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 (τ), and the epicentral distance (Δ), at 25 analogue stations of the NMSNET for 479 earthquakes with 2.5 ≤ mb ≤ 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 D ij =−0.14+1.63log10(τ ij )+0.031(Δ 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.