Special Report on the Westerwijtwerd Earthquake 22Nd May 2019
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Special Report on the Westerwijtwerd Earthquake 22nd May 2019 Datum May 2019 Editors Jan van Elk and Dirk Doornhof Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 2 Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 Contents 1 Summary.......................................................................................................................................... 5 2 Introduction ..................................................................................................................................... 6 3 Analysis of Measurements and Observations of the Westerwijtwerd Earthquake ........................ 7 3.1 Processing of the Earthquake records ..................................................................................... 7 3.2 Surface Ground Motions measured by KNMI Network .......................................................... 8 3.2.1 Peak Ground Accelerations and Velocities .................................................................... 11 3.2.2 Ground-Motion Durations ............................................................................................. 16 3.2.3 Spectral Accelerations and Comparison with the GMM ............................................... 20 3.3 Concluding Remarks Surface Ground Motions ..................................................................... 24 3.4 Determination of Hypocentre Location ................................................................................ 25 3.4.1 Standardised Operational Method ................................................................................ 25 3.4.2 Full Waveform Inversion (FWI) method, ....................................................................... 25 3.5 Production and Pressures ...................................................................................................... 28 3.6 Reported Building Damage .................................................................................................... 34 3.7 TNO Household Sensors ........................................................................................................ 35 3.8 Analysis .................................................................................................................................. 38 4 MRP status 22nd of May 2019 ........................................................................................................ 41 4.1 Activity rate ........................................................................................................................... 42 4.2 Earthquake density ................................................................................................................ 44 4.3 PGA and PGV ......................................................................................................................... 48 4.4 Damage state ......................................................................................................................... 48 4.5 Other patterns and considerations ....................................................................................... 49 4.5.1 Loppersum trends ......................................................................................................... 49 4.5.2 Probability earthquakes with higher magnitude and b-factor ...................................... 50 5 Intervention measures and their estimated effect on seismicity ................................................. 52 6 References ..................................................................................................................................... 53 7 Appendix A – Letter Evaluatie: Westerwijtwerd aardbeving ........................................................ 56 3 Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 4 Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 1 Summary This Special Report, which was announced in NAM’s letter of 24th May 2019 (NAM, 2019a), has been prepared in accordance with the Measurement and Control protocol (MRP) (NAM, 2017a). It addresses the earthquake on the 22nd May 2019 at 5:49 local time (CEST), near the village of Westerwijtwerd. The Westerwijtwerd earthquake had a magnitude of ML 3.4 on the Richter Scale, as established by KNMI. The earthquake was widely felt and caused (DS1) damage to buildings in the region around the epicentre. No falling objects (e.g. chimneys) or physical injuries have been reported. In this report, the measurements and observations obtained during the Westerwijtwerd earthquake are analysed in chapter 3. The assessment of these measurements and observations of ground motions and building damage showed that in all aspects, the Westerwijtwerd earthquake was in line with the current understanding of the field. No special characteristics of the Westerwijtwerd earthquake have been identified, or characteristics that deviate from current modelling results. On this basis, no revision of the Hazard and Risk Assessment (HRA) is required at this time. In chapter 4, the report provides an overview of the status and trend of all parameters used in the MRP framework. This showed the “waakzaamheidsniveau” (vigilance level) in the MRP has been reached for PGV. The other parameters are in the monitoring range. The expected effects of mitigation measures on seismicity and the parameters in the MRP are discussed in chapter 5. The analyses described above, which have been more extensive than those prepared in the first 48 hours after the earthquake, do not lead to a requirement to revise and restate NAM’s assessment of the situation, as described in its letters to Staatstoezicht op de Mijnen (SodM) and Ministry of Economic Affairs and Climate Policy (MEAC) of 24nd May 2019. 5 Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 2 Introduction This Special Report has been prepared in accordance with the Measurement and Control protocol (MRP) (NAM, 2017a), following the earthquake on the 22nd May 2019 at 5:49 local time (CEST and 3:49:00 UTC), near the village of Westerwijtwerd. This was the third largest earthquake in Groningen, th following the Westeremden earthquake of 8 August 2006, with a magnitude of ML 3.5 on the Richter th Scale, and the Huizinge earthquake of 16 August 2012, with a magnitude of ML 3.6. This earthquake has the same magnitude as the Zeerijp earthquake of 8th January 2018. During this earthquake, the largest ground acceleration, 0.042 g was recorded by one of the KNMI stations, station G170, located at 1.7 km from the epicentre. The largest PGV was registered by station G210, located at 3.6 km from the epicentre. This largest PGV was 1,0 cm/s. As a result of this PGV, the “waakzaamheidsniveau” (vigilance level) threshold of the Measurement and Control Protocol (MRP) was exceeded. The MRP requires the Risk Coordination Team (RCT)1 to meet within a few days when the ‘vigilance’ level is triggered. In this case NAM elected to treat the earthquake as a very significant event, based on the magnitude and the societal impact, and convened the RCT in line with the highest level in the escalation structure of the MRP. The RCT of NAM met that same day to analyse the event. The Westerwijtwerd earthquake had a magnitude of ML 3.4 on the Richter Scale, as established by KNMI. The earthquake was widely felt and caused (DS1) damage to buildings. Using the methodology developed by USGS, “Did you feel it?“ (USGS, 2011), the earthquake was felt at a large distance (~18km) from the epicentre, including parts of the city of Groningen. Some 115,000 houses are located in the area where the earthquake was felt. Especially in the area near Westerwijtwerd, where the largest vertical accelerations have been recorded, the earthquake was experienced as frightening. The number of buildings that have been exposed to a peak ground velocity larger than 2 mm/s (corresponding to the damage contour of the TU Delft) is about 44,500. Within the contour based on the SBR Guideline (masonry in normal state) some 6,500 buildings are located. On 28 May 2019, the Tijdelijke Commissie Mijnbouwschade Groningen (TCMG) reported to have received some 2,600 damage claims. After preliminary analysis of the available data, letters (NAM, 2019a) were prepared, detailing the initial assessment of the impact of the earthquake and providing the Minister of Economic Affairs and Climate and the regulator SodM. The current “Special Report on the Westerwijtwerd Earthquake – 22nd May 2019” presents the technical analysis carried out during the 2-week period following the Westerwijtwerd earthquake. In detail, some of the technical conclusions have been further refined since the 48-hour letter was prepared, although no fundamental new insights have come to light since then. It is possible that future studies carried out as part of the Studies and Data Acquisition Plan will generate new insights. 1 The MRP requires the RCT to meet within a few days when the ‘vigilance’ level is triggered. In this case NAM elected to treat the earthquake as a very significant event, based on the magnitude and the societal impact. NAM convened the RCT within 24 hours, in line with the highest level in the escalation structure of the MRP. 6 Special Report on the Westerwijtwerd Earthquake of 22nd May 2019 3 Analysis of Measurements and Observations of the Westerwijtwerd Earthquake 3.1 Processing of the Earthquake records In this section of the Special Report, the measurements and observations of the Westerwijtwerd Earthquake are reported and discussed. An initial analysis of these measurements