Ocean Networks Canada - subduction

Tsunami alert follows 8.2 quake off Chile

dwowens@uvic.ca — Wed, 02 Apr 2014 04:25:42 +0000

On April 1 at 4:46:45 PM Pacific Daylight Time (23:46:45 UTC), a magnitude 8.2 earthquake occurred off Chile's Pacific coastline, according to the US Geological Survey. Ocean Networks Canada instrumenatation captured both ground shaking and a very small tsunami as they crossed the northeast Pacific.

Map of the epicentre and 16 aftershocks along the subduction zone between the Nazsca and South American plates, 1 April 2014. Data provided by USGS and plotted using Google Earth. (Click to enlarge.)

At a depth of 20.0 km below the seabed, the shallow near-field quake struck 86 km northwest of the mining area of Iquique, hitting a region that has been rocked by numerous quakes over the past two weeks. According to the USGS, this earthquake occurred as the result of thrust faulting at shallow depths near the Chilean coast. The location and mechanism of the earthquake are consistent with slip on the primary plate boundary interface, or megathrust, between the Nazca and South America plates. In this area, the Nazca plate subducts eastward beneath the South America plate at a rate of 65 mm/yr. Subduction along the Peru-Chile Trench to the west of Chile has generated the uplift of the Andes mountain range.

Ocean Networks Canada's seismometer in Cascadia Basin recorded the tremors as they crossed the North Pacific. Seismic data clearly indicate arrival of the initial P waves approximately 750 seconds (12.5 minutes) after the earthquake, and following S waves about 1375 seconds (23 minutes) after the earthquake struck. Bottom Pressure Recorders on the NEPTUNE Observatory also detected passage of the tsunami in real time, as it crosses our observing stations in the northeast Pacific.

Data from the Cascadia Basin ocean-bottom seismometer indicating arrival of P and S waves. The top trace shows East-West motions, the centre trace shows North-South motions, and the lower trace shows vertical motions. (Click to enlarge.)

A 1.9-metre tsunami was recorded at a northern Chilean port Tuesday evening. The Pacific Tsunami Warning Center issued an alert for all of Latin America's Pacific coast. There was no threat issued to the Pacific coast along North America.

NOAA issued a forecast of tsunami heights as the energy propagated away from the source region, indicating heights up to 100 cm close to the epicenter, with rays of 2-10 cm wave heights extending across portions of the South Pacific Ocean toward New Zealand and archipelegos in the South-Central Pacific.

Tsunami wave energy propagation forecast issued by the Pacific Tsunami Warning Center, showing contours of maximum wave amplitudes (in cm) associated with the 1 April 2014 earthquake.

Travel times for tsunami propagation were also modeled by the National Tsunami Warning Center, with expected arrival of a small (2-4 cm) tsunami in coastal British Columbia beginning 15 hours after the event, around 7:00 AM Pacific Daylight Time.

Predicted travel times for tsunami waves generated by the 1 April 2014 earthquake. Three-hour intervals are marked by the heavy white lines, intermediary hours are marked by blue shades and dashed white lines indicate half-hour boundaries. Arrival in both New Zealand and British Columbia was predicted to begin approximately 15 hours after the initial earthquake.

Seafloor pressure traces from the CORK pressure instrument at Cascadia Basin, 1-2 April 2014. The upper plot shows initial passage of the earthquake just past 00 UTC (indicated by the blue lines), followed by passage of the small tsunami beginning at 14 UTC. The lower plot focuses on the tsunami onset period, 11-15:20 UTC. Wave amplitudes at this deep-water site (2660 m) were approximately 8 mm.

No stranger to seismic activity, Chile is one of the world¹s most earthquake-pronecountries. In 2010, a magnitude-8.8 quake and ensuing tsunami in central Chile killed more than 500 people and destroyed several hundred thousand homes along the coast.

Northern California Earthquake Strikes Southern Cascadia Subduction Zone

dwowens@uvic.ca — Mon, 10 Mar 2014 16:26:38 +0000

10 March 2014, 05:18 UTC (10:18 pm local time) a magnitude 6.8 earthquake struck the southern end of Cascadia subduction zone. The epicentre location of the earthquake was about 50 km west of the California coast. During the subsequent few hours many aftershocks followed in the same area (see map).

This map shows the location of the main shock and aftershocks at the Mendocino Triple Junction between the North America, Pacific and Gorda tectonic plates.

Ocean Networks Canada's NEPTUNE observatory is about 900 km to the north, and its seismometers in Cascadia Basin, Barkley Canyon and Clayoquot Slope recorded the powerful main shock as well as some of the larger (up to magnitude 4.6) aftershocks about 2 minutes after the event (see seismic recordings below).

