Special Report on the
Earthquakes near Uithuizen in
August, September & October 2022

Datum
Editors

October 2022

Special Report on the earthquakes near Uithuizen in August, September and October 2022

An earlier version of this Special Report has been shared with SodM on Friday 7th October 2022. This
version of this report covered seven earthquakes in August and September 2020 that all had their
epicentre in a small area near the village of Uithuizen.
The earthquake of 11th October 2022, also with an epicentre near Uithuizen, increased the
earthquake density to 0.46 earthquakes / (year * km2). With this last earthquake the monitoring level
for ‘Strong Seismicity’ was exceeded.
The current version of the report has been updated to include the earthquake on 11th October 2022.

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Special Report on the earthquakes near Uithuizen in August, September and October 2022

Contents
Summary and Conclusions ...................................................................................................................... 5
1

Introduction ..................................................................................................................... 6
1.1

Reason for this Special Report ............................................................................................ 6

1.2

Content of this Special Report ............................................................................................ 7

2

Analysis of recent earthquakes near Uithuizen............................................................... 8
2.1

Event rate............................................................................................................................ 8

2.2

Epicentres ........................................................................................................................... 9

2.3

Source Mechanism............................................................................................................ 12

3
Preliminary Analysis of the Surface Ground-Motions Recorded During the ML 1.3-2.7
Uithuizen Earthquakes of August and September 2022 ....................................................................... 15
3.1

Introduction ...................................................................................................................... 15

3.2

Peak Ground Accelerations and Velocities ....................................................................... 15

3.3

Ground-Motion Durations ................................................................................................ 50

3.4

Spectral Accelerations and Comparison with Ground-Motion Models............................ 50

4

Conclusions .................................................................................................................... 50
4.1

Event rate, epicentres and source mechanism................................................................. 50

4.2

Ground Motions................................................................................................................ 50

References

...................................................................... 51

Appendix A FWI analysis of the earthquake near Uithuizen on the 19th August 2022 with a
magnitude of 1.9 .......................................................................................................................................
Appendix B FWI analysis of the earthquake near Uithuizen on the 28th August 2022 with a
magnitude of 1.3 .......................................................................................................................................
Appendix C FWI analysis of the earthquake near Uithuizen on the 9th September 2022 with a
magnitude of 2.3 .......................................................................................................................................
Appendix D FWI analysis of the earthquake near Uithuizen on the 18th September 2022 with a
magnitude of 0.8 .......................................................................................................................................
Appendix E FWI analysis of the earthquake near Uithuizen on the 24th September 2022 with a
magnitude of 2.7 .......................................................................................................................................
Appendix F FWI analysis of the earthquake near Uithuizen on the 24th September 2022 with a
magnitude of 1.7 .......................................................................................................................................
Appendix G FWI analysis of the earthquake near Uithuizen on the 11th October 2022 with a
magnitude of 1.3 .......................................................................................................................................

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Special Report on the earthquakes near Uithuizen in August, September and October 2022

Summary and Conclusions
In August, September and October 2022 eight earthquakes occurred with an epicentre in a 500m by
150 m area near the village of Uithuizen.
Despite being 9 days apart, the first two of these earthquakes had almost identical epicentres. The
comparison of their source mechanism also indicated these earthquakes occurred at the same fault.
These eight earthquakes with an epicentre in a very small area had a large impact on the highest
observed earthquake density. On 24th September 2022 the monitoring level of the highest earthquake
density for ‘increased seismicity’ was exceeded, when earthquake density reached 0.38 earthquake
events per (year * km2). With the earthquake on 11th October 2022, this monitoring level reached 0.46
earthquake events per (year * km2) exceeding the monitoring level for ‘strong seismicity’.
The motions recorded for the earthquakes near Uithuizen are broadly consistent with the predictions
from the empirical PGV GMPE’s used in damage assessment.

