The most important factors in attempting to assess risk from earthquakes are their frequency and the time-dependence of seismic magnitude. Historical records, although they go back more than a millennium, do not offer sufficient statistical rigor for which tens or hundreds of thousand years are needed. So the geological record is the only source of information and for most environments it is incomplete, because of erosion episodes, ambiguity of possible signs of earthquakes and difficulty in precise dating; indeed some sequences are extremely difficult to date at all with the resolution and consistency that analysis requires. One set of records that offer precise, continuous timing is that from ocean-floor sediment cores in which oxygen isotope variations related to the intricacies of climate change can be widely correlated with one another and with the records preserved in polar ice cores. For the past 50 ka they can be dated using radiocarbon methods on foraminifera shells The main difficulty lies in finding earthquake signatures in quite monotonous muds, but one kind of feature may prove crucial; evidence of sudden fracturing of otherwise gloopy ooze (Sakagusch, A. et al. 2011. Episodic seafloor mud brecciation due to great subduction zone earthquakes. Geology, v.39, p. 919-922).
The Japanese-US team scrutinised cores from the Integrated Ocean Drilling Program (IODP) that were drilled 5 years ago through the shallow sea floor above the subduction zone associated with the Nankai Trough to the SE of southern Japan. Young, upper sediments were targeted close to one of the long-lived faults associated with the formation of an accretionary wedge by the scraping action of subduction. Rather than examining the cores visually the team used X-ray tomography similar to that involved in CT scans, which produce precise 3-D images of internal structure. This showed up repeated examples of sediment disturbance in the form of angular pieces of clay set in a homogeneous mud matrix separated by undisturbed sections containing laminations. The repetitions are on a scale of centimetres to tens of centimetres and were dated using a combination of 14C and 210Pb dating (210Pb forms as a stage in the decay sequence of 238U and decays with a half-life of about 22 years, so is useful for recent events). The youngest mud breccia gave a 210Pb age of AD 1950±20, and probably formed during the 1944 Tonankai event, a great earthquake with Magnitude 8.2. Two other near-surface breccias gave 14C ages of 3512±34 and 10626±45 years before present. These too probably represent earlier great earthquakes as it can be shown that mud fracturing and brecciation by ground shaking needs accelerations of around 1G, induced by earthquakes with magnitudes greater than about 7.0. So, not all earthquakes in a particular segment of crust would show up in seafloor cores, most inducing turbidity flow of surface sediment, but knowing the frequency of the most damaging events, both by onshore seismicity and tsunamis, could be useful in risk analysis. In its favour, the method requires cores that penetrate only about 10 m, so hundreds could be systematically collected using simple piston coring rigs where a weighted tube is dropped onto the sea floor from a small craft.
- Earthquakes from the ocean: Danger zones (nature.com)