Geologic and geodetic constraints on the seismic hazard of Malawi&rsquo;s active faults: The Malawi Seismogenic Source Database (MSSD)

Williams, Jack N.; Wedmore, Luke N. J.; Fagereng, Åke; Werner, Maximilian J.; Mdala, Hassan; Shillington, Donna J.; Scholz, Christopher A.; Kolawole, Folarin; Wright, Lachlan J. M.; Biggs, Juliet; Dulanya, Zuze; Mphepo, Felix; Chindandali, Patrick

doi:https://doi.org/10.5194/nhess-2021-306

Preprints

https://doi.org/10.5194/nhess-2021-306

Preprints

16 Nov 2021

Status: a revised version of this preprint is currently under review for the journal NHESS.

Geologic and geodetic constraints on the seismic hazard of Malawi’s active faults: The Malawi Seismogenic Source Database (MSSD)

Jack N. Williams^1,2,a, Luke N. J. Wedmore², Åke Fagereng¹, Maximilian J. Werner², Hassan Mdala³, Donna J. Shillington⁴, Christopher A. Scholz⁵, Folarin Kolawole⁶, Lachlan J. M. Wright⁵, Juliet Biggs², Zuze Dulanya⁷, Felix Mphepo³, and Patrick Chindandali⁸ Jack N. Williams et al.

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¹School of Environmental Sciences, Cardiff University, Cardiff, UK
²School of Earth Sciences, University of Bristol, Bristol, UK
³Geological Survey Department, Mzuzu Regional Office, Mzuzu, Malawi
⁴School of Earth and Sustainability, Northern Arizona University, Flagstaff, Arizona, USA
⁵Department of Earth Sciences, Syracuse University, Syracuse, New York, USA
⁶BP America, Houston, Texas, U.S.
⁷Geography and Earth Sciences Department, University of Malawi, Zomba, Malawi
⁸Geological Survey Department, Zomba, Malawi
^anow at: the Department of Geology, University of Otago, Dunedin, New Zealand

Received: 26 Oct 2021 – Discussion started: 16 Nov 2021

Abstract. Active fault data are commonly used in seismic hazard assessments, but there are challenges in deriving the slip rate, geometry, and frequency of earthquakes along active faults. Herein, we present the open-access geospatial Malawi Seismogenic Source Database (MSSD), which describes the seismogenic properties of faults that have formed during East African rifting in Malawi. We first use empirical observations to geometrically classify active faults into section, fault, and multi-fault seismogenic sources. For sources in the North Basin of Lake Malawi, slip rates can be derived from the vertical offset of a seismic reflector that is estimated to be 75 ka based on dated core. Elsewhere, slip rates are constrained from advancing a ‘systems-based’ approach that partitions geodetically-derived rift extension rates in Malawi between seismogenic sources using a priori constraints on regional strain distribution in magma-poor continental rifts. Slip rates are then combined with source geometry and empirical scaling relationships to estimate earthquake magnitudes and recurrence intervals, and their uncertainty is described from the variability of outcomes from a logic tree used in these calculations. We find that for sources in the Lake Malawi’s North Basin, where slip rates can be derived from both the geodetic data and the offset seismic reflector, the slip rate estimates are within error of each other, although those from the offset reflector are higher. Sources in the MSSD are 5–200 km long, which implies that large magnitude (M_W 7–8) earthquakes may occur in Malawi. Low slip rates (0.05–2 mm/yr), however, mean that the frequency of such events will be low (recurrence intervals ~10³–10⁴ years). The MSSD represents an important resource for investigating Malawi’s increasing seismic risks and provides a framework for incorporating active fault data into seismic hazard assessment in other tectonically active regions.

Jack N. Williams et al.

Status: final response (author comments only)

RC1:
'Comment on nhess-2021-306', Anonymous Referee #1, 16 Dec 2021

Content of the article :

In the manuscript, the authors present a seismogenic source database for Malawi and surrounding regions. The database contains information concerning both the 3D geometry of the faults and their activity. A special care is given to the estimation of the slip rate of each fault structure and the underlaying uncertainties. Three types of seismogenic sources are presented : sections, faults and multifaults. For each source, an estimation of the slip-rate and the magnitude of earthquakes that can be hosted by the source is given. All the results presented in this study are easily accessible in an online repository.