NEPTUNE seismic recordings of the 10 March 2014 earthquake. Three components, North (HHN), East (HHE) and Vertical(HHZ) of 3 seismometers show the various earthquake signals arriving at different times. The top 3 traces were recorded by the ocean bottom seismometer in Cascadia Basin; the second set of 3 traces are from the Barkley Canyon seismometer and the lowest 3 traces are from the seismometer at Clayoquot Slope The green bar indicates the origin time of the main shock; about 2 minutes later the fast travelling P-waves (indicated by the red bars) arrive at the NEPTUNE stations, and almost 2 minutes later the S-waves arrive (indicated by the yellow bars), immediately followed by low-frequency surface wave shaking. About 10 minutes after the main shock, high frequency water sound arrives, as clearly visible in the top two panels. (Click to enlarge.)

The southern end of the Cascadia subduction zone terminates at the northern end of the San Andreas fault, meeting at the Mendocino triple junction between three tectonic plates, the North American plate in the east, the Pacific plate in the south, and the Gorda plate to the north. The San Andreas fault zone compensates mostly the lateral motion between the North American and the Pacific plate, whereas the Cascadia subduction thrust fault takes up the motion of the Juan de Fuca plate going down (subducting) below North America.

The 10 March earthquake occurred at a depth of around 16.6 km, and its fault mechanism was taking up mostly lateral motion from the interplay between the Pacific and the Gorda plate. It was not a subduction thrust earthquake and therefore did not generate any tsunami. However, as every earthquake redistributes tectonic stresses in the area (hence the many aftershocks), we will keep an extra eye out for any other seismicity in that region.

Negligible Tsunami From Alaska Earthquake

dwowens@uvic.ca — Sat, 05 Jan 2013 08:00:00 +0000

A strong magnitude 7.5 earthquake struck west of Craig, Alaska at 12:58AM PST, 5 January 2013. Tsunami warning and alerts were issued for a broad section of the Alaskan and Canadian coastline, but no damaging waves were generated. According to the USGS, this earthquake was likely "related to that Haida Gwaii earthquake three months previously, and is an expression of deformation along the same plate boundary system."

The following plots show tide data from Port Alexander, AK (upper-right) and pressure data (lower-left) from Ocean Networks Canada CORK and bottom pressure recorder instruments at three stations on our network. Tide data indicate a crest-to-trough wave amplitudes of 20-30 cm at Port Alexander, with the initial waves arriving 41 minutes after the earthquake. The pressure plots show strong shaking at the seafloor (indicated by the blue areas) at all three instrument locations. The red lines within the blue areas are low-pass filtered versions of the blue lines (only signals with periods longer than about a minute are shown). Therefore, the relatively high frequency ground shaking and microseismic noise generated by ocean swells do not show up on the redline, but tsunami waves with a period of 10-20 minutes, if present, should be indicated by the red line. A tsunami was not detected by Ocean Networks Canada scientists who examined these data.

The tsunami generated by this earthquake was not nearly as large and devastating as those that struck Sri Lanka, Indonesia and Japan in recent years, because it likely occurred on a strike-slip (lateral) fault. Vertical displacement for strike-slip earthquakes is typically much less than may be expected from a major subduction earthquake. The following short video illustrates the distinction between strike-slip and subduction earthquakes.

For scientists like those at the Pacific Geoscience Centre and other institutions studying tsunami propagation in the northeast Pacific, this event will provide valuable insights.

Haida Gwaii Earthquake and Tsunami

rlat@uvic.ca — Sat, 27 Oct 2012 07:00:00 +0000

A powerful magnitude 7.7 earthquake struck central Moresby Island in the Haida Gwaii archipelago at 8:04PM PDT, 27 October 2012. Residents along the west coast—from Alaska to Vancouver—also felt several aftershocks up to magnitude 5.8. However, no major damage or injuries were reported. This was the largest temblor to hit Canada since 1949, when an 8.1-magnitude quake hit west of the Queen Charlotte Islands, in the same area.

The following plots show both seismic and bottom pressure data collected by Ocean Networks Canada instrumentation, located approximately 600 km south of the earthquake epicenter. The light-blue plots at upper-left illustrate changes in pressure, as measured by bottom pressure recorders at four node locations. In the top three plots, initial strong blue signatures, beginning at 3:04 UTC, indicate shaking of the seafloor as seismic energy passed through the region. As this energy dispersed, the recordings indicate changes in sea level above each bottom pressure recorder as waves emanating from the disturbance traveled across the ocean's surface. The fourth plot shows bottom pressure anomalies at Folger Passage, a shallow (100 m) near-shore station where surface swell typically conceals tsunami waves from easy detection by the casual viewer.

