Field Notes 
Forecasting aftershocks   Iran quake wreaks fear and ruin

Forecasting aftershocks

Aftershocks can pose a serious threat to populated areas already hit by an earthquake. In 1999, following the deadly magnitude-7.4 quake in Izmit, Turkey, a magnitude-5.8 aftershock struck the area, killing seven people and injuring 420. Three hours after the magnitude-7.3 Landers earthquake shook Southern California on June 28, 1992, a magnitude-6.4 aftershock shook the area, creating more property damage than the initial earthquake.

Starting in 1989, after the Loma Prieta earthquake, seismologists began issuing aftershock hazard assessments. With more accurate and complete seismic data available in near-real time over the Internet and a growing understanding of aftershock dynamics and hazard mapping, Stefan Wiemer of the Swiss Federal Institute of Technology in Zurich says he and colleagues have been able to create the next level of aftershock hazard assessments.

Wiemer and graduate student Matt Gerstenberger along with Egill Hauksson from the California Institute of Technology have developed a model that allows them to map which locations face the highest probability aftershock shaking over specific time periods. “New in our model is that we analyze the spatial differences in the aftershock behavior, and are therefore able to create a detailed map of the aftershock hazard,” Wiemer says.

In this 30-day aftershock hazard map for the 1999 magnitude-7.1 Hector Mine earthquake, the star represents the epicenter location. The colors represent the forecasted aftershock hazard with blue to purple colors indicating the highest hazard. This forecast, computed 4 days after the mainshock, correlates well with the locations of subsequent larger aftershocks, shown as triangles (magnitude-5.0 and higher) and circles (magnitude-4 and higher). Image courtesy of Stefan Wiemer.

As published in the current Bulletin of the Seismological Society of America, Wiemer and colleagues were able to create aftershock hazard maps following the magnitude-7.1 Hector Mine earthquake that shook southern California in 1999. Four days after the initial quake, they released a 30-day hazard map that accurately forecasted all of the magnitude-5.0 and higher aftershocks and all but one of the aftershocks between magnitudes 4.0 and 4.9.

Wiemer says the new model is a significant step toward a more accurate forecast of aftershocks. “The improved ability to understand and forecast aftershock hazard can be useful for scientists, decision makers and the general public to make educated decisions after a large earthquake and thus contribute to improving aftershock hazard mitigation.”

Lisa M. Pinsker

Iran quake wreaks fear and ruin

People in Iran are still attempting to recover from a devastating earthquake that struck the western region of the country on June 22. The earthquake that occurred at 7:28 a.m. in Iran recorded a magnitude of 6.3, according to the International Institute of Earth Engineering and Seismology in Iran. Tremors were felt in Iran’s capital, Tehran, which lies 225 kilometers east of the earthquake’s epicenter.

In the immediate vicinity of the earthquake, the damage was overwhelming. Villages have been completely destroyed in the main event and its ensuing aftershocks. The government estimates a death toll of approximately 250 people, with at least 1,300 injured and thousands homeless. The abundance of aftershocks immediately following the earthquake have the villagers in fear of more damage.

Earthquakes are a relatively common occurrence in this region, which lies along the Zagros fold and thrust belt where the Arabian and Eurasian plate boundaries collide. But this is the largest quake to have occurred since 1997, when an earlier earthquake of magnitude 7.1 (located 300 kilometers north of the present earthquake) killed an estimated 1500 people.

Poorly constructed houses contributed heavily to the present damage, and liquefaction of the soil in some areas aided destruction. Relief aid, though offered by many countries, including the United States, has been slow in arriving, leaving the homeless struggling to come to terms with the crisis.

Salma Monani

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