- Ground-motion model is (MOD1 and MOD3):
where: Y is in m∕s2, a = -2.666 ± 0.072, b = 1.158 ± 0.020, c = -2.312 ± 0.055, h = 1.734 ± 0.197,
σinter-event = 0.142, σintra-event = 0.358 and σ = 0.385 for MOD1 without site term and a =
-2.710 ± 0.064, b = 1.165 ± 0.021, c = -2.244 ± 0.044, h = 1.779 ± 0.158, e = 0.225 ± 0.004,
σinter-event = 0.151, σintra-event = 0.276 and σ = 0.315 for MOD3 with site term.
- Use 3 site classes using approach of Emolo et al. (2011) (see Section 2.342):
s = 1
- Site with significant positive mean residual.
s = 0
- Site with mean residual not significantly different from zero.
s = -1
- Site with significant negative mean residual.
Derive site terms for all stations in second step, based on residuals w.r.t. to overall model derived without
considering site effects. Only statistically-significant terms (using Z-test and 5% level) are retained. Assign
site corrections of either -1, 0 or 1 based on residuals. Find reduction in ϕ but not in τ when applying
- Use data from 29 stations (with 104 to 211 records each) of dense (20 × 10km2) Lawrence Berkeley
National Laboratory/Calpine-Geysers network (01/09/2007–15/11/2010) of induced (by water injection
and steam extraction) seismicity at The Geysers vapour-dominated geothermal field (N. California, USA).
Instruments I/O Sensor SM-6 geophones (f0 = 14Hz) then Oyo GS-11D 4.5Hz sensors.
- Select earthquakes with focal depths ≤ 5km since deep events assumed to be natural earthquakes.
- Select only waveforms with signal-to-noise ratio greater than 10 in frequency range 0.5–35Hz.
- Instrument correct data. Remove mean and trend baselines. Filter using zero-phase 4-pole Butterworth
filter with passband 0.7–35Hz. Extract parameters in time window starting at origin time and ending at
time corresponding to 98% of total energy in waveform, which were tapered with 0.1 taper width with a
cosine window. Finally differentiated and filtered again to obtain accelerations.
- Test another functional form including term for anelastic attenuation, for which report coefficients. Choose
final model due to: its simplicity, σ and R2.
- Examine inter-event and intra-event residuals (as scatter plots and histograms) w.r.t. Mw, rhypo and for
each station separately. Find no significant trends based on fitting lines to residuals.
- Examine standard deviations at each station before and after site correction (single-station σ, ϕSS,
analysis). Compute average value weighted by number of records per station, ϕSS. Compare to overall
total σ and find single-station σ is lower (0.2410 compared to 0.3849) and that the different between them
is smaller when using MOD3 (0.2377 compared to 0.3152). For one station (CLV) find ϕSS,S is higher than
σ, which relate to local geological conditions.
- Find slight positive trend in inter-event residuals w.r.t. focal depth, which relate to possible decrease in
stress with depth or variation in rigidity modulus due to heterogeneities in highly-fractured medium.