Quantifying the Media Bias in Intensity Surveys: Lessons from the 2001 Bhuj, India, Earthquake

Abstract

Many seismologists have looked at the 26 January 2001 Bhuj earthquake as a key modern calibration event that could be used to improve estimates of magnitudes of large historic mainshocks in stable continental regions. Since no instrumental data are available for important historic events such as the 1819 Allah Bund, India, and the 1811-1812 New Madrid, central U.S. mainshocks, calibration hinges on comparisons of the macroseismic effects of these earthquakes with those of comparable modern earthquakes for which a reliable, instrumentally determined moment magnitude is available. However, although such a comparison is conceptually straightforward, in practice it is complicated by potentially significant inconsistencies in methods used to quantify macroseismic effects in different regions and/ or times. For the Bhuj earthquake, extensive intensity data sets have been compiled and published from both media accounts and detailed direct surveys. Comparing the two provides a quantification of the previously suspected media bias, whereby earthquake effects can be exaggerated in media accounts. This bias is a strong function of intensity level, with substantial bias at the highest shaking levels and significantly less bias at low intensities. Because only sparse documentary data are in general available for older historic earthquakes, the results of this study suggest that their inferred intensity distributions might be similarly biased. We further use the surveybased intensity values to develop a new relationship between intensities and ground motions.

Spatial Variability of "Did You Feel It?" Intensity Data: Insights into Sampling Biases in Historical Earthquake Intensity Distributions

“…Hough and Pande (2007) consider the intensity distribution for the 2001 M w 7.6 Bhuj, India, earthquake, based on media accounts with a distribution determined from traditional ground-based surveys. They find qualitatively similar biases to those found in this study; that is, with higher intensities assigned in the media-based assessment, in particular for locations with light-to-moderate damage.…”

Toward a consistent model for strain accrual and release for the New Madrid Seismic Zone, central United States

“…Moreover, for the CEUS, the intensity attenuation relations are largely if not entirely constrained by low‐to‐moderate intensity values, including values from moderate earthquakes and intensities at regional distances for the 1929 Grand Banks earthquake. In a careful comparison of intensities determined for the 2001 Bhuj, India, earthquake from media reports and values from intensive ground surveys, Hough and Pande [2007] show that the tendency to infer inflated intensity values is significantly stronger for higher intensities (VI and above) than for weaker shaking levels.…”

Intensity, Magnitude, Location, and Attenuation in India for Felt Earthquakes since 1762

“…reports and have been shown to be biased relative to those estimated from direct surveying of damage (Hough and Pande, 2007). Second, the functional form of equation (1) is identical to the functional form assumed for attenuation of peak ground acceleration (Evernden et al, 1973).…”