EGU2020-5505
https://doi.org/10.5194/egusphere-egu2020-5505
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

How significant is vertical ground motion from low magnitude earthquakes?

Janneke van Ginkel1,2, Elmer Ruigrok2,3, and Rien Herber1
Janneke van Ginkel et al.
  • 1ESRIG, Groningen University, Groningen, the Netherlands (j.a.van.ginkel@rug.nl)
  • 2R&D Seismology and Acoustics, Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
  • 3Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands

Up to now, almost all of the ground motion modeling and hazard assessment for seismicity in the Netherlands focuses on horizontal motion. As a rule of thumb, the strength of vertical ground motions is taken as 2/3 of that of horizontal ground motions. In reality of course, amplifications and V/H ratios are site-dependent and thus vary regionally.  Recent studies have indeed shown that vertical ground motion is not always simply 2/3 of the horizontal motion. However, these studies are performed in areas with high magnitude (Mw>5.0) earthquakes and the question is whether vertical motion is relevant to be included in seismic hazard assessment for low magnitude earthquakes (to date, max Mw=3.6 in Groningen).

In the Netherlands, the top part of the soils is practically always unconsolidated, so the elastic waves generated by deeper (~3000m) seated earthquakes will be subject to transformation when arriving in these layers. Recordings over a range of depth levels in the Groningen borehole network show the largest amplification to occur in the upper 50 meters of the sedimentary cover. We not only observe a strong amplification from shear waves on the horizontal components, but also from longitudinal waves on the vertical component. A better understanding of vertical motion of low magnitude earthquakes aims to support the design of re-enforcement measures for buildings in areas affected by low magnitude seismicity. Furthermore, interference between the longitudinal -and shear waves might contribute to damage on structures.

This study presents observations of longitudinal wave amplification in the frequency band 1-10 Hz, corresponding to resonance periods of Dutch buildings. From 19 seismic events, with a minimum of magnitude two, we retrieved transfer functions (TFs) from the vertical component, showing a strong site response at certain locations. In addition, we calculate event V/H ratios and VH factors from the surface seismometer. These results are compared with the TFs and show a similar pattern in terms of site response. Furthermore, the sites with highest vertical amplification correspond to very low (800-900 m/s) P-wave velocities. Our study shows that vertical amplification is very site dependent. However, the question whether the vertical motion is significant enough to form a real hazard can only be answered through cooperation between seismologist and structural engineer.

How to cite: van Ginkel, J., Ruigrok, E., and Herber, R.: How significant is vertical ground motion from low magnitude earthquakes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5505, https://doi.org/10.5194/egusphere-egu2020-5505, 2020

Comments on the presentation

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Presentation version 1 – uploaded on 17 Apr 2020
  • CC1: Comment on EGU2020-5505, Hans-Balder Havenith, 01 May 2020

    Dear Janneke,

    in your presentation I do not see any information about the hypocentral depth of your events.

    I think this should have an influence on the Z-component... especially in your case, as the Groningen earthquakes pobably were quite shallow, no ? 

    yours

    Hans

    • AC1: Reply to CC1, Janneke van Ginkel, 01 May 2020

      Dear Hans,

      Indeed, the hypocentres in Groningen are shallow: at 3 km depth. This has for sure an influence on the earthquake waves, so the P-waves experience less attenuation compared to earthquakes at greater depth. 

      In my abstract I introduce the setting in Groningen, therefore it is not included in the presentation, but I can make a change if that gives more clarity.

      Thanks for your interest,

      Janneke

      • CC2: Reply to AC1, Hans-Balder Havenith, 01 May 2020

        ..no changes of depth?  2.5-3.5km ... I think for these shallow depths a few hundred meters of difference may have an impact on Z-component. 

        H

        • AC2: Reply to CC2, Janneke van Ginkel, 02 May 2020

          Since all induced earthquakes orgininate in the same producing gas reservoir, we are quite sure about the depths. The reservoir has everywere a similar depth of 3km, and hypocentrers of the earthquakes are in the top zone. Many collegues have looked at the origin depths, but alle are similar.

          • CC4: Reply to AC2, Daniel Bowden, 04 May 2020

            I agree earthquake depth and magnitude will be important for total amplitude, but this shouldn't matter for cases where the authors look at V/H ratio, no? Is there something else I'm missing?

            • AC3: Reply to CC4, Janneke van Ginkel, 04 May 2020

              Hi Daniel, because of the absence of a hard rock reference site, we only look at relative amplitude spectra by comparing the different sites.  Furthermore we did not observe any non-linear behaviour by comparing amplitude spectra with magnitude and with this low intensity events we assume linear behaviour. So no magnitude dependency. 

  • CC3: Comment on EGU2020-5505, Daniel Bowden, 04 May 2020

    Nice presentation! I like figure overlaying with geology/peat thickness. Do you specifically window your PGA measurements around P and S, for vertical and horizontal respectively? Is it always the case that PGAV is from P-waves?

    • AC4: Reply to CC3, Janneke van Ginkel, 04 May 2020

      Yeah, the peat is influencing the level of saturation of the soil, hence the P-wave velocities are strongly decreased by this. The PGAV is derived from the measurements on the vertical component of the seismometers and assume mainly P-waves would contribute. Currently we are working on wave propagation models, for better understanding of conversions due to non-vertical angles of incidence