Transcript ppt
Strain Release Along the Northern Costa Rica Seismogenic Zone Susan Y. Schwartz Department of Earth and Planetary Sciences UC Santa Cruz Modes of Strain Release 1950 M~7.7 Costa Rica Subduction ZoneInstrumenting the Plate Boundary with a Seismic, GPS and Fluid Flow Network Collaborators: Tim Dixon, LeRoy Dorman, Kevin Brown, Marino Protti, Victor Gonzales, Heather DeShon, Edmundo Norabuena, Andy Newman, Sue Bilek, Ernst Flueh EPR interface seismicity: 17-28 km CNS interface seismicity:12-26 km shallower dip 0 20 40 60 80 % Locked 100 1. Up-dip limit of seismogenic zone is defined by shallowest geodetic locking and is consistent with models of thermal control (100150o C) 2. Plate boundary earthquakes begin deeper than start of seismogenic zone Thermal Modeling by Spinelli and Saffer ( 2004) 300o C isotherm from Harris and Wang (2002) 3. Plate boundary earthquakes terminate shallower than continental Moho and 350 C isotherm Implications 1. Locked portion of plate boundary is accumulating strain to be released in next large earthquake 2. Something is weakening the plate boundary to allow the transition from locked with no seismicity to unlocked with seismicity at ~250o C What causes the onset of microseismicity at 15-17 km depth where modeled temperatures on the plate interface are ~200-250oC Stable sliding Stick-slip Fault zone weakening by increased pore-fluid pressure from low grade metamorphic reations in basalt Fluid flow excursions caused by 3 episodes of slow slip on the plate interface Brown et al. (2005) 2003 Geodetically observed slow slip event Interseismic Strain Accumulation Slow Earthquake Signal Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior 0 20 40 60 % Locked 80 100 Global Distribution of Episodic Aseismic Slip Modified from Obara and Hirose [2005] Modified from Dragert and Rogers [2004] 2005 Nias Earthquake Afterslip Northeast Japan strain accumulation, asperity and afterslip patterns DIFFERENT FRICTIONAL PROPERTIES CONTROL FAST VS. SLOW SLIP Velocity Weakening Velocity Weakening/ Strengthening Transition Strain Accumulation Slip- Abundant Microseismicity Coseismic Slip- Asperity Afterslip Slow Slip Events Different frictional properties control fast vs. slow slip No observations of slow slip in strongly coupled regions exist Hot or warm subduction zones have deep slow slip Cascadia SW Japan Mexico Cooler subduction zones or with thin overriding crust have shallow slow slip NE Japan (afterslip) Boso Japan Costa Rica Deep slow slip may require frictional transition at shallow depth (low pressure) or fluids generated from dehydration reactions (baslate-eclogite). New GPS/Seismic/Tilt Nicoya Network for Detection of Slow Slip Events Collaborators: Tim Dixon, Kim Psencik (UM), Marino Protti, Victor Gonzales (OVSICORI-UNA) Technical Support: Dan Sampson (UCSC), Jacob Sklar & Freddy Blume (UNAVCO) SFB574 Borehole Seismic: Ernst Flueh, Wolfgang Rabbel, Martin Thorwart & Nilay Dinc CONCLUSIONS: Strain along the northern Costa Rica plate boundary is released in large earthquakes and slow slip. Spatial separation exists between these two modes of strain release with strain presently accumulating just offshore the Nicoya Peninsula and slow slip occurring in regions up and down-dip of this. A dense network of cGPS, seismic and tilt stations has been installed to improve our understanding of slow slip at this plate boundary.