The lower five plots, in dark blue above, illustrate seismic energy as measured by four Ocean Networks Canada seafloor seismometers and one land-based seismometer located in Bella Bella on British Columbia's central coast. Earthquake onset is clearly indicated in all five plots, with Bella Bella recording the earliest onset, because it is located much closer to the earthquake epicenter, 265 km to the southeast. The Bella Bella seismometer is part of the Canadian National Seismograph Network, maintained by National Resources Canada's Pacific Geoscience Centre.

Numerous aftershocks were also detected following the initial earthquake, with over 50 magnitude 4+ aftershocks recorded in the initial 16 hours. Most of these aftershocks occurred beneath the seafloor, as indicated in the following USGS map.

Tsunami warnings originally issued for a large stretch of the North and Central coast, as well as the Haida Gwaii region and eastward to Hawaii, were later cancelled or downgraded. One wave that hit Langara Island, northern-most island in the Haida Gwaii archipelago, measured 69 cm. Tsunami wave heights as high as 76 cm were recorded in Kahului, Maui, HI, and harbour oscillations up to 1.2 m were measured in Hilo, HI.

The tsunami generated by this earthquake was not nearly as large and devastating as those struck Sri Lanka, Indonesia and Japan in recent years, because it occurred on a strike-slip (lateral) fault. Vertical displacement for strike-slip earthquakes is typically much less than may be expected from a major subduction earthquake. The following short video illustrates the distinction between strike-slip and subduction earthquakes.

For scientists like those at the Pacific Geoscience Centre and other institutions studying tsunami propagation in the northeast Pacific, this event will provide valuable insights. The tsunami generated by this earthquake was similar in size to other tsunamis, such as last year's Japan tsunami and the Chilean tsunami of February 2010, when they reached the northeast Pacific. Although tremendously large in their source regions, these tsunamis diminished significantly after crossing large ocean basins to reach coastal British Columbia. The Haida Gwaii tsunami is the first regional tsunami tracked by Ocean Networks Canada's sensor network; its local source region is expected to result in different wave responses in BC's coastal embayments. By comparing these two types of tsunamis (distant vs. local), scientists can begin to piece together a picture of how BC coastal zones may react to large tsunamis originating in this region.

Cascadia Subduction Zone

dwowens@uvic.ca — Mon, 15 Oct 2012 07:00:00 +0000

A subduction zone (Bebout et al., 1996) is an area where two plates are converging, with one plate moving beneath the other. As the down-going (subducting) plate moves deeper, it transports water into depth where it is heated and released. The heat from the mantle and core causes the surrounding rocks to melt and become fresh magma for volcanic eruptions. The down-going plate is recycled in the Earth’s mantle. At the Cascadia subduction zone the ocean crust of the Juan de Fuca plate is subducting beneath the continental crust of the North American plate. At subduction zones, there usually is an area where the two plates become locked. This means that they are not slipping past each other and frictional stress can build up, storing large amounts of energy. When this stress finally reaches a breaking point, it releases the energy that has been stored resulting in what is known as a “megathrust” earthquake.

The locked zones can hold for hundreds of years as the Cascadia subduction zone has done since 26 January 1700 when the last megathrust earthquake occurred in this area (Satake et al., 2003). The earthquake magnitude was estimated as 9.0 and it resulted in a tsunami that was recorded in Japan. Evidence of this earthquake can be confirmed by geological evidence (land level changes, tsunami traces, turbidite deposits), biological evidence (tree rings), and human records (Native American stories and Japanese records) (Satake et al., 1996; Satake et al., 2003; Satake and Atwater, 2007). Megathrust earthquakes tend to occur in this region approximately every 300-500 years.

References

Bebout, E., Scholl, W., Kirby, H., & Platt, P. (1996). Subduction top to bottom. Geophysics Monograph Series, 96, 384. doi:10.1029/GM096

Satake, K., & Atwater, B. F. (2007). Long-term perspectives on giant earthquakes and tsunamis at subduction zones. Annual Review of Earth and Planetary Sciences, 35(1), pp. 349-374. doi:10.1146/annurev. earth.35.031306.140302

Satake, K., Shimazaki, K., Tsuji, Y., & Ueda, K. (1996). Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700. Nature, 379, pp. 246-250.

Satake, K., Wang, K., & Atwater, B. F. (2003). Fault slip and seismic moment of the 1700 Cascadia earthquake inferred from Japanese tsunami descriptions. Journal of Geophysical Research, 108(B11), pp. 1-17. doi:10.1029/2003JB002521

Ocean Networks Canada - subduction

Tsunami alert follows 8.2 quake off Chile

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Northern California Earthquake Strikes Southern Cascadia Subduction Zone

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Negligible Tsunami From Alaska Earthquake

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Haida Gwaii Earthquake and Tsunami

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Cascadia Subduction Zone

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