Duration magnitude scale and site residuals for northern Morocco

The first empirical duration magnitude (M-D) formula is developed and tested for the Northern Morocco Seismic Network (NMSNET). This relationship is obtained by relating the IGN (Instituto Geografico National, Madrid) body-waves mbLg(IGN) to the duration (tau), and the epicentral distance (Delta), at 25 analogue stations of the NMSNET for 479 earthquakes with 2.5 less than or equal to mb less than or equal to 5.4, from March 1992 to February 2001. M-D estimates are significantly more precise while introducing a correction term for each of these stations, cSta(j). The magnitude for the i(th) event (M-D)(i) is the mean value of individual M-Dij=-0.14+1.63log(10)(tau(ij))+0.031(Delta(ij))+cSta(j). The cSta(j) corrections reduce considerably the local site effects which influence the recorded durations and cause stations to either overestimate, or underestimate M-D up to 0.5 magnitude units. Average station M-D residuals (-cSta(j)) are found to be independent of the distance from the epicenter to at least 10 degrees. It seems evident that regional geological features in the immediate behavior of stations have a systematic effect on the corresponding obtained residuals: older well-consolidated Precambrian crystalline rocks produce high negative residuals (shorter durations), younger unconsolidated sediments produce high positive residuals (longer durations), whereas, intermediate M-D site residuals appear to be the result of the effect of various factors, principally age and state of consolidation of the bedrock, combined with the local tectonic.