Why Building-Specific Sensors Are Replacing USGS ShakeMaps for Earthquake Damage Estimates
The intensity of ground shaking underneath a building, and the structural characteristics of the building itself are the two most important factors when estimating earthquake damage. Shaking intensity is affected by the magnitude of the earthquake, the proximity of the building site to the epicenter, local geology, soil type, wave directivity, and other factors.
The ShakeMap® system developed by the U.S. Geological Survey (USGS) provides near-real-time maps estimating the ground motion footprint of an earthquake following an event. ShakeMaps are generated automatically from networks of ground sensors deployed regionally across the U.S. along with feedback from ShakeMap users who contribute to the “Did You Feel It?” network. ShakeMaps are a valuable tool used by local and state governments to quickly view the regional impacts of large earthquakes, and by federal agencies to estimate the financial resources that will be needed to aid recovery
The Uncertainty of the ShakeMap System
ShakeMaps are intended to provide quick regional “snapshots” of the areas where the strongest shaking occurred after an earthquake. They are not intended to provide precise or accurate measurements of shaking at a specific building site, which can be widely different from that predicted by the ShakeMap contours.
USGS assigns a letter grade “A” to “F” to indicate the quality of its own ShakeMaps—from high to poor quality, respectively. The grades reflect the local density of sensor stations and the quality of knowledge about the area’s underlying faulting characteristics. By its own analysis, 80 percent of the 640 graded events since 2000 were assigned a grade of C or lower.1
Grades assigned by USGS to ShakeMaps, reflecting uncertainty and other factors affecting map quality
One important example of the shortcomings of the ShakeMap system is illustrated in the figure below, which shows the ShakeMap generated for the 1994 Northridge Earthquake. It demonstrates how the shaking intensity contours for spectral acceleration can be very different from readings produced by individual sensor stations within the USGS network.
During the Northridge event, a USGS sensor placed in the ground at the Cedar Hills Nursery in Tarzana produced abnormally high readings, the highest ground motion readings from that event—readings that, according to seismologists, reflected unprecedented, sustained shaking. At the same time, much smaller ground accelerations were observed at nearby sites. Subsequent studies found that “variations in site geology, topography, and other deeper geological factors caused a factor of seven difference in accelerations between Tarzana and a site 2 km distant,” making it “difficult to make a meaningful contour map of peak acceleration.”2
1994 Northridge Earthquake (6.7)
USGS ShakeMap for the 1994 Northridge Earthquake, showing ground acceleration contours and readings from individual sensor stations
The Challenges Associated with Using ShakeMaps for Building Damage Assessment
Until recently, lacking a better option, many companies and communities relied on data from ShakeMaps to make emergency response and business continuity decisions following earthquake events. Typically, based on the regional map of shaking intensity produced by ShakeMaps, building owners would hire structural engineers to inspect individual structures that were in areas of high shaking.
ShakeMaps have also been used by the insurance industry as a tool to trigger payouts for parametric insurance products, using ShakeMaps as a proxy for the loss. However, there is significant risk that the payout will be substantially different from the actual loss, due to the poor correlation between ShakeMap-predicted and actual shaking at a given site.
“The insurance industry has been nervous about using ShakeMaps as parametric triggers, because there is so much uncertainty,” says Evan Reis, Director of Science at Safehub, and Executive Director of the U.S. Resiliency Council (USRC), an organization dedicated to advancing tools for assessing the performance of buildings during earthquakes. “The USGS creates approximate ground shaking contours, which are mainly for public consumption and regional disaster response planning. They were not designed to be used as a trigger for insurance claims.”
Further, ShakeMaps are often updated over the course of days, weeks, or even months following an earthquake, as new data is collected and the USGS algorithm adjusts its estimate of shaking intensity. For example, for magnitude 6.5 or larger events occurring in 2021-2022, USGS produces on average seven ShakeMap versions for each event with the final version produced 48 days after the event. In some instances, the delay has been as long as one year or more, as shown below.
Delay between event occurrence and production of the final USGS ShakeMap for large earthquakes
The delay in producing final versions of ShakeMaps can be challenging for building owners and communities that need to make immediate decisions about where to allocate scarce resources. It can also be especially problematic for insurers that are required to pay out parametric insurance claims within days after an earthquake.
Building-Specific Sensor Technology Reduces Risk Bias and Uncertainty
Over the past five years, it has become possible to instrument individual buildings with earthquake sensors at a fraction of the cost of deploying sensors used in the USGS-affiliated networks. Over the
past 50 years, because of the high cost, the California Geological Survey has instrumented only 177 buildings through its Strong Motion Instrumentation Program. In comparison, over the past three years, Safehub has installed lower-cost sensors in nearly 700 buildings.
Given the uncertainties and delays associated with the ShakeMap program, a better solution for building owners and insurers who need to have building-specific shaking and damage data delivered
accurately and quickly, is to deploy sensors directly in buildings that are connected to a dedicated, customer-specific platform.
After a moderate-to-large earthquake, building owners will often evacuate a building until they receive a determination from a structural engineer that the building is safe to reenter. “After an earthquake event, you need to start assessing damage and determine how to manage repairs as soon as possible,” Reis says. “If you were to rely only on ShakeMaps, you could be ignoring specific buildings that are more damaged than the ShakeMap might indicate.”
The Safehub system enables building owners to make informed decisions about whether to evacuate or shelter-in-place, and how to prioritize building inspection. Further, the detailed data produced by the sensors can also produce valuable insights (such as hidden structural damage) to structural engineers once they arrive on site to make followup inspections.
According to Dr. Vitor Silva, seismic risk coordinator at the Global Earthquake Model (GEM) Foundation, a public-private partnership that provides models of earthquakes and their consequences,“building owners can now make better-informed decisions thanks to new building-specific sensor solutions that can provide sensor and strain data about damage to buildings that might otherwise be undetectable. Compared with USGS sensors that are installed in the ground, building-specific accelerometers installed at strategic locations throughout a building can immediately measure the impact of an earthquake on the building’s structural integrity.” Following destructive events, ground shaking or dynamic data provided by sensors can significantly reduce bias and error estimation by more than 60 percent, according to Dr. Silva.
When it comes to insurance, the accuracy and speed with which building-specific sensors can be delivered will transform earthquake parametric insurance.
The USGS ShakeMap system can be a valuable tool for producing a quick snapshot of the regional impacts of large earthquakes. But for building and business owners and the insurance industry, sensor
technology installed directly in individual buildings that are at risk is a superior tool for assessing the effects of earthquakes. Thanks to IoT sensors like Safehub and advanced analytics, building owners and insurers can access building-specific data immediately following an earthquake to inform emergency response, business continuity, and insurance policy-related decisions.
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