General comments :

In the introduction, the authors give a good description of the use of fault databases in the framework of seismic hazard and risk assessment. While the results of this paper, in the shape of a seismogenic source database, are a major component of seismic hazard assessment, there are not seismic hazard results themselves. Therefore, my opinion is that the title of this paper should be modified and the words “seismic hazard” should be removed, since no hazard results are presented in the paper.

In order to allow the results of these study to be used in a PSHA study, the way the weighting of the different source types should be done needs to be better discussed. It is not clear how the earthquake rate from the different source types should be combined. Should the weighting be done in order to fit a given MFD for the entire system? I would be useful if the authors added a section in the discussion part of the article to clarify this point. If possible, a comparison of their computed earthquake rates with the rates calculated using the earthquake catalogue could be added.

Specific comments :

Line 169 – I suggest modifying the term “statistical treatment” by “exploration”

Line 218 – Simplifying the surfaces of the faults is a potentially impactful hypothesis in terms of hazard assessment. The change in the surface can both affect the moment rate estimate for the fault and the distance taken into account in the GMPEs. While it is possible that the complexity observed in the fault trace might not be present at depth, the straight line is the other end-member of the possibilities for the fault surface. Why not let the final user of the database, the hazard modeller, choose the level of simplification to be applied? Especially since modern PSHA codes can now handle rather complicated geometries.

Line 230 – Would it be possible to add the uncertainty on the dip in the database ? This parameter can be source of large uncertainties in the hazard levels, and since the knowledge of the dip is not uniform in the system, adding the uncertainty on each fault could be useful.

Line 278 – Simplifying the fault system by removing splay faults also implies to consider that the whole deformation is accommodated by the main fault. Since the metrics used in GMPEs don’t usually take into account such details, the impact on the hazard would probably be minimal or within the simplification already made when using a GMPE. However, the impact of the simplification on the deformation should be commented in the text.

Line 441 – A point is missing.

Line 455 – Some underlaying assumptions behind these results should be stated here, even if there are discussed later in the article. These recurrence intervals are obtained assuming that the slip-rate is fully seismogenic. It is also assumed that each source can only host on magnitude (for one branch of the logic tree), but other magnitude frequency distributions could be possible.

Table 3 – In this table, it is not very clear if the values are for one specific fault or for the system as a whole. If it is for the system as a whole, can the different lines be read together? For example, is the table saying that the mean recurrence of a M6.8 earthquake is 10900 years?

The legend of the table should be better detailed.

Line 465 - The 5% threshold is probably too severe for this type of analysis. For some fault the two distributions are very similar, and the difference are minimal, sometime affecting only the width of the distribution, but the mean values are similar. The discussion in the following paragraph is probably more useful in order to understand the difference between the slip-rate estimates.

Figure 8 - The authors should add indexes to these figures, so each individual fault could be identified on the map in figure 2 and in the database. Additionally, the t-test result value could be added to the figure, helping to understand the reason why one is accepted and not the others.

Citation: https://doi.org/10.5194/nhess-2021-306-RC1
- AC1: 'Reply on RC1', Jack Williams, 05 Apr 2022
  