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1.2 Content of this Special Report
The current report describes aspects of the earthquakes near Uithuizen in August, September and
October 2022. On 24th September 2022 the largest earthquake density increased to 0.38 earthquake
events per (year *km2) and exceeded the monitoring level for ‘increased seismicity’.
With the earthquake of 11th October 2022 also with an epicentre near Uithuizen the earthquake
density increased to 0.46 earthquakes / (year * km2). With this last earthquake the monitoring level
for ‘Strong Seismicity’ was exceeded.

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2 Analysis of recent earthquakes near Uithuizen
2.1 Introduction
A total of eight earthquakes occurred near the village of Uithuizen in the Groningen field between
August and October 2022, with local magnitudes ranging from 0.7 to 2.7. The largest event occurred
on Saturday 24th September 2022 at 10:20 am (UTC; 12:20 local time) and is the first event of local
magnitude greater than 2.5—the smallest magnitude considered in the Groningen Hazard and Risk
Assessment—to occur since the 16th November 2021 ML3.2 Zeerijp earthquake.

2.2 Event rate
Since 2014, the number of earthquakes shows a declining trend. From a peak of more than 60
earthquakes per year (with magnitude larger than 1.2 on the Richter-scale) in September and October
2011, the earthquake rate has dropped to around 10 earthquakes per year in July 2022 (Fig. 2.1). More
recently it has increased to 20 earthquakes per year.

Figure 2.1

Number of earthquakes per year as stipulated in the Mijnbouwregeling artikel 1.3a.5.
These are earthquakes with a magnitude larger than ML ≥ 1,2 during the previous 6
months converted to an annual basis.

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Figure 2.2

Top: Earthquake density maps for earthquakes with a magnitude larger ML ≥ 1,2 during
the previous six months. The earthquake density is shown as an annual density. Bottom:
Graphs of earthquake magnitude versus date. Left map shows the earthquake density at
1st September 2022. The middle map shows earthquake density at 1st October 2022 and
the right map at 12st October 2022.

On 24th September 2022 the monitoring level of the highest earthquake density for ‘increased
seismicity’ was exceeded, when earthquake density reached 0.38 earthquake events per (year * km2).
This monitoring level increased on 11th October 2022 to 0.46 earthquake events per (year * km2),
exceeding the monitoring level for ‘strong seismicity’.

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Figure 2.3

Map of the deep subsurface with in green the Groningen gas field and surrounding gas
fields. The red dot shows the location of the earthquake near Uithuizen on 9th September
2022. All other earthquakes before 1st September 2022 have been indicated as yellow dots.
The size of the dot is an indication of the magnitude of the earthquake.
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Figure 2.5

Moment tensor solutions for the double-coupled parts (left to right, top to bottom: events
31, 32, 33, 34, 35 and 38.

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Figure 2.6

Moment tensor solutions for the double-coupled parts (left to right: events 39 and 42.

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3 Analysis of the Surface Ground-Motions recorded
during the ML 1.3-2.7 Uithuizen Earthquakes
3.1 Introduction
In this chapter of the report the ground-motions recorded for the earthquakes near Uithuizen will be
presented and discussed. The locations of the epicentres with respect to the B-network and Gnetwork, the two strong-motion networks operated by the KNMI in the Groningen field (Ref. 44 and
46), are as shown in Figure 3.1.
The purpose of this report is to provide an overview of the ground-motions recorded during the six
largest events in table 2.1 and 2.2, all of which occurred near Uithuizen, in terms of their amplitudes
and durations, and compare the recorded amplitudes of motion with predictions from the current
empirical Ground-Motion Prediction Equations (GMPEs; Ref. 42) used to estimate values of peak
ground velocity (PGV) due to earthquakes in the Groningen field, as well as the V6 and V7 Groningen
Ground-Motion Models (GMMs; Ref. 41, 42 and 43). The smallest two events are not included in the
analysis because of the very small amplitudes of their ground-motions.
A total of 83 records from each of the six events were accessed from the online portal of the KNMI
(KNMI, 1993; http://rdsa.knmi.nl/dataportal/), corresponding to 74 records from the surface
accelerographs of the G-network and nine records from the B-network. The records were processed
as described by Edwards and Ntinalexis (2021); the total number of usable records for each event, as
well as the distance ranges covered by the records, are presented in Table 3.1. Figure 3.2 shows the
usable recordings in the magnitude-distance occupied by the database of Ntinalexis et al. (Ref. 47).

