During July 2019, a sequence of earthquakes, including an Mw 6.4 foreshock and an Mw 7.1 mainshock, occurred near Ridgecrest, California. ShakeAlert, the U.S. Geological Survey (USGS) earthquake early warning system being developed for the U.S. West Coast, was operational during this time, although public alerting was only available within Los Angeles County. ShakeAlert created alert messages for the two largest events and for many of the larger aftershocks. In this study, we dissect log files and replay data through the system to reconstruct the sequence of events and analyze the performance of the system during that period. Although the system performed reasonably well overall, the sequence also revealed various issues and short comings that will be addressed in impending and future system upgrades. ShakeAlert detected and characterized both the Mw 6.4 and Mw 7.1 earthquakes within 6.9 s of their origin times and created alert messages that were available to ShakeAlert’s pilot users. No public alerts were sent out by the ShakeAlertLA cell phone app (the only publicly available alerting method at the time), because the predicted shaking for Los Angeles County was below the app’s alerting threshold of modified Mercalli intensity 4.0. For the Mw 6.4 event, this was accurate. For the Mw 7.1 event, public alerts for Los Angeles County were warranted, but ShakeAlert underpredicted the shaking levels, because both the point‐source and finite‐fault algorithms underestimated the magnitude of the earthquake by 0.8 units. A number of software and hardware issues that were responsible for the magnitude underestimation of the mainshock have been identified and will be addressed in future ShakeAlert releases. We also analyze the hypothetical alerting performance of ShakeAlert had public alerting been available throughout southern California.

中文翻译：

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2019年Ridgecrest地震序列期间ShakeAlert地震预警系统的性能

2019年7月，加利福尼亚里奇克雷斯特附近发生了一系列地震，包括6.4 Mw前震和7.1 Mw主震。尽管仅在洛杉矶县内提供公共警报，但在美国西海岸开发的美国地质调查局（USGS）地震预警系统ShakeAlert仍在运行。ShakeAlert为两个最大的事件以及许多较大的余震创建了警报消息。在本研究中，我们将剖析日志文件并通过系统重播数据，以重建事件序列并分析该期间的系统性能。尽管系统总体上表现良好，但该序列还揭示了各种问题和不足之处，这些问题和缺点将在即将到来的系统升级中解决。ShakeAlert在震源时间6.9 s之内检测到了6.4级和7.1级地震，并对其进行了特征描述，并创建了警报消息，可供ShakeAlert的试点用户使用。ShakeAlertLA手机应用程序（当时唯一的公开可用的警报方法）没有发出任何公共警报，因为洛杉矶县的预计震动低于该应用程序的修正后的Mercalli强度4.0的警报阈值。对于Mw 6.4事件，这是准确的。对于7.1兆瓦级地震，有必要对洛杉矶县进行公共警报，但ShakeAlert低估了震级，因为点源和有限故障算法均低估了地震震级0.8个单位。已经确定了造成主震幅度低估的许多软件和硬件问题，这些问题将在以后的ShakeAlert版本中解决。我们还分析了在整个加利福尼亚南部地区都可以使用公共警报的情况下，ShakeAlert的假设警报性能。