  We thank the reviewer for their constructive and positive review of our study . In the text below, we have copied their comments, and then replied to them in italicised text.
  General comments:
  In the introduction, the authors give a good description of the use of fault databases in the framework of seismic hazard and risk assessment. While the results of this paper, in the shape of a seismogenic source database, are a major component of seismic hazard assessment, there are not seismic hazard results themselves. Therefore, my opinion is that the title of this paper should be modified and the words “seismic hazard” should be removed, since no hazard results are presented in the paper.
  We agree, and if invited to submit a revised version of this manuscript, we will change the title to 'Geologic and geodetic constraints on the magnitude and frequency of earthquakes along Malawi's active faults: The Malawi Seismogeic Source Database (MSSD)'
  In order to allow the results of these study to be used in a PSHA study, the way the weighting of the different source types should be done needs to be better discussed. It is not clear how the earthquake rate from the different source types should be combined. Should the weighting be done in order to fit a given MFD for the entire system? I would be useful if the authors added a section in the discussion part of the article to clarify this point. If possible, a comparison of their computed earthquake rates with the rates calculated using the earthquake catalogue could be added.
  We thank the reviewer for their insightful comments, and agree that how different source types are weighted is a significant source of uncertainity when incorporating the MSSD into probabilistic seismic hazard analysis (PSHA). In revising this manuscript, we will refer the reader to a subsequent study in which we have used the MSSD in PSHA, and have explicity explored this uncertainity. Ths study is currently in review with Natural Hazards, and a pre-print of it can be found here:
  Williams, J., Werner, M., Goda, K., Wedmore, L., De Risi, R., Biggs, J., ... & Chindandali, P. (2022). Fault-based Probabilistic Seismic Hazard Analysis in Regions with Low Strain Rates and a Thick Seismogenic Layer: A Case Study from Malawi., 27 March 2022, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-1452299/v1]
  In summary, in this manuscript we used the MSSD to simulate stochastic earthquake event catalogs with a 2-million-year duration, and in which earthquakes occurred randomly on each MSSD source following a memoryless Poisson process. We then generated catalogs for all possible source type weighting combinations at increments of 0.1, and with the constraint that the weighting of any source type must be ≥0.1 (n=36). We then selected the combination that, for the magnitude range 6-7.6, produced a catalog with the closest b-value to the observed b-value in Malawi as derived from instrumental seismicity (1.02; Poggi et al 2017). Following this, we selected a weighting of 0.6-0.3-0.1 for section, fault, and multifault sources respectively, which is qualitatively consistent with the inference that there must be relatively frequent small magnitude section source events to maintain a b-value ~1. This is discussed further in Section 4.4 and Figure 7 in Williams et al 2022.
  Williams et al 2022, we also provide an analysis for how the moment rate (M_0) of the synthetic earthquake catalogs generated using the MSSD compared to the instrumental catalog M_0 in Malawi. This analysis concludes that although the M_0 of the synthetic catalogs are 2x higher than the observed M_0 in Malawi, this can be accounted for by the incomplete nature of the instrumental record in Malawi, which in turn is indicative of the low slip rate of its faults, the inference that they may be locked and/or host clustered earthquakes, and that the instrumental catalog has only a short (<60 years) duration ( see section 4.6 in Williams et al 2022).
  We note that given that the reviewer has raised these points on this study about the MSSD, not the PSHA, we had considered whether some of the above discussion could be incorporated into this current study. However, we concluded that this would exacerbate the length of this study, and that describing the MSSD and PSHA separately remains the best strategy for presenting this work. Nevertheless, in the revised manuscript, we will surmise the above discussion with reference to this (in review) PSHA, and how it shows how different source types in the MSSD can be used in future. In either case, our PSHA shows the importance of using geologic and geodetic data to constrain the activity of faults in Malawi, and hence the importance of the MSSD's development.
  Specific comments :
  Line 169 – I suggest modifying the term “statistical treatment” by “exploration”
  We will make this change in the revised manuscript
  Line 218 – Simplifying the surfaces of the faults is a potentially impactful hypothesis in terms of hazard assessment. The change in the surface can both affect the moment rate estimate for the fault and the distance taken into account in the GMPEs. While it is possible that the complexity observed in the fault trace might not be present at depth, the straight line is the other end-member of the possibilities for the fault surface. Why not let the final user of the database, the hazard modeller, choose the level of simplification to be applied? Especially since modern PSHA codes can now handle rather complicated geometries.
  
  We agree that the way in which we simplify the fault geometries from the Malawi Active Fault Database (MAFD) into the MSSD is subjective. However, many of the attributes that are assigned to sources in the MSSD are calculated using the simplified geometries (e.g., earthquake magnitude, recurrence interval in equations 4 and 5). Hence, if a final user wishes to change the source geometry, for consistency, these attributes will also need to be revised. For the MSSD to incorporate multiple interpretations of the MAFD would be impractical. In the revised manuscript we will therefore emphasise that alternative interpretations of the MAFD for seismic hazard assessment are possible. Furthermore, since the MAFD is also openly available (Williams et al 2022, https://zenodo.org/record/5507190#.YkypWy0RpB3), final users are welcome to choose their own level of source geometry simplification, although this will require changing other source attributes. In our revisions, we will specify that the simplified source geometries in the MSSD mean they should not be used in instances where accurate fault traces at seismogenic depths are available (e.g., site-specific engineering development, assessment of surface rupture hazards).
  
  Line 230 – Would it be possible to add the uncertainty on the dip in the database ? This parameter can be source of large uncertainties in the hazard levels, and since the knowledge of the dip is not uniform in the system, adding the uncertainty on each fault could be useful.
  
