Characterization of the San Andreas Fault by Fault-Zone Trapped Waves. Yong-Gang Li, John E. Vidale and Steven M. Day

Transcription

1 Characterization of the an Andreas Fault by Fault-Zone Trapped Waves Yong-Gang Li, John E. Vidale and teven M. Day We are interested in the an Andreas Fault Observatory at Depth (AFOD) program in an integration of scientific and operational activities under the various Earthcope components. AFOD provides us an opportunity to study the major plate boundary, i.e. the AF, using fault-zone seismic trapped waves. We have successfully recorded fault-zone trapped waves generated by microearthquakes and controlled sources at the Californian active fault zones, including the AF at arkfield, the an Jacinto fault near Anza, and rupture zones of recent major earthquakes at Landers and Hector Mine [Li et al., ]. Observations and numerical modeling of fault-zone trapped waves allowed us to delineate the fault geometry (width, depth, continuities, barriers, and branches) and physical properties (velocities, Q, crack densities, and oisson's ratio) of fault-zone rocks with high-resolution at the seismogenic depth. We have observed the postearthquake fault healing (fault strength regaining) with time at the Landers rupture zone in repeated seismic surveys. The evolving wave velocity increase with time, consistent with the closure of cracks that opened during the 99 M7.4 earthquake, hints tantalizingly at the governing physics. Knowledge of spatial and temporal variations in fault structure through fault-zone seismic wave study may help resolve these variations and predict the behavior of future earthquakes on the active faults in California and elsewhere. We propose to record fault-zone trapped waves generated by explosions and microearthquakes using the surface and borehole seismic arrays deployed at the an Andreas fault near the AFOD drilling site arkfield (Fig. ). We shall use fault-zone trapped waves to delineate the internal structure of the arkfield AF segment in 3-D and monitor the temporal variation of fault physical properties with high resolution. In the past two decades, seismological studies have revealed a low-velocity zone surrounding the surface trace of the an Andreas in central California [e.g. Michelini and McEvilly, 99], showing this zone to be as much as to -wide and with -wave velocities of.0 to 3.0 /s and Vp/Vs ratios of.0 to.3 in the nucleation zone of the 966 M6 earthquake. They suggest that the low Vs and corresponding high Vp/Vs ratios within the fault zone are caused by dilatant fracturing due to high pore-fluid pressures [Nur, 97]. Byerle [990], Rice [99], and Boore et al. [994] note that the high pore-pressures within a fault zone at seismogenic depths may be due in part to its greater permeability than adjacent blocks. Alternatively, the high pore pressure within the fault zone may be caused by porosity reduction due to progressive volumetric compacting [leep and Blanpied, 99], thermal expansion of fluids due to frictional heating [Lachenbruch, 980], or the dehydration of clay minerals [Raleigh, 977]. Fluid-filled fractures may also exist at shallow depths, as shown, for example, by the measured fluid pressure at.5 depth in the Varian well near arkfield. Here, the pressure is about Ma above hydrostatic [Johnson and McEvilly, 995 after F. Riley, 994]. As a result of the seismic velocity reduction due to intense fracturing, brecciation, liquid-saturation and possibly high pore-fluid pressure, the fault zone forms a natural low-velocity vertical waveguide. When a seismic source occurs in the fault zone, some seismic energy is trapped and focused within the waveguide, and propagates as normal modes. Trapped waves can be observed only when both the source and receiver are located within or close to the fault zone. The source can be earthquakes or controlled sources. ince the fault-zone trapped waves arise from coherent multiple reflections at the boundaries between the low-velocity fault zone and the high-velocity surrounding rock, the amplitudes and frequencies of trapped waves strongly depends on the fault geometry and physical properties. We can resolve the fault zone width of tens to several hundreds of meters using the records of fault-zone trapped modes. Thus, the fault-zone trapped waves can be used as a high-precision probe of the state of the fault zone at the seismogenic depth. In our