New research on the 9.0-magnitude Tohoku-Oki earthquake explores the intersection between Earth science, material properties and advanced modelling techniques. By combining aspects of these scientific fields, Ehsan Jamali Hondori in Park Jin-Oh na Univerzi v Tokiu so lahko ugotovili vlogo, ki so jo imeli podmorski sedimenti v tem smrtonosnem potresu, ki je prizadel Japonsko leta 2011. Raziskava bi lahko pomagala tudi pri odkrivanju prelomov, ki bi lahko povzročili podobne potrese v prihodnosti.
The duo’s work focused on a shallow plate boundary thrust fault, also known as a décollement, which is a very shallow and unstable zone of tectonic activity. This fault is positioned relative to the Japan trench subduction zone off the eastern coast of Japan, and its rupture led to the disturbance on the seafloor that created the tsunami waves associated with the Tohoku-Oki earthquake. This large-scale interpretation is well-accepted; however, what requires further investigation is the stability of the underlying sediments (layered particulates that have not yet become solid rock) that may have affected the rupture propagation of the fault.
Jamali Hondori and Park studied this stability using 2D seismic imaging, followed by a pore-fluid pressure calculation of the sediments at the décollement. The seismic imaging allows for a reconstruction of the geological structures, and the pore-fluid pressure describes the behaviour of the sediment particles as they are subjected to a high-pressure load coming from the ocean above the décollement.
Seizmični podatki in podatki o porni tekočini so bili zbrani ločeno, pri čemer so bili seizmični podatki v obliki seizmograma, podatki o porni tekočini pa so bili narisani glede na strižno deformacijo in oddaljenost od jarka.
Podatkovni jackpot
The earthquake epicenter was located remarkably close to site 2E, which is a survey location from a previous study. With this prime positioning relative to the décollement, Jamali Hondori and Park hit the jackpot when it comes to seismic depth images.
Seismic depth images of the décollement revealed the formation of an accretionary prism. This is a collection of displaced sediments that have been dredged up and jostled by the region’s tectonic movements. Measuring the relative velocities of the seismic waves through these sedimentary structures allowed the duo to conclude that the pore-fluid pressure of sediments led to destabilization, which in turn led to seismic activity near site 2E.
The research makes an important connection between the cause of the earthquake and how fluid drains from the sediments. This was evaluated by calculating a “fluid overpressure ratio”, which quantifies the drainage and amount of fluid still present in the sediments. Jamali Hondori and Park have shown that beneath site 2E, there is an active drainage path. As a result, the seepage of fluid from the sediments results in lower pore pressure conditions in this zone. At the décollement, however, highly pressurized pore fluid is trapped within the impermeable sediments. This causes fault instability and decreased friction, which favours rupture propagation.
Nestabilni sedimenti
Skratka, tektonska obremenitev in toplotni pritisk sedimentov, ko se premikajo vzdolž njihove podlage, sta verjeten krivec za nepričakovano velik prelom preloma potresa Tohoku-Oki. Z drugimi besedami, potres Tohoku-Oki je povzročil hidrostatični pritisk oceana, ki pritiska na usedline. To je nazadnje destabiliziralo sedimente v mikroskopskem obsegu in s tem ustvarilo obsežno tektonsko gibanje.
Če bi bila tlak pornega fluida in hidrostatični tlak enaka po velikosti, potresne obremenitve ne bi bilo. Namesto tega je razlika med obema vzrok za velik kozeizmični zdrs med potresom, kjer se kozeizmični nanaša na mehanski dogodek, ki sovpada s potresno aktivnostjo.
Raziskovalci so analizirali strižno in navpično efektivno napetost na mestu napake, poleg razmerja med izračunano in pričakovano navpično efektivno napetostjo - ki jo dvojica opisujeta kot efektivno razmerje napetosti. Ta analiza je razkrila nagnjenost preloma h kozeizmičnemu zdrsu in razpoku, kjer je nizko efektivno razmerje napetosti povzročilo zdrs preloma in s tem povezan cunami.
Horizontalni premik
Slabljenje valov namiguje na naravo Zemljine astenosfere
Nazadnje Jamali Hondori in Park zaključujeta, da so zapleteni vzorci lomljenja na prelomu ustvarili razmere, kjer je bil pritisk porne tekočine ključnega pomena pri določanju ravni strižne napetosti. Posledično raziskovalci poudarjajo, da je ta močan potres povzročil horizontalni premik, ki je sorazmeren z nihanji v stabilnosti sedimenta.
This research also has implications for our understanding of décollement thrust faults, illustrating the importance of the properties of the sediment beneath the fault. It could be possible to study the sediment profile of specific fault and predict powerful earthquakes before they occur. The ability to forecast nature-driven hazards would be invaluable to people living on coastlines, especially as climate change drives the frequency of increasingly serious natural disasters.
Raziskava je opisana v Znanstvena poročila.
