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| Tectonics of the Cascadia Subduction zone | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Abstract | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� The Pacific Northwest is threatened by the Cascadia subduction zone, which is occurring between the Juan de Fuca and the North American plates.� The Juan de Fuca Plate is subducting hot young oceanic lithosphere underneath the North American Plate at an approximate rate of 4.5 cm/year.� Although the Cascadia Subduction Zone is relatively quiet, it shows the characteristics of exhibiting strong interplate earthquakes.� Seismicity is high, 90% of all of the Pacific Northwest earthquakes, in the overriding North American and the Juan de Fuca plate in northwestern Washington.� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Introduction | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� The objective of this paper is to understand the tectonics of the Cascadia subduction zone in the Pacific Northwest.� It is very important to understand what happens in the Cascadia subduction zone because it is very active tectonically and it cause potential tectonic problems.� The plate tectonics of the Pacific Northwest is characterized by having an oblique ocean/continent subduction zone, where the Juan de Fuca plate is subducting underneath the North American plate, which is lower in density than the oceanic crust of the Juan de Fuca plate (Clague 1997). The top of the down-going Juan de Fuca plate is at a depth of 30 kilometers along the coast and 60-70 kilometers depth on the eastern part of the Cascadia subbduction zone (Currie et al. 2001).� �It has produced a geologically complex convergent margin.� The spreading rate along the Juan de Fuca ridge has been about 50-65 mm/year since about 20 million years ago.� It is apparent that there has been a shift in the Juan de Fuca-Pacific relative motion by 10-15o in the last 5.9 million years. According to various modals, the Moho is at 34-36 km depth and dipping at about 7o to the east-southeast. � | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� At the edge of the Juan de Fuca plate, there is the Fuan de Fuca trench, which is a minor-sized spreading zone in the Pacific Ocean and located several hundred kilometers offshore of the Pacific Northwest (Clague 1997).� However, it is not highly visible due to the sediments from the Columbia River.� A good amount of sediment is drained and accumulated into the trench off the coast of the Pacific Northwest (Parsons et al. 1998).�� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Seismic reflection | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� According to many scientific studies, the continental Moho is projected at a depth of 36 km in the eastern portions of the Pacific Northwest.� The Moho then persists seaward to approximately 40 km, which then disappears.� At the depth of 45 km, the oceanic Moho of the Juan de Fuca Plate dips steeply underneath the North American Plate (Bostock et al. 2002).� The Moho of the Juan de Fuca Plate is at a depth of 34-36 km (Tr�hu et al. 2002).� � | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Seismic refraction | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� The velocity of the upper crust in the North American Plate has been calculated to be 50 km/s.� The Moho is at about 7-8 km/s.� According to various scientific studies, the subducting lithosphere of the Juan de Fuca plate has a calculated velocity range of 6.5 km/s to 8.0 km/s (Tr�hu et al. 1994).� This indicates that velocity of the subducting slab of the Juan de Fuca Plate increases as it goes further in depth.� The oceanic crust of the subducting slab lies underneath of the Oregon Coast Range and western Willamette Valley (Tr�hu et al. 1994).� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Earthquakes | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� Normal stresses are driving the Juan de Fuca Plate into the North American Plate, which include the ridge push, slab pull, transform pull, and shear resistance (Govers and Mijer 2001).�� The earthquake activity is common in Washington and northern California but infrequent in Oregon (Fleming and Tr�hu 1999).� Large earthquakes tend to occur along the Cascadia plate boundary every 600 years (Peterson et al. 2002). Earthquakes can occur at depths at 100 km and within the mantle (Creager et al. 2002).� The earthquakes can occur within both the crustal and mantle portions of the old subducted Pacific plate.� However, in contrast, the width of the Cascadia seismogenic zone is very narrow at about 5 kilometers (Creager et al. 2002).� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| There are three types of earthquakes that occur in the Pacific Northwest.� Continental crustal earthquakes occur to a depth of 30 kilometers, whereas, any other earthquake that occurs at 80 kilometers is considered to be in the subduction zone (Zhao et al. 2000; Clague 1997). There are also earthquakes that occur in the subducting oceanic crust (Cassidy and Waldhauser 2002).� �Lateral faults have been identified in the accretionary wedge and some of them have cut into the Juan de Fuca basement (Govers and Mijer 2001).� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� Historic records suggest that large tsunamis can occur.� For instance, in January 1700, an earthquake with the magnitude of 9 occurred and caused huge tsunamis in Japan (Murray and Lisowski 2000; Savage et al. 2000).