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• 1. [Article] Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA

  A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in ...

  Citation × Citation Citation Abstract Copy Title: Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA Author: Peterson, Curt D., Kristensen, Kurt, Minor, Rick Year: 2014 A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems. Title: Large-Scale Fluidization Features from Late Holocene Coseismic Paleoliquefaction in the Willamette River Forearc Valley, Central Cascadia Subduction Zone, Oregon, USA Author: Peterson, Curt D., Kristensen, Kurt, Minor, Rick Year: 2014 A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems. Close

• 2. [Article] Central Cascadia Subduction Zone Creep

  Central Cascadia between 43ºN and 46ºN has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep ...

  Citation × Citation Citation Abstract Copy Title: Central Cascadia Subduction Zone Creep Author: Schmalzle, Gina M., McCaffrey, Robert, Creager, Kenneth C. Year: 2014 Central Cascadia between 43ºN and 46ºN has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust "decade-scale" locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicatecoastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland undercentral Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years(the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43ºN and 46ºN may be less perme-able compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43ºN and 46ºN where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip. Title: Central Cascadia Subduction Zone Creep Author: Schmalzle, Gina M., McCaffrey, Robert, Creager, Kenneth C. Year: 2014 Central Cascadia between 43ºN and 46ºN has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust "decade-scale" locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicatecoastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland undercentral Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years(the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43ºN and 46ºN may be less perme-able compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43ºN and 46ºN where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip. Close

• 3. [Article] Interseismic Locking on the Hikurangi Subduction Zone: Uncertainties from Slow-Slip Events

  lnterseismic locking on the Hikurangi subduction zone in New Zealand is examined in light of alternative assumed locking distributions and the impact of transients (slow-slip and volcanic sources) on temporal ...

  Citation × Citation Citation Abstract Copy Title: Interseismic Locking on the Hikurangi Subduction Zone: Uncertainties from Slow-Slip Events Author: McCaffrey, Robert Year: 2014 lnterseismic locking on the Hikurangi subduction zone in New Zealand is examined in light of alternative assumed locking distributions and the impact of transients (slow-slip and volcanic sources) on temporal and spatial resolution. The modern pattern of locking in the north is poorly resolved and, based on simulations of possible transient behavior, may be an ephemeral feature of the subduction cycle. While there appears to be some contemporary locking in the northern half of the Hikurangi subduction zone (HSZ), its location is model dependent, and hence, its relationship to structure, slow-slip, or any transition zone there is unclear. Simulations of site velocities using the 14 year history of transient events reveal that in the timescale of the interseismic period the northern half of the HSZ could be either locked or unlocked, and this may not be resolvable for decades. In the southern half, there is strong contemporary locking in the 15 to 40 km depth range, but again, the slow-slip history leads to uncertainty in the long-term pattern. Slow-slip events not only reduce the long-term locking by aseismic slip but also greatly hinder our ability to see it. It is within the range of possible models that the slip deficit rate at the HSZ is more uniform along strike, and the modern appearance is controlled by the particular pattern of transients over the past 10 to 20 years when the GPS data were collected. Similarly, uncertainties in surface velocities will be large at any subduction zone with large transients. Title: Interseismic Locking on the Hikurangi Subduction Zone: Uncertainties from Slow-Slip Events Author: McCaffrey, Robert Year: 2014 lnterseismic locking on the Hikurangi subduction zone in New Zealand is examined in light of alternative assumed locking distributions and the impact of transients (slow-slip and volcanic sources) on temporal and spatial resolution. The modern pattern of locking in the north is poorly resolved and, based on simulations of possible transient behavior, may be an ephemeral feature of the subduction cycle. While there appears to be some contemporary locking in the northern half of the Hikurangi subduction zone (HSZ), its location is model dependent, and hence, its relationship to structure, slow-slip, or any transition zone there is unclear. Simulations of site velocities using the 14 year history of transient events reveal that in the timescale of the interseismic period the northern half of the HSZ could be either locked or unlocked, and this may not be resolvable for decades. In the southern half, there is strong contemporary locking in the 15 to 40 km depth range, but again, the slow-slip history leads to uncertainty in the long-term pattern. Slow-slip events not only reduce the long-term locking by aseismic slip but also greatly hinder our ability to see it. It is within the range of possible models that the slip deficit rate at the HSZ is more uniform along strike, and the modern appearance is controlled by the particular pattern of transients over the past 10 to 20 years when the GPS data were collected. Similarly, uncertainties in surface velocities will be large at any subduction zone with large transients. Close

• 4. [Article] Earthquake and Tsunami Forecasts: Relation of Slow Slip Events to Subsequent Earthquake Rupture

  The 5 September 2012 Mw 7.6 earthquake on the Costa Rica subduction plate boundary followed a 62-y interseismic period. High-precision GPS recorded numerous slow slip events (SSEs) in the decade leading ...

