Scientists rely on the public's reporting of ground shaking to characterize the intensity of ground motion produced by an earthquake. How accurate and reliable are those perceptions?

A new study by Italian researchers suggests that a person's activity at the time of the quake influences their perception of shaking more than their location. Whether a person is at rest or walking plays a greater role in their perception of ground motion than whether they were asleep on the first or sixth floor of a building. People in motion had the worst perception.

"People are like instruments, more or less sensitive," said Paola Sbarra, co-author and researcher at the Istituto Nazionale di Geofisica e Vulcanologia in Rome, Italy. "A great amount of data and proper statistical analysis allowed us to make a fine-tuning of different conditions for a better interpretation of earthquake effects," said Sbarra.

The paper, co-authored by colleagues Patrizia Tosi and Valerio de Rubeis, is published in the March issue of the Seismological Research Letters (SRL).

Sbarra and colleagues sought to analyze two variables - how an observer's "situation" and "location" influenced their perception in order to improve the characterization of low macroseismic intensities felt near small earthquakes or far from larger ones. Contrary to their findings, the current European macroseismic scale, which is the basis for evaluating how strongly an earthquake is felt, considers location the stronger indicator for defining intensity.

The authors analyzed data submitted to "Hai-sentito-il-terremoto?," which is similar to the U.S. Geological Survey's "Did You Feel It?" website that analyzes information about earthquakes from people who have felt them. After an earthquake, individuals answer questions about what they felt during the quake, along with other questions regarding their location and activity.

Intensity measures the strength of shaking produced by the earthquake at a certain location. Intensity is determined from effects on people, human structures, and the natural environment.

The number of people who feel an earthquake is critical to determining intensity levels, and low intensity earthquakes generate fewer reports, making objective evaluation of shaking difficult.

Urbanization exposes French cities to greater seismic risk
Mar 08 - French researchers have looked into data mining to develop a method for extracting information on the vulnerability of cities in regions of moderate risk, creating a proxy for assessing the probable resilience of buildings and infrastructure despite incomplete seismic inventories of buildings. The research exposes significant vulnerability in regions that have experienced an "explosion of urbanization."

"Considering that the seismic hazard is stable in time, we observe that the seismic risk comes from the rapid development of urbanization, which places at the same site goods and people exposed to hazard" said Philippe Gueguen, co-author and senior researcher at Universite Joseph Fourier in Grenoble, France. The paper appears in the journal Seismological Research Letters (SRL).

Local authorities rely on seismic vulnerability assessments to estimate the probable damage on an overall scale (such as a country, region or town) and identify the most vulnerable building categories that need reinforcement. These assessments are costly and require detailed understanding of how buildings will respond to ground motion.

Old structures, designed before current seismic building codes, abound in France, and there is insufficient information about how they will respond during an earthquake, say authors. The last major earthquake in France, which is considered to have moderate seismic hazard, was the 1909 magnitude 6 Lambesc earthquake, which killed 42 people and caused millions of euros of losses in the southeastern region.

The authors relied on the French national census for basic descriptions of buildings in Grenoble, a city of moderate seismic hazard, to create a vulnerability proxy, which they validated in Nice and later tested for the historic Lambesc earthquake.

The research exposed the effects of the urbanization and urban concentrations in areas prone to seismic hazard.

"In seismicity regions similar to France, seismic events are rare and are of low probability. With urbanization, the consequences of characteristic events, such as Lambesc, can be significant in terms of structural damage and fatalities," said Gueguen. "These consequences are all the more significant because of the moderate seismicity that reduces the perception of risk by local authorities."

If the 1909 Lambesc earthquake were to happen now, write the authors, the region would suffer serious consequences, including damage to more than 15,000 buildings. They equate the likely devastation to that observed after recent earthquakes of similar sizes in L'Aquila, Italy and Christchurch, New Zealand.