Garry C. Rogers

Case for very low coupling stress on the Cascadia Ssubduction Fault

A fundamental problem in plate tectonics is the shear strength of major plate boundary faults. This translates to the question whether the generally observed small earthquake stress drops of 3–10 MPa on major faults release most of the accumulated stress or only a small fraction of it. There is strong evidence that the San Andreas fault, a major transform plate boundary, is weak (<20 MPa shear resistance). It is not yet clear whether subduction thrust faults are also weak. We present two types of evidence from the northern Cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault, comparable to that estimated for the San Andreas fault. First, the well-defined surface heat flow and heat generation allow negligible frictional heating on the plate interface. The average shear stress on the fault must thus be very low over a time scale of a few million years. Second, focal mechanism solutions for small crustal earthquakes in the southern Vancouver Island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress. This inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low. One explanation for the weakness of the fault is the presence of near-lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence. The conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked, accumulating strain energy toward a future great earthquake. The surface deformation responds to the small (<20 MPa) temporal changes of the stress field associated with the subduction earthquake cycle. This transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin. The weakness of the Cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards. For example, a subduction earthquake may induce large strike-slip earthquakes in the forearc that affect a large area.

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as Mw = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements—in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface—are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.

Current deformation and the width of the seismogenic zone of the northern Cascadia subduction thrust

Evidence has been obtained for the accumulation of elastic strain across the northern Cascadia subduction zone that may be released in a future very large subduction thrust earthquake. Vertical and horizontal strain rates across the southern Vancouver Island region have been determined through (1) long-term trends in tide gauge data, (2) changes in repeated accurate leveling surveys, (3) changes in repeated high-accuracy gravity profiles, and (4) horizontal shortening observed in repeated precise positioning surveys. The outer coast is uplifting at a rate of a few millimeters per year decreasing landward, and shortening is occurring across the 100-km-wide coastal region at a rate of about 0.1 microstrain per year (mm km−1yr−1). The results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault. The location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data. Most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern Vancouver Island and northern Washington, provided the locked portion is restricted to a 60-km-wide band offshore beneath the continental shelf and slope. This conclusion also results from modeling the coseismic subsidence on the outer coast of Vancouver Island about 300 years ago deduced from paleoseismicity data. The unusually narrow downdip extent of the subduction thrust seismogenic zone, that extends little if at all beneath the coast, is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments. The high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km. The distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of Vancouver and Victoria from this source. The narrow width may also limit the earthquake size; however, events of magnitude well over 8 are possible.

Spatial‐temporal patterns of seismic tremors in northern Cascadia

[1] We study in detail the two consecutive episodic tremor-and-slip (ETS) events that occurred in the northern Cascadia subduction zone during 2003 and 2004. For both sequences, the newly developed Source-Scanning Algorithm (SSA) is applied to seismic waveform data from a dense regional seismograph array to determine the precise locations and origin times of seismic tremors. In map view, the majority of the tremors occurred in a limited band bounded approximately by the surface projections of the 30-km and 50-km depth contours of the plate interface. The horizontal migration of tremor occurrence is from southeast to northwest with an average speed of 5 km/d. In cross section, tremors in both sequences span a depth range of over 40 km across the interface, with the majority occurring in the overriding continental crust. In particular, 50-55% of them are located within 2.5 km from the strong seismic reflector bands above the plate interface. The lack of vertical migration implies that a slow diffusion process in the vertical direction cannot be responsible for tremor occurrences. The source spectra of tremors clearly lack high-frequency content (>5 Hz) relative to local earthquakes. We propose two possible models to explain the relationship between slip and tremors. The first one regards ETS tremors as the manifestation of hydroseismogenic processes in response to the temporal strain variation associated with the episodic slip along the lower portion of the plate interface downdip from the locked zone. In the second model, tremors and slip are associated with the same process along the same structure in a distributed deformation zone across the plate interface. Neither model can be dismissed conclusively at this stage.

