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UCL Institute for Risk and Disaster Reduction


Earthquake surface measurements reveal new revelations about how faults rupture

Joanna PFaure Walker12 November 2018

PhD student Francesco Iezzi (Birkbeck College), supervised by Prof Gerald Roberts (Birkbeck College) and Dr Joanna Faure Walker (UCL IRDR), has published a paper that could revolutionalise how geologists and seismic hazard modellers use long established scaling relationships between fault lengths and surface rupture parameters.

The paper is freely available to all and can be found here.

What new observations have been made?

For five earthquakes studied, the surface fault slip (the amount the fault surface moved during the earthquake) and the throw (the vertical component of the slip) was higher where there was a bend along the length of the fault.

Following the central Italy August and October 2017 earthquakes that ruptured the ground surface, we made detailed high spatial resolution measurements of surface fault displacement along the length of the surface fault ruptures. A study of the amount of vertical and horizontal displacement that occurred along the length of the fault revealed that the throw and slip that occurred during the earthquakes increased where there are bends in the fault. This result is critical and has not been identified before for individual earthquakes.

Damage in Amatrice from the August 2016 Earthquake. Photograph take during EEFIT fieldwork by Dr Joanna Faure Walker.

Why does this occur?

We hypothesis that this occurs in order to maintain the horizontal strain (change in length relative to the original length) across a fault during an earthquake and the long-term horizontal strain-rate that accumulates from multiple earthquakes over thousands of years.

Are there other examples of this?

We then went back and studied other examples earthquakes where there was enough information to determine whether a similar pattern of higher throw and slip could be seen across bends in the fault. In the three further events studied in USA Basin and Range, Greece, and Mexico, we found the same relationship. So it seems this phenomenon occurs worldwide in normal (extensional) faults.

This was the first time that the change in vertical component of slip during an earthquake has been shown to be predictable. However, the observed relationship of increased throw across fault bends has been identified previously in long-term displacements that have accumulated over 15 thousand years as a result of multiple earthquakes in Italy (Faure Walker et al., 2009, Wilkinson et al., 2015). Before now, it was not known whether this increase was caused by there being more earthquakes across the bends or more movement during individual events.  We now know that there can be more slip during individual events, however we do not know whether this is the only mechanism for creating a long-term higher throw-rate across the bends.

What does this mean for earthquake science?

This paper suggests that slip during an earthquake will change where there is a bend along the length of the fault and this change can be quantified and predicted using the proposed theory. This means that close to the fault, earthquakes may be more damaging near a bend in the fault. This finding suggests that we cannot use fault scaling relationships between fault length and expected slip in earthquakes without consideration of fault geometry. This paper can also explain much of the scatter seen in existing plots of maximum surface slip against fault length because when collecting the data as input for such relationships, consideration was not given about whether the measurements were taken across fault bends or not.

These changes in slip along faults in individual earthquakes related to the fault geometry should be included in probabilistic seismic hazard assessments (PSHA).

What other research in the IRDR relates to this?

This work contributes to the IRDR and colleagues’ work on investigating fault behaviour to improve our understanding of earthquake hazard. Recent papers have demonstrated the importance of including detailed fault geometry and slip-rates in seismic hazard calculations (Faure Walker et al., 2018) and Coulomb stress transfer calculations (Mildon et al., 2016, 2017).

Iezzi et al (2018), Coseismic Throw Variation Across Along‐Strike Bends on Active Normal Faults: Implications for Displacement Versus Length Scaling of Earthquake Ruptures, Journal of Geophysical Research, https://doi.org/10.1029/2018JB016732 

More data needed for better earthquake hazard and risk calculations

Joanna PFaure Walker6 November 2018

New research demonstrates the importance of having detailed measurements at multiple sites along a fault of how fast the fault is moving and how the surface orientation of the fault changes. To access the full paper click here.

Why do we need fault measurements?

Measurements of fault slip rate and the geometry of the fault (it’s 3d orientation) can be used to calculate earthquake recurrence intervals to give probabilities of how likely earthquakes of different magnitudes are to occur. We also need these measurements to model how much ground shaking there will be at given locations. Hazard maps of expected ground shaking can be used to inform building codes and identify where buildings including homes and schools might need retrofitting to improve their resistance to earthquake shaking.

There are other methods available for creating earthquake hazard maps, such as using historical records of earthquake shaking. However, these records unlikely go back far enough in time to capture all faults capable of hosting large earthquakes because some faults will not have hosted earthquake within the time period covered by such records. Therefore, the hazard from some faults would be missing in hazard maps based solely on historical seismicity leading to underestimations in earthquake hazard.

What new insights have been revealed in the research publication?

