Immature faults, such as those in the Eastern California Shear Zone, generate widespread fracturing at the surface during earthquakes. A new displacement model characterizes the hazard posed to infrastructure by this distributed deformation.
By Alba M. Rodriguez Padilla, Ph.D. candidate, UC Davis (@_absrp)
Citation: Rodriguez Padilla, A.M., 2023, New model tackles immature faults and their widespread fractures, Temblor, http://doi.org/10.32858/temblor.325
The past two decades have witnessed five earthquakes that left impressive surface ruptures in the deserts of eastern California and northern Mexico. The Landers (1992), Hector Mine (1999), El Mayor-Cucapah (2010), and Ridgecrest (foreshock and mainshock, 2019) events were carefully mapped using a combination of field observations and remote sensing, illuminating the distribution of surface deformation for these events. All five quakes were characterized by pervasive cracking at the surface that extended kilometers away from the fault that hosted the earthquake. The distant fractures are a feature of something called “distributed deformation.”
These distributed deformation examples contrast with the most recent mapped earthquakes on the San Andreas Fault (Parkfield 1966 and 2004 events) or the Turkey earthquakes earlier this year. In these latter events, fractures are localized around the mapped surface ruptures (Sarmiento et al., 2021; Reitman et al., 2023). In other words, Parkfield’s and Turkey’s earthquakes did not create widely distributed deformation at the surface.
Engineers and policymakers rely on models that describe where the surface is likely to break to assess the hazard posed by deformation during an earthquake. These are called displacement hazard models. Fractures that occur kilometers away from the surface rupture appear to occur more often on immature faults, though quantitatively establishing the maturity of a fault or fault zone remains challenging. Thus, widely distributed fracturing needs to be accounted for in these models.
In a new paper published in BSSA, my advisor at the University of California Davis, Michael Oskin, and I use displacement measurements and surface rupture maps from the five above-mentioned events in eastern California and northern Mexico to create a displacement hazard model that accounts for widely distributed fracturing generated in earthquakes on immature fault zones. This work is possible thanks to a new database that compiles surface rupture maps and displacement measurements, among other information, for more than 60 earthquakes (Sarmiento et al., 2021).
Immature fault zones
Immature faults, such as many of the faults in the Eastern California Shear Zone and the Walker Lane Belt (in both California and Nevada), have experienced less than about 25 kilometers of cumulative displacement in their history and tend to appear to us as discontinuous surface features that are broken down into smaller segments (e.g., Dolan and Haravitch, 2014). Beyond fracture extent, the degree of geological maturity of a fault shows some correlation with rupture velocity and aftershock productivity. The fact that fault maturity correlates with some important hazard-related earthquake features suggests that maturity should be a consideration when assessing the hazard posed by large earthquakes on a fault (Guo et al., 2023).
The faults in the Eastern California Shear Zone (which produced the Landers, Hector Mine and Ridgecrest earthquakes) and the Baja California Transtensional Rift (which produced the El Mayor-Cucapah earthquake) share the common factor that they have not experienced large cumulative displacements and they occurred in the desert, where deformation may be easily seen and mapped. These factors make these fault systems ideal for characterizing the displacement hazard associated with distributed fracturing from earthquakes on immature faults, using data from these five recent events.
A new model
The distributed fracturing that is characteristic of earthquakes on immature faults threatens infrastructure at the surface and the shallow subsurface. Consider a fracture generated during an earthquake that cuts across a pipe. A displacement on the fracture of one centimeter is unlikely to compromise the integrity of the pipe. A displacement of one meter sure will.
Importantly, the hazard does not just depend on the presence of a fracture, but the displacement that a given fracture hosts, regardless of whether the fracture is newly created in an event or preexisting and therefore reactivated. Our displacement hazard model is presented as the probability of finding a displacement on a fracture that exceeds some displacement threshold at some distance away from a fault that hosts an earthquake.
The model allows end users, such as engineers, to select the displacement value relevant for the type of infrastructure of interest and use it as the model’s threshold. For example, if a pipe breaks when it experiences a displacement larger than 30 centimeters, an end-user can compute the probability of the displacement on a fracture exceeding 30 centimeters and look at how that probability changes with distance from the fault. The model also depends on earthquake magnitude because observations show that average and maximum displacements during an earthquake correlate with the event’s magnitude.
This model joins a suite of preexisting displacement hazard models that engineers can use. However, the focus on widespread fracturing ensures that end-users can account for enhanced hazards posed by earthquakes on immature faults.
Apply when appropriate
Because the data used to develop our new model are based on immature faults, application of this model to account for the hazard from distributed fracturing in mature fault systems, such as the San Andreas (California) or Anatolian (Türkiye) faults, would not be appropriate. This is a general, preliminary model that neither accounts for the direction of the displacement (e.g., vertical versus horizontal), nor how displacement depends upon distance along the rupture, as displacement tends to peak near the middle of the rupture and taper away to zero at the rupture tips.
As new data from surface rupturing earthquakes on immature faults is collected, we will be able to include these features in future models, continuing to improve our ability to assess distributed displacement hazard in zones with emerging fault systems, such as parts of the Malborough Fault system in New Zealand, young fault systems in Taiwan, and some nascent faults in southeast China.
Dolan, J. F., & Haravitch, B. D. (2014). How well do surface slip measurements track slip at depth in large strike-slip earthquakes? The importance of fault structural maturity in controlling on-fault slip versus off-fault surface deformation. Earth and Planetary Science Letters, 388, 38-47.
Guo, H., Lay, T., & Brodsky, E. E. (2022). Seismological Indicators of Geologically Inferred Fault Maturity. Authorea Preprints.
Padilla, A. M. R., & Oskin, M. E. Displacement Hazard from Distributed Ruptures in Strike-Slip Earthquakes. BSSA, 2023.
Reitman, N., Briggs, R., Barnhart, W.D., Jobe, J.A. et al., Fault Rupture Mapping of the 6 February 2023 Kahramanmaraş, Türkiye, Earthquake Sequence from Satellite Data, USGS Digital Object Identifier Catalog, 10.5066/P985I7U2
Sarmiento, A., Madugo, D., Bozorgnia, Y., Shen, A., Mazzoni, S., Lavrentiadis, G., Dawson, T., Madugo, C., Kottke, A., Thompson, S., Baize, S., Milliner, C., Nurminen, F., Boncio, P., and Visini, F. (2021). Fault Displacement Hazard Initiative Database, UCLA B. John Garrick Institute for the Risk Sciences, Report GIRS-2021-08, doi: 10.34948/N36P48.
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