Figure 3.1

Left: Epicentres of the nine earthquakes (black star) together with epicentres of previous
earthquakes of ML ≥ 2.5 (magenta stars) and of ML 1.8-2.4 (grey stars). Right: G-network
(green) and B-network (red) stations; open circles indicate G-stations without borehole
geophones.

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Figure 3.3

Geometric mean horizontal components of PGA (upper) and PGV (lower) recorded during
the six Uithuizen earthquakes (red) and in previous earthquakes (blue) plotted against
local magnitude

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Figure 3.4

Horizontal components of PGA recorded during the Uithuizen earthquakes and previous
earthquakes plotted against epicentral distance

The largest PGA and PGV values of each of the six events were recorded at the same station
and component: station G610 and component H1 (NS). The largest PGA of the ML2.7 event
was 64.57 cm/s2, which is the eighth-largest value that has been recorded by the KNMI
networks since 2006, but significantly smaller than the largest PGA which was recorded at the
EW component of the BGAR station during the ML3.4 Zeerijp earthquake that occurred in
January 2018, with a value of 108.68 cm/s2. The largest PGV of the ML2.7 event was 1.13 cm/s,
the 23rd largest recorded since 2006 and also significantly smaller than the largest PGV
recorded to date, a 3.46 cm/s on the NS component of the MID1 station during the August
2012 ML3.6 Huizinge earthquake. The largest PGA and PGV values of the six Uithuizen events
are presented in Table 3.2.

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Figure 3.6

Horizontal components of acceleration and velocity recorded at the G340 station during
event 31; the upper frame shows the accumulation of Arias intensity (energy) over time.

Figures 3.7 – 3.12 show the horizontal components of PGA and PGV obtained within 6 km of the
epicentre, from which it can be appreciated that the very strong polarisation often observed in
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Special Report on the earthquakes near Uithuizen in August, September and October 2022
Groningen recordings (e.g., Ref. 40) is also apparent in records of these events, especially at stations
G040 and G610. Additionally, the strong directivity of the ground-motions is also apparent, with
significantly larger amplitudes recorded primarily to the south and secondarily to the east of the
epicentres.

Figure 3.7

Horizontal components of PGA (upper) and PGV (lower) recorded during event 31 (ML1.9)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.

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Figure 3.8

Horizontal components of PGA (upper) and PGV (lower) recorded during event 32 (ML1.3)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.

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Figure 3.9

Horizontal components of PGA (upper) and PGV (lower) recorded during event 33 (ML2.3)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.

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Figure 3.10 Horizontal components of PGA (upper) and PGV (lower) recorded during event 38 (ML2.7)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.

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Figure 3.11 Horizontal components of PGA (upper) and PGV (lower) recorded during event 39 (ML1.7)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.

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Figure 3.12 Horizontal components of PGA (upper) and PGV (lower) recorded during event 42 (ML1.3)
at epicentral distances of less than 6 km; units are cm/s2 and cm/s, respectively.
As already shown in Figures 3.4 and 3.5, the amplitudes decay rapidly with distance although the effect
of simultaneous arrivals of direct and critically refracted/reflected waves leads to an increase in
amplitudes at some locations between 12 and 20 km from the epicentre. However, these effects do
not lead to significant absolute amplitudes at those distances, and it is clear from Figures 3.7 – 3.12

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that, outside the epicentral area, the motions are of very low amplitude: < 0.01g for PGA and < 0.1
cm/s for PGV.

