On the estimation of focal depth from macroseismic data

Abstract

Gassmann's method (Jahresbericht des Schweltzerischen Erdbebendlensts, 1925) for determining focal depth from the areas enclosed within the various isoseismal lines, leads (after correcting a misprint in the original paper) to the formula in which J is the maximum intensity, Rj is the radius of the area enclosed within isoseismal j, h is the depth of the focus below the surface of the ground, p is an empirical absorption‐coefficient, and M is the logarithmic modulus log10 e. This formula has been tried on the two strong earthquakes at Helena, Montana, on October 18 and 31, 1935. The fit is not good; the exponential variation of the energy with distance appears to be too strong.

Location and Felt Reports for the 25 April 2010 mbLg 3.9 Earthquake near Alice, Texas: Was it Induced by Petroleum Production?

“…7). The 1997 observation of a surface crack is consistent with a shallow focal depth, as is the relatively small area of the highest-intensity felt regions for both the 1997 and 2010 earthquakes (e.g., Blake, 1941).…”

Earthquakes at intermediate depths in south-west New Zealand

“…3) shows intensity 4 occurring over a much greater range of epicentral distances than would be found in a shallow-focus earthquake. The quantitative relationship between focal depth and the isoseismal patterns discussed by Blake (1941), and more fully by Gutenberg and Richter (1942) Earthquakes with intermediate or deep foci are in the main restricted to certain types of unstable region. They are characteristic of the more active continental margins and island arcs, but are rare in other environments, the only important exceptions being a small area of strong intermediate activity in the Hindu Kush and another in Rumania.…”

Seismic hazard assessment in the Ionian Islands based on observed macroseismic intensities

“…After trials of different relations between magnitude and epicentral intensity, and of different intensity-attenuation relations, the three catalogues of calculated intensities were obtained by transforming magnitude into epicentral intensity, in the Mercalli-Cancani-Sieberg (MCS) scale, with the Stamelou (1986) relations for quakes with focus depths less than 20 and 40 kin, while for the deeper events the Karnik (1969) relation was applied. The Blake (1941) intensity-attenuation relation was used with the parameter proposed by Stamelou (1986) for the same two classes of shallow events; the deeper events were elaborated in the same way as those in the intermediate interval. The local catalogues computed in this way for the period 1600-1989 contain 1814 shakings at Argostoli (1667 in this century), 1005 at Corfu (861 this century), and 1388 at Leukas (1242 in this century), with local intensity larger than, or equal to, 3 MCS.…”