Geological analysis of subduction zones
Geological analysis of subduction zones
New studies of the San Andreas fault by geologist has been around for 30 years. Early investigations focus mainly on the short segments of the fault because only limited data were available (Griscom, Jackens C9). The concern of the studies are due to the recognition of the San Andreas fault to be the sources of the strong earthquake in San Francisco on 1906 and at nearby areas 1857 and 1989 (Griscom, Jackens C9).
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As the basis of the study the investigator used gravitational and magnetic systems to measure and gather important data. “Gravity and magnetic data reflect respectively the density and magnetization of the rocks beneath the surface” (Griscom, Jackens C9)
Recent studies of geologic and geophysical data for the San Andreas fault system suggests that the eastward migration of the plate boundary was due to a series of eastward jumps of the Mendocino Triple Junction. “The history of this triple junction is one of successive eastward jumps, with sustained periods at each position while significant strike-slip motion occurred on the various transform fault systems, including the San Andreas fault.” (Griscom, Jackens C9) . According to the investigation done by Griscom and Jackens, the triple junction which is situated in the north end of the early Pilarcitos fault extension have jumped eastward of more than 100 km to the Andreas fault which is now 32o20′ N of the North American plate. Furthermore, above the south edge of the Juan de Fuca plate, the North American plate which has a shape of a thin lip that gradually thickens eastward of possibly only about 30 km at the Coast Range fault. They found out that south of the Juan de Fuca plate, asthenospheric material that filled the slab window should lie beneath the North American plate at a depth comparable to that of the upper surface of the Juan de Fuca plate. They point out that because the North American plate has been moving relatively southward across this boundary for many millions of years, the top of the asthenosphere probably is shallow beneath much of the Coast Ranges in central California, and the thin west lip of the North American plate may be decoupled from much of the mantle, although some under-plating by mantle material is likely.
The studies further concludes that the deformation along the boundaries of the Pacific and North American plates takes place over a zone 50 to 100 km because of the decoupled lithosphere. This is wider than previously believed that it’s just within the San Andreas fault. This is because “the lithosphere of southern California near the San Andreas fault system is thin and may be decoupled from much of the mantle which is relatively thin” (Griscom, Jacken C9), and the reason why the plate boundary has been able to migrate eastward from the base of the continental slope to its present position at the San Andreas fault. Griscom and Jacken explains it may also both why certain structural blocks southwest of the fault in southern California have been able to rotate clockwise by as much as 70°-90° during and after the Miocene and how extensional basins formed between these blocks.
Furthermore, they explained that it can help why the seismicity of the San Andreas fault generally does not extend below 12-km depth. The gravitationally predicted depth to the base of the North American plate is within the limits of at least within 70 km of the San Andreas fault. If given a velocity of 5 cm/year for the movement of the Pacific plate relative to the North American plate, the factor for heat flow should increase by 2 (approximately 200 km south of the edge of the Juan de Fuca plate. This is because it requires 4Ma for the heat anomaly to reach a surface of 20 km depth.
How and why the San Andreas transform boundary formed and developed replacing at least a part of the once-continuous subduction zone maybe explained by strike-slip movement along faults of the San Andreas system million years ago. According to the investigation done by Griscom and Jackens physical properties of these narrow fault zones are different from those in surrounding rock masses. They said that the difference were due to the existence of fractured or pulverized rock, exotic rock slivers in the fault zone that may have been transported along the fault from other places.
The study shows that there were three lithospheric plates that meet at the Mendocino Triple Junction. These three are Juan de Fuca, Pacific & North American. This is were the trench meets and to transform faults. “Along this trench to the north, the Juan de Fuca plate is subducting eastward beneath the North American plate” (Griscom, Jackens C9). Along the trench going north, the Juan de Fuca plate is subducting to the east beneath the North American plate. “As this incipient San Andreas transform fault lengthened over time, eastward subduction continued to the north of the migrating triple junction”(Griscom,Jackens C9).
The fate of the Juan de Fuca and other neighbors are determined by the recent and present geologic evidence. The geologic patterns of geologic evolution shows a great possibility of seismic destruction in the future. “The earthquake affected coastal areas of British Columbia, Washington and Oregon and is recorded in oral histories of native peoples. The geology of earthquakes on the Juan de Fuca subduction zone is reviewed, along with the evidence that there was a major quake in 1700, that there have been others in previous centuries, and that there will be more in the future.”(Earle 1).
