Precariously balanced rocks (PBRs) are formations discovered all through the world the place a slender boulder is balanced precariously on a pedestal boulder. They kind as blocks preserved on cliffs, or when softer rocks erode and depart the tougher rocks behind. They also can kind when landslides or retreating glaciers deposit them in unusual positions.
Despite their delicate balancing act, many PBRs — just like the Brimham Rocks in Yorkshire, or Chiricahua National Monument in Arizona — have survived earthquake shaking over hundreds of years. They can due to this fact inform us the higher restrict of earthquake shaking that has occurred since they have been first fashioned — shaking that, have been it sturdy sufficient, would have triggered them to topple.
By tapping into historic geological knowledge locked inside Californian PBRs, Imperial College London researchers have damaged floor on a brand new method to spice up the precision of hazard estimates for giant earthquakes by as much as 49 %.
Earthquake hazard fashions estimate the chance of future earthquakes in a given location. They assist engineers resolve the place bridges, dams, and buildings must be constructed and the way strong they need to be — in addition to informing earthquake insurance coverage costs in high-risk areas.
The findings are printed right this moment (October 1, 2020) in AGU Advances.
Lead writer Anna Rood, from Imperial’s Department of Civil and Environmental Engineering, stated: “This new approach could help us work out which areas are most likely to experience a major earthquake. PBRs act like inverse seismometers by capturing regional seismic history that we weren’t around to see, and tell us the upper limit of past earthquake shakes simply by not toppling. By tapping into this, we provide uniquely valuable data on the rates of rare, large-magnitude earthquakes.”
Current earthquake hazard estimates rely largely on observations like proximity to fault strains and the way seismically lively a area has been previously. However, estimates for rarer earthquakes which have occurred over intervals of 10,000 to 1,000,000 years are extraordinarily unsure as a result of lack of seismic knowledge spanning these timescales and subsequent reliance on rocky assumptions.
By counting uncommon cosmic ray-generated atoms in PBRs and digitally modeling PBR-earthquake interactions, Imperial researchers have created a brand new methodology of earthquake hazard validation that may very well be constructed into current fashions to finetune their precision.
To faucet into the seismology of the previous, the researchers got down to decide the fragility (chance of toppling as a result of floor shaking) and age of PBRs at a website close to to the Diablo Canyon Nuclear Power Plant in coastal California.
They used a way referred to as cosmogenic floor publicity relationship — counting the variety of uncommon beryllium atoms fashioned inside rocks by long-term publicity to cosmic rays — to find out how lengthy PBRs had existed of their present formation.
They then used 3D modeling software program to digitally recreate the PBRs and calculate how a lot earthquake floor shaking they may stand up to earlier than toppling.
Both the age and fragility of the PBRs have been then in contrast with present hazard estimates to assist increase their certainty.
They discovered that combining their calculations with current fashions lowered the uncertainty of earthquake hazard estimates on the website by 49 %, and, by eradicating the ‘worst-case-scenario’ estimates, lowered the typical dimension of earthquakes estimated to occur as soon as each 10,000 years by 27 %. They additionally discovered that PBRs might be preserved within the panorama for twice so long as beforehand thought.
They conclude that this new methodology reduces the variety of assumptions, and due to this fact the uncertainty, utilized in estimating and extrapolating historic earthquake knowledge for estimates of future threat.
Study co-author Dr. Dylan Rood, of Imperial’s Department of Earth Science and Engineering, stated: “We’re teetering on the edge of a breakthrough in the science of earthquake forecasting. Our ‘rock clock’ techniques have the potential to save huge costs in seismic engineering, and we see them being used broadly to test and update site-specific hazard estimates for earthquake-prone areas — specifically in coastal regions where the controlling seismic sources are offshore faults whose movements are inherently more difficult to investigate.”
The crew is now utilizing their methods to validate hazard estimates for southern California — one of the vital hazardous and densely populated areas of the United States.
Anna stated: “We’re now looking at PBRs near major earthquake faults like the San Andreas fault near Los Angeles. We’re also looking at how to pinpoint which data — whether it be fault slip rates or choice of ground shaking equations — are skewing the results in the original hazard models. This way we can improve scientists’ understanding of big earthquakes even more.”
Reference: “Earthquake hazard uncertainties improved using precariously balanced rocks” by A. H. Rood, D. H. Rood, M. W. Stirling, C. M. Madugo, N. A. Abrahamson, Ok. M. Wilcken, T. Gonzalez, A. Kottke, A. C. Whittaker, W. D. Page, Peter J. Stafford, 1 October 2020, AGU Advances.