Thursday, December 09, 2021

Early warning may not save lives during earthquakes

By EMN Updated: Nov 22, 2014 11:13 pm


[dropcap]P[/dropcap]roposals for an earthquake Early Warning System (EWS) for India — similar to the one successfully tested during the recent quake in California — is mired in a scientific controversy.
The EWS detects the initiation of an earthquake and issues warning alerts of possible forthcoming ground shaking.
When a 6.1 magnitude earthquake shook the ground below California’s Napa Valley Aug 24, the EWS gave a 10 -seconds alert to residents of San Francisco, about 50 km from the epicentre.
India’s Ministry of Earth Sciences (MoES) is considering a similar system for Uttarakhand in the foothills of the Himalayas. Seismologists at the Indian Institute of Technology (IIT) in Roorkee are tasked with building a prototype. And EWS is one the themes for discussion at a Dec 11 symposium on earthquake engineering to be held at the institute.But Pune-based seismologist Arun Bapat, an associate of the US-based International Earthquake and Volcano Prediction Centre in Orlando, Florida, is sceptical. “EWS is a myth, it cannot mitigate seismic disaster,” Bapat told IANS.
Bapat, who had predicted the Dec 26, 2004, tsunami long before it happened, says any EWS for Uttarakhand will not be able to save lives within a 100-km radius. He has communicated his views to ministry secretary Shailesh Nayak, he said.
Energy released during an earthquake travels around the earth as two types of waves : primary or P-waves which are non-destructive and secondary or S-waves which cause the intense shaking and most damage.
But travelling nearly twice as fast as S-waves, the P-waves arrive at the seismometer first. EWS simply exploits this difference in travel time between the two types of waves by sending out a warning to locations ahead of the arrival of destructive S-waves there.
The warning time is usually in seconds, enough to stop trains, shut down heavy industries, airports and power plants (including nuclear) but not enough for people to take cover, Bapat says. “This is especially true if one takes into account the element of total surprise, the hallmark of all earthquakes.”
If installed in northwest (Himachal/Uttarakhand) or in northeast Himalayas, EWS could provide at best 22 to 35 seconds’ warning time to residents of Delhi and Guwahati “but both the city authorities have to update the computer system and alarm issuing mechanism”, Bapat says. After detection of the P-waves, he says, it takes about five seconds for the computer to determine that a local tremor will develop into a damaging earthquake.
During this time, the destructive S-waves will already have travelled at least 17 km. Therefore, there is a “blind zone,” about 20 km in all directions from the epicentre where no warning can be issued, he says.
If the earthquake starts on an unknown fault — not covered by the network of seismometers — the lead time to recognize the danger and issue the first alert will be longer and hence the “blind zone” larger, he says. “This is a major drawback with EWS considering that the maximum destruction from earthquake occurs within 40-70 km radius from the epicentre.” In the US, the entire San Francisco Bay Area would lie within the “blind zone” whenever any of the major faults along the San Andreas Fault ruptures, says Bapat, adding that the situation around the world was not very different.
“The seismically active zone running from Iran to Malaysia and on to Indonesia will not be able to avert the greatest calamities with EWS.”.
What Bapat says about the blind zone was factual, admits William Ellsworth, a seismologist at the US Geological Survey. “But it does not invalidate the value of a warning system if it is part of a larger programme to reduce earthquake risk,” Ellsworth told IANS in an email.
He said the larger programme should ideally include public education on what to do before an earthquake strikes, during the event and after the shaking stops.
Ellsworth said that India faced a significant seismic hazard from great earthquakes that will some day rupture the faults that are uplifting the Himalayas. “EWS has a role to play as part of earthquake risk management, but it is not a solution in itself.” Says Vineet Gahalaut, a top seismologist at the National Geophysical Research Institute in Hyderabad: “Japan currently uses its EWS to issue warnings three seconds after an initial rupture and it has been extremely useful in the recent Tohoku earthquake. “For India, it is a must.”
Ramesh Singh, a geologist at Chapman University in California, agrees. “Seismometers are not costly and connecting signals with supercomputer can be done without much expense,” Singh told IANS. “Roorkee is running a pilot project based on the arrival of P and S waves. This is a good idea to develop.”
Bapat argues: “It is no doubt a good idea but EWS fails totally to achieve its declared goal within the ‘blind zones’ where expected levels of destruction are the highest.
“Even outside the ‘blind zones’, where warnings of 5 to 30 seconds can be given, the life-saving value of such alerts is questionable,” he says.

By EMN Updated: Nov 22, 2014 11:13:06 pm