Collect magazine articles on earthquakes and volcanoes in project work
Answers
When an earthquake begins, the fast-moving primary compressional wave (P-wave) front is picked up by nearby CISN or PNSN sensors. Data from these sensors are monitored by the ShakeAlert system in real-time, which quickly assesses if a P-wave trigger is present, determines the characteristics of the earthquake (origin time, magnitude and epicenter), and sends an alert about the shaking intensity expected from the slower-moving secondary transverse wave (S-wave) front to devices away from the epicenter. The P-waves move about twice as quickly as the more destructive S-waves, but are usually not damaging. As the distance from the epicenter increases, the P-waves have more opportunity to out-run the S-waves, and thus the warning times given ahead of potentially damaging shaking are mainly a matter of how far you are away from the origin.
But simply generating an alert is not enough. Time is lost in transmitting alerts to consumer devices and emergency alert systems, which serve to actually transmit the alert to people and facilities that then need time to act on the alerts. So, speed is of the essence. The project is maturing technologically to improve the timing of early warning alerts. For example, our algorithms can now quickly assess P-waves and provide details about the impending shaking, sometimes before the S-wave even reaches the surface. This is especially critical in California, where much of the population resides directly on the faults we are informing them about.
We also continuously monitor system performance to catch oddities that should be addressed. Large earthquakes hundreds or thousands of kilometers away don’t cause heavy shaking locally, but the P- and S-waves of those ruptures are still picked up by the local sensors. The algorithms are calibrated to ensure that these large teleseismic events are not mischaracterized as smaller local events. The system must also be able to deal with stations going through their own calibration sequences, locating earthquakes on the edges of our network, and separating the individual P-wave triggers of foreshocks, aftershocks and mainshocks that could happen one right after the other (and thus have overlapping seismic signals).