For example, geodesists use high-precision Global Navigation Satellite System (GNSS) instruments distributed across the western United States to determine the velocities of the Earth’s surface relative to the stable and very slowly deforming eastern United States. These activity rates can be measured both by geologists, with knowledge of how much displacement has occurred along a fault over a given length of time (like our previous example of a displaced sediment layer with a known age), and by geodesists. In addition to the fault locations, we need to know how active each fault is, which reflects how frequently a fault has earthquakes and how big those earthquakes may be. Unfortunately, earthquakes don’t just happen on mapped faults – they occur all the time in places where there is no fault on the map – but all those earthquakes are also taken into account in hazard models, as indicators of where and how often earthquakes could occur away from known faults. These observations can be made in the field or using remotely sensed imagery, like aerial imagery and other techniques. For example, if we know that a certain sediment deposit is 1,000 years old, and we see that this sediment has been cut and moved by the fault, we know that an earthquake on the fault reached the surface in the last 1,000 years. Earthquakes on these faults disrupt the surface features, and the date the earthquake occurred can be estimated by the age of the disrupted features. The location of faults is known through observations of topographic and landscape disruptions caused by movement on said faults. Faults and EarthquakesĮxisting faults and locations of past earthquakes are the starting point for a seismic hazard model. Larger versions of each photo can be found at: 2001 M6.8 Nisqually, Washington, 2014 M6.0 South Napa, California, 1994 M6.7 Northridge, California, 1975 M7.7 Kalapana, Hawaii, 2011 M5.8 Mineral, Virginia, and 2018 M7.1 Anchorage, Alaska. Damage from various earthquakes around the U.S. Let’s look at what goes into the NSHM in more detail to better appreciate the improvements and progress for the 2023 model.Ī map from the 2018 United States Seismic Hazard Model showing highest hazard in red and lowest hazard in gray. The USGS is currently working on the updated hazard model for the 50 states targeted for completion and release in 2023. The distinction between hazard and risk is that hazard refers to natural phenomena from earthquakes (e.g., ground shaking), whereas risk refers to consequences of the hazard (e.g., dangers the hazard poses to life and property). The model and related maps have a surprising variety of uses, from guiding engineering design of structures to helping determine earthquake insurance rates, and informing government officials, emergency managers, and public officials about potential risks to their communities. Additionally, engineering analyses inform us about how built structures could react to the ground shaking forecasted by the model. Geological Survey (USGS) uses the most current data and state-of-the-art forecasting methods to create an updated National Seismic Hazard Model (NSHM). The level of tsunami danger is being evaluated for other US and Canadian Pacific coasts in North America, according to the NWS.Faults, earthquakes and geology – are the ingredients that go into making a seismic hazard model. “Based on all available data there is no tsunami threat,” the National Weather Service Pacific Tsunami Warning Center said. Kodiak is near the northwestern tip of Kodiak Island, which is the largest island in Alaska and is the second-largest island in the US.Ī tsunami watch had been issued for Hawaii but was later canceled. Police in Kodiak, the largest town on the island of Kodiak, advised residents after the quake to move to high ground, adding that the high school was open as an evacuation location.Īs the alerts changed from warnings to advisories, the Kodiak Police Department said in a message, “Kodiak has been downgraded to Tsunami Advisory status however we are not all clear.” “A tsunami was generated by this event, but no longer poses a threat,” the center said. When it comes to earthquakes, size matters but so does the terrain Credit: USGS / Ben Brooks Ben Brooks/USGS/JPL/NASA A USGS Earthquake Science Center Mobile Laser Scanning truck scans the surface rupture near the zone of maximum surface displacement of the magnitude 7.1 earthquake that struck the Ridgecrest area.
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