A recent article entitled “A Seismic Change in Predicting How Earthquakes Will Shake Tall Buildings” that appeared in the New York Times on June 27, has generated some concern regarding the performance of tall buildings during earthquakes. The article cites statements made during the eleventh U.S. National Conference on Earthquake Engineering — which several RMS earthquake engineering experts attended, stating that there are large changes being introduced to ground motion models. Ground motion models predict the intensity of ground shaking at a site.
New findings into the effect of a magnitude 7.0 earthquake originating from the 74 mile-long (119 kilometer) Hayward Fault in the California Bay Area, suggests that fire following earthquake alone could see more than 52,000 single-family homes burn. Earlier this month, the United States Geological Survey (USGS) released new results for their HayWired scenario, a scientifically plausible magnitude 7.0 earthquake on the Hayward fault. The hypothetical HayWired earthquake occurs at 4:18 p.m. on April 18, 2018, the anniversary of the magnitude 7.8 earthquake which struck San Francisco in 1906. The mainshock ruptures the fault along its length for about 52 miles (83 kilometers). The impact of such an event on one of the most densely populated and interconnected areas of the U.S. West Coast — with a population of about seven million people — would be disruptive.
During 2017, Oklahoma and southern Kansas experienced Mw 3+ earthquakes more frequently than in California. Although the annual count of earthquakes in the Oklahoma/Kansas area has declined since peaking in 2015, the rate of earthquake occurrence is still extremely high. More than 50 events (Mw 3+) have occurred since the start of 2018, including events of Mw 4.6 and Mw 4.5 during the first weeks of April, with shaking strong enough to be felt in Oklahoma City.
In Oklahoma and other parts of the Central and Eastern United States, research studies have linked the increased occurrence of earthquakes to oilfield wastewater injection. Damage has mostly been minimal, but the ground shaking occurring from events greater than or equal to Mw5 could have produced extensive damage had comparable events occurred in more greatly populated areas of the state.
In 2016, the United States Geological Survey (USGS) released their first short-term forecast for seismic hazard — this was meant to capture the increased (but potentially transient) nature of this hazard, and the forecast emphasized that the seismic hazard in Oklahoma is comparable to that of California. Since the first one-year forecast in 2016, the USGS has released subsequent forecasts for 2017 and now for 2018.
RMS recently hosted a two-day workshop for the United States Geological Survey (USGS) on March 7-8, based at our Newark headquarters in California. The aim of the workshop was to both discuss and receive feedback from the scientific community on the interim update to the ongoing 2018 USGS National Seismic Hazard Map Project (NSHMP).
RMS works actively with the USGS and the community and has previously hosted USGS workshops to help facilitate scientific discussions and to improve understanding of earthquake hazard. The workshop was well attended by over 150 people from academia, the scientific community, and industry. A summary of the workshop can be found here.
Oklahoma, Colorado, and Texas have all experienced unusually large earthquakes in the past few years and more earthquakes over magnitude 3 than ever before.
Over a similar time frame, domestic oil and gas production near these locations also increased. Could these earthquakes have been induced by human activity?
Figure 1: The cumulative number of earthquakes (solid line) is much greater than expected for a constant rate (dashed line). Source: USGS
According to detailed case studies of several earthquakes, fluids injected deep into the ground are likely a contributing factor – but there is no definitive causal link between oil and gas production and increased earthquake rates.
These larger, possibly induced, earthquakes are associated with the disposal of wastewater from oil and gas extraction. Wastewater can include brine extracted during traditional oil production or hydraulic fracturing (“fracking”) flowback fluids – and injecting this wastewater into a deep underground rock layer provides a convenient disposal option.
In some cases, these fluids could travel into deeper rock layers, reduce frictional forces just enough for pre-existing faults to slip, and thereby induce larger earthquakes that may not otherwise have occurred. The 2011 Mw 5.6 Prague, Oklahoma earthquake and other recent large midcontinent earthquakes were located near high volume wastewater injection wells and provide support for this model.
However, this is not a simple case of cause and effect. Approximately 30,000 wastewater disposal wells are presently operated in the United States, but most of these do not have nearby earthquakes large enough to be of concern. Other wells used for fracking are associated with micro-earthquakes, but these events are also typically too small to be felt.
To model hazard and risk in areas with increased earthquake rates, we have to make several decisions based on limited information:
- What is the largest earthquake expected? Is the volume or rate of injection linked to this magnitude?
- Will the future rate of earthquakes in these regions increase, stay the same, or decrease?
- Will future earthquakes be located near previous earthquakes, or might seismicity shift in location as time passes?
Induced seismicity is a hot topic of research and figuring out ways to model earthquake hazard and possibly reduce the likeliness of large induced earthquakes has major implications for public safety.
From an insurance perspective, it is important to note that if there is suspicion that the earthquake was induced, it will be argued to fall under the liability insurance of the deep well operator and not the “act of God” earthquake coverage of a property insurer. Earthquake models should distinguish between events that are “natural” and those that are “induced” since these two events may be paid out of different insurance policies.
The current USGS National Seismic Hazard Maps exclude increased earthquake rates in 14 midcontinent zones, but the USGS is developing a separate seismic hazard model to represent these earthquakes. In November 2014, the USGS and the Oklahoma Geological Survey held a workshop to gather input on model methodology. No final decisions have been announced at this time, but one possible approach may be to model these regions as background seismicity and use a logic tree to incorporate all possibilities for maximum earthquake magnitude, changing rates, and spatial footprint.
Figure 2: USGS 2014 Hazard Map, including zones where possibly induced earthquakes have been removed. Source: USGS