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 Zone-Instrumenting 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

% Locked

0 20 40 60 80 100

1. Up-dip limit of seismogenic zone is defined by shallowest geodetic locking and is consistent with models of thermal control (100-150o C)

2. Plate boundary earthquakes begin deeper than start of seismogenic zone

3. Plate boundary earthquakes terminate shallower than continental Moho and 350 C isothermThermal Modeling by Spinelli and Saffer ( 2004)

300o C isotherm from Harris and Wang (2002)

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)

Interseismic Strain Accumulation

Slow Earthquake Signal

2003 Geodetically observed slow slip event

% Locked

0 20 40 60 80 100

Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior

Global Distribution of Episodic Aseismic Slip

Modified from Dragert and Rogers [2004]Modified from Obara and Hirose [2005]

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

2005 Nias Earthquake AfterslipNortheast Japan strain accumulation,

asperity and afterslip patterns

Different frictional properties control fast vs. slow slip

No observations of slow slip in strongly coupled regions exist

Hot or warm subduc-tion 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.