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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 

Five members of the IRDR visit Amatrice as part of the EEFIT mission

ZoeMildon21 October 2016

View along the main street of Amatrice

View along the main street of Amatrice

Six weeks after the earthquake that struck Amatrice, central Italy, EEFIT (Earthquake Engineering Field Investigation Team) deployed a team to the region to investigate the damages. The team involved five members of the IRDR; Prof. David Alexander, Dr Joanna Faure Walker, Dr Carmine Galasso and PhD students Zoe Mildon and Serena Tagliacozzo.

Zoe taking measurements along the surface rupture, Mt Vettore

Zoe measuring the surface rupture

Joanna and Zoe’s main aim of the trip was to map the surface ruptures from the earthquake. Slip at depth along the fault plane that generated the earthquake came to the surface, and could be seen as offset soils and open cracks along the slope of Mt Vettore. By measuring the orientation and offset of the rupture, they hope to gain a better understanding of the earthquake process. In addition, they worked together with Domenico Lombardi (Uni. Manchester) to look at the environmental effects of the earthquake, such as landslides, rock falls and ground cracks. They were using the Environmental Seismic Intensity Scale (ESI 2007) which aims to provide a measure of the intensity of shaking during an earthquake, similar to the Modified Mercalli Scale, but from only considering effects to the environment.

Carmine’s primary interest was to investigate strong ground motion signals recorded at various seismic stations around the epicentral area. Areas of particular interest included the three stations closest to the earthquake that recorded the highest PGA (Peak Ground Acceleration). One of these was close to the Umbrian town of Norcia that recorded among the highest ground motion measurement, yet the town was relatively undamaged. Three stations ~50km north-west of the epicentre also recorded unusually strong ground motions and these were visited as well to determine if there were any site specific effects that may explain these high measurements. He also worked with other members of the EEFIT team to do rapid surveys of building damage.

Interview for Italian news, Carmine is front left.

Interview for Italian news, Carmine is on the left of the reporter, Serena is to the right

David and Serena were interested in investigating the social effects of the disaster and how local communities were responding to it. They started by interviewing relief workers from various agencies, including the Civil Protection and Red Cross (Croce Rossa). They also visited L’Aquila, 40km to the south-east of the Amatrice epicentral area, as the city experienced a similar magnitude earthquake in 2009 and they were interested in the progress of reconstruction and the availability of the services to displaced communities.

All members also visited the town of Amatrice and surrounding villages to observe the damage. We would like to thank the Civil Protection Authorities and Vigil del Fuoco for their help and assistance during this trip.

Further detail about other members of the EEFIT trip and activities can be found at the mission blog. An EEFIT report will be released in the near future and there will be a presentation organised for late November to present the initial findings.

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