Carmen Rivera, 65, on morning walk with her dog Ash in Ridgecrest, California, after a 6.4 earthquake this month. Photo / LA Times
Confusion and frustration that followed a big California jolt have highlighted the hurdles an earthquake early warning system here might face, a scientist says.
When a 7.1 quake was felt across much of central and southern California this month, Los Angeles residents took to social media questioning why it hadn't registered on the just-launched ShakeAlertLA app.
Officials had to point out that shaking in LA County hadn't reached a point that was likely to result in major damage, as seismologists warned that lowering the threshold could leave people too accustomed to the alerts.
Unlike Japan, Mexico and now parts of the US, New Zealand doesn't have a nationally-funded earthquake early warning system.
These systems work by detecting and reacting to the fastest-travelling P-waves before the slower but more damaging S-waves arrive, giving people a few crucial seconds to prepare.
Although building an earthquake early warning system would require a massive technology upgrade to GeoNet's current capability, Massey University's Dr Julia Becker and colleagues have been surveying Kiwis how they might use one.
In any case, Victoria University of Wellington seismologist Professor John Townend said the California episode provided some eye-opening lessons.
"The system did work – but there was a disagreement over what the developers had built it to do, and what the public was expecting it to do," he said.
Earthquake early-warning systems like the one under development in California are generally intended to issue warnings if an earthquake exceeds a specific size, referred to as its magnitude, or if the calculated shaking intensity – which varies from place to place – exceeds a certain threshold.
Neither of these conditions was met in Los Angeles during the Californian earthquakes last week, Townend said.
"The feedback from residents of Southern California highlights how important it is for the operators of early warning systems to communicate what it is their systems are actually meant to do.
"It was a thought-provoking thing to watch from my perspective. Several jurisdictions are making good progress in this sort of area, but the recent Californian earthquakes provide a reminder that you still have to make sure people understand how these systems work and when and what kind of warning messages they might receive."
Townend added there were still limitations to even the most sophisticated systems, like that of Japan's billion-dollar array.
"Of course – and this is true in any part of the world – not all earthquakes will have the characteristics that enable an early warning system to usefully warn people," he cautioned.
"For example, the February 22 earthquake in Christchurch happened so close to the downtown area that there was just no time to alert anyone, even if you had detected the first waves and realised what was going on.
"But there are other configurations that might work well. If the Alpine Fault began rupturing from somewhere near Haast, say, towards Nelson, or the Hikurangi subduction zone started unzipping, this would probably take somewhere in the order of 100 seconds to happen and some parts of the country are far enough away for a useful warning to be issued.
"So you have a decent chunk of time to do something in certain parts of the country, for certain types of earthquake.
"On the other hand, in an earthquake on the Wellington Fault, right beneath the city, you wouldn't be in the same situation.
"In some cases, the initial shaking you feel provides the only warning you have of more to come, and particularly if you're near the coast and the shaking is long and strong then you have to be prepared for a possible tsunami.
"There's another question to consider regarding the hierarchy of responses you might take in different situations: it might be possible to shut off power systems or computers or trains quickly, but in many cases you won't be able to do much more than tell people to drop-cover-hold, pull over while driving, stand back from a surgical operation, or turn of the gas."
New Zealand's current GeoNet capability included hundreds of seismic instruments on land, a range of tsunami gauges measuring water level, and geodetic data fed in by more than 180 continuous GPS (CGPS) stations.
In 2013, a GNS Science report used a scenario similar to the March 1947 tsunami earthquake off the coast north of Gisborne to assess GeoNet's detection capabilities and potential required updates to the network.
After testing a range of detection and classification algorithms with the simulated data, the report authors concluded such an earthquake could be detectable by the network in real-time.
However, it found a large portion of the geodetic sensor network would need to be upgraded to stream the data and provide accurate information.
"The GeoNet continuous GPS network is presently far from being readily available for a tsunami early warning system," the report found.
At the time the report was written, only 37 of the CGPS sites provided data in real time, and a real-time processing procedure wasn't available.
Creating an earthquake early warning system would require a "substantial effort" from GeoNet staff, a "significant increase" in funding, along with the development of procedures and technology to process data in real time.
Kiwi company Jenlogix, however, operated a network of P-wave-detecting Palert units, now used by several councils, universities, district health boards, ports, and power companies.
The units all streamed data to a central server, but also had triggers than could activate local emergency systems – such as stopping a lift at the next floor and open the doors, or turning off the gas supply.
The data was also made available to others such as GeoNet and the Rapid Alert system of KiwiRail.
In any event, Townend noted, the next big earthquake would be a surprise.
"We can't predict earthquakes and early warning systems aren't a panacea. We still each need to know what to do if the ground starts to shake — and have plans in place for what to do once the shaking has stopped."
Magnitude: The term "magnitude" refers to an earthquake's size, which is governed by the amount of slip and a fault and which can be related to the amount of energy released as seismic waves. It is calculated from measurements made with seismometers. Each earthquake has a single magnitude.
Intensity: This describes the effects of shaking and varies from place to place depending on the distance from the earthquake, the local geology, and soil conditions. One earthquake can produce greatly varying shaking intensities in different places, and the shaking will have different effects on different types of buildings, bridges, or other infrastructure.
One issue seemed harder to agree on than most - how big the crowd was.