Why earthquake hazard maps often fail and what to do about it

Abstract

The 2011 Tohoku earthquake is another striking example – after the 2008 Wenchuan and 2010 Haiti earthquakes – of highly destructive earthquakes that occurred in areas predicted by earthquake hazard maps to be relatively safe. Here, we examine what went wrong for Tohoku, and how this failure illustrates limitations of earthquake hazard mapping. We use examples from several seismic regions to show that earthquake occurrence is typically more complicated than the models on which hazard maps are based, and that the available history of seismicity is almost always too short to reliably establish the spatiotemporal pattern of large earthquake occurrence. As a result, key aspects of hazard maps often depend on poorly constrained parameters, whose values are chosen based on the mapmakers' preconceptions. When these are incorrect, maps do poorly. This situation will improve at best slowly, owing to our limited understanding of earthquake processes. However, because hazard mapping has become widely accepted and used to make major decisions, we suggest two changes to improve current practices. First, the uncertainties in hazard map predictions should be assessed and clearly communicated to potential users. Recognizing the uncertainties would enable users to decide how much credence to place in the maps and make them more useful in formulating cost-effective hazard mitigation policies. Second, hazard maps should undergo rigorous and objective testing to compare their predictions to those of null hypotheses, including ones based on uniform regional seismicity or hazard. Such testing, which is common and useful in similar fields, will show how well maps actually work and hopefully help produce measurable improvements. There are likely, however, limits on how well hazard maps can ever be made because of the intrinsic variability of earthquake processes.

Introduction

Until March 11, 2011, residents of Japan's Tohoku coast were proud of their tsunami defenses (Onishi, 2011a, Onishi, 2011b, Onishi, 2011c). The 10-meter high sea walls that extended along a third of the nation's coastline – longer than the Great Wall of China – cost billions of dollars and cut off ocean views. However, these costs were considered a small price to pay for eliminating the threat that had cost many lives over the past centuries. In the town of Taro, people rode bicycles, walked, and jogged on top of the impressive wall. A school principal explained, “For us, the sea wall was an asset, something we believed in. We felt protected.”

The defenses represented what an affluent technological society could do. Over a period of years, most recently in 2010, an agency of the Japanese government, advised by some of Japan's leading seismologists, had calculated precisely what kinds of earthquakes could be expected in different parts of the country. The largest hazard was assumed to be from thrust fault earthquakes to the east, where the Pacific plate subducts at the Japan Trench and the Philippine Sea plate subducts at the Nankai Trough. For the area of the Japan Trench off Miyagi prefecture on the Tohoku coast, the hazard mappers stated that there was a 99% probability that a magnitude 7.5 earthquake would occur in the next 30 years (Earthquake Research Committee, 2009, Earthquake Research Committee, 2010). This forecast, as well as similar detailed seismicity forecasts for all other regions, was used to produce the national seismic hazard map that predicted the probability that the maximum ground acceleration (shaking) in any area would exceed a particular value during the next 30 years. Larger expected shaking corresponds to higher predicted seismic hazard. A similar approach was used to forecast the largest expected tsunami. Engineers, in turn, used the results to design tsunami defenses and build structures to survive earthquake shaking.

All this planning proved inadequate on March 11, when a magnitude 9 earthquake offshore generated a huge tsunami that overtopped the sea walls, causing over 19,000 deaths (including missing; official police data as of December 2011) and at least $200 billion damage (Normile, 2012), including crippling nuclear power plants. This earthquake released about 150 times the energy of the magnitude 7.5 quake that was expected for the Miyagi-oki region by the hazard mappers. Somehow, the mapping process significantly underpredicted the earthquake hazard. The complex decision making process involved for the Fukushima nuclear power plant is reviewed by Nöggerath et al. (2011).

The hazard map, whose 2010 version is shown in Fig. 1, predicted less than a 0.1% probability of shaking with intensity “6-lower” (on the Japan Meteorological Agency intensity scale) in the next 30 years. In other words, such shaking was expected on average only once in the next 30/0.001 or 30,000 years. However, within two years, such shaking occurred.

How this discrepancy arose has become a subject of extensive discussion among seismologists (Kerr, 2011). We raised three issues in a recent short opinion article (Stein et al., 2011): 1) What went wrong for Tohoku? 2) Was this failure an exceptional case, or does it indicate systemic difficulties in earthquake hazard mapping? 3) How to improve this situation? Here we discuss these issues in more detail.