Category Archives: geochronology

Cascadia subduction zone: Tectonic Earthquakes of the Pacific Northwest

IRIS and the US Geological Survey have recently produced an educational video about tectonic earthquakes in the region of the US Pacific Northwest. The project was funded by the National Science Foundation.

The Video


YT link for the embedded video below.
mp4 link for the embedded video below.

mp4 embedded video:


YT embedded video:

I recently collected a core with a thick sandy deposit that is hypothesized to be the sedimentary deposit that was the result of tsunami deposition following the 1700 A.D. Cascadia subduction zone earthquake. Here is my post about that core.

Here is a composite of the two cores that I collected. The top of the core is on the left.

  • YT link for the embedded video below:

Earlier this year was the 315th anniversary of the 1700 AD Cascadia subduction zone earthquake and tsunami. I compiled some information about that earthquake and tsunami. I included some information about the plate tectonics of the region. Here is the post for that anniversary.

Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).

This map (McCrory et al., 2006) shows the secular (ongoing modern) rates of motion for the Juan de Fuca and Gorda plates relative to the North America plate (Wilson, 1998; McCrory, 2000). Red triangles denote active arc volcanoes.

Here is a view of the subduction zone showing the landscape and the plate configuration within the Earth. The cross section is located near the southern Willamette Valley. This is schematic and does not completely match the real geography. Note how the downgoing plate melts and the rising magma leads to volcanism of the Cascade volcanoes (a volcanic arc).

Here is a version of the CSZ cross section alone (Plafker, 1972).

This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.

Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.

This figure shows the regions that participate in this interseismic and coseismic deformation at Cascadia. Atwater et al., 2005. Black dots on the map show sites where evidence for coseismic subsidence has been found in coastal marshes, lakes, and estuaries.

This shows how the CSZ is deforming vertically today (Wang et al., 2003). The panel on the right shows the vertical motion in mm/yr.

This figure, also from Wang et al. (2003), shows their estimate of how the coseismic vertical motion may happen. Contours are in meters.

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.

Here is a photo of the ghost forest, created from coseismic subsidence during the Jan. 26, 1700 Cascadia subduction zone earthquake. Atwater et al., 2005.

Here is a photo I took in Alaska, where there was a subduction zone earthquake in 1964. These tree snags were living trees prior to the earthquake and remain to remind us of the earthquake hazards along subduction zones.

This shows how a tsunami deposit may be preserved in the sediment stratigraphy following a subduction zone earthquake, like in Cascadia. Atwater et al., 2005. If there is a source of sediment to be transported by a tsunami, it will come along for the ride and possibly be deposited upon the pre-earthquake ground surface. Following the earthquake, tidal sediment is deposited above the tsunami transported sediment. Sometimes plants that were growing prior to the earthquake get entombed within the tsunami deposit.

Here is a new animation of the tsunami that was triggered during the 1700 AD CSZ earthquake. This is just a model and has considerable uncertainty associated with it. From the US NWS Pacific Tsunami Warning Center (PTWC).

This is the timeline of prehistoric earthquakes as preserved in sediment stratigraphy in Grays Harbor and Willapa Bay, Washington. Atwater et al., 2005. This timeline is based upon numerous radiocarbon age determinations for materials that died close to the time of the prehistoric earthquakes inferred from the sediment stratigraphy at locations along the Grays Harbor, Willapa Bay, and Columbia River estuary paleoseismic sites.

Offshore, Goldfinger and others (from the 1960’s into the 21st Century, see references in Goldfinger et al., 2012) collected cores in the deep sea. These cores contain submarine landslide deposits (called turbidites). These turbidites are thought to have been deposited as a result of strong ground shaking from large magnitude earthquakes. Goldfinger et al. (2012) compile their research in the USGS professional paper. This map shows where the cores are located.

Here is an example of how these “seismoturbidites” have been correlated. The correlations are the basis for the interpretation that these submarine landslides were triggered by Cascadia subduction zone earthquakes. This correlation figure demonstrates how well these turbidites have been correlated. Goldfinger et al., 2012

This map shows the various possible prehistoric earthquake rupture regions (patches) for the past 10,000 years. Goldfinger et al., 2012. These rupture scenarios have been adopted by the USGS hazards team that determines the seismic hazards for the USA.

I have a paper that also discusses the paleoseismology and sedimentary settings in Cascadia (and Sumatra). Patton et al., 2013.

Here is my abstract:

Turbidite deposition along slope and trench settings is evaluated for the Cascadia and Sumatra–Andaman subduction zones. Source proximity, basin effects, turbidity current flow path, temporal and spatial earthquake rupture, hydrodynamics, and topography all likely play roles in the deposition of the turbidites as evidenced by the vertical structure of the final deposits. Channel systems tend to promote low-frequency components of the content of the current over longer distances, while more proximal slope basins and base-of-slope apron fan settings result in a turbidite structure that is likely influenced by local physiography and other factors. Cascadia’s margin is dominated by glacial cycle constructed pathways which promote turbidity current flows for large distances. Sumatra margin pathways do not inherit these antecedent sedimentary systems, so turbidity currents are more localized.

