Integrated Geophysical Analysis over the Cascadia Subduction Zone

Asif Ashraf Author

04/04/2021 Added

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This study combines multiple geophysical datasets to determine physical rock properties and crustal architectures of the Cascadia Subduction Zone.

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[00:00:01.450]Hello everyone.
[00:00:02.310]My name is Asif Ashraf.
[00:00:03.760]I am a second year master student
[00:00:05.310]at the Department of Earth and Atmospheric Sciences.
[00:00:08.570]Today, I'm going to present my research
[00:00:10.660]which is the Integrated geophysical analysis
[00:00:13.210]over the Cascadia subduction zone.
[00:00:15.600]So for the outline of my presentation,
[00:00:17.380]first,
[00:00:18.213]I will give you the introduction of the study area,
[00:00:19.660]then I'll talk about the objectives of this study,
[00:00:22.110]then I'll go over the background information
[00:00:24.880]which is basically trying to construct (mumbles).
[00:00:27.598]Then I will go to the dataset that I used
[00:00:29.624]for the study and the results I got from those data.
[00:00:32.890]And finally, the conclusion.
[00:00:35.550]So my study area is the Cascadia subduction zone.
[00:00:38.430]The Cascadia subduction zone consists of three microplates.
[00:00:41.410]So in the North we have the Explorer plate in the middle,
[00:00:43.750]on the Fuca plate.
[00:00:45.060]And in the South, we have the Gorda plate.
[00:00:47.200]Now the entire zone extends from British Columbia
[00:00:49.820]in the North to Northern California in the South.
[00:00:53.141]Now for the motivation for this study,
[00:00:55.530]we actually have an imminent threat
[00:00:57.680]of a mega thrust earthquake in the subduction zone.
[00:01:00.490]Also, we can see a variance in seismic patterns
[00:01:04.310]over different parts of the subduction zone.
[00:01:07.340]Before the objectives of this study,
[00:01:09.346]is that, we want to map major tectonic structures off
[00:01:14.372]Juan de Fuca plate using multiple geo-physical datasets.
[00:01:18.910]Then we also want to develop 2D integrated models,
[00:01:23.790]that go through two different seismicity zones.
[00:01:29.020]So the first tectonic structure of the Juan de Fuca plate
[00:01:31.910]that is of our interest is the propagator wake.
[00:01:35.080]After propagator wake actually formed through
[00:01:37.350]the propagation of spreading ridge.
[00:01:40.080]So for example, if this spreading ridge moves
[00:01:42.910]from this point to this point
[00:01:45.670]then we will see a V-shaped fracture zone
[00:01:50.110]on the oceanic crust.
[00:01:52.340]And this V-shaped fracture zone can be evident
[00:01:55.220]from the offset in the magnetic Isochrons.
[00:01:59.503]Then the second Tectonic structure that is
[00:02:02.040]of our interest are the Seamounts.
[00:02:04.460]So in this Bathymetry Map we can clearly see
[00:02:07.060]these Bathymetric Seamounts.
[00:02:09.230]Now over here, the seismic reflection lines also
[00:02:13.200]show these Bathymetric seamounts.
[00:02:15.650]But there are also a number of buried seamounts
[00:02:18.850]which are of significant crustal structures
[00:02:22.260]for this oceanic Juan de Fuca plate.
[00:02:24.220]Now, we also want to map this.
[00:02:28.950]So for the data sets that we used
[00:02:32.090]for this study are the topography
[00:02:34.670]or bathymetry and the gravity data, the magnetic data,
[00:02:38.140]and of course the seismic reflection data sets.
[00:02:40.200]And these are used for the spatial geophysical analysis.
[00:02:43.520]So these potential field datasets are,
[00:02:46.090]the Raw data.
[00:02:48.310]But we have actually applied correction to this data.
[00:02:51.010]Then we calculated the residual anomaly,
[00:02:53.890]which actually highlighted the shallow structures.
[00:02:57.140]Comparatively shallow structures, which is of our interest.
[00:03:01.380]Then we actually filter, we actually applied several filters
[00:03:06.190]to this data to highlight the features of our interest.
[00:03:11.483]To develop the 2D integrated models,
[00:03:13.620]we have also used the topography, the gravity,
[00:03:16.520]the magnetic, and seismic datasets.
[00:03:19.458]The seismic datasets are actually used
[00:03:21.547]to constrain our modeling.
[00:03:24.660]So this slide shows the Seismic Reflection Datasets
[00:03:27.280]that we used to constrain the non-subducting part
[00:03:30.960]of the oceanic crust.
[00:03:32.510]Now for the subducting part of the oceanic crust,
[00:03:35.030]we have actually used seismic refraction datasets.
[00:03:42.410]So in results section, the first thing I want to
