the active zone, with some indication of convergence of foci toward the location of the initial shocks. The early aftershock distribution extended 20 km south of the main shock; cracking occurred to 33 km south of the main shock; and intense aftershock activity for the entire sequence extended 27 km south of the main shock. At least 95 per cent of the earthquakes, including the three largest, have P-wave radiation patterns consistent with right lateral transcurrent motion on the San Andreas fault. Earthquakes of the sequence fall very closely along the fault trace. About 75 per cent of the total strain release for the sequence can be accounted for by earthquakes in the main shock region, the principal shock (M = 5.5) contributing only 25 per cent of the total. The sequence is characterized by a high incidence of large aftershocks, an extensive area of aftershock activity, and average focal depths near 5 km-three properties apparently related, and distinguishing two types of sequence traits in central California.

Catalog of Nevada earthquakes, 1852-1960

AbstractNevada earthquakes for the period from about 1852 to 1961 are tabulated on I.B.M. punch cards. During this period 1,173 events with Nevada epicenters were felt, 586 others with Richter magnitudes above 4.0 were recorded and were probably felt by some residents, and approximately 220 were reported in nonspecific terms (e.g. “several aftershocks were felt”). High seismicity of this region is indicated by the fact that on an equal-area basis, during the period 1934-1960, Nevada has had the highest incidence of earthquakes per unit area of any of the conterminous Western States. The seismic activity shows a distinct tendency, statistically inadequate, for a 20-year cycle of activity, with peaks at about 1852(?), 1872, 1894, 1916, 1932-1933, and 1954. The general increase in number of events reported each year correlates well with the increase in Nevada population, but is probably modified by the development of: increasingly sophisticated methods of recording, spacing of seismographic stations, more intensive earthquake investigations of the United States Coast and Geodetic Survey, and changing population density and distritution.A method is developed for correlating between Richter magnitude, earthquake intensity, and size of felt area. This permits estimation of earthquake magnitude for shocks that pre-date instrumental methods of recording.The areal distribution of the earthquake epicenters does not indicate a noticeable change in pattern during this brief historic period, except possible recent intensification of activity near Lake Mead. Seismic activity is concentrated in three zones: a Western Nevada-Eastern California Zone which includes the northern Sierra Nevada frontal faults, Fish Lake Valley-Furnace Creek-Death Valley fault systems and Walker Lane; a 118° Meridian Seismic Zone which contains most of the historic surface faults; and a weak Southern Nevada Transverse Zone that includes the Lake Mead activity.

On Andean structure, Part II: Earthquake risk in Chile

Abstract The problem of earthquake risk in Chile is discussed in terms of seismic history, geotectonic provinces and regional variations in earthquake incidence. Seismic zoning is impractical because all of Chile above Parallel 42° South has been within the epicentral area of some destructive earthquake. The probability concept of earthquake risk is discussed in reference to the seismicity of Santiago. It is found that the earthquake risk at Santiago for an acceleration of 0.1 g is 62.5% in any 10-year period. For the same interval, the risk of a “destructive” earthquake (intensity 8 1/2) is of the order of 20%.