According to Earle the Juan de Fuca plate are locked with the North American plate and it being deformed, compressed laterally, and roughly pushed down and forced up in some areas. He added that when the amount of strain built up in the rocks of the crust exceeds the friction on the locked zone an earthquake will occur. Both Juan de Fuca and Gorda plates are being formed along their respective spreading ridges and are being pushed (“subducted”) underneath the North American Plate along the subduction zone.
The Juan de Fuca Plate is subducting at a rate of around 4.5 cm/year. This is an indication of further movement of the plate as in the evolution patterns.
As the Gorda, Juan de Fuca, and Explorer plates descend along the subduction zone, they are warmed by heat flowing from the surrounding mantle. The upper parts of the plates carry water in fractures, seafloor sediments, and the altered minerals of the oceanic lithosphere itself. As the plates heat up this water is expelled and rises into the “wedge” of asthenosphere that lies above the subduction zone. The presence of the water lowers the melting temperature of the asthenospheric rock and enables it to partially melt to produce a variety of basalt and basaltic andesite magmas. At depths of about 80 to 100 kilometers the high temperature of the asthenosphere enables perhaps 20 to 30% of the peridotite there to melt in the presence of water. When this melt becomes sufficiently abundant it separates itself from the surrounding partially-molten peridotite and rises buoyantly towards Earth’s surface, forming the magmas that sustain volcanism in the High Cascades.
Advanced Geologic group like the Pacific Geoscience Centre of the Geological Survey of Canada monitors defomation of the crust using a series of GPS stations. An example of this at Ucluelet on the west coast of Vancouver Island. It is moving eastward with respect to the rest of the North American Plate, at a rate of close to 11 mm per year and being pushed down to almost 2mm/year. In other words, at Ucluelet the plate is being deforned or squeezed because the North American Plate is locked against the descending Juan de Fuca Plate.
Another indication of the evolution pattern and the extent of the subduction is estuary of the Copalis River in southwestern Washington. The land along the coast in this region subsided as a result of the 1700 earthquake. The area was then flooded with seawater, killing the trees and other vegetation. Also, the abandoned native fishing camp near to Lincoln City, Oregon. The fire pits were orginally dug into dune sand near to the beach. Over subsequent decades they were naturally filled in and covered with soil. Sand from the tsunami associated with the 1700 earthquake then covered up the topsoil. The tsunami sand has since been covered with tidal mud.
In Port Alberni on Vancouver Island. The large water pipeline crossing the view was partly destroyed by the tsunami generated by the huge 1964 Alaska earthquake. Sand from that tsunami is present at a depth of about 20 cm in the tidal flat area on the near side of the pipeline. Another layer of tsunami sand, derived from the 1700 earthquake, is present at depths of 50 to 70 cm.
Excavations into both terrestrial soil profiles and marine sediments have shown evidence of numerous past earthquakes on the Juan de Fuca subduction zone. Based on the various data sets it is evident that the average repeat time for earthquakes in this area is around 500 years. The maximum time is probably around 1000 years and the minimum time may be as little as 200 years.
It has been 300 years since the last major earthquake on this subduction zone. There could be another such quake tomorrow, or it may not come for several hundred more years.
The recent studies on the East Pacific Rise and the Juan de Fuca Ridge have emphasized the rapid rates and magnitudes of changes observed in hydrothermal systems in response to volcanic and tectonic events.
A major breakthrough to conduct such studies occurred in 1993 when a real-time acoustic monitoring system for low-level seismicity along the Juan de Fuca Ridge using a network of permanent deep-ocean hydrophone arrays (SOSUS) owned by the U.S. Navy became operational where seismic event was detected north of Axial Seamount.
Griscom A., Jachen R.,(1991) “CRUSTAL AND LITHOSPHERIC STRUCTURE
FROM GRAVITY AND MAGNETIC STUDIES”, Chapter 9,[online], http://education.usgs.gov/california/pp1515/chapter9.html
Earle, S., (2000),”The 1700 Juan de Fuca Earthquake”, Malaspia University-College , [online], http://www.mala.bc.ca/~earles/1700quake/1700p6.htm