previous study of the AF at arkfield, observations and modeling of fault-zone trapped waves at 4-6 Hz from microearthquakes allow us to reveal a ~00-m-wide low-velocity waveguide on the AF where the Vs is reduced by 30-40% [Li et al., 990; Li and Leary, 990]. Fig. shows the fault-zone trapped waves with relatively large amplitudes and long period following waves recorded at MM station for 4 similar microearthquakes occurring on the AF at arkfield in 987. ynthetic dispersion, amplitude spectra and waveforms of trapped modes using a computer code developed by Li [988] in terms of the structural model at arkfield are shown in Fig.. The synthetics fit to observations quite well, indicating the model applicable for the internal structure of the AF near arkfield. Trapped wave inferred lowvelocity waveguide corresponds to the presumably highly-damaged zone of recurrent M6 earthquake episodes, and thus is coincident with the principal slip plane of the AF. Fault-zone trapped waves were also successfully excited by near-surface explosions within the AF at arkfield [Li et al., 997a]. We used explosion-excited trapped waves to study the shallow structure of the fault zone and discriminate most active slip plane. Fig. 3 shows seismograms recorded at a 0-station array across the AF at arkfield town for 3 explosions detonated within and outside the fault zone in 995 (Fig. ). We observed trapped waves only for shot 0 on the AF and the stations located at the main and northern fault strands which were broken in the 966 M6 event, indicating the existence of a low velocity waveguide on them. In contrast, trapped waves did not appear at stations far away from the faults for 0. We also noted that trapped waves were not clear at the station located on the southern fault strand, which did not break in the 966 event, indicating the lack of a low-velocity waveguide on it. imulations of explosion-excited trapped waves revealed a 60-m-wide low-velocity zone along the AF at shallow depth. We also discriminated the recently active fault strands at the an Jacinto fault near Anza, California, using faultzone trapped waves from earthquakes recorded at seismic arrays installed at the fault strands (Fig. 4). We observed fault-

2 zone trapped waves at the Casa Loma fault northwest of Anza which ruptured in the 98 M6.9 earthquake, but not at the Hot prings fault which did not break in at least several hundred years [Li et al., 997b]. Fig. 5a shows seismograms at arrays across the CLF and HF for a M3 earthquake (event in Fig. 4) occurring near Hemet in 995. rominent faultzone trapped waves appeared at the CLF but not at HF, indicating that the strong low-velocity waveguide exists on the CLF but not on the HF. Finite-difference synthetic trapped waves revealed a 0-m-wide low-velocity zone along the CLF where Vs is reduced by ~5% from the wall-rock velocity. Trapped waves recorded at arrays across the Buck Ridge, Clark Lake, and Coyote Creek faults southeast of Anza revealed m wide low-velocity waveguides on them [Li and Vernon, 00a]. Fig. 5b illustrates observed and synthetic seismograms at the BRF for an earthquake (event in Fig. 4) occurring near Anza Gap in 999. Locations of earthquakes showing trapped waves delineate the most seismically active fault strands of the JFZ in a region with complicated slip planes near Anza (Fig. 4). The BRF connects the CVF at seismogenic depth and extends through Anza slip gap to connect the CLF. The CCF disconnects from the CLF at the south edge of Anza Gap. We interpret the low-velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the Anza region. Observations and numerical modeling of fault zone trapped waves generated by aftershocks and near-surface explosions have allowed us to delineate the fine internal structure of the Landers rupture zone [Li et al., 999; 000]. The rupture zone is marked by a low velocity and low Q waveguide 50 m wide at the surface, tapering to m at 0 depth. hear velocities of the rupture zone are reduced by 40-50% from those of the surrounding rock. Within the rupture zone, the shear-velocity increases from.0 /s to.5 /s and Q increases from 0 to 60 with depth. From the view point of fracture mechanics, the distinct low-velocity waveguide on faults may be a remnant of the process zone, which is inelastic deformation around the propagating crack tip during rupture. The strength of the fault zone may vary