Sn to Sg conversion and focusing along the Atlantic Margin, Morocco: Implications for earthquake hazard evaluation

Abstract

Digital data from a telemetered, short‐period seismic network in Morocco provide a new perspective for understanding the cause of severe shaking and macroseismic reports in Morocco produced by large, offshore earthquakes located along the Azores‐Gibraltar seismic zone. Even though the earthquake epicenters are 500–1000 km away from the Moroccan coast, historical records show that such events are capable of producing considerable damage in inland areas. We analyze 15 earthquakes that occurred in this region. The records show multiple S phases with varying frequencies and amplitudes. The S phase with the largest amplitude, usually misinterpreted as Sn, has a phase velocity of 4.2–4.4 km s−1. We show that these S arrivals can best be explained as Sn to Sg converted phases. Calculated locations of the conversion points for these phases exhibit two distinct zones almost parallel to the Atlantic coastline: one is located along the passive continental margin and the other is located about 100 km inland from the coastline. We interpret these two zones to be regions where a sudden change in crustal thickness occurs. Such zones act to focus and magnify the amplitudes of seismic phases. This interpretation explains the unusually strong felt reports within Morocco from such distant offshore events, and it has a significant effect on earthquake hazard evaluation and mitigation studies.

Sedimentary Basins and the Blockage of Lg Wave Propagation in the Continents

“…However, its group arrival time is too early to be the Lg that propagates directly from Novaya Zemlya with group velocity of 3.5 km/sec. This phase is interpreted to be an Lg phase produced by mode conversions of Sn at the interface of the Barents Sea and the Kola Peninsula, similar to those observed at coastlines in other regions for oshore events recorded on land (e.g., ISACKS and STEPHENS, 1975;BARAZANGI, 1977;CHINN et al, 1980;SEBER et al, 1993). The point at which the direct Sn from Novaya Zemlya intersects the Kola Peninsula is at a distance of about 13°from NORSAR, as shown in Figure 3(A).…”

Geophysical and geological interpretation of discrepancies in site residuals in northern Morocco: a simple qualitative approach

“…This structure is responsible for focusing seismic waves towards these stations (Seber et al 1993). The influence of this structure is reflected in a notable increase in recorded durations, and consequently may justify the positive values of residuals of these stations (average 0.12).…”

“…Another late seismic wave phase recorded is also observed, and the travel time duration of this L. AIT BRAHIM E T A L . The large-amplitude, low-velocity, S phase is interpreted as the result of the Sn to Sg conversion (Seber et al 1993). This conversion is attributed to the thickness of the crust along the continental margin and the coastal zones (Seber et al 1993).…”

Duration Magnitude Scale and Site Residuals for Northern Morocco

“…Since then, this domain is a homogeneous stable and competent block with an isostatically compensated crust (BENSARI, 1987;SEBER et al 1996), and it is characterized by poor seismic activity and a particular amplification of the seismic signals. The amplification effect observed at these stations can be attributed to the low impedance of younger, less consolidated quaternary sediments (BEAUDET, 1961;SU et al, 1992) at station sites, or it can be viewed as a systematic result of the presence of a probable zone which causes the signal to be amplified before being acceded by the Mesetian domain (SEBER et al, 1993). Thickness of sedimentary layers at these sites is not important compared to the thickness of the Palaeozoic substratum.…”