Constraining Earthquake Source Paramters in Rupture Patches and Rupture Barriers S51A-2635 on Gofar Transform Fault, East Pacific Rise from Ocean Bottom Seismic Data Pamela Moyer1 ([email protected]), Margaret Boettcher,1 Jeffrey McGuire2, and John Collins 2 1 University of New Hampshire, 2 Woods Hole Oceanographic Institution
area of Why are earthquakes on oceanic this study Gofar Discovery Quebrada How are source parameters determined 2015 Earthquake locations on Gofar transform fault −4.4˚ Rupture 2008 M transform faults interesting? for earthquakes on Gofar transform fault? Barrier Rupture wZone 6.0 ocean bottom seismometer Rupture 2007 M o We use spectral analysis techniques and an omega-squared source model determined by an Empirical Green’s Function (EGF) 6.0 Many of the largest earthquakes on oceanic transform faults occur 4.4 ocean bottom seismometer Barrier Rupture wZone 2010 plus strong-motion accelerometer method to obtain earthquake source parameters. Stress drop is calculated from Eshelby [1957] using the Madariaga [1976] −4.5˚ by repeatedly re-rupturing the same portion of the fault on a model assuming a circular rupture with the weighted average corner frequency and seismic moment for each earthquake : fairly regular basis Δσ = stress drop On Gofar transform fault on the East Pacific Rise (EPR), 2005 −4.6˚ M0 = seismic moment Mw ~6.0 earthquakes occur every ~5 years Rupture Year o r = source radius 4.6 Barrier Rupture 7 M 0 where k During the last five seismic cycles, the largest earthquakes on Patch = r = f = corner frequency 2000 Rupture 3 c −4.7˚ Earthquakes with stress drop values Gofar have ruptured the same area of the fault (rupture patch) Barrier 16 r fc β = shear wave velocity Earthquakes used as EGFs in spectral analysis while only small earthquakes occur in the intervening fault k = a constant that depends on the rupture model −106.3˚ −106.2˚ −106.1˚ −106˚ −105.9˚ −105.8˚ segments (rupture barriers) 1995 Magnitude (Mw) o o o o o In 2008, an ocean bottom seismometer (OBS) deployment 5 5.4 5.8 6.2 106.4 106.2 106 105.8 105.6 successfully captured the end of a seismic cycle on Gofar Total 1990 50 1. Corner frequency from spectral analysis transform fault 106 105.5 105 104.5 104 103.5 103 102.5 Foreshock Zone Longitude (W) Aftershock Zone The EGF spectral ratio method is used to eliminate path and site effects by division in the frequency domain of a large magnitude earthquake The 2008 OBS experiment recorded an extensive foreshock 100 Swarm Zone (the mainshock) by a smaller magnitude earthquake (the EGF, considered a point source) to obtain corner frequency of the mainshock. sequence localized within a 10 km rupture barrier, the M 6.0 w 150 mainshock and its aftershocks in a ~20 km rupture patch, and Location of M5.2 aftershock 02/23/2008 12:38:48 ML 3.2 an earthquake swarm located in a second rupture barrier 200
) -2 1 2
ïÝ Day of 2008 Centroid of 2008 2 mainshock 10 0.1 10 10 M6.0 mainshock f = 2.3 Hz f 250 Time of Mw 6.0 c 1+ Centroid of 2007 (f) f 01 c2 Magnitude (Mw) Ω 1 Error limits 300 M6.2 mainshock ( f ) = 2
55.4 5.86.2 Acc (m/s At crossing modified from -0.1 Ω (f) 2 02 f 350 1+ 5 ïÝ McGuire et. al. [2012] fc Depth (m) 05/08/2008 03:22:13 ML 2.0 1 -3 ï ï ï ï ï ï ï (top) Locations of over 20,000 earthquakes that occurred along Gofar Relative 10% threshold EGF 1 Norm. Variance transform fault from August - December 2008. 10-3 100
Earthquakes Mw > 5.0 on the Quebrada, Discovery, and Gofar ) x10 2 0 transform faults since 1990. Events with overlapping rupture 0 (bottom) Earthquake time and space distribution showing the rate of S-wave 10 Corner Frequency (Hz) areas are shown in a constant color. seismicity in the foreshock, mainshock and aftershock, and swarm zones. -1 acceleration model to spectral ratio spectra The best-fit corner frequency (star) Acc (m/s 0 5 10 15 20 25 30 100 101 100 101 is varied to estimate error from Research Objective multitaper to spectral Deployment of ocean bottom seismometers at Gofar transform Time (s) Frequency (Hz) Frequency (Hz) increased misfit. frequency domain division fault. The ball at the end of the arm is a broadband seismometer. We investigate whether inferred variations in frictional behavior leading to rupture patches and rupture barriers along Gofar transform fault affect the rupture processes of 3.0 < M < 4.5 earthquakes by The yellow ball is a strong-motion accelerometer. The orange w balls contain batteries, wire, and flotation devices. determining source parameters of earthquakes recorded during the 2008 OBS deployment. 09/10/2008 07:42:06 ML 4.5 0.2 0.2 0.2