Moment Magnitude–Local Magnitude Calibration for Earthquakes off Canada's West Coast

Local magnitude ( M L ) values of earthquakes off Canada9s west coast are known to be underestimated by at least 0.5 magnitude units compared with other magnitude scales. Moment magnitude ( M w ), derived from moment tensor analysis, provides the most robust estimate of the magnitude of earthquakes. Moment tensor analysis of regional seismic data in western Canada is now possible due to the installation of more than 40 three-component broadband stations in western Canada, the U.S. Pacific Northwest, and southeast Alaska. Moment tensor solutions are now possible down to M ∼4.0. More than 230 regional moment tensor solutions have been calculated off Canada9s west coast at the Geological Survey of Canada for 1995–2002. These solutions, along with 14 previous solutions by Oregon State University in 1994–1995 and 13 Harvard solutions for 1984–1993, allow a systematic M w – M L calibration for earthquakes in this region. The study area extends from the Queen Charlotte Islands region in the north to the area off the west coast of southern Vancouver Island. At the northern end of the study area, where there is little oceanic crust in the source-receiver travel path, M w is systematically larger than M L by 0.28 ± 0.08 magnitude units. At the southern end of the study area, where there is a significant amount of ocean crust in the source-receiver travel path, M w is systematically larger than M L by 0.62 ± 0.08 magnitude units. Calibration of M L with M w will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.

Moment Magnitude–Local Magnitude Calibration for Earthquakes in Western Canada

Local magnitude ( M L ) is the primary magnitude scale calculated for western Canada by the Geological Survey of Canada (gsc). Moment magnitude ( M w ), derived from moment tensor analysis, provides a more robust estimate of the magnitude of earthquakes but is more demanding to calculate. Moment tensor analysis of regional seismic data for earthquakes with magnitudes larger than 3.5 in western Canada is now possible owing to the installation of more than 40 three-component broadband stations in western Canada, the Pacific Northwest of the United States, and southeast Alaska. More than 100 regional moment tensor solutions have been calculated in the Canadian Cordillera and Vancouver Island/Puget Sound region for 1996–2004 at the gsc. These solutions, along with 45 prior solutions, allow the calibration of M w – M L throughout much of western Canada. Continental crust events in the Canadian Cordillera and Vancouver Island/Puget Sound region are found to have M w = M L for earthquakes with M L ≥ 3.6. In contrast, earthquakes located within the subducting slab in the Vancouver Island/Puget Sound region, where there are complex source–receiver travel paths, have M w systematically larger than M L by nearly 0.6 magnitude units. The calibrations of M w with M L are an important result that will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.

An explanation for the double seismic layers north of the Mendocino Triple Junction

We propose that the gently eastward dipping double planed seismic zone observed at 15–25 km depths in the southern Cascadia subduction zone, just north of the Mendocino triple junction, is a direct consequence of the thermally controlled rheology. As the oceanic lithosphere subducts to a depth of about 15 km, the temperature regime causes a brittle-plastic transition to occur within the oceanic crust. Thus, a ductile layer forms in the lower oceanic crust, sandwiched between the brittle upper crust and brittle upper mantle. The very high strain rates near the triple junction caused by the northward push of the Pacific plate on the Gorda plate increase the seismicity and thus accentuate the double seismic zone in this region. This model explains the focal mechanisms observed in the seismic zone and their spatial change. The double seismic layers clearly define the position of the subducting Gorda plate, previously uncertain in the Cape Mendocino region.

A Revised Earthquake Chronology for the last 4,000 Years Inferred from Varve-Bounded Debris-Flow Deposits beneath an Inlet near Victoria, British Columbia

