Influence of Incidence Angle on Low-Frequency (0.2 to 1 Hz) Site Responses: An Analysis of Site Response From the Atlantic Coastal Plain of the Eastern US

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Areally extensive sedimentary sequences like the Atlantic Coastal Plain (ACP) or Mississippi Embayment strata of the central and eastern U.S. can have a strong effect on ground motions. The most pronounced effect is amplification of ground motions at fundamental frequencies defined by the vertical resonance of S-waves between the surface and the often strong reflector at the base of the sediments. For near-vertical arrivals, resonances are initiated by waves reflected down from the surface, and for thick sedimentary sequences frequencies can be below 1 Hz. For waves arriving from the side, such as from regional events in adjacent areas of the continent, vertical resonance involves arrivals being scattered or refracted as S-waves. Here I use receiver arrays on the ACP strata, which is characterized by resonance peaks at 0.2 Hz and higher, to examine the response from near-vertically incident arrivals compared to waves coming from regional events with a strong component of horizontal arrivals. Data come from the USARRAY sites and temporary stations deployed for active-source or receiver function experiments. Near-vertical arrivals are analyzed from teleseisms, which include body-wave arrivals that refract to near-vertical incidence. Regional earthquakes include events from the central and eastern U.S. and from the Caribbean region, which should include a substantial component of arrivals at shallow angles. Initial results from a limited number of events such as the 2011 Mineral, Virginia, earthquake suggest slightly higher amplification from the regional events at low frequencies (<0.4 Hz), but similar amplifications at other frequencies. Possible interpretations include the influence of surface waves from the regional events in comparison to teleseismic arrivals dominated by other phases, or different durations of shaking at the bedrock reference sites versus the ACP sites. In either of these cases, the different amplifications may be showing the impact of choices made in the site response methodology, especially the choice of analysis window length and the phases included.

Presenting Author: Thomas Pratt

Authors