Microearthquakes located on the Blanco Fracture Zone with sonobuoy arrays

Johnson, Simon R.; Jones, Paul Roy

doi:10.1029/jb083ib01p00255

Cited by 27 publications

(13 citation statements)

References 19 publications

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“…Spreading is supported by basement doming and igneous intrusions within the basin's sediment fill that were inferred from seismic reflection data [ DeCharon , 1989; Embley and Wilson , 1992]. The basins' inner valley floor is subsiding 1.8 cm/a for the last 6600 years [ Griggs and Kulm , 1973], it is extending at a rate comparable to the Pacific–Juan de Fuca motion rate for the past 12,000 years [ DeCharon , 1989], and it is seismically active (Figure 16) [also Johnson and Jones , 1978]. The depression's morphology with a central basin bordered by inward facing back‐tilted normal fault blocks [ Embley and Wilson , 1992] is consistent with spreading.…”

Section: Discussionmentioning

confidence: 99%

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

2008

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[1] The Blanco Transform Fault Zone (BTFZ) forms the $350 km long Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along-fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull-apart basin. The longest transform ($150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone ($9 km) than other BTFZ transform segments and accommodates the largest (M w 6.4-6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull-apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional ( 15%) compared to transform areas (35-100%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.

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Section: Discussionmentioning

confidence: 99%

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

2008

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“…However, there is a break in the Blanco transform (e.g. Johnson & Jones 1978) that may limit the maximum area and thus the magnitude to a smaller value. Similar unknown breaks or offsets may limit the maximum area for other of the transforms.…”

Section: -0mentioning

confidence: 96%

Seismicity and rates of relative motion on the plate boundaries of Western North America

Hyndman¹,

Weichert²

1983

Geophysical Journal International

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The consistency of earthquake data and plate tectonic models and other estimates of fault slip, may be tested by estimating rates of motion from the earthquakes using the concept of seismic moment. The contribution of individual events may simply be summed, but a generally better estimate of long-term average slip rate is obtained by integration over magnitudefrequency of occurrence relations. Estimates of fault motion rates associated with earthquakes are possible within about a factor of 2 using this approach if the major sources of uncertainty are given careful consideration; e.g. incompleteness and inaccuracies in the earthquake data; empirical moment-magnitude relations and the effect of their stochasticity; fault 'widths' or depth extents; the recurrence relations; maximum magnitudes and the form of truncation at the maximum magnitude. A formulation for the recommended magnitude density truncation is developed. Application of the latter method to the earthquake data of the offshore transform faults of the Juan de Fuca ridge system, the Queen Charlotte fault zone and the northern Vancouver Island area in each case gives good agreement with rates from plate tectonic models. For the southern San Andreas fault and Gulf of California area there is also good agreement with previous estimates obtained by summing the contributions of individual events. However, the displacement rate in the margin convergence zone of southern British Columbia, Washington and Oregon computed from the seismicity is at least a factor of 10 lower than from plate models and from other convergence estimates, and primarily aseismic slip is suggested. The fault motions as a function of time computed from the seismicity records have also been plotted and compared to the longterm average rates. The plots permit estimates of the minimum present accumulated elastic strain, and show if there are any temporal relations among earthquake displacements on different fault zones. htroduc tionA basic understanding of the origin and nature of earthquakes in a region and detailed evaluation of the associated seismic risk requires development of valid tectonic models. Tectonic

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“…The locations for the events were taken from the International Seismological Center (ISC), the United States Geological Survey's Preliminary Determination of Epicenters (PDE), Oregon State University, University of Washington, and University of California, Berkeley earthquake catalogues. The northeastward bias in epicentral locations, readily apparent from events recorded after July, 1963 (Figure 1), is due to inaccurate velocity models assumed for oceanic transform faults (Bergman and Solomon, 1988), and an asymmetric coverage of the northeast Pacific Ocean by land-based seismograph stations (Northrop, 1970;Johnson and Jones, 1978). Although the epicenters are displaced northeastward relative to the transform, it is still apparent from Figure 1 that the trend of the epicenters parallel to the transform is offset in a right-lateral sense through the Cascadia Depression (Embley et aI., 1987).…”

Section: Data and Analysismentioning

confidence: 99%

Relationship between the seismicity and geologic structure of the Blanco Transform Fault Zone

1991

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Abstract. Morphologic studies of an oceanic transform, the Blanco Transform Fault Zone (BTFZ), have shown it to consist of a series of extensional basins that offset the major strike-slip faults. The largest of the extensional basins, the Cascadia Depression, effectively divides the transform into a northwest segment, composed of several relatively short strike-slip faults, and a southeast segment dominated by fewer, longer faults. The regional seismicity distribution (mb -> 4.0) and frequency-magnitude relationships (b-values) of the BTFZ show that the largest magnitude events are located on the southeast segment. Furthermore, estimates of the cumulative seismic moment release and seismic moment release rate along the southeast segment are significantly greater than that of the northwest segment. These observations suggest that slip along the southeast segment is accommodated by a greater number of large magnitude earthquakes. Comparison of the seismic moment rate, derived from empirical estimates, with the seismic moment rate determined from plate motion constraints suggests a difference in the seismic coupling strength between the segments. This difference in coupling may partially explain the disparity in earthquake size distribution. However, the results appear to confirm the relation between earthquake size and fault length, observed along continental strike-slip faults, for this oceanic transform.

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Microearthquakes located on the Blanco Fracture Zone with sonobuoy arrays

Cited by 27 publications

References 19 publications

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

Seismicity and rates of relative motion on the plate boundaries of Western North America

Relationship between the seismicity and geologic structure of the Blanco Transform Fault Zone

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