A reanalysis of the varve chronology from hydraulic piston sediment cores was carried out to establish better uncertainty estimates on ages of prehistoric debris-flow deposits (DFDs) in the last 4000 yr. Saanich Inlet is an anoxic fiord located in southeast Vancouver Island near the city of Victoria, British Columbia. It contains annually laminated (varved) marine mud deposited in anoxic conditions. Interlayered with these Holocene varves are massive layers of coarser sediments deposited by submarine debris flows. It has been previously interpreted that these flows were induced by earthquake shaking. Two of the DFDs correspond to known earthquakes: A.D. 1946 Vancouver Island (M 7.3) and the A.D. 1700 Cascadia plate-boundary subduction earthquake (M 9). Based on varve counts, 18 DFDs (310, 410–435, 493–582, 767–887, 874–950, 1001–1133, 1163–1292, 1238–1348, 1546–1741, 1694–1811, 1859–2104, 2197–2509, 2296–2483, 2525–2844, 2987–3298, 3164– 3392, 3654–4569, 3989–4284 yr ago from A.D. 2010 datum) were correlated among two or more cores during this time period, suggesting an average return period of strong shaking from earthquakes of about 220 yr. Nine of the DFDs overlap with the age ranges for great plate-boundary earthquakes that have been determined by other paleoseismic studies: coastal subsidence and offshore turbidity deposits. The remaining nine events give an average return period of about 470 yr for strong shaking from local earthquakes. The peak ground acceleration calculated from a recurrence relation based on statistics from local earthquakes for a 470-yr period is 0.30g, which corresponds to the upper range of Modified Mercalli Intensity (MMI) VII (seven). Historical data from Vancouver Island and other areas show that this level of shaking (MMI VII) is sufficient to trigger submarine landslides.

The northern limit of the subducted Juan de Fuca plate system

Analysis of data recorded at an array of three-component broadband seismograph stations deployed on northern Vancouver Island and the adjacent British Columbia mainland, at the northern end of the Cascadia subduction zone, provides the first constraints on the S wave velocity structure of this region and permits us to define the northern limit of the subducted Juan de Fuca plate system. During a 2-year period, more than 80 teleseisms were recorded at our five stations. The method of receiver function analysis was used to constrain the S velocity structure to upper mantle depths. Beneath the northern three stations, a relatively simple continental crust is interpreted with a well-defined Moho near 37–39 km depth. An upper crustal S velocity discontinuity at these stations is interpreted as the top of the high-velocity rocks of the Wrangellia terrane. In contrast, more complicated structure dominated by pronounced low-velocity zones dipping to the NE are interpreted beneath our southern two stations. The shallower low-velocity zone is 6–8 km thick, has an S velocity contrast of 0.6–1.1 km/s, and lies within the continental crust. This feature is similar to a pronounced low-velocity layer (the E zone) imaged beneath southern Vancouver Island. The deeper low-velocity zone is interpreted as the subducted oceanic crust. We interpret the pronounced change in S velocity structure that we observe as the northern limit of the subducted oceanic plate beneath Vancouver Island. This change coincides with significant changes in topography, heat flow, gravity, and geochemistry.

Characterization of Active Faulting Beneath the Strait of Georgia, British Columbia

Southwestern British Columbia and northwestern Washington State are subject to megathrust earthquakes, deep intraslab events, and earthquakes in the continental crust. Of the three types of earthquakes, the most poorly understood are the crustal events. Despite a high level of seismicity, there is no obvious correlation between the historical crustal earthquakes and the mapped surface faults of the region. On 24 June 1997, a M L = 4.6 earthquake occurred 3–4 km beneath the Strait of Georgia, 30 km to the west of Vancouver, British Columbia. This well-recorded earthquake was preceded by 11 days by a felt foreshock ( M L = 3.4) and was followed by numerous small aftershocks. This earthquake sequence occurred in one of the few regions of persistent shallow seismic activity in southwestern British Columbia, thus providing an ideal opportunity to attempt to characterize an active near-surface fault. We have computed focal mechanisms and utilized a waveform cross-correlation and joint hypocentral determination routine to obtain accurate relative hypocenters of the mainshock, foreshock, and 53 small aftershocks in an attempt to image the active fault and the extent of rupture associated with this earthquake sequence. Both P -nodal and CMT focal mechanisms show thrust faulting for the mainshock and the foreshock. The relocated hypocenters delineate a north-dipping plane at 2–4 km depth, dipping at 53°, in good agreement with the focal mechanism nodal plane dipping to the north at 47°. The rupture area is estimated to be a 1.3-km-diameter circular area, comparable to that estimated using a Brune rupture model with the estimated seismic moment of 3.17 × 10 15 N m and the stress drop of 45 bars. The temporal sequence indicates a downdip migration of the seismicity along the fault plane. The results of this study provide the first unambiguous evidence for the orientation and sense of motion for active faulting in the Georgia Strait area of British Columbia.