Addressing limitations in existing ‘simplified’ liquefaction triggering evaluation procedures: application to induced seismicity in the Groningen gas field
Abstract: The Groningen gas field is one of the largest in the world and has produced over 2000 billion m3 of natural gas since the start of production in 1963. The first earthquakes linked to gas production in the Groningen field occurred in 1991, with the largest event to date being a local magnitude (ML) 3.6. As a result, the field operator is leading an effort to quantify the seismic hazard and risk resulting from the gas production operations, including the assessment of liquefaction hazard. However, due to the unique characteristics of both the seismic hazard and the geological subsurface, particularly the unconsolidated sediments, direct application of existing liquefaction evaluation procedures is deemed inappropriate in Groningen. Specifically, the depth-stress reduction factor (rd) and the magnitude scaling factor relationships inherent to existing variants of the simplified liquefaction evaluation procedure are considered unsuitable for use. Accordingly, efforts have first focused on developing a framework for evaluating the liquefaction potential of the region for moment magnitudes (M) ranging from 3.5 to 7.0. The limitations of existing liquefaction procedures for use in Groningen and the path being followed to overcome these shortcomings are presented in detail herein.
Summary (2 min read)
2.1 Overview of the simplified procedure
- The Dutch National Annex to the Eurocode for the seismic actions (i.e., NPR 9998 2017), recommends the use of the Idriss and Boulanger (2008) variant of the simplified liquefaction evaluation procedure, but allows other variants to be used if they are in line with the safety philosophy of the NPR 9998-2017.
- As a result, the Idriss and Boulanger (2008) variant and the updated variant (Boulanger and Idriss 2014) have been used in several liquefaction studies in Gronginen, resulting in predictions of potentially catastrophic liquefaction effects that have severe implications for buildings and for infrastructure such as dikes.
2.2 Depth-stress reduction factor: rd
- The grey lines were computed by Cetin (2000) from equivalent linear site response analyses performed using a matrix of 50 soil profiles and 40 motions.
- The black lines are the median (thick line) and median plus/minus one standard deviation (thinner lines) for the Cetin (2000) analyses.
3.2 Magnitude Scaling Factor: MSF
- The proposed MSF have lower values for smaller magnitude events, relative to Idriss and Boulanger (2008) relationship, and therefore will result in a higher predicted CSR*.
- Accordingly, any assessments in the trends in the changes to CSR* need to consider both the use of both the Lasley et al. ( 2016) rd relationship and the newly proposed MSF, which were consistently developed.
4.1 Groningen-specific rd and MSF relationships
- Once developed, the Groningen-specific rd and MSF relationships can be used in conjunction with the CRRM7.5 curve shown in Figure 7 to compute the FSliq at depth in profiles in Groningen subjected to induced earthquake motions.
- The computation of liquefaction hazard curves that will be used to determine whether the hazard due to liquefaction is significant enough to require the consequences from liquefaction to be assessed is discussed next.
4.2 Planned output from the liquefaction hazard study
- The optimal LPIish thresholds corresponding to different severities of surficial liquefaction manifestations are dependent on the liquefaction triggering procedure used to compute FSliq and the characteristics of the profile.
- Without liquefaction case history data to develop Groningen-specific thresholds, the thresholds proposed by Iwasaki et al. (1978) will be conservatively (Maurer et al. 2015c ) used in the pilot study with the LPIish framework (i.e., LPIish < 5: no to minor surficial liquefaction manifestations are predicted; LPIish > 15: severe surficial liquefaction manifestations are predicted).
5 Discussion and conclusions
- The framework of the liquefaction hazard pilot study is in complete accord with the safety philosophy of the NPR 9998-2017 and is particularly well suited to the specific nature of the timedependent induced seismicity being considered.
- The results of the study will form the basis on which decisions will be made regarding the need for implementing mitigation measures.
- The liquefaction hazard study is benefiting significantly from the broader efforts to assess the regional seismic hazard in Groningen, to include the development of a regional velocity model (Kruiver et al. 2017a, b) , site response model (Rodriguez-Marek et al. 2017) , and ground-motion prediction model (Bommer et al. 2017 ).
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Cites background from "Addressing limitations in existing ..."
...No liquefaction was reported per se, but muddying of a creek near Lake Ellesmere was reported....
...The Avon-Heathcote Estuary is also a former mouth of the Waimakariri River and, thus, likely has similar deposits as Lake Ellesmere (Green et al. 2018c); these deposits may also be similar to the tidal flats that are purported to have severely liquefied during the 1865 Barrow-in-Furness earthquake…...
...The Avon-Heathcote Estuary is also a former mouth of the Waimakariri River and, thus, likely has similar deposits as Lake Ellesmere (Green et al. 2018c); these deposits may also be similar to the tidal flats that are purported to have severely liquefied during the 1865 Barrow-in-Furness earthquake (Musson 1998)....
...Lake Ellesmere is shallow and is better described as a lagoon or estuary than a lake....
...The macroseismic epicenter of the 1870 earthquake that impacted Christchurch is estimated to be under Lake Ellesmere, which is approximately 25 km S-SW of the CBD and is a former mouth of the Waimakariri River, which currently empties into Pegasus Bay north of Christchurch....
Cites background or methods from "Addressing limitations in existing ..."
...This actually required an additional step backward to develop a revised liquefaction triggering model for tectonic earthquakes due to potential biases in the rd and MSF relationships inherent to existing variants of the simplified model (Lasley et al. 2016, 2017; Green et al. 2019)....
...(2016, 2017) and Green et al. (2019) to develop the revised rd, neq, and MSF for tectonic earthquakes....
...10 shows a comparison of the Groningen-specific MSF relationship for Zone 602, as an example, and the worldwide relationships proposed by Idriss and Boulanger (2008) (MSFIB08) and Green et al. (2019) (MSFWUS)....
...Consistent with the revised MSF relationship given in Green et al. (2019), the Groningen-zone-specific MSF relationship is given by the following expression: Table 3....
...In the next section, the Groningen-specific relationships, rd-Gron and MSFGron, in conjunction with the revised CRR curve (Green et al. 2019), are used to assess the liquefaction hazard across the Groningen liquefaction study area shown in Fig....
"Addressing limitations in existing ..." refers background or methods in this paper
...The word “simplified” in the procedure’s title originated from the proposed use of a form of Newton’s Second Law to compute cyclic shear stress (τc) imposed at a given depth in the soil profile, in lieu of performing numerical site response analyses (Whitman 1971; Seed and Idriss 1971)....
...1994) and for historical reasons the amplitude of the equivalently damaging sinusoidal loading is set equal to the 0.65 times the maximum value of the erratic/random loading (e.g., Whitman 1971; Seed and Idriss 1971)....
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