Transform faults, also known as strike-slip faults, occur where the tectonic plates of the Earth’s crust move horizontally past each other in opposite directions and establish shear stresses at depth within the crust. Examples are found onshore at many places around the world, including California, Turkey, New Zealand, and Haiti. Rupture of these faults can have devastating consequences on population centres. For example, the 1906 earthquake in San Francisco damaged more than 80% of the city, and the 2010 Haiti earthquake caused more than 100,000 deaths.
Earthquake-prone areas generally experience events cyclically, but it is currently impossible to precisely predict earthquake recurrence. This is because these areas are tectonically complex and often comprise a co-dependent system of faults. The San Andreas Fault that underlies southern California has the potential to affect over 20 million people, and experts such as Dr Wei believe that an earthquake in this region is long overdue. Improving our ability to estimate where and when an earthquake may strike, and at what magnitude, is therefore of great importance.
Transform faults on land are easier to access and are of particular interest because of their power to cause damage as mentioned earlier. However, these transform faults are tectonically more complex and the interval between earthquake events is longer than research timescales or human lifetimes. This makes it difficult to study these fault systems and understand the mechanisms that control their behaviour.
Oceanic transform faults as analogues
One approach to this challenge currently being embraced by Dr Wei’s research group is to use oceanic faults as analogues for the continental setting. Oceanic transform faults possess the same fundamental physics as continental settings yet they are tectonically much simpler than continental faults and earthquake cycles at oceanic transform faults generally have a much shorter periodicity (sometimes as little as five years). This means that several earthquake cycles on these faults have been observed with modern instruments. This is remarkable considering the fact that we have no complete modern record of an entire earthquake cycle of any major fault zones on land.
Oceanic earthquakes are also more predictable due to the presence of clear foreshock activity that precedes the main earthquake. Dr Wei describes these settings as a simpler case to “crack the code” of earthquake behaviour at transform faults. Insights learned from ocean floor faults – such as the East Pacific Rise – may have valuable implications for earthquake hazards on land.
Capturing and modelling earthquake behaviour
One intriguing observation of earthquake cycles is at the East Pacific Rise where there is a 150 km fault system (Gofar and Discovery faults), comprising eight fault segments that have ruptured at similar times over the past five earthquake cycles since the 1990s. This indicates that they are acting in phase with each other and, though the mechanisms for this synchronicity are not fully understood, they are likely reliant upon similar rates of slip and a positive coupling between fault blocks. Dr Wei hopes that by studying the synchronisation of earthquakes, a better understanding of how and where earthquake events are triggered could be achieved.
As a first step, Dr Wei and his team have simulated the earthquake cycles of two nearby fault blocks considering brittle failure. They found that one large earthquake may trigger another by transferring stress from one fault segment of crust to another. This indicates, firstly, that both the affected fault blocks have reached the end of their earthquake cycle and are close to rupture and secondly, these stress transfers may lead to synchronisation when the two segments are close.
In the next few years, Dr Wei hopes to create a next-generation integrated model that also includes the viscoelastic stress transfer below the brittle crust. This integrated, realistic, and comprehensive model will then be validated by applying it to well-monitored oceanic transform fault settings, to recreate observed behaviour. Ultimately, it is hoped that – through enhanced modelling – a more quantitative and predictive understanding of how earthquakes behave could be achieved. It is also hoped that the model could be used to predict earthquakes at less well-understood continental settings.
The silent earthquakes
In addition to looking at the behaviour of large earthquakes, Dr Wei’s research has also considered the phenomena of “shallow creep”, which occurs along transform faults including the San Andreas Fault in California and the North Anatolian Fault in Turkey. Shallow creep takes place in the top 3–5 km and at a rate much slower than regular earthquakes: these so-called “silent” earthquakes are not felt. However, creep is an important way to release stress on faults and might affect future earthquakes.
Rates of movement during periods of creep may be up to 15 mm per year and they can take place either continuously or episodically, varying throughout the earthquake cycle. Dr Wei uses geodetic data such as interferometric synthetic aperture radar (InSAR) data from satellites and creepmeters (instruments designed specifically for this purpose) to measure creep. Dr Wei’s research has derived models to better explain observed creep behaviour, by inferring the presence of unstable layers at relatively shallow depths. The properties of these layers are linked to the creep characteristics. Understanding the role of creep in the earthquake cycle is also important again for better understanding earthquake occurrence.
