P-WAVE TRAVEL-TIME RESIDUAL MAP FOR EASTERN NORTH AMERICA

A significant region of abnormally low S-wave velocity has been identified in the northeastern United States by a number of authors including Li et al (2003), Schmandt and Lin (2014), Menke et al. (2016), and Levin et al (2017). The area has been named the Northern Appalachian Anomaly (NAA). This low S-wave region may be a remnant of the passage of the Great Meteor hotspot through New England. Alternatively, the NAA may be caused by a small-scale eddy in the flow regime in the asthenosphere.

In this paper, we present a P-wave travel time residual map for the northeastern part of North America. The impetus for this project was the occurrence of a large (Mw 7.1) deep (609 km depth) earthquake in the Peru-Brazil region on August 24, 2018. The P-waves exhibit extremely strong dilatation first motion to stations in the northeastern US. Because the event was so deep, P-waves to ENA traverse the low-velocity zone only once, reducing attenuation. In addition, the seismograph network was quite dense, a legacy, in part, of the EarthScope program.

P-wave first arrivals across ENA can be measured to high accuracy due to the high signal to noise ratio. Station corrections were computed as the difference between the travel time through a 40-km crust and the travel time through the observed crustal thickness as measured by the EarthScope Automated Receiver Survey (EARS). P-wave travel-time residuals were computed relative to the ak135 model (Kennett et al, 1995). The final correction was to subtract the mean residual from the data, to account for unknown time offsets such as origin time and location uncertainty.

P-wave residuals measured at nearly 300 stations were plotted in map view using ArcGIS. Strong negative residuals, indicating a fast upper mantle occur in the Canadian Shield, as expected. New York State is generally fast, but not as fast as the Shield. Vermont and New Hampshire exhibit both fast and slow residuals. Maine has exclusively slow residuals. In terms of the NAA, the P-wave residual pattern is less well-defined and is shifted northeastward into Maine. Residuals in Pennsylvania are near zero, with generally positive (slow) residuals in the southern Appalachians (Virginia to Georgia). We note that our results are in good agreement with the 40-year-old paper by Taylor and Toksoz (1979).