� This earthquake indicates that the plate is locked at shallow depths at approximately 2 km beneath the sea floor at the Juan de Fuca trench (Savage et al. 2000) and 20 kilometers of the North American plate boundary (Thatcher 2001; Williams and McCaffrey 2001).��� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Discussion | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| There is definitely some major plate tectonic dynamics occurring in the two plates.� It appears to be some possible constraints occurring between the Juan de Fuca plate and the North American plate.� There are potential constraints between the Juan de Fuca plate and the Pacific plate.� There is a mid-ocean ridge, Juan de Fuca ridge, occurring between the Pacific Plate and the Juan de Fuca plate.� This mid-ocean ridge creates more oceanic crust of the Juan de Fuca plate.� At the present time, the oceanic crust is about 5 Ma. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| The computerized cross-sections illustrate that there is some constraints in the subduction zone.� The accretionary wedge can act as elastic band when an earthquake occurs, it can flex up and down.� In Cross-section 3, the majority of the earthquakes are shallow and within the oceanic and crustal layers of the Cascadia subduction zone.� However, some earthquakes can occur within the subducting asthenosphere of the Juan de Fuca plate.� According to Table 1, the average earthquake depth is slightly under 6 km. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Figure 1 shows that certain portions of the Pacific Northwest are experiencing active seismic activity.� These portions include the northern Cascades and the northern part of California.� This is due to where the subduction zone is experiencing intense movements from the Juan de Fuca plate.� Oregon, on the other hand, is more silent in terms of tectonic activity.� Strong earthquakes can occur in Oregon but overall it is relatively aseismic. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Figure 2 illustrate variations of reflection in the Cascadia subduction zone.� In Figure 2a, the thin reflection lines are due to the responses from the subduction zone underneath the two respective crustal layers in the North American plate and the Juan de Fuca plate.� The thick lines in Figure 2b are from the northern portion of the Cascadia subduction zone near Vancouver, in respect to the oceanic crust rather the continental crust.� Figure 3 illustrate that the majority of the Cascadia subbduction zone has low velocities in the crust layer.� This is probably due to low crustal depths and high densities in the crust.� This low velocity profile is most likely from the oceanic crust of the Juan de Fuca plate, which is located off the coast of Oregon and Washington.� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| The northern portion of the Cascadia subduction zone shows a different slope in its subduction of the Juan de Fuca when comparing it the southern portion.� Figure 4 shows a more gradual slope in the northern portion of the Cascada subduction zone.� According some data, the southern portion of the Cascadia subduction zone has a sharper slope than the northern portion.� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� Figure 5 illustrates the locked zone off the coast of Oregon and Washington is not due to slipping but frictional resistance on the fault interface (Thatcher 2001) and it recovers after earthquakes (Wang et al. 2000).� Table 1 illustrate that the majority of the larger earthquakes occurred before 1970.� However, some of the recent earthquakes can be classified as moderate to large earthquakes but they are fewer in number than the pre-1970 period (Rogers and Crosson 2002).� The rate of moderate-sized earthquakes is currently very low in the Cascadia subduction zone for the past 135 years (Peterson et al 2002).� � | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� Almost 90% of the Pacific Northwest earthquakes occurred in the state of Washington.� This indicates that the ocean/continent subduction zone underneath the state of Washington is experiencing more tectonic constraints than under Oregon and British Columbia.� Also, throughout history, more volcanic eruptions have occurred in Washington than any part of the Pacific Northwest.� The majority of these eruptions are from Mt. St. Helens, which has been classified as the most active volcano in the Cascades.� From the earthquake record, it was indicated that the majority of the deep earthquakes occur underneath the state of Washington.� The cross section illustrates that the subducting slab is at an approximate depth of 20-80 km.� As a result, the intermediate and deep earthquakes in table 1 occurred in the subducting slab of the Juan de Fuca Plate. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| more images | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Conclusion | images (figures 1-5) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ����������� The Cascadia subduction zone has the tendency to be unpredictable in terms of its tectonic activity.� There is no telling when an earthquake occurs or a volcano erupts.� However, earthquakes and volcanoes illustrate what is happening with the subduction zone that is underneath the Pacific Northwest.� Otherwise, it can be difficult to determining what is occurring with the Cascadia subduction zone.� | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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