The May 2015 magnitude 7.9 earthquake near Japan’s Ogasawara Islands presented a seismological enigma. Its aftershock, pinpointed at a depth of 680 kilometers, shattered the record for the deepest earthquake ever recorded. This depth places the event firmly within Earth’s lower mantle, a region where extreme pressure and temperature typically preclude the brittle fracturing that causes earthquakes. This anomaly prompted scientists to question the conventional understanding of deep earthquakes and explore alternative mechanisms for their genesis.
Earthquakes are most common in the crust and upper mantle, becoming progressively rarer below 200 kilometers. At depths exceeding 500 kilometers, the immense pressures and temperatures cause rocks to behave plastically, deforming rather than breaking. This ductile behavior inhibits the formation of the extensive fault networks that characterize shallower earthquakes. Furthermore, the efficient stress redistribution at these depths further diminishes the likelihood of subsequent seismic activity, including aftershocks. The 2015 Ogasawara event, therefore, challenged this established framework, prompting a thorough re-examination of the seismic data.
Initial studies of the Ogasawara earthquake sequence reported a foreshock and a potentially record-breaking deep aftershock. The event, located within the Izu-Bonin subduction zone, was attributed to the Pacific plate subducting beneath the Philippine Sea plate. However, a reassessment of the data revealed a significant discrepancy. The location of the record-breaking aftershock appeared strangely isolated, exhibiting a large seismic gap relative to other earthquakes in the region. This isolation contradicted the expected pattern of seismicity along the entire subducting slab, raising doubts about the tectonic origin of the deep event.
The researchers, led by Hao Zhang of the University of Southern California, meticulously re-analyzed the seismic data from the 2015 Ogasawara earthquake sequence. Their findings challenged the initial interpretation of a tectonic origin for the record-breaking deep aftershock. Instead, they proposed a novel explanation: a mineralogical transformation within the subducting slab triggered the seismic event. This hypothesis hinges on the behavior of olivine, a common mineral in Earth’s mantle.
Olivine, stable at shallower depths, becomes unstable under the extreme pressure and temperature conditions of the lower mantle. It undergoes a phase transition, transforming into denser minerals like ringwoodite, wadsleyite, and majorite. However, within cold, subducting slabs, olivine can persist in its metastable state to greater depths than it would under normal mantle conditions. This delayed transformation is key to the proposed mechanism. When the metastable olivine finally undergoes the phase transition at depth, it generates significant stress and releases energy, potentially triggering deep earthquakes.
The colder temperatures within subducting slabs facilitate the preservation of metastable olivine to greater depths, making these regions more susceptible to this type of mineralogical transformation-induced seismicity. The researchers argue that the isolated nature of the 2015 Ogasawara deep aftershock, coupled with the absence of extensive aftershock sequences typical of tectonic earthquakes, supports their hypothesis of a mineralogical origin. While acknowledging the challenges in definitively ruling out a tectonic origin, they contend that their findings represent the most compelling evidence to date for a different mechanism driving some deep earthquakes.
This new interpretation of the 2015 Ogasawara earthquake has significant implications for our understanding of Earth’s deep interior dynamics. It suggests that mineralogical transformations, driven by the unique conditions within subducting slabs, can play a crucial role in generating deep earthquakes. This mechanism provides an alternative explanation for seismic events occurring in regions where conventional tectonic models struggle to account for the observed seismicity. Furthermore, it highlights the complex interplay between mineralogy, temperature, pressure, and stress in shaping the behavior of Earth’s deep interior.
The findings of this study challenge existing models of deep earthquake generation and offer a new perspective on the processes at play within subducting slabs. By recognizing the potential for mineralogical transformations to trigger seismic events, scientists can refine their models and improve their ability to predict and interpret deep earthquakes. This improved understanding is crucial for assessing seismic hazards and mitigating the risks associated with these powerful events. Moreover, it contributes to a broader understanding of the dynamic processes that shape Earth’s interior, from the movement of tectonic plates to the evolution of the mantle.
The implications of this research extend beyond the specific case of the 2015 Ogasawara earthquake. It provides a framework for understanding other deep earthquakes that exhibit similar characteristics, such as isolated occurrences and limited aftershock sequences. By incorporating the role of mineralogical transformations, seismologists can develop more comprehensive models that capture the full range of processes contributing to deep seismicity. This, in turn, will enhance our ability to interpret seismic data and glean insights into the complex dynamics of Earth’s interior.
Further research is needed to fully explore the implications of this proposed mechanism. Detailed studies of the mineralogical composition of subducting slabs, combined with sophisticated numerical models, can help refine our understanding of the conditions that favor mineralogical transformation-induced earthquakes. Investigating other deep earthquake events with similar characteristics will further test the validity of this hypothesis and contribute to a more comprehensive picture of deep Earth processes.
The study of deep earthquakes represents a frontier in seismology. These events, occurring in a realm far removed from direct observation, offer a unique window into the dynamics of Earth’s deep interior. By unraveling the mechanisms driving these enigmatic tremors, scientists gain valuable insights into the processes that shape our planet and contribute to a deeper understanding of the forces that sculpt Earth’s dynamic landscape. The 2015 Ogasawara earthquake, with its record-breaking depth and intriguing characteristics, has spurred a reevaluation of deep earthquake mechanisms and paved the way for a more nuanced understanding of the complex interplay between mineralogy and seismicity in Earth’s lower mantle.
