Many of the Pacific Northwest's stunning features are a result of its vast and complex geology. Situated along the Ring of Fire, shifting tectonic plate boundaries off the Pacific Coast are responsible for the region's distinct terrain. Immediately off the Washington Coast sits the Juan De Fuca Plate, one of the smallest tectonic plates on earth. It's wedged between the Pacific Plate and the North American plate and contains nearly all plate boundary types, including divergent, convergent, and transform boundaries. This makes the Jaun De Fuca Plate a prime location for geologic study.

This assignment tasked students with mapping earthquake data on the Jaun De Fuca plate. Data was retrieved from the United States Geological Survey in a ten year period from 2010 to 2020. The plotted earthquake data illustrates the outline of many of the plate boundaries off the Pacific Coast.

I began the assignment by retrieving the relevant data from the USGS earthquake catalog. I limited my earthquake data to between the latitudes of 38.891 and 52.107 and the longitudes of -122.168 and -131.836. This encompassed the entirety of the Juan De Fuca plate while keeping the processing requirements minimal. I downloaded this data in a .csv format which can be easily processed in Python.

Once the data was read into my Python file, I performed some minor processing to prepare it for plotting. Time values are converted into datetime and converted from UTC to Pacific time. This process is actually dependent on the time zone of the machine running the code, so time values will vary slightly depending on where this program is executed.

To fulfill the first assignment requirement, I plotted earthquake magnitude versus time over a ten year period. To make the graph more readable and visually appealing I made the scatter plot entries larger for greater magnitudes and applied a color gradient to further illustrate the data trend.

To create the main earthquake map I retrieved a map image from OpenStreetMap and created a bounding box the same size as my earthquake data's outermost lat/long positions. I plotted each earthquake based on its latitude and longitude and once again made the scatter points larger based on their magnitude. Creating the map for April 2015 was simple as I only needed to reduce the number of data points to a one month period.

Earthquake Locations	Tectonic Plates

Earthquake Locations	Tectonic Plates

The largest clusters of earthquakes are along the tectonic plate boundaries to the north and south of the Juan De Fuca plate. These locations actually contain even smaller tectonic plates, the Explorer Plate and South Gorda Plate, which broke off of the Juan De Fuca Plate millions of years ago. In the south, the Blanco Fracture Zone, Gorda Ridge, and Mendocino Fracture Zone have hundreds of earthquakes of various magnitudes. To the north, Explorer Ridge and the Sovanco Fracture Zone have a cluster of earthquakes with a greater magnitude than other regions.

The large amount of earthquakes along the Blanco Fracture Zone are likely due to the transform boundary. As the Juan De Fuca Plate moves east towards the Cascadia Subduction Zone, it slides along a portion of the Pacific Plate which protrudes inwards towards the coast. The earthquakes along the Mendocino Fracture Zone are likely due to a similar transform boundary. In this case it's the Gorda Plate making contact with the Pacific plate.

The larger earthquakes on the northern end of the Juan De Fuca Plate are likely due to the transform boundary between the Pacific Plate and the North American Plate. This trend, while outside of the scope of my data, is visible north and south of the Juan De Fuca Plate.

The scatter of earthquakes inland of the Juan De Fuca plate are likely a result of the covergence boundary at the Cascadia Subduction zone. This is where the Juan De Fuca Plate slides under the North American Plate. These boundaries are known to create up to 80% of all earthquakes, and are also responsible for the numerous volcanos in the Pacific Northwest.

The magnitude of earthquakes between 2010 and 2020 ranged between 2.5 and 6.8. The average magnitude was 3.24, indicating that the vast majority of earthquakes were small.

During this time period there was a cluster of earthquakes along the divergence boundary between the Pacific Plate and the Juan De Fuca Plate. This boundary is divided into the Cobb, Coaxial, and Cleft Segement, and the earthquakes appear to be primarily located on the Coaxial Segment.

These earthquakes can be attributed to the eruption of the Axial Seamount Volcano on April 23rd 2015. The volcano is active and last erupted in 2011 and 1998.

Identify a divergent boundary and a transform boundary on the map you selected in part I

1074 earthquakes occured in the Gorda Ridge divergence zone between 2010 and 2020. The magnitudes ranged from 2.5 to 6.8 and averaged 3.26. This is exactly the same range as the full data set and a slightly higher average.

The divergence zone has a much higher density of small magnitude earthquakes compared to medium and high intensity earthquakes over time. A vertical line is visible in early 2014 indicating a large earthquake that was followed by smaller aftershocks.

On a map this region is represented by a line of earthquakes tracing the plate boundary.

553 earthquakes occured in the Blanco Fracture Zone between 2010 and 2020. The magnitudes ranged from 2.5 to 6.3 and averaged 3.62. This is the same lower bound for range but a lessened maximum value compared to the main data set. The average is notably higher however indicating stronger earthquakes in this region.

Unlike the main data set and Gorda Region, the density of low-magnitude earthquakes in the Blanco Fracture Zone is roughly equivilant to the medium-magnitude earthquakes. This is also indicated by the higher average magnitude.

Similar to the Gorda Region, the Blanco Fracture Zone appears as a line of earthquakes tracing the plate boundary.

My Part I data demonstrates the sheer number of earthquakes that occur in a ten year period. In a reasonably small region of the earth the count was over 3,200 earthquakes, which is much more than one would expect. The magnitude vs. time graph demonstrates that the vast majority of earthquakes have a magnitude of 4 or less which is explains why they are seldom noticed. When plotted on a map, the earthquakes trace the fault lines, and are especially noticeable along transform boundaries. The divergence zone on the western end of the Juan De Fuca Plate has a surprisingly small amount of earthquakes, with the exception of the April 2015 Axial Seamount eruption. The convergence boundary at the Cascadia Subduction Zone also lacks a defining line of earthquakes, but instead is visible in a scatter of inshore earthquakes. This is likely due to the Juan De Fuca plate traveling under the North American Plate and pushing magma upwards in the process. This cycle is responsible for the Northwest's famous volcanoes.

In Part II a divergent and transform boundary are compared by analyzing the Gorda Ridge and Blanco Fracture Zone. While it's difficult to discern a clear pattern in the magnitude vs. time graph, the transform boundary had fewer, stronger earthquakes on average. In comparison, the divergence boundary had almost twice as many earthquakes, most of which were very small.

Global Volcanism Program, 2013. Volcanoes of the World, v. 4.8.6. Venzke, E (ed.). Smithsonian Institution. Downloaded 02 Mar 2020. https://doi.org/10.5479/si.GVP.VOTW4-2013

“Why Study Cascade Volcanoes?,” Volcano Hazards Program, Mar. 2019.

A. Qassim, “Easy Steps To Plot Geographic Data on a Map — Python,” Towards Data Science, Aug. 2019.

Gulick, S., Meltzer, A., and S. Clarke (1998) Seismic structure of the southern Cascadia subduction zone and accretionary prism north of the Mendocino triple junction. Journal of Geophysical Research-Solid Earth, 103(B11).

USGS Data:
https://earthquake.usgs.gov/earthquakes/map/

Open Street Map:
https://www.openstreetmap.org/#map=5/43.386/-124.194