  We will follow the reviewer’s recommendation in the revised manuscript and add dip uncertainty as attributes in the MSSD. It should, however, be noted that our model of source geometry only considers the intermediate dip estimate.
  Line 278 – Simplifying the fault system by removing splay faults also implies to consider that the whole deformation is accommodated by the main fault. Since the metrics used in GMPEs don’t usually take into account such details, the impact on the hazard would probably be minimal or within the simplification already made when using a GMPE. However, the impact of the simplification on the deformation should be commented in the text.
  We will address this comment in the revised manuscript by noting that in removing subsidiary splays, the slip rates reported in the MSSD are essentially the cumulative slip rate from both the main fault and all its associated splays.
  Line 441 – A point is missing.
  We will correct this in the revised manuscript.
  Line 455 – Some underlaying assumptions behind these results should be stated here, even if there are discussed later in the article. These recurrence intervals are obtained assuming that the slip-rate is fully seismogenic. It is also assumed that each source can only host on magnitude (for one branch of the logic tree), but other magnitude frequency distributions could be possible.
  We agree and will be more explicit about the assumptions of seismic coupling and fault rupture types when describing our results in the revised manuscript.
  Table 3 – In this table, it is not very clear if the values are for one specific fault or for the system as a whole. If it is for the system as a whole, can the different lines be read together? For example, is the table saying that the mean recurrence of a M6.8 earthquake is 10900 years? The legend of the table should be better detailed.
  The reviewer is correct to point out that in the initial submission, Table 3 was not described in sufficient detail. In the revised manuscript, we will clarify in the legend that the values we provide are calculated from the intermediate estimates of all MSSD sources of the given type (e.g., the section magnitudes minimum, mean, and maximum values consider the magnitudes of all section sources in the MSSD). We will also emphasise that the recurrence intervals are calculated assuming that each source only ruptures in that type.
  Line 465 - The 5% threshold is probably too severe for this type of analysis. For some fault the two distributions are very similar, and the difference are minimal, sometime affecting only the width of the distribution, but the mean values are similar. The discussion in the following paragraph is probably more useful in order to understand the difference between the slip-rate estimates.
  We acknowledge that it is surprising how many of the t-tests reject the null hypothesis that the two distributions come from probability distributions with the same mean but unequal variances. Our interpretation of this result is that since the variance of each slip rate distribution is high, many (10,000) Monte Carlo simulations must be run to achieve stable results. In other words, if less simulations are run, the result of the t-test changes each time we perform the analysis (see Figure 1 below). This large number of simulations entails that the p-value is very sensitive to even small differences in the mean between the two distributions, and hence the null hypothesis is rejected in cases when the mean values of each distribution qualitatively appear to be quite similar. In this respect, it is worth noting that z-tests may be considered more appropriate for large data sets. However, in this case, the variance of both sample distributions should be the same, which is not true for the two samples we are are comparing; hence our preference to retain the two-sample t-test.
  Figure 1: Sensitivity of T-test results to the number of Monte Carlo simulations (100, 1000, or 10000) that are performed when sampling slip rate distributions for the comparison described in Section 3.5 in the main text. Here each set of Monte Carlo simulations and the T-test is repeated 500 times. We then determine the number of tests that are accepted depending on whether 100, 1000, or 10,000 simulations are performed when sampling slip rates. Results from the T-test are stable when either 0 or 500 tests are rejected.
  As discussed for the following comment, we will report p-values for each test in Fig 8 in the revised text. However, for 10 out of 11 cases, the p value is <0.01, and so changing the significance level will not influence our finding that in all but one case, the null hypothesis is rejected. We will incorporate the above discussion in the revised manuscript.
  Figure 8 - The authors should add indexes to these figures, so each individual fault could be identified on the map in figure 2 and in the database. Additionally, the t-test result value could be added to the figure, helping to understand the reason why one is accepted and not the others.
  We agree, and will make the necessary revisions to the figure/
  
  Citation: https://doi.org/10.5194/nhess-2021-306-AC1
RC2:
'Comment on nhess-2021-306', Luigi Ferranti, 19 Feb 2022

The manuscript by Williams et al.: “Geologic and geodetic constraints on the seismic hazard of Malawi’s active faults: The Malawi Seismogenic Source Database (MSSD)” represents a comprehensive contribution to parametrize seismogenic sources in this section of the EAR and helps assessing the resulting hazard.

The steps for building the database are clearly illustrated and uncertainties explored in details.

