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New USGS hazard map shows increased earthquake risks

Friday, December 20th 2019, 1:08 pm - USGS latest National Seismic Hazard Model shows increased earthquake risks for parts of the continental United States

It’s no surprise that some areas of the U.S. west coast are increasingly facing a higher risk of a major earthquake occurring. According to the United States Geological Survey (USGS), much of California, and some areas of western Nevada, are among the areas with a higher chance of experiencing a significant ground shaker.

Every five years, the USGS releases an updated seismic hazard map which marks very clearly in red, those zones that are especially vulnerable to earthquake activity. Over the past five years, a new set of earthquake observations across the U.S. has helped improve model performance. For instance, data from the Ridgecrest earthquake which occurred back in July of this year showed geologists that quakes can be very complex, but at the same time the information gathered during this event was extremely valuable to update the hazard models which calculate the impact they can have in a given area.

Map 1 Seismic Risk Large USGS USGS Seismic Risk Updated Map

USGS scientists have been incorporating the latest updates to their National Seismic Hazard Model, in order to better predict the near future of earthquake impact. More detailed information of past and recent quakes, new ground shaking computer models, improved representation of ground shaking in areas located over unstable terrain and more detailed information on shaking depending on building or soil type, are just a few of the main updates included in the recent version.

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The USGS National Seismic Hazard Model Project Team has included a broad spectrum of shaking scenarios since different shaking wavelengths affect structures differently. For instance, a shorter wavelength, which moves faster, will have a greater influence on a shorter structure, while the longer, slower-moving wavelengths, will have more of an impact on tall buildings and bridges.

The geology of the ground is also greatly improved in these contemporary models since soil type and structure is decisive when it comes to factoring in this information to asses potential hazards in a given area. Buildings, bridges, dams, railways, schools and other structures also have specific material codes. These come from the work of civil engineers and are included within the Seismic Hazard models so that the output can be of great value to help entire communities prepare for a significant earthquake.

Photo 1 USGS San Andreas Fault San Andreas Fault. Courtesy: USGS

Beyond improving computer models that alert us of potential hazards during an earthquake, we have another major problem as urban centres continue to expand. Population in large metropolitan areas just continues to grow, and rapidly. In urban areas of California, the number of people exposed to unstable grounds has also been on the rise decade after decade. The Bay area in northern California and the greater Los Angeles area are among two of the most densely populated areas facing this threat.

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Overall, there are more people living in areas where seismic hazards are a reality than ever before, and this number will only continue to grow. Today, 1 in 10 U.S. people live in areas where the risk of experiencing intense ground shaking is high. The western third section of the U.S. is one of those areas where the chance of experiencing a damaging earthquake is at least 36 to 74 per cent, and across a large swath that includes almost all of California, above 74 percent. Other areas at moderate to high risk are found in areas of Missouri, Tennessee, Arkansas, Kentucky or Illinois.

Map 2 UCERF3 postcard California Rupture Zones. Courtesy: USGS

Californians -- especially those living in the Bay area -- are perfectly aware that sooner or later, within a time frame of now to 100 years, the ground will shake intensely. In the meantime, all the extra time they can get should be used to improve as much as possible the resistance of all structures, but also the quality of those seismic hazard models which can help save many lives.



Source: USGS

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