Chaney, Ronald C.

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Missouri University of Science and Technology (Missouri S&T): Scholars' Mine

Missouri University of Science and Technology Scholars' Mine International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics 1991 - Second International Conference on Recent Advances in Geotechnical Earthquake Engineering & Soil Dynamics 15 Mar 1991, 11:10 am - 11:20 am Fault Processes and Liquefaction in the Marine Environment Ronald C. Chaney Humboldt State University, Arcata, CA Follow this and additional works at: https://scholarsmine.mst.edu/icrageesd  Part of the Geotechnical Engineering Commons Recommended Citation Chaney, Ronald C., "Fault Processes and Liquefaction in the Marine Environment" (1991). International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. 2. https://scholarsmine.mst.edu/icrageesd/02icrageesd/session15/2 This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conferences on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu. Proceedings: Second International Conference on Recent Advances In Geotechnical Earthquake Englneertng and Soli Dynamics, March 11-15, 1991 St. Louis, Mlssourt, Panel Discussion Session No. VI Fault Processes and Liquefaction in the Marine Environment Ronald C. Chaney Professor, Humboldt State UnlversHy, Arcata, CA INTRODUCTIOO Earthquake geotechnology for the design of offshore structures involves many issues concerned with the esti-mation of magnitude, acceleration and sediment stability. In the follOW'ing sections fault processes and attenuation of earthquake ground motion for subduction zone and thrust faults will be discussed. In addition, evidence of liquefaction in the marine environment will also be presented. Fault Processes and Attenuation of Earthquake Ground Motion for Subduction Zone and Thrust Faults The cascadia Subduction zone offshore is being subjected to a north-south directed compression, Spence (1987a, b). This compression is a result of the Pacific plate pushing the Cascadia subduction zone in a northwest direction via the Blanco and Mendocino Faults. Faults in the Gorda Plate are believed to be aligned along in situ zones of weakness which parallel the magnetic anomalies and are orientated approximately north-south further offshore, then fanning to a more northeast-southwest direction irrmediately offshore (McPherson, 1989) . This pattern of faults mimics the magnetic anomaly trends, as shCMn in Fig. 1. Earthquake epicenters in the Gorda Plate propagate along the magnetic anomalies. There are a number of attenuation relationships that have been developed for subduction zone and thrust faults (Joyner and Boore, 1988; Kawashima et al., 1984). The distance (D) used in developing these relationships is typically from the surface rupture to the site ~r ~J+ ~I+ PACIFIC PLATE + 12& + 125 + 124 Fig. 1 - Break-up of the Gorda Plate: three intraplate earthquake sequences (Ref. Smith et al., 1981; t·lcPherson, R. c., 1989). which the acceleration isbeing recorded. This is a reasonable approximation for high angle thrust or strike slip faults, Fig. 2b, but may not be realistic for low angle thrust faults or subduction zones as shCMn in Fig. 2a. In the case of low angle thrust faults the rupture plane may pass under the site. In this case the distance from the rupture plane to the site can potentially be less than the distance from the site to the surface rupture. The result of using the smaller of the two distances will be an increased acceleration level. 2139 A) Low Angle Thrust Faults or Subduction Zones D D' Zone of Energy Release B) High Angle Thrust Faults or Strike Slip D Accel. Log D (Surface Fault Break) C) Typical Form of Attenuation Relationship Fig. 2 - Determination of Acceleration At a Site Based on Distance. Site location for a normal and thrust faulting environment will influence its expected acceleration during a seismic event. Attenuation curves for normal and thrust faulting as a function of whether the site is on the upper or lCNJer block are presented in Fig. 3. A review of Fig. 3 shows that for distances to the fault less than 100 km, the upper block gives a higher peak acceleration as compared to the lower block, Bureau (1978). !00 50 30 10 5 3 Distance to fault (km) -d-30 50 { ---Upper Block ---Lower Block Fig. 3 - Attenuation Curves for Normal and Thrust Faulting (Ref. Bureau, 1978). Liquefaction in the Marine Environment When the Humboldt County Earthquake (M=7.0) struck the Northern California coast on November 8, 1980 liquefaction failures occurred at Big Lagoon and offshore of the Klamath River. At Big Lagoon extensive soil failure occurred all along a 3 mile long and approximately 300 foot wide natural sand spit as shown in Fig. 4. The types of soil failures experienced ranged from lateral spreading to sand boils. The sand boils were observed on both the dry sand spit and subrrerged in the lagoon. Similar sand spits have been used for both comrrercial and residential development in many seismic areas. G> c:: 0 N BIG LAGOON, CALIF. Bottom Topography ___ Extent of Saline Bottom Loyer 0.5km Fig. 4 - Liquefaction of Sand Spit At Big Lagoon, California During the Magnitude 7.0 Earthquake on Noverr~r 8, 1980. The liquefaction failure offshore of the Klamath R1ver was controlled initially by sediment type. The seaward boundary of the failure zone coincides with the contract between muddy sand and sandy clayey silt. The variety and the specific nature of features observed ou sonographs indicate several modes of sediment failure. The modes that are indicated are liquefaction, lateral spreading and sediment collapse and flowage (Field and Hall, 1982; Field, 1984) as shown in Fig. S. Subsurface liquefaction is inferred at shallCM' depths as result of excess pore pressures generated in sand by the earthquake, refer to Fig. 6. 2140 Fig. 5 - Conceptual block diagram showing variety and types of features observed in the failure (Field, 1984) • PROGRESSION OF FAILURE PROCESSES SANDY MUD MUDDY SAND _ --~---~----r~sm -.: ..... '>--··-··-····~-·····- 1) LIOUIFACTION SAND 2) LATERAL SPREADING COLLAPSE FLOWS -/ \ -.. ~..-----~·····-- 3) COLLAPSE and SEDIMENT FLOWS Fig. 6 - Schematic illustration showing conceptual Progression of failure from subsurface Liquefaction (top) to lateral spreading (middle) and ending with sediment collapse and flow (bottom) • Interpretation based on sonographs and high-resolution seismic reflec-tion data. This progression is in part sequen-tial but may have been largely contemporaneous. Lateral spreading followed liquefaction but collapse and sediment flows may have occurred coincident with spreading. (Ref. Field, 1982). 2141 CCN:LUSIOOS Based on the preceeding discussion, the following conclusions can be made: (1) Determination of acceleration levels resulting from law angle thrust faults based on distance from surface breaks may be law. (2) Construction of shore facilities on sandspits can experience liquefaction and lateral spreading. (3) Liquefaction and lateral movement occurs on very law angle (l;o 0 ) sea beds. REFEREOCES Bureau, G. J. (1978). "Influence of Faulting on Earthquake Attenuation," Proceedings Specialty Conference Earthquake Engineering and Soil Dynamics, American Society of Civil Engineers, Vol. 1, Pasadena, pp. 290-307. Field, M. E. (1984). "The Submarine Landslide of 1980 off Northern California," United States Geological Circular 938, pp. 65-72. Field, M. -E., and Hall, R. K. (1982). "Sonographs of Submarine Sediment Failure Caused by the 1980 Earthquake Off Northern California," Geo-Marine Letters, Vol. 2, pp. 135-141. Joyner, W. B. and Boore, D. M. (1988). Measure-ment, Characterization, and Prediction of Strong Ground Motion," Specialty Conference on Earthquake Engineering and Soil Dynamics, American Society of Civil Engineers, 43-102. Kawashima, K., Aizawa, K., Takahashi, K. (1984). "Attenuation of Peak Ground Motion and Absolute Acceleration Response Spectra," Proceedings of the Eighth World Conference On Earthquake Engineer-ing, Vol. 11, pp. 257-264. McPherson, R. C. (1989). "Seismicity and Focal Mechanisms Near Cape Mendocino, Northern California: 1974-1984," thesis in partial fulfillment of the requirements for the degree Master of Science, Humboldt State University, Arcata, California. Smith, S. W., McPherson, R. C., and Severy, N. I. ( 1981) • "Breakup of the Gorda Plate," Abstract, Earthquake Notes, Vol. 52, p. 42. Spence, W. (1987a). "Implications of the slowing of Subduction at Cascadia," Abstract, EOS, Vol. 68, No. 44, November 3, pp. 1467-1468. Spence, W. (1987b). "Anamolous Subduction and the Origin of Stresses at Cascadia," preprint of paper for open-file report, December 16. 

Fault Processes and Liquefaction in the Marine Environment

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Fault Processes and Liquefaction in the Marine Environment

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