The use of earthquake rate changes as a stress meter at Kilauea volcano Nature, V. 408, 2000 By J. Dietrich, V. Cayol, and P. Okubo Presented by Celia Schiffman

Do changes in stress correlate with changes in earthquake rates? Stress changes and EQ rates are not linearly correlated EQ nucleation process is dependent on time and stress (lab. observations) Estimating stresses that drive EQ’s versus stress changes resulting from EQ’s. Kilauea: –Frequent stressing events –Independent observations of deformation –High rates of seismic activity –Changes in seismicity/eruptions/subsurface magma movement –Rift-zone magmatic expansion/detachment faulting

Formulas

Stress step --> characteristic aftershock sequence (i.e. immediate jump in seismicity then decay according to Omori’s decay law [1/t])

Two methods to estimate stress change from EQ rates 1)Stress as a function of time in a specific volume -Calculate time series at grid points from earthquake rate data -Calculate stress changes over succesive time intervals using stress steps at the midpoint of each time interval

Two methods to estimate stress change from EQ rates 2) Spatial distribution of stress changes for a stress event (EQ or intrusion) -Use eq 1 to solve for constant stressing at Sr, take a stress step (corresponding to the stress event), then constant stressing at Sr again. -Eq counts are made for subregions sorted by fault orientation km depths -Each volume needs at least 8 EQ’s -Grid nodes spaced 1 km apart

Timeline Pre 1975: –Eruptive activity 1975: –M7.2 EQ : –Intrusion (1977) –Rapid deformation (up to 25 cm/yr extension) –Intense seismicity –Aseismic creep on detachment –Rift opening at 40 cm/yr : –5 fold slowing of stressing rates 1983: –Eruption Post 1983: –Nearly continuous rift eruption –Deformation decreased to 4 cm/yr

Location

Observations Deformation data sets Number of earthquakes

-Eq rates are low-pass filtered -Assume Eq’s occur on faults that are optimally oriented in the stress field -Artifacts from random fluctuations in EQ rate and possible catalog inconsistencies during swarm events -Slowing of stressing rates from (0.3->0.15) Next: Compare to BEM’s constructed from independent estimates of stress changes Calculate stress changes based on eq’s

Model variables: depth, height, width, dip and opening of dike Dip and depth of detachment fault Width of creeping portion of fault Outputs: Predicted surface deformations Stress changes Pick best fitting values for variables based on deformation data

Pre-1983 eruption Post-1983 eruption Boundary Element Models

Statistics: Slope=1.1 Correlation=0.8 Problems Non-linear relationship not clearly demonstrated Is it really necessary? Would a linear fit work just as well? Need geodetic data (GPS) to better constrain changes in stress to see if step-function, or linear