3.3 Ground-Motion Durations
The maximum amplitude of ground shaking, whether represented by PGA or PGV, provides a simple
indication of the strength of the motion but the potential for adverse effects—such as damage to
masonry buildings or triggering liquefaction—also depends on the duration or number of cycles of the
motion.
A feature that has been consistently observed in the Groningen ground motions is a very pronounced
negative correlation between PGA and duration, with high amplitude motions consistently associated
with shaking of very short duration (Ref. 38). The same pattern is observed in the recordings of the
Uithuizen earthquakes, as shown in Figure 3.13. The shortest duration recorded during the Uithuizen
events is only 0.435 seconds, is associated with a PGA of 11.20 cm/s2 and belongs to the H1 component
of station G030. The horizontal components of both acceleration and velocity from this station are
shown in Figure 3.14, which also shows the build-up of Arias intensity (which is a measure of the energy
in the motion) over time. The strong concentration of the energy in a single pulse of motion in the H1
component is immediately apparent. Durations of the signals recorded in the G610 station (Fig. 3.15),
where the largest PGA and PGV values of all six events correspond, are slightly longer and range, on
the H1 component, from 1.695 to 1.870 seconds. The reason, in this case, is a strong P-wave arrival
about 1.5 seconds before the S-wave peak, which elongates the calculated durations (Figures 3.14 3.20).

Figure 3.13 Pairs of PGA and significant duration for individual components of the Uithuizen records,
with symbols indicating the rupture distance of the recording.

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Figure 3.14 Horizontal components of acceleration and velocity recorded at the G030 station during
event 38; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.15 Horizontal components of acceleration and velocity recorded at the G610 station during
event 31; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.16 Horizontal components of acceleration and velocity recorded at the G610 station during
event 32; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.17 Horizontal components of acceleration and velocity recorded at the G610 station during
event 33; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.18 Horizontal components of acceleration and velocity recorded at the G610 station during
event 38; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.19 Horizontal components of acceleration and velocity recorded at the G610 station during
event 39; the upper frame shows the accumulation of Arias intensity (energy) over time.

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Figure 3.20 Horizontal components of acceleration and velocity recorded at the G610 station during
event 42; the upper frame shows the accumulation of Arias intensity (energy) over time.

3.4 Spectral Accelerations and Comparison with Ground-Motion Models
Additional insight into the nature of the ground motions can be obtained from the 5%-damped
acceleration response spectra and Fourier spectra. The spectra from the G610 recordings are shown
in Figures 3.21 – 3.26. The spectral shapes are consistent with previous observations in the field. The
divergence between the red and black curves in both frames shows that the horizontal polarisation of
the recordings seen previously for PGA and PGV (Figures 3.7 – 3.12) persists across the entire range of
usable response periods.

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Figure 3.21 Fourier and response spectra from the G610 record of event 31

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Figure 3.22 Fourier and response spectra from the G610 record of event 32

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Figure 3.23 Fourier and response spectra from the G610 record of event 33

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Figure 3.24 Fourier and response spectra from the G610 record of event 38

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Figure 3.25 Fourier and response spectra from the G610 record of event 39

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Figure 3.26 Fourier and response spectra from the G610 record of event 42
For this analysis, the key question of interest is whether the motions recorded in this earthquake are
consistent with the current GMM and empirical PGV GMPEs being used in the Groningen field. The
current GMM is the V7 GMM (Ref. 39) and event 38 is within its range of applicability (2.5 - 7.25) and
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we have simply calculated the total residuals at the surface for different ground-motion parameters.
In each case, the residual is the natural logarithm of the ratio of the observed (recorded) to the median
predicted value, so a residual of 0.7 indicates that the recorded value was underestimated by a factor
of 2 by the model and a residual of -0.7 would indicate over-prediction by a factor of 2. Figure 3.27
shows the residuals of spectral accelerations at 0.01 seconds with respect to the V7 GMM plotted
against rupture distance. The scatter is very considerable but similar to the scatter of the data used in
the V7 GMM development, while the residuals are centred above the zero line, which suggests an
under-prediction by the model. A trend of the residuals with distance can be observed, with larger
residuals observed at short distances. At longer periods (Figures 3.28 – 3.29), the scatter is smaller,
however, the residuals are better centred on the zero line, indicating that the median predictions of
the model provide an overall good fit to the data at those periods. It is important to note that the V7
GMM is calibrated to match the average spectral accelerations over the periods between 0.01 and 1
seconds, and not spectral accelerations at individual periods. The residuals with respect to the average
spectral acceleration are shown in Figure 3.32, where it can be observed that the distance trend seen
in the short periods is not present, and overall, the model under-predicts the recorded groundmotions, albeit only to a small degree.