2. Seismic moment from spectral analysis ) 2 G04 BN1 G04 BN2 G04 BNZ The low frequency level of the far-field displacement spectra is modeled using Boatwright [1980] and a best-fit 0 0 0 Do earthquake source parameters reveal differences Q obtained from a velocity model through the foreshock zone [Roland et al., 2012] to find seismic moment. (right) Stress drop Acc (m/s Rupture −0.2 −0.2 −0.2 −4.4˚ 0 5 10 15 20 0 5 10 15 20 in fault zone properties on Gofar transforms fault? results for earthquakes Barrier 0 5 10 15 20 -2 Time (s) Time (s) Time (s) along Gofar transform 7.6 MPa Rupture 10 dB seismic moment (M0) from 160 Source parameters of 77 earthquakes (3.0 ≤ M ≤ 4.5) show higher stress drop in the Patch 1 1 1 w fault showing a higher Rupture low frequency spectral level 10 10 10 aftershock zone of the 2008 M 6.0 earthquake (red) compared to zones which host 13.4 MPa w weighted average Barrier 140 swarms of microearthquakes (yellow and blue) stress drop in rupture −4.5˚ 7.0 MPa 02/23/2008 12:38:48 ML 3.2 ft
) Q 0 0 0 120
patches than rupture Ω (f) Higher stress drops in rupture patches indicate higher seismic coupling with stronger fault zone 2 0.1 e 10 10 10 barriers reflecting f = 0 material compared to lower stress drops in rupture barriers where hydrothermal circulation in a ( ) 1 100 differences in fault S-wave n highly fractured fault zone accommodates slip mostly through aseismic processes −4.6˚ Frequency (Hz) displacement -1 zone properties. -3 f -1 10 -1 80 Acc (m/s 10 spectra + 10 10 Circle size indicates -0.1 1 fc Rupture Rupture Rupture earthquake magnitude. 60 Barrier Patch Barrier 0 5 10 15 20 25 30 model to 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 Mw 3.0 earthquake displacement Time (hrs) Time (hrs) Time (hrs) −4.7˚ Time (s) integrate to M 4.5 earthquake km spectra w displacement (bottom) Spectrograms for a ML 4.5 earthquake in the foreshock zone show 0 10 20 0 1 10 10 resonant signals that corrupt low frequencies during integration making spectral −106.3˚ −106.2˚ −106.1˚ −106˚ −105.9˚ −105.8˚ Frequency (Hz) modeling difficult for large magnitude (Mw > 4) earthquakes.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Damaged Relatively Intact Highly Damaged Displacement Spectra Spectral Ratio Log10 Stress Drop (MPa) Fault Zone Fault Zone Fault Zone 3. Seismic moment from relative surface wave amplitudes 1015 Ocean Bottom Seismometers 6 The relative magnitude method of McGuire [2008] using long period (20 seconds) differential amplitudes of Earthquakes with Global CMT catalog magnitudes 2 8 * * * * * * * 102 surface waves recorded on broadband OBSs on Discovery is used to find seismic moment of M > 4.0 earthquakes. −4˚ Earthquakes with updated relative magnitudes 1 7 w 3 4 5
14 6.5 Discovery 10 Number of Earthquakes: 77 cross-correlation x500 coefficient = 0.927 A 6
(f) 1 0 10 M 5.5 1013 w
Relative Ω (f) cross-correlation 5 −4.5˚ x333 coefficient = 0.994 Aftershock Aftershock 1 spectral ratio 4.5 displacement spectra 10 100 MPa Relative M 1012 Gofar Foreshock Foreshock 4 spectral ratio displacement spectra cross-correlation L2 Best Fit x333 3.5 Model to Model to 10 MPa coefficient = 0.973 displacement spectra spectral ratio 3.5 4 4.5 5 5.5 6 6.5 −106.5˚ −106˚ −105.5˚ −105˚ −104.5˚ −104˚ 11 Mws Stress Drop (MPa) 10 10-1 100 101 10-1 100 (top) Bathymetry of Gofar and Discovery transform faults showing OBS locations (triangles).
Corner Frequency (Hz) 0 6.5 Frequency (Hz) Frequency (Hz) 10 cross-correlation 1.0 MPa coefficient = 0.960 B 0.1 MPa x500 6 (left) Displacement spectra for M 3.3 earthquakes in the (left, A) Comparison of magnitude determined by teleseismic surface wave and relative w 5.5 8.0 MPa 7.0 MPa 8.3 13.4 9.7 7.6 MPa 9.4 MPa w foreshock zone (yellow) and aftershock zone (red) highlighted D08 BHZ D05 D04 D01 BNZ amplitude methods. Events with Global CMT catalog magnitudes shown in red. in stress drop results figure (far left). 3 3.5 4 4.5 5 ï ï ï ï Magnitude 0 50 100 150 200 250 Longitude Time (s) 4.5 (left, B) Comparison of magnitude determined by spectral and relative amplitude methods. Events with (right) Spectral ratios of the same earthquakes. Note the higher Relative M Earthquake from 7 Dec. 2008 recorded on Discovery OBSs Global CMT catalog magnitudes shown in red. The very similar estimates of seismic moment for M > 5 (top) Cartoon summarizing seismic behavior, fault geometry, and material corner frequency from the ratio for the aftershock (red) leading Corner frequency versus magnitude with lines of constant stress 4 (red) with a Global CMT catalog magnitude of M 5.3 cross- earthquakes using these independent techniques (e.g. low frequency spectral level recorded on OBS properties along Gofar transform fault [Froment et al., 2014]. to a higher stress drop. drop. The color of the circle indicates where along the fault the w correlated with an event from 1 Sep. 2008 with an unknown 3.5 accelerometers on Gofar with the surface wave amplitudes recorded on broadband stations on Discovery) earthquake is located (following Froment et al., 2014). The least 3.5 4 4.5 5 5.5 6 6.5 (bottom) Stress drop results from spectral analysis (black circles). magnitude (blue). The ratio in amplitude is the ratio of seismic Spectral M shows that the different OBS sensors are well-calibrated. squares best-fit line indicates nearly self-similar scaling w The weighted average stress drop is shown for each zone. moments between events.