The techniques Dr Wei uses to study earthquake behaviour have other intriguing possibilities. North Korea is notoriously secretive about its defence capability. However, it cannot hide its nuclear testing programme from the seismographs and satellites that constantly monitor and survey our planet. Denied access means there is limited information on the local geology and seismic data acquired from outside the country have limitations when attempting to constrain the exact location and depth at which tests have occurred. Dr Wei’s research has used InSAR satellite data to pinpoint test locations more accurately and presents compelling evidence for surface deformation linked to nuclear tests that took place in North Korea in January 2016 and September 2017.
Clearly, there is still much to learn in assessing the hazards associated with earthquakes but Dr Wei’s research is pulling several pieces of this complex puzzle together. The techniques and skills he and his team have developed also allow them to research aspects of global security, helping secure our safety in multiple ways.
Earthquakes kill more people than any other natural hazard. For example, the 2004 Indian Ocean earthquake and tsunami killed more than 200,000 people. Any progress towards understanding earthquakes is useful in the long run. Other than the societal importance, earthquakes are complex and unpredictable, making them interesting. You need to guess where they will happen next and study them when they happen. Earthquakes occur around the world and earthquake study is multi-disciplinary. So, I have the chance to meet scientists around the world and be exposed to all kinds of interesting instruments and methodology.
How has your research advanced our understanding of earthquake behaviour?
One major goal of my research is to understand the earthquake cycles on oceanic transform faults in a quantitative and predictive way. We are making steady progress towards this goal. For example, we have shown that the distance and fault zone properties between earthquake segments control their interaction and therefore the synchronisation of them. One implication of this research is that more accurate mapping of fault segments should lead to a better estimation of their behaviour. Also, my research on shallow fault creep in California suggests that shallow heterogeneity of rocks should be taken into account for any formal estimate of earthquakes on these faults.
The application of your research to nuclear monitoring is fascinating. Can you explain more?
Monitoring nuclear tests is of great interest to both the scientific community and the government. Currently, the seismic method is the main approach to locate and characterise nuclear tests because of denied access. However, the seismic method suffers from large uncertainty on the absolute location (~a few km) and yield estimate (~50%). It would be extremely valuable to develop other complementary and independent methods. For the last few years, my group have been using InSAR data to measure ground displacement caused by nuclear tests in North Korea. Using changes in the ground, we can locate the nuclear tests within a few hundred metres and provide a reasonable estimate of the yield.
Will we ever reach a time when we can accurately predict when an earthquake will occur?
Probably not and definitely not in my lifetime. One reason that we cannot predict earthquakes like predicting the weather is that we cannot measure the stress level underground everywhere on the fault. I do not see any chance to solve this fundamental problem in the near future. Also, the majority of large earthquakes do not have any precursory phenomenon, making this silver bullet approach fail. However, in the last few decades, tremendous efforts have been made to build early warning systems for large earthquakes around the world. These systems may issue a warning a few seconds to minutes before strong shaking arrives, which might be a more practical way to protect people.
Which areas of the world could benefit most from this research?
There are three major transform fault systems around the world: the San Andreas Fault in California, the North Anatolian Fault in Turkey, and the Alpine Fault in New Zealand. The research on earthquake cycles of transform faults will benefit these areas the most. However, there is a still a long way to go to directly apply our results to these fault systems, which is typical for any fundamental research. In addition, the research could potentially have implications to other places such as subduction zones because the numerical codes can be applied to these environments too.
Dr Wei’s research focuses on tectonic geodesy, fault mechanics, natural hazards, and nuclear monitoring.
National Science Foundation (NSF)
US Air Force Research Lab (AFRL)
Yajing Liu (McGill), Randy Watts, Kathleen Donohue, Yang Shen, Tao Wei (University of Rhode Island), YoungHee Kim (Seoul National University), Yoshihiro Kaneko (GNS Science), Pia Victor (Helmholtz-Zentrum Potsdam), Jeffery McGuire (Woods Hole Oceanographic Institutions), Roger Bilham (University of Colorado, Boulder), Hongfeng Yang (Chinese University of Hong Kong), Jiancang Zhuang (Institute of Statistical Mathematics of Japan), Shiyong Zhou (Peking University), Jihua Fu (China Earthquake Administration)
Dr Wei received his BS in Geophysics from the Peking University, China in 2004. He later received a PhD in Earth Sciences from the University of California, San Diego in 2011. Since then, he has worked as a postdoc at the Woods Hole Oceanographic Institution and as an Assistant Professor at the University of Rhode Island.
Dr Meng “Matt” Wei
Graduate School of Oceanography
University of Rhode Island
Kingston, RI 02881
T: +1 401 874 6530