over the earthquake cycle [Vidale et al., 994; Marone et al., 995]. Inferred healing is consistent with state- and ratedependent healing models [Dieterich, 978]. Rupture models that involve variations in fault-zone fluid pressure over the earthquake cycle have been proposed [ibson, 977; Blanpied et al., 99]. Knowledge of spatial and temporal variations in fault structure may help resolve these variations and predict the behavior of future earthquakes. To monitor the postearthquake variations of fault zone physical properties after the Lander earthquake [Li et al., 00], we conducted seismic surveys at the Landers southern rupture zone in 994, 996, and 998 (Fig.6a). Fig. 6b shows seismograms recorded at Line across the Johnson Valley fault for explosion 4 detonated on the rupture zone near the Landers mainshock epicenter in 994, 996, and 998. We found that the shear velocity of the fault zone rock increased by ~.% between 994 and 996, and increased further by ~0.7% between 996 and 998 (Fig. 6c). This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 99 earthquake. The observed fault-zone strength recovery is consistent with a decrease of ~0.03 in the apparent crack density within the fault zone. The ratio of decrease in travel time for to waves changed from 0.75 in the earlier two years to 0.65 in the later two years between 994 and 998, suggesting that cracks near the fault zone are partially fluidfilled and have became more fluid saturated with time. Recently, we studied the complex multiple-faulting pattern of rupture zone of the M7. Hector Mine earthquake with fault-zone trapped waves generated by near-surface explosions and aftershocks, and recorded at linear seismic arrays deployed across the surface rupture [Li et al., 00]. 3-D finite-difference simulations of fault-zone trapped waves indicate a 75 to 00-m-wide low velocity and low Q waveguide along the ruptured Lavic Lake fault where velocities vary from.0 to.5 /s at depths of 0-0, reduced by ~40-50% from wall-rock velocities, and Q is ~0-60. The pattern of aftershocks for which we observed trapped waves shows that this low-velocity waveguide has two branches in the northern and southern portions of the rupture zone at seismogenic depth (Fig. 7a). To north, although only the rupture segment on the LLF broke to the surface, a rupture segment on a buried fault also extended northward from the mainshock epicenter. To south, the rupture on the LLF intersected the Bullion fault and bifurcated while the rupture on the BF was minor. The analysis of fault-zone trapped waves helps delineate a more complex set of rupture planes than the surface breakage, in accord with the complex pattern of aftershock distribution and geodetic evidence. Fig.7b exhibits recorded and synthetic seismograms at Array 3 across the south LLF for shot detonated within the rupture zone. Trapped waves are prominent at stations within the rupture zone. Fig. 7c illustrates seismograms recorded at Array across the north LLF for 3 aftershocks occurring on the north LLF, the buried fault, and away from the faults. We observed trapped waves for events occurring on the two faults but not for the event between them, showing the existence of low-velocity waveguide on rupture segments along the LLF and the buried fault. Our simulations of dynamic rupture using a finite-element code show that generic models are able to produce the general features of the northern part of the rupture, indicating that such a faulting bifurcation is physically plausible and consistent with observations. Encouraged by the results from our previous studies of active faults in California using fault-zone trapped waves, we would further study the AF at arkfield through the Earthcope-AFOD program. In cooperation with scientists of the UG, Duke University, Virginia Tech, and Germany, we plan to deploy a tight seismic array of 00 portable seismometers with 0-0 m station spacing across the AF near the AFOD drilling site to record explosion-excited fault-zone trapped waves in 00 Fall (Fig. ). Trapped waves recorded at this dense array and the borehole seismic string will be used for a high-resolution delineation of damaged zones of recurrent M6 earthquake episodes at arkfield. We shall also deploy a small array at the same place as in 995 to record signals from the shot 0 in 00 for study of the possible changes in fault properties between 995 and 00.