[00:03:45.932]show you are the developed 2D models.
[00:03:48.168]Washington Model, shown in this slide,
[00:03:51.760]is the Northern one that go
[00:03:52.603]through the high seismicity zone.
[00:03:54.680]Now models basically show the crustal architecture
[00:03:57.155]and the physical properties
[00:03:59.810]of subsurface rocks of the subduction zone.
[00:04:03.510]It has a lot of details and new findings
[00:04:05.940]but for now I want to divert your attention
[00:04:08.130]to the most significant one,
[00:04:09.980]which is the presence
[00:04:11.090]of lower density zones in the oceanic crust.
[00:04:17.100]Now these lower density zones are shown
[00:04:19.278]by either gray or this orange shading in the model.
[00:04:25.410]So when an oceanic crust approaches a subduction zone,
[00:04:29.350]there should be a gradual increase
[00:04:30.920]in density as it moves further away
[00:04:34.150]from the oceanic center. Eh, from the spreading center.
[00:04:37.430]But in our modeling, we have found several zones
[00:04:41.938]of oceanic Juan de Fuca crust that have lower density
[00:04:43.620]than the surrounding crust
[00:04:45.000]which is not the geological norm.
[00:04:49.060]We can also see their presence in the Oregon model
[00:04:51.220]which is how the model that go through lowers
[00:04:53.776]Seismicity zones.
[00:04:54.660]So a total of 12, lower density zones,
[00:04:59.270]have been identified
[00:05:00.960]through our modeling.
[00:05:03.460]Now I will show you the result of our
[00:05:06.022]geophysical and spatial analysis.
[00:05:07.760]Now, in addition of the previously interpreted propagator awakes
[00:05:14.306]We have also interpreted several newly
[00:05:15.780]pseudofault lineaments.
[00:05:20.250]Now these pseudofault lineaments,
[00:05:21.560]are mapped from filtered potential field data
[00:05:24.460]but primarily disturbances within the sea floor
[00:05:27.220]magnetic stripes indicate their potential location.
[00:05:30.100]However, there are several other factors that go
[00:05:32.520]into their interpretation.
[00:05:35.180]Now, in the figure on left
[00:05:39.150]these are the lines that show our 2D modeling lines.
[00:05:44.010]And the white lines in between actually the extent
[00:05:48.370]of our Modeled lower density zone.
[00:05:52.320]Now Newly almost all of the pseudofault
[00:05:55.888]lineaments and propagator wakes correspond
[00:05:58.920]to model lower density zone.
[00:06:01.330]For example, if you see this propagator wake
[00:06:04.960]actually go through this model lower density zone.
[00:06:09.270]Now this is one of our newly identified sort of lineament
[00:06:13.290]and you can see that also go
[00:06:14.610]through on of the lower density zone.
[00:06:19.710]In our spatial analysis
[00:06:20.810]we have also included interpretation
[00:06:22.690]of several Buried Seamounts.
[00:06:24.340]Now Based on the characteristic gravity signature,
[00:06:26.240]from the bathymetry seamount
[00:06:29.370]and also the seismic seamount,
[00:06:31.630]We have actually identified several buried seamounts
[00:06:35.110]throughout the Juan de Fuca plate.
[00:06:38.700]Now In conclusion,
[00:06:41.350]the first thing I want to say
[00:06:43.300]that both previously identified propagator wakes
[00:06:46.580]and newly mapped pseudofault lineaments coincide
[00:06:50.210]with lower density regions within the Juan de Fuca plate.
[00:06:54.990]And the second conclusion of our study is
[00:06:57.280]that the seamounts appear to be clustered
[00:06:59.710]around both propagator wakes and pseudofault lineaments.
[00:07:03.570]For example, you can clearly see
[00:07:06.360]that the sea mounts are actually clustered
[00:07:09.010]around our newly identified for the full lineament.
[00:07:13.620]Now, these two observations suggest
[00:07:16.650]that the propagator wakes, that are these,
[00:07:21.140]and the pseudofault lineaments that are these,
[00:07:24.280]are zones
[00:07:25.330]of crustal weakness within the oceanic plate
[00:07:29.630]although both the pseudofault lineaments
[00:07:32.520]and propagator wakes show, similar physical property
[00:07:36.380]our observations exist that they have different origin.
[00:07:40.140]We think the newly identified pseudofault lineament
[00:07:45.570]are actually triggered by offset
[00:07:48.000]between the Juan de Fuca spreading ridges.
[00:07:53.070]I want to thank everyone who have attended
[00:07:55.670]this presentation
[00:07:57.260]and this is the reference slide.
[00:07:58.930]If anybody wants to check.

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Integrated Geophysical Analysis over the Cascadia Subduction Zone

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