Cascadia Volcanic Arc © ™ ® (Part of Pacific Ring of Fire), Intelligent Grandfather's Guide © ™ ® : Volcanoes, Plate Tectonics, and Earthquakes

Mount Baker Stratovolcano slumbers behind
Simon Fraser University
(For emphasis, image is not proportional)

UPDATED
At depths shallower than 30 km (19 mi) or so, the westwardly moving and rising North America Tectonic Plate is locked (stuck) against the easterly moving and sinking (subducting)
Explorer, Juan de Fuca and Gorda Tectonic Plates. Pressure and stress are building in this area (Cascadia 'Subduction' Zone), which is stuck, locked by friction, while strain slowly builds up, pushing up land and mountains on the North American Tectonic Plate as these seismic forces act.

Two years ago scientist put finely tuned GPS laser detectors along a 300 kilometre stretch, on the tops of mountains from the Olympic Peninsula in Washington State, north to the tops of mountains in the Insular Mountain range, north-west of Campbell River, one half way up the east coast of Vancouver Island, BC. YES ! Indeed, the mountains are rising, pushed up from below ! THAT IS NOT A GOOD THING !

Those stuck tectonic plates will break apart when the fault's frictional strength is exceeded and the rocks slip past each other along the fault in a megathrust earthquake. Below 30 km (19 mi) the plate interface exhibits episodic tremor and slip.

The width of the Cascadia subduction zone varies along its length. That depends on the angle of attack of the North American Tectonic Plate after first coming into contact with, and starts rising above the sinking (subducting) Explorer, Juan de Fuca and Gorda Tectonic Plates.
depending on the angle of the sinking Explorer, Juan de Fuca and Gorda Tectonic Plates (subducted) oceanic plate, which heats up as it is pushed deeper beneath the continent. As the edge of the plate sinks and becomes hotter and more molten, the subducting rock eventually loses the ability to store mechanical stress; earthquakes may result. On the Hyndman and Wang diagram (not shown, click on reference link below) the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.[9]

The Cascadia subduction zone runs from triple junctions at its north and south ends. To the north, just below Haida Gwaii, it intersects the Haida Gwaii Fault [formerly, the Queen Charlotte Fault] and the Explorer Ridge. To the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino Fracture Zoneat the Mendocino Triple Junction.

Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 9.0. They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture known as a megathrust earthquake. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. The Cascadia Subduction Zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches from mid-Vancouver Island to Northern California.[10]

Because of the great length of the fault, the Cascadia Subduction Zone is capable of producing very large earthquakes if rupture occurs along its entire length. Thermal and deformation studies indicate that the region 60 kilometers (about 40 miles) downdip (east) of the deformation front (where plate deformation begins) is fully locked (the plates do not move past each other). Further downdip, there is a transition from fully locked to aseismic sliding.[10]

In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters (0.8 inches) over a 50 kilometer by 300 kilometer (about 30 mile by 200 mile) area. The movement was the equivalent of a 6.7 magnitude earthquake.[11] The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.[12]

In 2004, a study conducted by the Geological Society of America analyzed the potential for land subsidence along the Cascadia subduction zone. It postulated that several towns and cities on the west coast of Vancouver Island, such as Tofino and Ucluelet, are at risk for a sudden, earthquake initiated, 1–2 m subsidence.[13]

San Andreas Fault connection[edit]

Studies of past earthquake traces on both the northern San Andreas Fault and the southern Cascadia subduction zone indicate a correlation in time which may be evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. The 1906 San Francisco earthquake seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake.[14]

The Cascadia Subduction Zone (CSZ) is a 1,000 km (620 mi) long dipping fault that stretches from Northern Vancouver Island to Cape Mendocino in northern California. It separates the Juan de Fuca and North America plates. New Juan de Fuca plate is created offshore along the Juan de Fuca Ridge.[7][8]

The Juan de Fuca Tectonic Plate moves eastward, toward, and eventually is shoved beneath, the continent North American Tectonic Plate? (A better way to state this, for clearer understanding, would be to state that the western moving North American Tectonic Plate butts up against the Pacific Plates, and rises above them. evidenced by the GPS mapping of mountain peaks rising, from the Olympic Peninsula in Washington State all the way north to the mountains north-west of Campbell River. RISING MOUNTAINS are NOT A GOOD THING. Eventually, they will DROP back down, most likely in five to twelve minutes, during the Cascadia Megaquake). In between five to twelve minutes, the North American west coast will all drop down one to three metres. Places like Chesterman Beach and Long Beach in British Columbia will liquefy and Tofino and Ucluelet, BC will also drop down one – three metres, below the level of the Pacific Ocean. In addition, the incoming Megatsunami as a result of the Cascadia Megaquake could be 12 stories (120 feet) high.

The North American Tectonic Plate only surfaces offshore about 80 kilometre, under the Pacific Ocean at what has been called the Cascadia Subduction Zone, The zone separates the Juan de Fuca Plate, Explorer Plate, Gorda Plate, and North American Plate. Here, the oceanic crust of the Pacific Ocean has been sinking beneath the continent for about 200 million years, and currently does so at a rate of approximately 40 mm/yr.[7][8] “Sinking beneath the continent mis-characterizes and does not lead to greater clarity”. My opinion, starting about 200 kilometres east of the 'Cascadia Fault' and about 160 kilometres inland to the east, the western moving North American Tectonic Plate butts up against the Pacific Plates, and rises above them.

At depths shallower than 30 km (19 mi) or so, the Cascadia zone is locked by friction while strain slowly builds up as the subduction forces act, until the fault's frictional strength is exceeded and the rocks slip past each other along the fault in a megathrust earthquake. Below 30 km (19 mi) the plate interface exhibits episodic tremor and slip.