3 36 00' evtb-e Array 00 Drill ite 03 0 MM Array A-A' M Middle Mountain 35 55' Cholame Hills an Andreas Fault Zone AFs AFm AFn Array 995 T0 0 5 T0 A arkfield 0 35' 0 30' 0 5' Fig. Maps showing locations of explosions (grey stars)and portable seismometers at arkfield in 995 experiment, and proposed dense seismic array (solid line) and explosions (black stars) in 00. MM - Middle Mountain. AFm - main fault strand, AFn - northern strand, AFs - southern strand, A - southwestern fracture zone. The circle with cross denotes the AFOD drilling site. ynthetic Fundamental First high-mode Franciscan Block (350 m) Q ~30-60 (00-60 m) Granitic Block.6/s.4/s.8/s.8/s Fig. 3 Top: Vertical component seismograms recorded at 0 stations for explosions 0, 0 and 03 in 995 Fig. ). tations T05 was deployed on the AFm and T08 was on the AFn, at which prominent fault-zone trapped waves were recorded for explosion 0 detonated within the fault zone. T06 was 50m north of the AFm trace, showing weaker trapped waves for the sam shot. However, there was lack of trapped waves at all stations for 0 and 03 detonated ~3 away from the fault zone. Bottom: ynthetic (solid) and recorded (dotted) amplitude spectra and seismograms (<4 Hz filtered) at stations T05 and T06 for 0. The model is shown in Fig. 3, but Vs = 0.85 /s and Q~30 within the fault zone, and Vs =.5 /s and Q~30 outside the fault zone for the top layer. The low-velocity waveguide at surface is 60 m wide. Fig. Top: Vertical component seismogram recorded at station MM for events B-E (Evt B) occurring in 987at depths of 4-5 within the AF ~0 northwest of Midddle Mountain station (Fig. ). Trapped wavees are marked by III. F: ynthetic trapped. Bottom: Velocity model of the AF near arkfield used for computation of synthetic fault-zone trapped waves, dispersion and amplitude spectra of fundamental and firs high modes.

4 33 50' an Jacinto (5a) rofiles across the an Jacinto Fault Zone 33 0' Hemet HF CLF JF Elsinore Fault Zone an Jacinto Fault Zone Anza Gap Anza _ Horse Canyon Array C Coyote Mountain BRF an Andreas Fault Zone Array A 7 00' 6 0' Fig. 4 Map shows locations of seismic arrays installed in 995 and 999 across fault strands of the an Jacinto fault zone near Anza and earthquakes recorded at seismic arrays. Color and open symbols denote events with and without fault-zone trapped waves, respectively. Diamonds, squares, and circles are events recorded at arrays A, B, and C in 999, respectively. Triangles are events in 995. Magnitudes of events, ranging from M.0-3.5, are proportional to the symbol size. Blue colors denote deeper events. BRF, Buck Ridge fault; CCF, Coyote Creek fault; CLF, Casa Loma fault; CVF, Clark Valley fault; HF, Hot prings fault. haded zones denote the waveguides inferred by trapped waves. (5b) Toro eak Array B CCF Borrego Valley Clark Lake Borrego prins rofiles across the Buck Ridge Fault near Anza CVF Borrego Mountain Fig.5 (a) Observed and synthetic seismograms at arrays across the CLF and HF for a M3 event (Fig. 4)occurring within the JFZ near Hemet. Model parameters: waveguide width on the CLF is 0 m where Vs =.6 /s while a weak waveguide of 40 m wide and Vs = 3.0 /s on the HF. Vs = 3.3 /s for wall rocks. (b) 3-D finite difference synthetic (blue lines) and observed (red lines) seismograms at array B across the BRF for a M.6 event (Fig. 4)occurring on the BRF at 4 depth near Anza Gap. Model parameters are: waveguide width is 00 m at surface and tapers to 75 m at 8 depth. velocities range from.6 /s to.7 /s reduced by 0-30% from wall rock velocities, and Q values range from 40 to 80 within waveguide at depths of 0-8. The source is a double-couple source within the waveguide. (6a) 34 0' Mojave Desert HVF KF 3 Line Landers JVF (6b) Rupture Zone rofiles across the Rupture Zone of the 99 Landers Earthquake Fig. 6 (a) Map showing locations of seismic arrays at Lines and 3 across the southern rupture zone of the 99 Landers earthquake, and shots 3, 4, 5. The surface-fault slip is shown in the inset. JVF, Johnson Valley fault; KF, Kickapoo fault; HVF, Homestead Valley fault; and MF, into Mountain fault. (b) Vertical-component seismograms at Line for shot 4 detonated in 994, 996, and 998. Trapped waves were prominent within the low-velocity rupture zone (~50 m wide). (c) Travel time decreases in percent for, and fault-zone trapped (6c) Traveltime change (dt/t %) Ratio of to time shift.5 (A) 34 0' 0 4 lip (m) 994 to 996 FZ (B) M7.5 5 Line MF Yucca Valley 6 30' 6 0' West East Traveltime change (dt/t %) Ratio of to time shift.5 (A) 996 to 998 FZ (B) West East waves determined from cross-correlations of seismograms recorded at Lines and 3 for shots 4 and 5 in repeated experiments. The mean ratio of travel time changes for to (solid line) was 0.75 with a standard deviation (dashed lines)of 0.06 between 994 and 996, but decreased to 0.65 with a standard deviation of 0.05 between 996 and 998.