The Gorda plate is deforming due to north-south compression between the Pacific and Juan de Fuca plates. There have been many papers written about this. The most recent and comprehensive review is from Jason Chaytor (Chaytor et al., 2004). Here is a map of the Cascadia subduction zone, as modified from Nelson et al. (2006) and Chaytor et al. (2004). I have updated the figure to be good for projections in a dark room (green) and to have the correct sense of motion on the two transform plate boundaries at either end of the CSZ (Queen Charlotte and San Andreas faults).

Here is the Chaytor et al. (2004) map that shows their interpretation of the structural relations in the Gorda plate.

Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

The Blanco fracture zone is also an active transform plate boundary. The BFZ is a strike slip fault system that connects two spreading ridges, the Gorda Rise and the Juan de Fuca Ridge. Here is a map that shows the tectonic setting and some earthquakes related to the BFZ from April 2015. There are some animations on this web page showing seismicity with time along the BFZ, over the past 15
years.

References:

Plate Tectonics: 200 Ma

Gibbons and others (2015) have put together a suite of geologic data (e.g. ages of geologic units, fossils), plate motion data (geometry of plates and ocean ridge spreading rates), and plate tectonic data (initiation and cessation of subduction or collision, obduction of ophiolites) to create a global plate tectonic map that spans the past 200 million years (Ma). Here is the facebook post that I first saw to include this video.

Here is a view of their tectonic map at a specific time.


Gibbons et al. (2015) created several animations using their plate tectonic model. I have embedded both of these videos below. Other versions of these files are placed on the NOAA Science on a Sphere Program website.

They also compare their model with P-wave tomographic analytical results. P-Wave tomography works similar to CT-scans. CT-scans are the the result of integrating X-Ray data, from many 3-D orientations, to model the 3-D spatial variations in density. “CT” is an acronym for “computed tomography.” Both are kinds of tomography. Here is a book about seismic tomography. Here is a paper from Goes et al. (2002) that discusses their model of the thermal structure of the uppermost mantle in North America as inferred from seismic tomography.

Here is an illustration from the wiki page that that attempts to help us visualize what tomography is.


P-Wave tomography uses Seismic P-Waves to model the 3-D spatial variation of Earth’s internal structure. P-Wave tomography is similar to Computed-Tomography of X-Rays because the P-wave sources are also in different spatial locations. For CT-scans, the variation in density is inferred with the model. For P-Wave tomography, the variation in seismic velocity. Typically, when seismic waves travel faster, they are travelling through old, cold, and more dense crust/lithosphere/mantle. Likewise, when seismic waves travel slower, they are travelling through relatively young, hot, and less dense crust/lithosphere/mantle.

Regions of Earth’s interior that have faster seismic velocities are often plotted in blue. Regions that have slower velocities are often plotted in red.

Here are their plots showing the velocity perturbation (faster or slower). I include the figure caption below the image.



Plate reconstructions superimposed on age-coded depth slices from P-wave seismic tomography (Li et al., 2008) using first-order assumptions of near-vertical slab sinking, with a) 3.0 and 1.2 cm/yr constant sinking rates in the upper and lower mantle, respectively, following Zahirovic et al. (2012), and b) 5.0 and 2.0 cm/yr upper and lower mantle sinking rates, respectively, following Replumaz et al. (2004). Both end-member sinking rates indicate bands of slab material (blue, S1–S2) offset southward from the Andean-style subduction zone along southern Lhasa, consistentwith the interpretations of Tethyan subducted slabs by Hafkenscheid et al. (2006). However, although the P-wave tomography provides higher resolution than S-wave tomography, the amplitude of the velocity perturbation is significantly lower in oceanic regions (e.g., S2) and the southern hemisphere due to continental sampling biases. Orthographic projection centered on 0°N, 90°E.

Earthquake in Papua New Guinea!

In the last ~24 hours, we have had a few earthquakes in northeastern Papua New Guinea, all in the M 5 range.

    Here are the USGS web sites for these earthquakes:

  • 2015.09.11 M 5.5
  • 2015.09.11 M 5.3
  • 2015.09.12 M 5.0

Here is a map that shows these three earthquakes. I have plotted the USGS moment tensors for the M 5.3 and M 5.5 earthquakes. I placed a moment tensor / focal mechanism legend in the upper right corner of the map. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.

These moment tensors show compression in the ~northeast direction. This is consistent with the northwest striking plate boundary fault to the southwest of these earthquakes.


Here is a generalized tectonic map from Hamilton (1979), posted on the Oregon State University website.

Here is a more detailed geologic and seismicity map from Baldwin et al. (2012). These M ~5 earthquakes are plotted near the Ramu-Markham fault which is an onland extension of the New Britain trench (south verging subduction zone plate boundary), a plate boundary fault system separating volcanic arc / ophiolite rocks on the north from the New Guinea Mobile Belt rocks to the south.


This map has some cross sections to help us interpret the complicated geology of the region (Baldwin et al., 2012). We can note that the different maps of this region all plot the plate boundaries in different places, probably because they are all using different sources of information (e.g. the GPS geodetic map below, which only uses motions of points).