Figure 3.27 Residuals of Sa(T) with respect to the central branch of the V7 GMM at 0.01 seconds

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Figure 3.28 Residuals of Sa(T) with respect to the central branch of the V7 GMM at 0.1 seconds

Figure 3.29 Residuals of Sa(T) with respect to the central branch of the V7 GMM at 0.2 seconds

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Figure 3.30 Residuals of Sa(T) with respect to the central branch of the V7 GMM at 0.5 seconds

Figure 3.31 Residuals of Sa(T) with respect to the central branch of the V7 GMM at 1 second

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Figure 3.32 Residuals of period-averaged Sa with respect to the central branch of the V7 GMM
The V7 GMM superseded and replaced the V6 GMM (Ref. 41); the extensive additions, improvements
and changes that the V7 GMM has in comparison to V6 are described in detail in Bommer et al. (Ref.
39). However, for completeness, and because the V6 GMM was used in the TNO-SDRA on which the
current operational strategy for the Groningen field is based, we repeat the comparisons of Figures
3.27 – 3.32 for the V6 GMM in Figures 3.33 – 3.38 below. The observations that can be made are similar
to those made for the V7 model residuals, with the distance trend at the short periods, however, being
more prominent. The V6 database was not included in Figures 3.33 – 3.38 for comparison, as it was
processed and compiled following different methods than the records of this event.

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Figure 3.33 Residuals of Sa(T) with respect to the central branch of the V6 GMM at 0.01 seconds

Figure 3.34 Residuals of Sa(T) with respect to the central branch of the V6 GMM at 0.1 seconds

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Figure 3.35 Residuals of Sa(T) with respect to the central branch of the V6 GMM at 0.2 seconds

Figure 3.36 Residuals of Sa(T) with respect to the central branch of the V6 GMM at 0.5 seconds

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Figure 3.37 Residuals of Sa(T) with respect to the central branch of the V6 GMM at 1 second

Figure 3.38 Residuals of period-averaged Sa with respect to the central branch of the V6 GMM
The current empirical PGV model was also developed in 2021 (Ref. 42). Events 31, 33 and 38 are within
the applicability range of the GMPEs (ML1.8-3.6), while event 39 is marginally outside; therefore, we
have calculated the total, inter- and intra- event residuals for each of these four events. Figures 3.39 3.42 show the within-event model residuals of the three component definitions of PGV predicted by
the GMPEs, plotted against hypocentral distance. In all cases, nearly all residuals of the Uithuizen

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earthquake recordings are within two within-event standard deviations of the zero line, which suggests
that the model captures well the variability of the data.

Figure 3.39 Event- and station-corrected within-event residuals of three component definitions of PGV
with respect to the equations of the empirical PGV GMPE (Bommer et al., 2021) for event
31. Residuals of the Uithuizen earthquake recordings are shown in green and of other
events in blue. The within-event standard deviation (φSS) is shown in red dashed lines.

Figure 3.40 As with Figure 39, for event 33.

Figure 41. As with Figure 39, for event 38.

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Figure 3.42 As with Figure 39, for event 39.
Figure 3.43 compares the inter-event residuals (event-terms) of the Uithuizen earthquakes to those of
the previous events of the database. These event terms effectively represent the average offset of the
recorded motions from each earthquake compared to the median prediction from the empirical model
for the event magnitude, with a positive event-term indicating a stronger-than-average earthquake, a
negative value a somewhat weaker-than-average earthquake. The event-terms of the Uithuizen
earthquakes lie all within one standard deviation of the expected mean, indicating that the PGV values
recorded are within the prediction range of the model.

