We employed the log-periodic power law singularity (LPPLS) methodology to systematically investigate the 2020 stock market crash in the U.S. equities sectors with different levels of total market capitalizations through four major U.S. stock market indexes, including the Wilshire 5000 Total Market index, the S&P 500 index, the S&P MidCap 400 index, and the Russell 2000 index, representing the stocks overall, the large capitalization stocks, the middle capitalization stocks and the small capitalization stocks, respectively. During the 2020 U.S. stock market crash, all four indexes lost more than a third of their values within five weeks, while both the middle capitalization stocks and the small capitalization stocks have suffered much greater losses than the large capitalization stocks and stocks overall. Our results indicate that the price trajectories of these four stock market indexes prior to the 2020 stock market crash have clearly featured the obvious LPPLS bubble pattern and were indeed in a positive bubble regime. Contrary to the popular belief that the 2020 US stock market crash was mainly due to the COVID-19 pandemic, we have shown that COVID merely served as sparks and the 2020 U.S. stock market crash had stemmed from the increasingly systemic instability of the stock market itself. We also performed the complementary post-mortem analysis of the 2020 U.S. stock market crash. Our analyses indicate that the probability density distributions of the critical time for these four indexes are positively skewed; the 2020 U.S. stock market crash originated from a bubble that had begun to form as early as September 2018; and the bubble profiles for stocks with different levels of total market capitalizations have distinct temporal patterns. This study not only sheds new light on the makings of the 2020 U.S. stock market crash but also creates a novel pipeline for future real-time crash detection and mechanism dissection of any financial market and/or economic index.

我们采用对数周期幂律奇异性 (LPPLS) 方法，通过包括威尔希尔 5000 总市场指数在内的四大美国股市指数，系统地调查了 2020 年美国股市在不同总市值水平的股市崩盘情况。标普 500 指数、标普中型股 400 指数和罗素 2000 指数，分别代表股票整体、大盘股、中盘股和小盘股。在 2020 年美国股市崩盘期间，所有四个指数在五周内都下跌了三分之一以上，而中盘股和小盘股的跌幅均远大于大盘股和股票的整体跌幅。我们的研究结果表明，这四个股指在 2020 年股市崩盘之前的价格轨迹具有明显的 LPPLS 泡沫模式，并且确实处于积极的泡沫状态。与普遍认为 2020 年美国股市崩盘主要由 COVID-19 大流行造成的观点相反，我们已经表明，COVID 只是起到了火花的作用，而 2020 年美国股市崩盘源于股市本身的日益系统性的不稳定. 我们还对 2020 年美国股市崩盘进行了补充事后分析。我们的分析表明，这四个指标的临界时间概率密度分布呈正偏态；2020年美国股市崩盘源于早在2018年9月就开始形成的泡沫；不同总市值水平的股票的泡沫曲线具有不同的时间模式。这项研究不仅揭示了 2020 年美国股市崩盘的原因，而且为未来的实时崩盘检测和任何金融市场和/或经济指数的机制剖析创造了新的管道。