The width of the Cascadia subduction zone varies along its length, depending on the angle of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As the edge of the plate sinks and becomes hotter and more molten, the subducting rock eventually loses the ability to store mechanical stress; earthquakes may result. On the Hyndman and Wang diagram (not shown, click on reference link below) the "locked" zone is storing up energy for an earthquake, and the "transition" zone, although somewhat plastic, could probably rupture.[9]

The Cascadia subduction zone runs from triple junctions at its north and south ends. To the north, just below Haida Gwaii, it intersects the Queen Charlotte Fault and the Explorer Ridge. To the south, just off of Cape Mendocino in California, it intersects the San Andreas Fault and the Mendocino Fracture Zone at the Mendocino Triple Junction.

Cascadia earthquake sources

Earthquake effects[edit]

Megathrust earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude9.0. They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture known as a megathrust earthquake. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. The Cascadia Subduction Zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches from mid-Vancouver Island to Northern California.[10]

San Andreas Fault connection[edit]

Earthquake timing[edit]

Great earthquakes

estimated year		interval
2005 source[15]	2003 source[16]	(years)
about 9 pm, January 26, 1700 (NS)		780
780–1190 CE	880–960 CE	210
690–730 CE	550–750 CE	330
350–420 CE	250–320 CE	910
660-440 BCE	610–450 BCE	400
980–890 BCE	910–780 BCE	250
1440–1340 BCE	1150–1220 BCE	unknown

The last known great earthquake in the northwest was the 1700 Cascadia earthquake. Geologicalevidence indicates that great earthquakes (> magnitude 8.0) may have occurred sporadically at least seven times in the last 3,500 years, suggesting a return time of about 500 years.[5][2][3] Seafloor core evidence indicates that there have been forty-one subduction zone earthquakes on the Cascadia subduction zone in the past 10,000 years, suggesting a general average earthquake recurrence interval of only 243 years.[6] Of these 41, nineteen have produced a "full margin rupture," wherein the entire fault opens up.[5] By comparison, similar subduction zones in the world usually have such earthquakes every 100 to 200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.[17]

There is also evidence of accompanying tsunamis with every earthquake. One strong line of evidence for these earthquakes is convergent timings for fossil damage from tsunamis in the Pacific Northwest and historical Japanese records of tsunamis.[18]

The next rupture of the Cascadia Subduction Zone is anticipated to be capable of causing widespread destruction throughout the Pacific Northwest.[19]

Forecasts of the next major earthquake[edit]

Prior to the 1980s, scientists thought that the subduction zone did not generate earthquakes like other subduction zones around the world, but research by Brian Atwater and Kenji Satake tied together evidence of a large tsunami on the Washington coast with documentation of an orphan tsunami in Japan (a tsunami without an associated earthquake). The two pieces of the puzzle were linked, and they then realized that the subduction zone was more hazardous than previously suggested.

In 2009, some geologists predicted a 10% to 14% probability that the Cascadia Subduction Zone will produce an event of magnitude 9.0 or higher in the next 50 years.[20] In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher[21][22]

Geologists and civil engineers have broadly determined that the Pacific Northwest region is not well prepared for such a colossal earthquake. The earthquake is expected to be similar to the 2011 Tōhoku earthquake and tsunami, because the rupture is expected to be as long as the 2004 Indian Ocean earthquake and tsunami. The resulting tsunami might reach heights of approximately 30 meters (100 ft).[20] FEMA estimates some 13,000 fatalities from such an event, with another 27,000 injured. It predicts that a million people will be displaced, with yet another 2.5 million requiring food and water. An estimated 1/3 of public safety workers will not respond to the disaster due to a collapse in infrastructure and a desire to ensure the safety of themselves and their loved ones.[6] Other analyses predict that even a magnitude 6.7 earthquake in Seattle would result in 7,700 dead and injured, $33 billion in damages, 39,000 buildings largely or totally destroyed, and 130 simultaneous fires.[4]

The Cascade Volcanic Arc is a continental volcanic arc that extends from northern California to the coastal mountains of British Columbia.[1] The arc consists of a series of Quaternary age stratovolcanoes that grew on top of pre-existing geologic materials that ranged from Miocenevolcanics to glacial ice.[1] The Cascade Volcanic arc is located approximately 100 km inland from the coast, and forms a north-to-south chain of peaks that average over 3,000 m (10,000 ft) in elevation.[1] The major peaks from south to north include:

Lassen Peak and Mt. Shasta (California)
Crater Lake (Mazama), Three Sisters, Mt. Jefferson, Mt. Hood(Oregon)
Mt. Adams, Mount St. Helens, Mt. Rainier, Glacier Peak, Mt. Baker (Washington)
Mt. Garibaldi and Mt. Meager massif (British Columbia)

The most active volcanoes in the chain include Mount St. Helens, Mt. Baker, Lassen Peak, and Mt. Hood. St. Helens captured worldwide attention when it erupted catastrophically in 1980.[1] St. Helens continues to rumble, albeit more quietly, emitting occasional steam plumes and experiencing small earthquakes, both signs of continuing magmatic activity.[1]

Most of the volcanoes have a main, central vent from which the most recent eruptions have occurred. The peaks are composed of layers of solidified andesitic to dacitic magma, and the more siliceous (and explosive) rhyolite.

The volcanoes above the subduction zone include:

Cascadia Volcanic Arc © ™ ® (Part of Pacific Ring of Fire), Intelligent Grandfather's Guide © ™ ®

Sunday, November 4, 2018

Volcanoes, Plate Tectonics, and Earthquakes

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Below Image: Kulshan Stratovolcano© / Mount Baker Stratovolcano©