This database extends the previous database available only for the southern part of the rift (SMSSD) to the whole Malawi rift (south, central and north), and increases the estimates of source parameters by adopting an updated geodetic model which results in a useful reduction of parameters uncertainties. I find particularly interesting the comparison between system-based and geologic-based (the offset of a 75-ka seismic reflector in Lake Malawi) estimates of slip rate and recurrence, which offers confidence in adopting the system-based approach elsewhere (central and northern sectors) where geologic information is scarce. I also agree with the possibility of very large (>7.5 Mw) but infrequent extensional earthquakes in this strong and thus elastically thicker continental crust although the hazard is clearly posed by intermediate and more frequent earthquakes.

In summary, the presented compilation poses a strong basis for future detailed studies aiming at more detailed filed and geophysical characterization of fault geometries and segmentation patterns and of estimations of aseismic release on some faults. I have no observations on the manuscript structure and arguments. Two typos are indicated below.

Line 138: invert “lower aseismic crust” with “aseismic lower crust”

Line 379: “and there a range”, correct with “and there is a range”

Citation: https://doi.org/10.5194/nhess-2021-306-RC2
- AC2: 'Reply on RC2', Jack Williams, 05 Apr 2022
  
  In the text below, we have copied the reviewers comments, and then replied to them in italicised text.
  The manuscript by Williams et al.: “Geologic and geodetic constraints on the seismic hazard of Malawi’s active faults: The Malawi Seismogenic Source Database (MSSD)” represents a comprehensive contribution to parametrize seismogenic sources in this section of the EAR and helps assessing the resulting hazard.
  The steps for building the database are clearly illustrated and uncertainties explored in details.
  This database extends the previous database available only for the southern part of the rift (SMSSD) to the whole Malawi rift (south, central and north), and increases the estimates of source parameters by adopting an updated geodetic model which results in a useful reduction of parameters uncertainties. I find particularly interesting the comparison between system-based and geologic-based (the offset of a 75-ka seismic reflector in Lake Malawi) estimates of slip rate and recurrence, which offers confidence in adopting the system-based approach elsewhere (central and northern sectors) where geologic information is scarce. I also agree with the possibility of very large (>7.5 Mw) but infrequent extensional earthquakes in this strong and thus elastically thicker continental crust although the hazard is clearly posed by intermediate and more frequent earthquakes.
  In summary, the presented compilation poses a strong basis for future detailed studies aiming at more detailed filed and geophysical characterization of fault geometries and segmentation patterns and of estimations of aseismic release on some faults. I have no observations on the manuscript structure and arguments. Two typos are indicated below.
  We acknowledge and thank the reviewer for their positive comments on our study.
  Line 138: invert “lower aseismic crust” with “aseismic lower crust”
  Line 379: “and there a range”, correct with “and there is a range”
  In the revised manuscript, we will correct both these typos as suggested by the reviewer
  
  Citation: https://doi.org/10.5194/nhess-2021-306-AC2

Jack N. Williams et al.

Data sets

Malawi Seismogenic Source Database Williams, Jack N.; Wedmore, Luke N. J.; Fagereng, Åke; Werner, Maximilian J.; Biggs, Juliet; Mdala, Hassan; Kolawole, Folarin; Shillington, Donna J.; Dulanya, Zuze; Mphepo, Felix; Chindandali, Patrick R. N.; Wright, Lachlan J. M.; Scholz, Christopher A. https://zenodo.org/record/5599617#.YXhT2i0Rpz8

Malawi Active Faut Database Williams, Jack; Wedmore, Luke; Scholz, Christopher A; Kolawole, Folarin; Wright, Lachlan J M; Shillington, Donna J; Fagereng, Å; Biggs, Juliet; Mdala, Hassan; Dulanya, Zuze; Mphepo, Felix; Chindandali, Patrick; Werner, Maximilian J https://zenodo.org/record/5507190#.YXhYrC0Rpz_

Jack N. Williams et al.

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Short summary

We use geologic and GPS data to constrain the magnitude and frequency of earthquakes that occur along active faults in Malawi. These faults slip in earthquakes as the tectonic plates either side of the East African Rift in Malawi diverge from one another. Low divergence rates (0.5–1.5 mm/yr) and long faults (5–200 km) imply that earthquakes along these faults are rare (once every 1,000–10,000 years) but could have high magnitudes (M 7–8). These data can be used to assess seismic risk in Malawi.


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