For reasons of geography, apparently, and the low number of people who have downloaded the app, and earthquake warning only went about to about 40 people on Tuesday in San Luis Obispo County ahead of a 4.3M earthquake.

The earthquake was originally measured as a 4.8M quake, which triggered the new statewide warning system to send out an alert to those likeliest to be affected — the MyShake app's threshold for issuing warnings is 4.5M. But, as KPIX reports, it served as an early real-world test of the system, which just officially launched in October.

The quake happened just before 10:30 a.m. Tuesday and it was centered seven miles south-southeast of Cholame and 22 miles southwest of Paso Robles. According to the USGS, only around 500 people felt the quake in that vicinity.

The MyShake app, developed at UC Berkeley over the last decade, issues cellphone alerts to everyone who has it, a few seconds before a 4.5M or larger quake is about to be felt. The app is meant allow people to run for cover if necessary to avoid injury in a large earthquake, and it relies on a statewide network of seismic activity sensors.

An earlier version of the app, called ShakeAlertLA, was promoted in Los Angeles County and would have been put to good use in July, when two large quakes shook Southern California — only those quakes were centered far outside the county to the east, and therefore the warnings did not go out for county residents who nonetheless felt the quakes.

Previously: Earthquake Early Warning System Launches 13 Years After Project Began