Precise measurements gathered from GPS stations over time have helped us better understand
earthquake hazards. For example, Prior to March 11, 2011 scientists noted that hundreds
of GPS receivers in Northern Honshu had been recording steady westward movement of the
stations along the coast. Because the shallow portions of the subducting and overriding
plates are locked by friction, the leading edge of the overriding plate was being forced
west, uplifting slightly as it was compressed, storing elastic energy.
Elastic rebound during the earthquake caused stations along the coast in Miyagi Prefecture
to jump east by as much as 14.5 feet or 4.42 meters. The abrupt uplift of the eastern edge
of the overlying plate near the Japan Trench produced the catastrophic tsunami.
We now know that a mirror-image great earthquake and tsunami happened on the Cascadia subduction
zone on January 26, 1700. Evidence for the earthquake of 1700 comes from Native American
oral histories, dating of ghost forests of the Pacific Northwest coast, and arrival of
a well-documented orphan tsunami that arrived in Japan nine hours after the earthquake.
Over the past decade, the compression of the Cascadia continental margin has been confirmed
by GPS data collected by EarthScope’s Plate Boundary Observatory. The coastline, near
the Juan de Fuca—North America plate boundary, is moving northeast more rapidly than inland
areas, while GPS stations east of the Cascade Mountains are not moving.
Now let’s zoom in to the GPS station at Cape Mears lighthouse on the Oregon coast
and see how it is moving. The graph shows the position of this GPS station
each year compared to its starting location in 2000. The sequence of data points documents
the northeast motion of Cape Mears at 0.4 inches, or 10.5 mm, per year. From 2000 to
2013, Cape Mears has moved northeast about 5.1 inches, or about 130 mm. Since the Cascadia
earthquake in 1700, Cape Mears has moved by almost 11 feet, or 3.3 meters.
What does this imply about the leading edge of the Cascadia continental margin?
Let’s look again at a cross section with graphs reflecting the movement of each GPS
station. We’ll use a spring to represent the elastic nature of the overriding plate.
Coastal stations are pushed northeast by the Juan de Fuca Plate at a faster rate than inland
areas so elastic energy builds within the leading edge of the overriding plate. Just
as the westward motion of GPS stations in Japan recorded growing stress on that subduction
zone prior to the 2011 earthquake, the steady movement of GPS stations in coastal Cascadia
also indicates growing stress. Although friction between the Juan de Fuca and North American
Plates has prevented our spring from releasing, when that friction is ultimately overcome,
the resulting earthquake will cause Cape Meares to jump southwest, back near the position
after the great Cascadia earthquake of 1700. Here’s what that elastic rebound process
would look like in map view. Although we can’t predict when or where the next earthquake
will happen along the subduction zone, we are certain that one will eventually occur.
Understanding the meaning of these GPS observations should motivate us to build an earthquake
and tsunami resilient Pacific Northwest.