Earthquakes famously can't be predicted. They still can't. But early-warning systems exist, and now a paper Wednesday published in Science Advances explains how best to utilize them, given their limitations.
One is that nobody knows how much warning time is physically possible at differing degrees of ground motion. But if we assume no delay between the warning and your response (zero latency), the main question becomes how long it takes for the motion of the ground to shake you. And that becomes a question of what you want the early-warning system to achieve. If you want the system to be very, very sure you're in danger before it warns you, your warning time will be shorter.
So, you can choose to be warned more frequently than is quite convenient and protect yourself unnecessarily – or to be warned only when major quake is in progress, but you'll have less time to react.
No, households don't have sensors, institutions do, but you get the idea.
"Users who set relatively high thresholds for taking action are less likely to receive timely and actionable information," warns the paper, which was written by Sarah Minson of the U.S. Geological Survey with colleagues, including from the California Institute of Technology.
The group's calculations are of keen interest to Israelis, who live perched on the great Syrian-African fault line known in part as the Great Rift Valley. The Dead Sea and the Sea of Galilee both sit smack within that fault line, which has been known to generate major quakes.
The last Big One was in 1927, killing an estimated 287 people in Israel and Transjordan, but there are little'uns practically on a daily basis. We wouldn't want to be warned for each piddling jerk of the Earth, but we can reasonably predict that more big ones will be coming, and will likely involve mass casualties. (True, the Israeli army has told kiddies that quakes are God's will, but the predictions are based on geology and precedent.)
Speed of light
Early warning systems do not predict quakes. There is still no way to do that. Another thing early warning systems don't do is generate false alarms.
They're always right, because what they do is tell you that a quake is already happening, Dr. Ron Avni of Ben-Gurion University of the Negev explains to Haaretz.
Their entire purpose is to tell you as fast as possible that a quake is in process. Even if only warned seconds in advance, you have a better chance of protecting yourself than if not warned at all, Avni points out.
Why do we have to choose between earlier warning, and more accurate warning? Must we? Yes.
Thing is, rupture onset is not diagnostic of ultimate rupture size, the geological evidence indicates. A minor rip may end in a huge quake. Sensors can be set to warn you at the minor onset (more frequent), or as the huge quake bears down on you (less frequent but more accurate).
Understanding how early quake warning systems work is helpful to understanding our options.
A quake starts. Sensors in the ground (typically equipped with pendulums) detect it. Crucially, the warning system's signals can move practically at the speed of light, which is much faster than ripples along the planet's crust.
So, the sensors transmit the warning instantaneously, more or less, while the shaking takes more time to propagate.
Now, what about the accuracy? The whole warning thing can work because all quakes involve three types of waves that are unleashed at about the same time but travel through the rock at different speeds, Dr. Avni explains.
Crucially, the fastest-moving are the least-harmful: these are low-amplitude waves called "P" (standing for "primary" or "press"). The P waves are vertical up-and-down motion that may unnerve you, but usually don't do much damage, Avni explains.
The second type of wave, called "S" (for "secondary" and for their characteristic "shear" effect) involves horizontal movement too and that's what causes the damage. The third type is called surface waves, and between the secondary and the surface waves, buildings can get shaken to bits.
Secondary S and surface waves spread more slowly than primary P waves.
"The farther we are from the epicenter of the quake, the greater the time difference between the impact of P and S waves," Avni explains: the farther we are, the more time we have to hide, if we are warned upon the propagation of the P wave, not the S wave.
Missile alerts and Mother Nature
All well and good. Probably we would prefer to be warned the microsecond a P wave sets off, before the S wave can laboriously follow it and drop ceiling beams on our heads.
But if we get warned every time the Dead Sea rift or San Andreas burps, we'd spend our lives cursing under our desks.
The point is to calibrate the systems to only warn us when a quake is big enough to warrant protective measures, which most geologists consider to be from 5.5 on the Richter scale and up, Avni says. This is where the new paper gives guidance.
Its conclusion: If you are willing to be alerted at lower ground-motion thresholds, you'll get more robust warnings with longer average warning times than if the system is calibrated to higher ground-motion thresholds.
You can choose to not be bothered unless the risk is acute, but you'll likely have less time to take action.
Israel scattered sensors throughout Jordan Valley, Avni says. But because Israel is so small, much like our missile alert systems, quake warning systems don't give us much notice. (And in another quirk of missile attacks and Mother Nature: during quakes, families may find their best resort is to hide in the bomb shelter, Avni suggests. Good to know.)
It's true that the farther one is from the epicenter, the more time one has to react to early warning. But in tiny Israel, the differences are not vast.
If the quake originates under the Dead Sea and one is in Jerusalem or Be'er Sheva, one has mere seconds to duck under a desk or head for the courtyard before the primary P waves hit and then the destructive S waves a few seconds later.
Finally, warning systems are good because the sensors can automatically trigger emergency reactions, from shutting down nuclear reactors to automatically opening the gates at fire stations and ambulance services, Avni points out. Smart elevators can be programmed to stop at the nearest floor. Schools can shoo the kids out to the yard or stairwells. Or people could ignore the whole thing like many Israelis ignore missile sirens, but in this case, there's no Iron Dome system to run interference.