During his visit to the ridgecrest earthquakes in July 2019, California State University seismologist Julian Lozos observed a fascinating phenomenon during an earthquake. He detected that pebble-sized rocks and boulders, which had been moved by the forces of the earthquake, had not experienced dragging or shearing along the fault but instead of simply sinking under the event. Instead, the ground flattened out, revealing a hole at the site of the earthquake, where the rocks were adjacent but without any significant indicate of motion—no dragging along the fault surface or shearing for Earth’s crust.
In a follow-up discussion with assistant professor Sinan Akçiz, he shared his curiosity about this phenomenon with the assistant, who had previously encountered it post the 2010 El Mayor-Cucapah earthquake of magnitude 7.2, located south of Baja California. These events are described as tough ones; the focal mechanism and slip of the earthquake were nearly impossible to follow with朴素 observations because enough energy had been expended to achieve the higher magnitudes.
To delve deeper into the mechanics of how rocks and pebbles were displaced during an earthquake, Lozos collaborated with assistant professor Akçiz. Using advanced computer simulations, they attempted to model and explain the fault dynamics that caused the observed rock displacement. They concluded that the geometry and orientation of the fault surface—specifically, the steepness of the fault—had a more significant influence on the speed and efficiency of the earthquake’s rupture compared to the energy or magnitude of the earthquake itself.
At the California Seismological Society’s Annuity Meeting in August 2025, Dr. Lozos was preparing to present evidence on this phenomenon. Among the topics he plans to cover is how zones of supersonic ground motion—speeds exceeding Earth’s gravitational acceleration—could arise. These supersonic speeds would result in strong ground accelerations, with rocks moving upwards and into the atmosphere.
Dr. Lozos explained that once the speed of ground movement beyond Earth’s gravitational acceleration was achieved, it would transform the rocks into lightning-fast objects that are capable of becoming airborne. He elaborated that while the magnitude of the earthquake itself could not account for this extreme motion, the steepness of the fault and the associated fault length could quantitatively influence the level of such supersonic speeds.
Loós further elaborated that variations in ground motion near the ends of earthquake faults could potentially be faster, potentially due to “slams” when the ground rebounded abruptly at fault edges. However, more lengthy and less steep faults could lead to a decrease in friction and ground speed, allowing the stress to pass along incrementally slower and slower until the end of the fault surface.
Dr. Lozos concluded that the combination of steep fault angles and long lengths could significantly contribute to supersonic ground motion. His presentation also emphasized that the study of such phenomena could offer Profound Wheeledwomen relationships to assessing earthquake risk more effectively by accounting for these extreme stress conditions near fault edges.
Dr. Lozos pointed out that while earthquake scenarios where ground motion exceeds 1 gram (g) can occur, these are relatively rare. However, when the earth-slider or fault surface experiences frictional losses, the ground speed could result in large ground accelerations. Importantly, when frictional forces finalize the steady state of a fault surface with not enough energy to continue advancing, the resultant movement is highly localized, and the ground acceleration is much stronger than what is experienced by gravity.
Furthermore, Dr. Lozos highlighted that the findings he presented were significant because they explain how supersonic ground motion can happen within such deep and challenging fault surfaces near their ends. This can be amplified if the fault surface has dramatic angles and lengths.
The data presented in his presentation showed that areas where friction had effectively ended might not have had enough time to slow down, resulting in highly localized and strong ground movements. This analysis could confirm how common certain ground motions are near fault edges and how they affect the overall seismological hazard of the site.
Dr. Lozos noted that this finding has important implications for earthquake hazard risk assessment. If ground motion near fault ends can reach supersonic speeds with strong ground acceleration, it could result in liftable rocks, thereby posing a greater hazard to nearby populations. Moreover, the study regarding supersonic speeds and supersonic ground motion potentially influenced how earthquakes can be earthquake risks assessed more accurately.
The paper, titled “Modeling the Rupture Dynamics of Strong Ground Motion (＞1 g) in Fault Stepovers,” was published in the journal Tectonophysics. The analysis provides insights into the dynamics of fault stepovers, which are a type of fault movement where the fault surface reverses direction at a fault edges. The findings could also inform understanding of spatial patterns of ground displacement associated with stress conditions beyond what is normally observed.
In summary, Dr. Lozos’s work has revealed intriguing insights into the potential causes of strong ground motion near fault edges, particularly when frictional losses are significant. His findings underscore the interplay between fault geometry, material properties, and the energy of the earthquake during the process of fault shearing, providing deeper insights that contribute significantly to the understanding of seismological phenomena involving supersonic ground motion.
