Author: Miranda Wiebe
First of all, why should you care?
Turbidites are sedimentary rocks that act as a unique “tape recorder” of past earthquakes -- earthquakes that occurred long before humans were around to feel them. This is due to the fact that earthquakes trigger the formation of turbidites, and so the non-turbidite rock deposits in between turbidites can give a sense of the length of time between earthquakes.
Having this record of past earthquakes enables scientists to make statistically significant predictions about future earthquakes. The ability to predict future earthquakes enables humans to better prepare for the immense destruction that these natural disasters can cause.
What is a turbidite?
Turbidites are the consequence of turbidity currents, which are essentially underwater landslides that occur due to sediment accumulation along an underwater shelf. Turbidity currents spread sediment throughout sea channels, as shown in the video below:
Source: Oregon State
How do we identify a turbidite?
Turbidite sediment deposits display “normal grading,” or decreasing grain size upwards from the base of a deposit. Turbidites distinguish themselves from other deposits by following the “Bouma sequence” (Fig. 1). The Bouma sequence is characterized as:
Figure 1: A turbidite deposit in a rock, from the Pigeon Point Formation, Pescadero Beach, California. This turbidite shows layers A-D of the Bouma sequence. Source: Wikipedia.
What causes a turbidite?
In order to trigger the turbidity current that produces a turbidite, there must be some force applied to the area. In many cases, that force is an earthquake. Earthquakes can also trigger shelf collapse, which is another method of generating a turbidity current.
How can turbidites be used to extrapolate earthquake records?
If we take the example of the Cascadia margin, located along the Oregon-Washington coast, we see that an extensive turbidite record can be used to reconstruct the history of earthquakes in the region. (Definition: in this case, a margin is the area along which oceanic crust is crashing into continental crust).
Adams (1990) first suggested that the best explanation of the turbidities are the 13 earthquakes that occurred along the Cascadia margin. When investigating the patterns of timing between turbidite deposits, Adams (1990) found that they matched well with those of great earthquake cycles.
Furthermore, the thickness of the sedimentary layer that tops the turbidites indicates that the last turbidity current occurred roughly 300 years ago, which is consistent with the most recent extensive earthquake event on the Cascadia margin (Adams 1990; Sanders 2005).
Research preformed more recently by faculty at Oregon State University supports the hypothesis put forward by Adams (1990) (Goldfinger et al. 2003). Fig. 2 shows locations of samples taken from the turbidites off the Cascadia margin, both by Adams (1990) and the Oregon State researchers.
Figure 2: Map showing locations of turbidite core samples taken off the coast of Washington and Oregon, on the Cascadia margin. Cores from Adams (1990) are in grey, cores from Oregon State researchers are in yellow. From Oregon State University (2003).
How can turbiditic data help us predict future earthquakes?
Using his turbiditic data to extrapolate patterns in earthquake frequency, Adams (1990) was able to predict that in the next 50 years, there is a 2-10% chance of a great earthquake occurring. Using similar statistical methods, Oregon State University (2003) found that the probability of a full margin rupture along the Cascadia margin in the next 50 years is 7-11%. Similar predictive statistics can be utilized at almost any other continental margin which contain complete turbiditic sequences.
What does this mean for New England?
Historically, New England has suffered earthquakes much smaller in magnitude than Oregon and Washington.
However, turbidite deposits in New England turbidites can be used to determine the timing of past earthquakes in the region. For example, the most severe earthquake on record is the Cape Ann earthquake which occurred in 1755 (Monecke 2018). In Lynn, Massachusetts’s “Sluice Pond,” there are lake sediments deposited in 1740 and 1810 that are interpreted to be turbidites deposited during the intense ground shaking in 1755 (Monecke 2018).
These findings imply that organic-rich sediments in small ponds around New England contain a historical record of earthquakes in the region (Monecke 2018). Perhaps these records may one day be used to predict future earthquakes in the region, which, if the correct magnitude and location, could cause a tsunami in New England (Koebler 2013).
Want to learn more?
Adams, J., 1990, Paleoseismicity of the Cascadia subduction zone; evidence from turbidites off the Oregon-Washington margin. Tectonics., v. 9(4), p. 569.
Goldfinger, C., Nelson, C.H., and Johnson, J.E., 2003, Deep-Water Turbidites as Holocene Earthquake Proxies: The Cascadia Subduction Zone and Northern San Andreas Fault Systems: Annali Geofisica, v. 46, p. 1169-1194.
Koebler, J., 2013, Study: Boston, New England at Greatest Tsunami Risk in US: https://www.usnews.com/news/articles/2013/04/19/study-boston-new-england-at-greatest-tsunami-risk-in-us (Accessed September 2018).
Monecke, K. et al., 2018, The 1755 Cape Ann Earthquake Recorded in Lake Sediments of Eastern New England; an Interdisciplinary Paleoseismic Approach.: Seismological Research Letters Pre-Issue Publication, p. 1212-22.
Oregon State University, 2003, Cascadia Paleoseismic History Based on Turbidite Stratigraphy: http://activetectonics.coas.oregonstate.edu/cascadia_turbs.htm (accessed September 2018).
Sanders, R., 2005, Sinking coastline may precede large subduction zone quakes: https://www.berkeley.edu/news/media/releases/2005/01/20_marshes.shtml (accessed September 2018).