Sn to Sg Conversion at the U.S. Atlantic Continental Margin

Abstract:

Isacks and Stephens [1975] observed a secondary phase with high frequency Lg characteristics that arrived soon after the Sn wave on seismograms generated by events in the West Indies. They concluded that an Sn-to-Lg conversion occurred at the continental margin, where the crust suddenly thickens. A later study on conversion along the continental margin was done by Seber et al. [1993] in Morocco. They noted that historically Morocco has experienced more damage from earthquakes occurring at the Azores-Gibraltar seismic zone (e.g. the M 8.7-9.0 Lisbon earthquake) at distances up to 500-1000 km than from those within the country. They conclude in their study that there are two parallel Sn-to-Sg conversion zones along the coast and interior of Morocco, where Sg is equivalent to Lg at shorter distances. We have seen similar Sn-to-Sg conversions for a M 5.2 event occurring ~1400 km off the Atlantic Coast on Dec. 23, 2013 using EarthScope’s Transportable Array (TA). We perform a travel time back-projection based on the geometry of the raypaths, similar to Seber et al. [1993], to determine the location of the conversion points for several Atlantic events and compare with seismograms generated by continental events. We also investigate the possibility of a second conversion at the crustal boundary between the Appalachians and the Coastal Plain. The MAGIC Array is used in tandem with TA to closely observe the propagation characteristics of the converted wave as it travels through the continent. With the sizeable increase in station coverage we are in a position to study this conversion in greater detail. Understanding the causes of Sn-to-Sg conversion and the conditions needed to produce it can lead to insight into the geometry and characteristics of the continental shelf and inland crustal boundaries. Learning about this conversion is also needed to determine seismic hazard along coastal areas, where high amplitude converted shear waves can cause unexpected levels of damage.