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Quality Assessment of Puerto Rico Seismic Network Post-Hole Stations

Trahan, Kimberly; Vanacore, Elizabeth

Undergraduate Research I | Fall 2021 | University of Puerto Rico- Mayaguez | Un-published

Abstract:

This paper is a quality assessment of the new post-hole seismometers that were installed through the island of Puerto Rico. The goal was to improve seismic monitoring. In order to conduct the quality assessment, we compared the performance of the post-holes to the vault seismometers via two methods: 1. comparing and analyzing the power spectral densities (PSDs) of the stations co-located and have overlapping operational times, and 2. comparing the signal-to-noise ratio (SNR) of local and teleseismic events recorded at both the vault and post-hole seismometers. The results observed in this study show that while the post-hole seismometers do perform better than the vault seismometers, the difference is not that much greater. For greater results, the post-holes would have to be installed deeper. This quality assessment is going to support the IRIS initiative for creating a standardized practice for post-hole installations by providing it with quantitative data.

Keywords: Quality Assessment, Puerto Rico, Post-hole seismometer, Vault seismometer, PSD, SNR

Characterization of the 2020 Mw 5.7 Magna, Utah Seismic Sequence Wavefield Decay

Trahan, Kimberly; Vanacore, Elizabeth; Lecocq, Thomas

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Undergraduate Research II | Spring 2022 | University of Puerto Rico- Mayaguez | Un-published,

Presented at AGU'22 as a poster and oral presentation. Presented at SSA'23 as a poster.

Abstract:

On March 18, 2020, an M5.7 earthquake occurred in Magna, Utah; the event struck a populated, urban area which raised the concerns of aftershocks. An aftershock forecast was provided by the USGS and the University of Utah Seismic Network (UUSS) using the earthquake catalogue. This study was conducted to observe and analyze the full wavefield of the aftershock sequence to see if it followed an Omori-Utsu Law-like trend. The RMS ground motion displacements were derived using the SeismoRMS python code of (Lecocq, et al., 2020) from Power Spectral Densities recorded by the UUSS stations located around the epicenter. An Omori-Utsu Law-like noise decay was observed at frequency bands of 4.0-14.0Hz and 4.0-20.0Hz at the stations located within 40km from the epicenter, as well as at a station located 113km away. The observations presented here indicate that the full wavefield may be used as a Omori-Utsu proxy at local seismic stations, but may have limitations due to the impacts of cultural noise as well as local geologic structures that influence the propagation of the seismic wavefield. The data collected in this study will be combined with data from other locales to see if the observable trend is endemic across all significant earthquake events irrespective of magnitude or tectonic setting. Ultimately the goal is to develop constitutive equations that may potentially provide an avenue for the development aftershock forecasts that do not rely on earthquake catalogue completeness or a network of seismic stations.

Keywords: Seismology, aftershock decay, seismic sequence, Magna earthquake, Omori-Utsu Law

AGU Abstract ID: 1122904

AGU Session: ANSS (Seismology)

SSA Session: Session: Earthquake Source Parameters: Theory, Observations and Interpretations [Poster]

Full Wavefield Aftershock Decay from RMS Ground Motion Displacements: An Omori-Utsu Law Proxy

Vanacore, Elizabeth; Friedman, Coralis; Trahan, Kimberly; Lecocq, Thomas

Puerto Rico Seismic Network (PRSN) | Presented at AGU'22 as a poster presentation.

Abstract:

During the COVID-19 pandemic in 2020, global observations of the reduction of cultural noise using the Root Mean Square (RMS) ground motion displacements derived from power-spectral densities made headlines. However, for stations in Southwestern Puerto Rico the signal due to cultural noise reduction was masked by a dominant exponential decay signal. Analysis of this decay suggested that the observed wave field was dominated by the local earthquakes associated with the 2019- 2020 Southwest Puerto Rico Seismic Sequence. This Omori-Utsu like decay, specifically at frequencies >4Hz, was additionally observed at seismic stations within local distances of the recent Ridgecrest and Manga Utah earthquake sequences suggesting this is an endemic feature of aftershock sequences. Based on these three data sets, here we present a summary analysis of this full wave field decay signal as an Omori-Utsu proxy. An equation with an Omori-Utsu form may be used to fit the observations such that DisplacementRMS(t)=k/(t+c)p . Here k is defined as the maximum displacement associated with the main shock, max(DisplacementRMS). The constant c is related to the underlying stress and the wave field signal lost due to attenuation, the latter is similar to catalogue magnitude completeness. The constant p here is between ~0.5-1.5 similar to the classic Omori-Utsu equation. This equation fit indicates that RMS ground motion displacements may act as a proxy for Omori-Utsu decays without the need for a local earthquake catalog. While promising, in its current form the proxy does have some limitations. The ground motion displacement observations are influenced by geologic factors such as site amplification. Consequently, a record of the main shock is currently necessary for the use of this proxy as geologic factors are more influential in observations than factors such epicentral distance. Additionally, given the frequency bands of local earthquakes and cultural noise overlap, over time cultural noise may become greater than the decay signal especially in urban environments. Despite these limitations, the consistent observations of this full wave field decay in various sequences and the Omori-Utsu proxy equation presented here indicate that the full wave field decay may be a useful observation for the future of aftershock forecasting.

AGU Abstract ID: S22D-0196