-->

Amplification of Seismic Ground Motion Hazard in the Fort St. John – Dawson Creek Area

Lead Researcher(s):  P. A. Monahan

Project ID:  2018-052

Key Research Organization(s):  Petrel Robertson Consulting Ltd.

Project Location:  Northeast BC

Strategic Focus Area:  Energy-Oil and Gas

Summary



With a focus on the populated Fort St. John-Dawson Creek area of the Peace Region in British Columbia’s Northeast Region, where earthquakes caused by hydraulic fracturing and water disposal by the petroleum industry are causes of public concern, this project provides new information on how and where seismic waves travelling through soils and sediments below the surface may be amplified. It provides necessary information for regulators and industry to reduce risk to infrastructure and disturbance to the public by improving protocols.    

The Need

Induced earthquakes are widely felt by residents of the region. Although only the largest produce ground motions that would be expected to cause damage, others are also disturbing by generating noise and causing concerns about the effects of shaking. The presence of certain sediments and geological conditions can amplify seismic waves, potentially increasing the threat to public infrastructure and increasing the disturbance to the general public.

The presence of soft soils at the surface, and other geological conditions found in the Peace Region, have the potential to amplify ground motion. In a manner similar to ocean waves slowing down and increasing in height as they approach the shore, seismic shear-waves increase in amplitude as they slow down in near-surface sediments. This has the effect of amplifying ground motion. Amplification can also occur due to resonance, by which specific frequencies of ground motion can reverberate between surface layers. Buildings with the same natural frequency as the site will be particularly affected.

With an increase in seismic activity from hydraulic fracturing and water disposal at natural gas sites in northeastern BC, it is important to understand how and where this can occur so that any risk can be reduced and managed.

This project is built on a regional study completed in early 2019 by collecting more detailed subsurface information for a smaller area, where the majority of the population in the Montney Play (the name given to the large sedimentary basin that hosts natural gas resources in this area) is concentrated.

Project Goals

This project fits under Geoscience BC’s Strategic Objective of ‘Facilitating Responsible Natural Resource Development’ and our goal to:

  • Maintain joint research with partners examining seismicity induced by hydraulic fracturing in northeastern BC to provide new science to better understand induced seismicity, mitigate risks and further improve regulation and industry practices.

Specifically, this project collected and interpreted additional subsurface geological and shear-wave velocity (Vs – the wave that shakes the ground back and forth perpendicular to the direction the wave is moving) data to investigate the controls and map the susceptibility to amplification of seismic ground motions due to shallow geological conditions in the Fort St. John-Dawson Creek area. In order to achieve this goal, the following steps were taken:

  • Preparation of new geological maps that reflect the thicknesses and properties of unconsolidated deposits; and
  • Acquisition of new Vs data in order to improve the Vs model for the shallow geological materials, and to provide data for future studies, particularly at seismograph stations.

Project Benefits

This project provides more detailed mapping (than the previous project 2016-062) for predicting ground motion amplification and assessing the induced seismic hazard to public and natural gas industry infrastructure in an area of the Montney Play where population and infrastructure are more concentrated.

It also provides basic Vs data to refine ground motion prediction equations and to inform ongoing research into seismicity induced by natural gas development.

The maps and data provide industry, regulators, communities and Indigenous groups with new data to understand the risks associated with ground motion in the Fort St John-Dawson Creek area.

Project outputs can be used by regulators and industry operators who are managing natural gas development to improve processes and protocols to reduce public disturbance and the risk of damage resulting from hydraulic fracturing and fluid injection by the natural gas industry.

Survey Area

This project took place in the Peace Region, including areas around Fort St. John and Dawson Creek in BC’s Northeast Region. All project activity was in the territories of Treaty 8 First Nations.

What was Found?

High resolution digital topographic mapping was used to revise existing surficial geological mapping because most surficial geological units have distinct topographic and geomorphic features.

A depth to bedrock contour map was created using a subsurface database comprising: 1,323 industry cased hole gamma-ray logs; 1,533 water well logs; and 1,955 geotechnical and scientific boreholes.

The project provides an update on a Vs model for shallow geological materials created for a previous study. For this, seven vertical seismic profile (VSP) logs in existing boreholes and 21 multichannel analysis of surface waves (MASW) tests were acquired and were supplemented with Vs data from 21 sites acquired for the previous study and new data at five additional sites.

Geological field work and interviews with local residents helped to confirm sites for data acquisition and to observe surficial geology.

To investigate the potential for seismic amplification, surface and subsurface data and Vs data were compared with earthquake recordings.

The project’s primary conclusion is that, within the project area, susceptibility to amplification of seismic waves is widespread, occurring where the thickness of sediment over bedrock exceeds five metres. Where the thickness exceeds 15 metres, amplification can occur over a wide range of frequencies (broadband amplification, the ocean wave analogue described above). However, amplification of specific frequencies due to resonance can occur where the thickness of sediment is between five and 15 metres, and the highest amplification may occur in these areas. A depth to bedrock map and a map showing susceptibility to amplification of seismic ground motions are provided by the study.

Deliverables