The Baldwin et al. (2012) map above uses geology and GPS geodesy, but this map only has plate motions plotted (from Paul Tregoning at The Australian National University). The map plots, “linear velocities of GPS sites in PNG, showing absolute motions of the numerous tectonic plates.” Go to his website where he presents some related papers.


This is an interesting map from a Baldwin et al. (2004). This shows how the Woodlark Basin is formed by the spreading center there. This map shows the Trobriand trough meet the New Britain trench and note how they verge in opposite directions. Compare this with the oblique map and cross section above.


Here is another map (from a GSA Bulletin article) that also shows how the NBT transitions into the RMFZ.


There was a M = 7.0 earthquake in northwest Papua New Guinea in late July 2015. I report about that earthquake and it also is compressional and related to the east-west striking north-south convergence along the north side of Papua New Guinea. Below is a map showing my interpretations of that earthquake.


The New Britain trench is a very seismically active region. Below are a couple reports on the seismic activity from the past year or two.

Here is a summary map showing some of the seismicity in the New Britain trench region. The first one shows the seismicity and moment tensors plotted with the slab contours from Hayes et al. (2012). The second map shows how the NBT transitions eastward to the South Solomon Sea trench. Note how the moment tensors show that the slip partitioning rotates with the strike of the subduction zone faults.



Rolf Aalto: Large Natural Floodplains: Overlooked Links in the Global Source-to-sink Continuum

Rolf Aalto is giving a presentation at the HSU Geology Coloquium on Tuesday 4/15/2014 in Van Matre Hall, room 109, at 5 PM.

This is Dr. Aalto’s website at Exeter.


This is what Dr. Aalto is interested in:
Rolf researches rivers and erosion across 6 continents, including: South America (Beni, Mamore, Orinoco & Ucayali Rivers), North America (Sacramento-CA, Feather-CA & Salmon-ID Rivers & Rio Grande-NM), Australasia (Strickland & Fly Rivers PNG), Europe (Danube River Romania), and Asia (Mekong River, Cambodia & exploratory sites in China). He leads the Exeter Radiometry Lab, which features world-class analytical facilities for tracing and dating particle movement throughout a wide range of fluvial dispersal systems. He develops novel field surveying/sampling and laboratory techniques to quantify processes across a range of fluvial environments as well as working to enhance remotely sensed data (SRTM and Aster). His currently funded research projects include a Critical Zone Observatory (CRB-CZO), a NSF-Margins project studying fluvial and biogeochemical processes in Papua New Guinea, a NSF project studying the evolution of the Sacramento River system, a NERC project investigating the evolution of the Beni River System in Bolivia, and a NERC project studying the evolution of the Mekong River.

Rolf’s teaching focuses on the application of GIS, modelling, and laboratory methods to solving problems within River Basin Science. Students rate his modules highly, especially for ‘development‘, and graduates report employability exceeding 90%. He is delighted to lead 2nd year field trips to California and Washington State (USA), New Zealand, and South Africa.

Here is some background information from his website:
Rolf obtained his undergraduate degrees from UC Berkeley, where he was inspired to study fluvial processes in a module taught by Prof. William Dietrich (at that time working in Papua New Guinea), completing an honors thesis studying floodplain sedimentation in a specially designed flume. He completed a MSc degree at the University of Washington (Seattle), working with Prof. Thomas Dunne as a Research Assistant to calculate sediment fluxes along the Amazon River and writing a thesis on ”Discordance between suspended sediment diffusion theory and observed sediment concentration profiles in rivers.” While developing ideas for his PhD and seeking funding to pursue his research ambitions on tropical rivers, Rolf was awarded a NASA Earth System Science Fellowship. A scouting campaign to collect samples along rivers in Bolivia laid the framework for Rolf to write a major NSF research grant, culminating in his dissertation “Geomorphic Form and Process of Sediment Flux within an Active Orogen.”

Rolf next worked as a Post Doc at UC Berkeley, returning full circle to study fluvial processes in PNG. He was hired as Research Faculty at Washington with Prof. David Montgomery (a MacArthur ‘Genius‘), funded by a NASA Post Doctoral grant to investigate the SRTM dataset and a CALFED project on the Sacramento River. He was then promoted to Assistant Professor (he remains an Affiliate Associate Professor) at UW where he developed a laboratory and graduate program, wrote four successful NSF research proposals, and initiated new projects in Amazonia, Romania, California, PNG, Venezuela, Greenland, and SE Asia. Rolf has also consulted professionally since 1995 on a range of topics related to geomorphic hazards and river restoration (as a Licensed Engineering Geologist). To date he has written/co-written successful research proposals worth >$10.0 million USD and led/co-led the execution of this research by diverse international & interdisciplinary teams working across a wide range of logistically challenging environments throughout the world.

In 2007 Rolf joined Exeter’s internationally acclaimed River Basin Science group to further develop world-class analytical facilities for tracing and dating sediment movement throughout a wide range of fluvial dispersal systems. He collaborates extensively with Prof. Nicholas (physics-based models of large river systems), Prof. Quine (erosion and biogeochemical evolution of soils), and Dr. Aragao (vegetation and fire in the tropics).