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| Seismic record from the Guerrero April 18 earthquake From Earthquake-Report.com |
A magnitude 7.2 earthquake struck in the Acapulco/Mexico City region early this morning, a Good Friday holiday morning when many residents had apparently slept in or gone away on vacation. The earthquake lasted about 30 seconds. The epicenter was in the state of Guerrero, north of Acapulco. The U.S. Geological Survey has an automatic damage estimator here. The USGS estimates deaths between 1-100, and economic losses between 1-100 million; Max Wyss's estimator at WAPMER predicts 0-50 killed. In spite of damage reports there have been no reports (10 hours later) of fatalities, so there are likely to be very few if any as more information comes in. Why?
One reason that many buildings in the area are built to be quake-resistent because of the history of previous earthquakes. But another reason is that Mexico has an early warning system for earthquakes, and the news has reported that a warning went out about 2 minutes before the quake (I have not been able to independently verify this).
Mexico instituted a Seismic Alert System (SAS) for Mexico city as an experimental project in August 1991. By monitoring the arrival of the faster compressional ("P") waves, warnings can be issued before the arrival of the stronger shear ("S) waves. The system gives, on average, about 60 seconds warning for earthquakes generated in the Guerrero Gap. The quake was the result of a thrust motion where the Cocos plate is being subducted below the North American plate at a rate of about 65 mm/year. The Guerrero Seismic Gap is a ~200 km long segment of this plate boundary that has experienced no significant earthquakes since 1911 (M7.6 at that time). It is thought that an earthquake of magnitude greater than 8 is possible if the entire gap were to rupture at the same time.
The development of the SAS was sponsored by the Mexico City Government, beginning operation in 1991. By the end of the first year, it was experimenting with providing warning to some public elementary schools, and was opened as a public service on commercial radio stations in 1993 after a successful alert that gave 65-73 seconds advance warning during two Guerrero earthquakes (M5.8 and M6) on May 14, 1993. Extensive planning for dissemination and education for the public followed. By 1998 the SAS detected 681 seismic events, 12 of which were strong enough to trigger the general early warning signals in Mexico City, one false one, and one earthquake well detected, but not warned. In the one false alarm, phenomena that were feared--such as panic that could cause injuries--did not occur, but it was realized that many members of the public had not been trained, and training of the public remained a high priority.
The advantages of an early warning system are numerous: Casualties and fatalities are reduced by making people aware that strong ground shaking is imminent. Tsunami calculations can be initiated earlier. Traffic such as trains or subways can be stopped or slowed. The disadvantages or risks are the alerts may not be quick enough in areas close to the focus of the earthquake and subjected to strong shaking, that there can be false alarms, and that the technique is not good if there are multiple earthquakes close in time or location. Hence, there are tradeoffs between speed and accuracy. Continuous citizen education and awareness must be maintained, and a wide variety of channels of communication must be used to ensure wide dissemination. The private sector must be incorporated into the early warning system so that appropriate services/operations can be shut down for safety.
2 comments:
As a resident of Mexico City I can tell you that all in all the Seismic Alert System has delivered results that exceed expectations. From commercial and government-operated radio and TV stations that relay its signal for the general public to unofficial smartphone apps, Mexico City dwellers have fully embraced the system after many years of information and education and several frightening shakes. I recommended in a blog of mine a few years back that they incorporated a technology that has worked in the U.S. for decades to warn people about hurricanes an other hazzrds, the weather radio by using locally the same frequencies as the NOAA. They finally did a few years ago with good results too. These units are more accessible than their original radio receptors. Overall, the system is far from perfect, because it was designed to go off in two modes: Preemptive and Public. Preemptive is for quakes up to 5.9 in magnitude and only warns the subscribers of the system and the owners of a weather radio. Public is for 6.0 and up and goes to radio and TV stations in Mexico City and Toluca City in neighboring Estado de Mexico state. Thus the signal does not always reach the population as desired and criticism arises from the general public blaming the SAS for not going off. Changes in earthquake-detection methodology have also been advised by certain well-reputed seismologists in order to avoid triggering of the alert in the case of minor quakes. Last Friday, in spite of the quake reaching a quite high magnitude, the SAS did not go off in Public Mode. Thus it did not reach commercial radio stations and government-operated radio and TV stations. It stayed in Preemptive mode with a considerable delay. Eventhough the SAS told the press that it had warned the population 68 seconds before the earthquake hit Mexico City, the signal actually went off only 10 seconds before as can be seen in YouTube videos where one of Mexico´s largest TV network Televisa´s news people announces the seismic alert system has gone off and only 10 seconds later the earthquake hits the studio. I myself became aware through my weather radio only 10 seconds before. Of course, 10 seconds are better that nothing at all.
However it still requires lots of investments (by the Mexican local and federal governments) in order for it to fully achieve its goals. Yet the seismic culture as you indicate, has become highly developed in Mexico City -while not so well in the rest of the country affected by this natural phenomenon- and you will see people walking out of buildings in a very orderly fashion. However, this took many decades of training with a first impact on newer generations directly from school. Most of this training is accompanied of a coordinated reaction with the Seismic Alert System. The results can be seen today. The change in culture has made people more conscious and -excuse me- less hysterical due to years and years of drills and education, which are fortunately being made extensive to the rest of the country, mainly along the Pacific coastline. When I was a student, some 30 years ago, none of this was true, but the 1985 earthquake changed things, including building codes in Mexico City, which are very strict today and almost comparable to those from Japan and Chile. Last Friday´s earthquake was deemed "unusual", and it was not only for being a shallow one. Quakes in Mexico City mostly feel like a strong rocking movement, with deep strokes instead of short, fast vibrations, as it often happens in California, for instance. Therefore, in Mexico City things are rarely thrown out of shelves except in high rises. I was in my two-storie home and when the quake reached its peak around second 45, things began flying offshelfs. I was amazed by this as it had never happened before, not even in the 1985 earthquake, where everything stayed in place in spite of the monstruous shaking. The quake stopped at around 01:40 but the rocking movement of the muddy underground kept on for another minute leaving lessons for everyone and hopefully for the people with the Seismic Alert System as well.
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