The paper, entitled “Variable fault geometry suggests detailed fault slip rate profiles and geometries are needed for fault-based probabilistic seismic hazard assessment (PSHA)” demonstrates that relying on only one or a few measurements of how fast the fault is moving along a fault and projecting these measurements along the entire fault may lead to underestimating the uncertainty in the earthquake hazard calculations. Crucially, there may be locations where the hazard is underestimated, meaning people could be at more risk than suggested by simpler models (the converse is also possible). Therefore, earthquake hazard assessments based on fault parameters need to either use detailed measurements including measurements of how fast the fault is moving at multiple sites along the fault or to incorporate how the lack of such data increases the uncertainty in calculated earthquake hazard assessments.

Why are detailed measurements not being already used?

In many regions it is difficult to constrain the fault slip rate (how much the fault has moved in a given time) or throw rate (vertical component of slip rate) along a fault at even one location, let alone several. However, there are regions where this is possible so as more data is collected, this detail should help to improve earthquake hazard assessments both in those regions and worldwide.

Where can I find out more?

Faure Walker J., Visini F., Roberts G., Galasso C., McCaffrey K., and Mildon Z., (2018) Variable fault geometry suggests detailed fault slip rate profiles and geometries are needed for fault-based probabilistic seismic hazard assessment (PSHA), Bulletin of the Seismological Society of America, doi: 10.1785/0120180137

The Fault2SHA Working Group is an ESC (European Seismological Commission) group of researchers in both universities and civil protection authorities collaborating to increase incorporation of fault data in seismic hazard assessments and to improve our understanding of how such data should be used.

Fault2SHA has successful session at ESC 2018 in Malta

Joanna PFaure Walker7 September 2018

The Fault2SHA ESC (European Seismological Commission) Working Group hosted a session on Wednesday 5th September at the ESC 2018 Meeting held in Valletta, Malta. Oona Scotti represented the group in her keynote on the opening day of the conference, in which she addressed “Modelling fault systems in PSHA: Challenges Ahead”. The Fault2SHA Working Group, for which I am on the Executive Committee, links different researchers working on faults and seismic hazard assessment (SHA) in Europe and beyond. This collaboration has brought together field geologists, fault-modellers and probabilistic seismic hazard modellers. The group provides a forum in which data, results, modelling capabilities, and improvements in scientific understanding can be shared. If you want more information, and to join, see Fault2SHA. The next Fault2SHA workshop will be in Kaust, Saudi Arabia, in November 2018 and the next meeting will run on 3rd-5th June 2019 in Barcelona, Spain.

I lead the Fault2SHA Central Apennines Laboratory. Our team comprises researchers from Italy (Paolo Boncio, Bruno Pace, Laura Peruzza, Francesco Visini), France (Lucilla Benedetti, Ooona Scotti) and the UK (Joanna Faure Walker, Gerald Roberts). At ESC in Malta, I introduced the central Apennines Laboratory and our current activities to the wider working group. The Central Apennines, as well as being a beautiful place to conduct fieldwork with the opportunity to obtain detailed datasets, suffers from large magnitude earthquakes. Indeed, earthquakes in the Central Apennines have featured widely in the UK press due hosting the two deadliest earthquakes in Europe of the last ten years: the 2009 L’Aquila sequence and the 2016 Amatrice-Norcia sequence.


The Fault2SHA Central Apennines Laboratory, which formed in January 2018, held an in-person meeting in July at the University of Chieti-Pescara, Italy. The photograph shows (from left to right) Oona Scotti, Francesco Visini, Joanna Faure Walker, Bruno Pace, Laura Peruzzi, Lucilla Benedetti, and Paolo Boncio.

During the Fault2SHA ESC session, I presented a second talk and a poster about my research investigating the importance of incorporating detailed fault geometry for understanding seismic hazard. The oral presentation demonstrated the importance of incorporating detailed fault geometry and loading on faults between earthquakes in Coulomb Stress Transfer modelling, a process that causes the stress on faults to change in response to an earthquake on a neighbouring fault. This was based on work carried out by Zoe Mildon (former IRDR PhD student, now a lecturer at the University of Plymouth) in collaboration with Gerald Roberts, Shinji Toda and myself (see Midon et al. 2016 and Mildon et al. submitted preprint). The poster displayed the importance of detailed fault geometry and slip-rate data for calculating earthquake probabilities and ground shaking intensities. I further represented Zoe for her poster within the session on earthquakes in regions of distributed deformation, that showed surface ruptures from the 1997 Colfiorito Earthquake in the central Apennines was due to primary earthquake slip (see Mildon et al., 2016 for details).

I thank all those at the conference with whom I had interesting discussions and I look forward to seeing all of our research progress.