5 34 40' Eastern Mojave Desert Lavic Lake 3 4 Hector Mine urface Rupture 4 showing FZTW in 000 without FZTW in 000 showing FZTW in 999 without FZTW in 999 rofiles at Array 3 across the outh Lavic Lake Fault for Explosion red - obs Vertical arallel blue - syn LLF M7. BF Bullion Mountains Array 34 30' 34 0' ELF HVF CF Aftershock on N. LLF Quackenbush Array 3 Array urface Rupture Gypsum Ridge Bullion Wash ' 6 0' 6 00' Fig.7a Map showing locations of seismic arrays and explosions at the Hector Mine rupture zone. Black circles and triangles denote aftershockes recorded in 000 while grey circles and triangles denote aftershocks recordedc in 999. Circles and triangles denote events showing and without fault-zone trapped waves. Dots denote aftershocks occurring in November of 999 and October of 000. BF - Bullion fault. CF - Calico fault. ELF - Emerson Lake fault. HVF - Homestead Valley fault. JVF - Johnson Valley fault. LLF - Lavic Lake fault. The shaded fault segments are trapped wave inferred rupture zone. rofiles at Array across the North Lavic Lake Fault Aftershock between Faults Aftershock on Buried Fault Evt arallel Evt 3 arallel Evt 4 arallel ' ' Fig. 7b Recorded and 3-D finite-difference synthetic seismograms at Array 3 across the rupture zone on the south Lavic Lake fault, which ruptured in the 999 Hector Mine earthquake, for shot. eismograms have been (-6 Hz) filtered and are plotted using a fixed scale for all traces in each panel. Model parameters used for the best fit to observations given by Li et al. [00] are depth-dependent: the low-velocity waveguide on the LLF is m wide, Vs is.0-.5 /s, and Q is 0-60 within the rupture zone in the depth range of 0-0. The velocities within the rupture zone are reduced by 40-50% from wall-rock velocities. Wall-rock Q is in the 0-0 depth range. In the example, we used an explosion source within the rupture zone at the depth of 30 m. Fig7c Fault-parallel component seismograms recorded at Array for aftershocks (events, 3, and 4 in Fig. 7a) occurring on the north Lavic Lake fault, the northly buried fault and between them, respectively. eismograms have been (<7 Hz) filtered and are plotted using a fixed scale for all traces in each panel. Fault-zone trapped waves were recorded for events and 4 occurring on the LLF and the buried fault, but not for event 3 occurring betwenn the two faults, indicating that the low-velocity waveguides exist on the LLF which breaks to the surface in the Hector Mine earthquake and the buried fault which might ruptured in the HW quake although it did not break to the surface. 0 --> References: Blanpied, M. L., D. A. Lockner, and J. D. Byerlee, An earthquake mechanism based on rapid sealing of faults, Nature, 359, , 99. Byerlee, J., Friction, overpressure and fault-normal compression, Geophys. Res. Lett. 7, 09-, 990. Dieterich, J. 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