Figure 43

Inter-event residuals of three component definitions of PGV with respect to the equations
of the empirical PGV GMPE (Bommer et al., 2021b). Residuals of the Uithuizen earthquake
recordings are shown in green and of older events in blue. The inter-event standard
deviation is shown in red dashed lines.

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4 Conclusions
4.1 Event rate, epicentres and source mechanism
In August, September and October 2022 (19th August to 11th Oktober), eight earthquakes took place in
a small area around the village of Uithuizen. Four of these earthquakes have magnitudes ML ≥ 1.5.
During the equilibration of the reservoir pressure the earthquakes are expected to occur mainly to the
North-West of Loppersum. The location of earthquakes near Uithuizen are consist with this.
Remarkable is the very small area of some 500 m by 150 m, where seven of these earthquakes took
place over a period of two months.
Based on locations of epicenters and source mechanism, the first two of these, although 9 days apart,
had their epicentre most likely at the same fault.
On 24th September 2022 the monitoring level of the highest earthquake density for ‘increased
seismicity’ was exceeded, when earthquake density reached 0.38 earthquake events per (year * km2).
With the earthquake on 11th October 2022, this monitoring level reached 0.46 earthquake events per
(year * km2) exceeding the monitoring level for ‘strong seismicity’.

4.2 Ground Motions
The ML 1.3-2.7 Uithuizen earthquakes of August, September and October 2022 have generated a large
number of ground-motion recordings. The largest component of PGA recorded is 0.06 g and the largest
value of PGV—which is generally considered a better indicator of the damage potential of the
motion—recorded is 1.13 cm/s, which is significantly smaller than the largest value of the Groningen
ground-motion database, the 3.46 cm/s recorded in the ML3.6 2012Huizinge earthquake.
An important observation is that the motions recorded in the Uithuizen earthquakes are broadly
consistent with the predictions from the empirical PGV GMPEs used to assess damage claims.
While the ground-motions recorded during the ML2.7 event appear to be, on average, lower than those
recorded in previous events of similar magnitudes (Figures 3.2 and 3.3), the V7 and V6 GMM groundmotion models, which are currently deployed in the seismic hazard and risk modelling for Groningen,
under-predict the amplitudes by a small degree. A possible explanation for this can be found when
examining the location of the epicentre with respect to the recording networks (Figure 3.1) and the
direction of the largest amplitudes (Figures 3.7 - 3.12). The majority of recording stations is located
southeast of the epicentres, which coincides with the direction of the largest amplitudes. Conversely,
the number of stations in the opposite direction is small. This could likely lead to a biased recording
sampling of the radiation pattern of the event, with areas with larger amplitudes being over-sampled.

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46. Ntinalexis, M., J.J. Bommer, E. Ruigrok, B. Edwards, R. Pinho, B. Dost, A.A. Correia, J. Uilenreef, P.J. Stafford & J. van Elk (2019). Groundmotion networks in the Groningen field: usability and consistency of surface recordings. Journal of Seismology 23(6), 1233-1253.
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52

Appendix A

FWI analysis of the earthquake near
Uithuizen on the 19th August 2022 with a
magnitude of 1.9

Appendix B

FWI analysis of the earthquake near
Uithuizen on the 28th August 2022 with a
magnitude of 1.3

Appendix C

FWI analysis of the earthquake near
Uithuizen on the 9th September 2022 with
a magnitude of 2.3

Initially KNMI estimated the magnitude of this earthquake at 2.4 and revised this later to 2.3. This
report was completed before this revision of the magnitude and therefore still states the initial
estimate of the magnitude.

Appendix D

FWI analysis of the earthquake near
Uithuizen on the 18th September 2022
with a magnitude of 0.8

Appendix E

FWI analysis of the earthquake near
Uithuizen on the 24th September 2022
with a magnitude of 2.7

Appendix F

FWI analysis of the earthquake near
Uithuizen on the 24th September 2022
with a magnitude of 1.7

Appendix G FWI analysis of the earthquake near
Uithuizen on the 11th October 2022 with a magnitude of
1.3