Estimating the rupture extent of low frequency. earthquakes near Parkfield, CA

Transcription

1 Estimating the rupture extent of low frequency T10:07: T10:08: earthquakes near Parkfield, CA ground velocity (m/s) e 8 PB.B073..EH1 10:07:39 10:08:09 10:08:39 Jessica Hawthorne (Leeds) Amanda Thomas (Oregon), Pablo Ampuero (Caltech) 6 April 2017

2 ground velocity (m/s) Tremor e 8 PB.B073..EH T10:07: T10:08: :07:39 10:08:09 10:08:39 Many small earthquakes On the plate interface below the seismogenic zone Low-frequency: most energy in 1-10 Hz band Constituent earthquakes have long durations?

3 Low frequency earthquakes have long durations. Why? ground velocity low frequency earthquake stack earthquake 0.5 seconds in Cascadia (Bostock et al., 2015) 0.2 seconds near Parkfield (Thomas et. al., 2016)

4 Low frequency earthquakes have long durations. Why? ground velocity low frequency earthquake stack They re spatially big, with durations earthquake radius 0.8 shear wave speed 1200 m diameters earthquake 0.5 seconds in Cascadia (Bostock et al., 2015) 0.2 seconds near Parkfield (Thomas et. al., 2016)

5 Low frequency earthquakes have long durations. Why? ground velocity low frequency earthquake stack They re spatially big, with durations earthquake radius 0.8 shear wave speed 1200 m diameters earthquake 0.5 seconds in Cascadia (Bostock et al., 2015) 0.2 seconds near Parkfield (Thomas et. al., 2016) 8 4 Or LFEs have different rupture dynamics? distance along fault (m) slip rate / plate rate

6 LFE families near Parkfield, CA 5 km 36 N SCYB RMNB SMNB LCCB 35.9 N W CCRB MMNB B073 B075 VCAB FROB W B076 VARB JCSB EADB 4000 and 8000 detections in the catalog of Shelly et al, 2009 HRSN and PBO borehole data, Do these LFEs have diameters > 1 km, as suggested by 0.2-s durations? B078 GHIB B072 B079

7 Want to use variation in travel time within the source region to estimate the LFEs spatial extents d d/vrupt d/vp Apparent stfs are 2 1 d d/vrupt Similar at periods longer than the travel time across the earthquake Different at shorter periods d/vrupt + d/vp

8 apparent source time function 2000 Synthetic 1000 apparent source time functions m diameter m diameter 4000 station direction Measure 1000 phase similarity: R = 1 ŝ a 60 0 N 5 ŝ a stations coherent phase moveout Similar at periods 0.8 longer than the travel 0.6 time across the 0.4 earthquake 0.2 Different at shorter periods diameter 1.3 wavespeed frequency (Hz)

9 Empirical Green s function phase removal Expect apparent stfs (s tk and s ik ) to vary among stations when wavelength < rupture diameter. But we only have the observations: d tk = s tk g k and d ik = s ik g k

10 Empirical Green s function phase removal Expect apparent stfs (s tk and s ik ) to vary among stations when wavelength < rupture diameter. But we only have the observations: ˆdtk = ŝ tk ĝ k and ˆd ik = ŝ ik ĝ k

11 Removing the Green s functions phases Have the observations: ˆd tk = ŝ tk ĝ k and ˆd ik = ŝ ik ĝ k Cross-correlate at each station ˆx k = ˆd ik ˆd tk = zero phase {}}{ ŝ ik ŝ }{{ tk ĝ } k ĝk same across stations? ˆd phase ψ

12 Removing the Green s functions phases cro energy 10 1 cross-correlation phases Have the observations: ˆd tk = ŝ tk ĝ k and ˆd ik = ŝ ik ĝ k diameter (m) Cross-correlate diameter (m) at each station frequency (Hz) coherent energy fraction ˆx k = ˆd ik ˆd tk inter-station zero phase {}}{ = ŝ ik ŝ }{{ tk ĝ } k ĝk same across stations? 0.2when x k phases are the same, wavelength is larger than LFE frequency (Hz)

13 00 energy / template energy Template-normalized LFE energies diameter (m) inter-station coherent LFE frequency (Hz) 300 cross-correlation phases diameter (m) frequency (Hz) total LFE energy: observed minus noise LFE energy coherent across stations: E c = ˆx k ˆx l ˆd tk ˆdtl 2 coherent energy fraction

14 Template-normalized LFE energies diameter (m) inter-station coherent energy fraction frequency (Hz) frequency (Hz) cross-correlation phases diameter (m) frequency (Hz) 300 total LFE energy: observed minus noise LFE energy coherent across stations: E c = ˆx k ˆx l ˆd tk ˆdtl 2 coherent energy fraction

15 Averaging over 2600 LFEs: Family template-normalized energy coherent energy / LFE energy inter-station coherent LFE diameter (m) inter-station coherent 2 5 frequency (Hz) Coherent fraction high out to 10 Hz Diameter around m Factor 3-4 smaller than 1.2 km expected for 3 km/s rupture velocity

16 Averaging over 3800 LFEs: Family template-normalized energy coherent energy / total inter-station coherent LFE diameter (m) inter-station coherent point source adjusted for noise 2 5 frequency (Hz) Coherent fraction high out to > 5 Hz Diameter < 700 m Decoherence could be due to timing, tapering, nearby LFEs Factor > 2 smaller than 1200 m expected for 3 km/s rupture velocity

17 So why are the rupture durations long? 8 4 short normal rupture propagation minimum earthquake size distance along fault (m) slip rate / plate rate Long rise time from nucleation? Slow propagation? Complex rupture patterns? Attenuation?

18 Conclusions ground velocity (m/s) e 8 PB.B073..EH T10:07: T10:08: :07:39 10:08:09 10:08:39 Low-frequency nature of tremor does not arise because the asperities are big. High inter-station coherence out to 10 Hz implies average diameters < 400 m and < 700 m Rupture extents are a factor of >2-4 smaller than expected for 0.2-s durations and near-shear-wave rupture velocities Suggest different rupture dynamics or a role for attenuation