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\journal{International Review of Financial Analysis}

\begin{document}

\begin{frontmatter}

\title{Faster, Wider, More Fragile? The Evolution of Intraday Shock
Propagation in U.S. Equity Markets, 1992--2026}

\author[uqo]{Simon-Pierre Boucher\corref{cor1}}
\ead{simon-pierre.boucher@uqo.ca}
\cortext[cor1]{Corresponding author.}
\affiliation[uqo]{organization={D\'epartement des sciences administratives,
            Universit\'e du Qu\'ebec en Outaouais},
            addressline={Pavillon Alexandre-Tach\'e, 283 boulevard Alexandre-Tach\'e},
            city={Gatineau},
            postcode={J9A 1L8},
            state={Qu\'ebec},
            country={Canada}}

\begin{abstract}
We study how intraday shocks travel across U.S.\ equities using
\DataRowsBn{} billion one-minute bars covering the full listed and delisted
cross-section from 1992 to 2026. Three facts emerge. First, markets absorb
idiosyncratic shocks: across \NEventsRealMM{} million isolated $\pm6\sigma$
one-minute events, abnormal market-wide propagation---measured against matched
within-stock placebos---is economically zero at horizons of 1 to 120 minutes,
in every decade. Second, transmission is concentrated and has intensified:
the shocked stock's twenty closest lagged-correlation neighbors respond
strongly (+\NetDiffFive$\times10^{-4}$ abnormal jump probability at five
minutes), and this network channel tripled between the 1990s and the 2020s
while market-wide spillover stayed at zero. Third, tail synchronization
migrated to the market-wide minute: ordinary extreme minute returns became
roughly \ExtremeDropFactor{} times less frequent than in the 1990s, the Epps
curve flattened (one-minute pairwise correlation \EppsNinetySixOne{} in 1996
versus \EppsFifteenOne{} in 2015), and severe co-jump minutes spike by orders
of magnitude in 2008 and 2020. Modern markets absorb the idiosyncratic and
transmit the systemic.
\end{abstract}

\begin{keyword}
Intraday shock propagation \sep market fragility \sep high-frequency data \sep
contagion \sep co-jumps \sep Epps effect \sep market microstructure
\JEL G10 \sep G12 \sep G14
\end{keyword}

\end{frontmatter}

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\section*{CRediT authorship contribution statement}
\textbf{Simon-Pierre Boucher:} Conceptualization, Methodology, Software,
Validation, Formal analysis, Investigation, Data curation, Writing -- original
draft, Writing -- review \& editing, Visualization.

\section*{Declaration of competing interest}
The author declares no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this
paper.

\section*{Data availability}
The underlying market data were originally sourced from Finnhub.io
(\url{https://finnhub.io}). The aggregated monthly archive used in this study is publicly accessible;
the exact pinned snapshot, full replication code, manifests, checksums, and
machine-readable results accompany the paper.

\section*{Acknowledgments}
[To be completed.]

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