  Citation × Citation Citation Abstract Copy Title: Earthquake and Tsunami Forecasts: Relation of Slow Slip Events to Subsequent Earthquake Rupture Author: Dixon, Timothy H., Jiang, Yan, Malservisi, Rocco, McCaffrey, Robert, Voss, Nicholas, Protti, Marino, Gonzalez, Victor Year: 2014 The 5 September 2012 Mw 7.6 earthquake on the Costa Rica subduction plate boundary followed a 62-y interseismic period. High-precision GPS recorded numerous slow slip events (SSEs) in the decade leading up to the earthquake, both up-dip and down-dip of seismic rupture. Deeper SSEs were larger than shallower ones and, if characteristic of the interseismic period, release most locking down-dip of the earthquake, limiting down-dip rupture and earthquake magnitude. Shallower SSEs were smaller, accounting for some but not all interseismic locking. One SSE occurred several months before the earthquake, but changes in Mohr–Coulomb failure stress were probably too small to trigger the earthquake. Because many SSEs have occurred without subsequent rupture, their individual predictive value is limited, but taken together they released a significant amount of accumulated interseismic strain before the earthquake, effectively defining the area of subsequent seismic rupture (rupture did not occur where slow slip was common). Because earthquake magnitude depends on rupture area, this has important implications for earthquake hazard assessment. Specifically, if this behavior is representative of future earthquake cycles and other subduction zones, it implies that monitoring SSEs, including shallow up-dip events that lie offshore, could lead to accurate forecasts of earthquake magnitude and tsunami potential. Title: Earthquake and Tsunami Forecasts: Relation of Slow Slip Events to Subsequent Earthquake Rupture Author: Dixon, Timothy H., Jiang, Yan, Malservisi, Rocco, McCaffrey, Robert, Voss, Nicholas, Protti, Marino, Gonzalez, Victor Year: 2014 The 5 September 2012 Mw 7.6 earthquake on the Costa Rica subduction plate boundary followed a 62-y interseismic period. High-precision GPS recorded numerous slow slip events (SSEs) in the decade leading up to the earthquake, both up-dip and down-dip of seismic rupture. Deeper SSEs were larger than shallower ones and, if characteristic of the interseismic period, release most locking down-dip of the earthquake, limiting down-dip rupture and earthquake magnitude. Shallower SSEs were smaller, accounting for some but not all interseismic locking. One SSE occurred several months before the earthquake, but changes in Mohr–Coulomb failure stress were probably too small to trigger the earthquake. Because many SSEs have occurred without subsequent rupture, their individual predictive value is limited, but taken together they released a significant amount of accumulated interseismic strain before the earthquake, effectively defining the area of subsequent seismic rupture (rupture did not occur where slow slip was common). Because earthquake magnitude depends on rupture area, this has important implications for earthquake hazard assessment. Specifically, if this behavior is representative of future earthquake cycles and other subduction zones, it implies that monitoring SSEs, including shallow up-dip events that lie offshore, could lead to accurate forecasts of earthquake magnitude and tsunami potential. Close

• 5. [Article] Slow Slip Events and Time-Dependent Variations in Locking Beneath Lower Cook Inlet of the Alaska-Aleutian Subduction Zone

  We identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface ...

  Citation × Citation Citation Abstract Copy Title: Slow Slip Events and Time-Dependent Variations in Locking Beneath Lower Cook Inlet of the Alaska-Aleutian Subduction Zone Author: Li, Shanshan, Freymueller, Jeffrey, McCaffrey, Robert Year: 2016 We identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface deformations inferred from GPS for pre-2004 and 2010–2011 are similar to each other, as are 2004–2010 and post-2011. We estimate the slip distribution on the Alaska-Aleutian subduction plate interface accounting for upper plate block rotations and interpret this toggling between two deformation patterns as caused by transient slip. We find that by allowing negative slip deficit rates (i.e., creep rates in excess of relative plate motion), the data in Lower Cook Inlet are fit significantly better during pre-2004 and 2010–2011, suggesting the occurrence of slow slip events (SSEs) there during those time periods. The earlier SSE lasted at least 9 years (observations in that area began in 1995) with Mw ~7.8. The latter SSE had almost the same area as the earlier one and a duration of ~2 years with Mw ~7.2. During 2004–2010 and post-2011, the inversions result in only positive slip deficit rates (i.e., locking) in Lower Cook Inlet. Slip rates are nearly constant during the Lower Cook Inlet SSEs, and the events start and stop abruptly. Both of these properties contrast with observations of SSEs in Upper Cook Inlet and elsewhere. The Lower Cook Inlet SSEs are consistent with previously proposed duration-magnitude scaling laws and demonstrate that slow slip events can last as long as a decade. Title: Slow Slip Events and Time-Dependent Variations in Locking Beneath Lower Cook Inlet of the Alaska-Aleutian Subduction Zone Author: Li, Shanshan, Freymueller, Jeffrey, McCaffrey, Robert Year: 2016 We identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface deformations inferred from GPS for pre-2004 and 2010–2011 are similar to each other, as are 2004–2010 and post-2011. We estimate the slip distribution on the Alaska-Aleutian subduction plate interface accounting for upper plate block rotations and interpret this toggling between two deformation patterns as caused by transient slip. We find that by allowing negative slip deficit rates (i.e., creep rates in excess of relative plate motion), the data in Lower Cook Inlet are fit significantly better during pre-2004 and 2010–2011, suggesting the occurrence of slow slip events (SSEs) there during those time periods. The earlier SSE lasted at least 9 years (observations in that area began in 1995) with Mw ~7.8. The latter SSE had almost the same area as the earlier one and a duration of ~2 years with Mw ~7.2. During 2004–2010 and post-2011, the inversions result in only positive slip deficit rates (i.e., locking) in Lower Cook Inlet. Slip rates are nearly constant during the Lower Cook Inlet SSEs, and the events start and stop abruptly. Both of these properties contrast with observations of SSEs in Upper Cook Inlet and elsewhere. The Lower Cook Inlet SSEs are consistent with previously proposed duration-magnitude scaling laws and demonstrate that slow slip events can last as long as a decade. Close