Guided Waves from Sources Outside Faults: An Indication for Shallow Fault Zone Structure?

Self Cite

S U M M A R YLocal and teleseismic earthquake waveforms recorded by a 180-m-long linear array (BB) with seven seismometers crossing the Clark fault of the San Jacinto fault zone northwest of Anza are used to image a deep bimaterial interface and core damage structure of the fault. Delay times of P waves across the array indicate an increase in slowness from the southwest most (BB01) to the northeast most (BB07) station. Automatic algorithms combined with visual inspection and additional analyses are used to identify local events generating fault zone head and trapped waves. The observed fault zone head waves imply that the Clark fault in the area is a sharp bimaterial interface, with lower seismic velocity on the southwest side. The moveout between the head and direct P arrivals for events within ∼40 km epicentral distance indicates an average velocity contrast across the fault over that section and the top 20 km of 3.2 per cent. A constant moveout for events beyond ∼40 km to the southeast is due to off-fault locations of these events or because the imaged deep bimaterial interface is discontinuous or ends at that distance. The lack of head waves from events beyond ∼20 km to the northwest is associated with structural complexity near the Hemet stepover. Events located in a broad region generate fault zone trapped waves at stations BB04-BB07. Waveform inversions indicate that the most likely parameters of the trapping structure are width of ∼200 m, S velocity reduction of 30-40 per cent with respect to the bounding blocks, Q value of 10-20 and depth of ∼3.5 km. The trapping structure and zone with largest slowness are on the northeast side of the fault. The observed sense of velocity contrast and asymmetric damage across the fault suggest preferred rupture direction of earthquakes to the northwest. This inference is consistent with results of other geological and seismological studies.

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 95%

Internal structure of the San Jacinto fault zone at Blackburn Saddle from seismic data of a linear array

Ben‐Zion

Ross

et al. 2017

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“…We note that many detections including those producing the clearest candidate FZTW (green and red circles) are generated by events located considerably away from the fault. This implies a relatively shallow trapping structure that can be excited by wave energy of regional events that enter the FZ layer from below Fohrmann et al 2004). Fig.…”

Section: Resultsmentioning

confidence: 99%

“…15) the wide distribution of events generating FZTW (e.g. Fohrmann et al 2004) and the constant moveout (Fig. 16) between the direct S and trapped waves (e.g.…”

Section: Discussionmentioning

confidence: 99%

Internal structure of the San Jacinto fault zone at Jackass Flat from data recorded by a dense linear array

Qiu

Ben‐Zion

Ross

et al. 2017

S U M M A R YThe internal structure of the Clark fault in the trifurcation area of the San Jacinto fault zone is imaged using seismograms recorded by a dense linear array (Jackass Flat, JF) crossing the surface trace of the fault and an adjacent array (TR) to the SW. Delay times between phase arrivals associated with ∼3500 local earthquakes and nine teleseismic events are used to estimate velocity variations within the arrays. The teleseismic P waves travel faster beneath the TR than the JF array, in contrast to larger scale tomographic results. Statistical analysis of local P-wave delay times indicates that the entire JF array, with an aperture of ∼400 m, is inside a low-velocity damage zone. This low-velocity zone is bounded on the NE side by a shallow bimaterial interface generating fault zone head waves, and it contains an inner zone of more intense damage generating fault zone trapped waves. The P-wave velocity contrast across the local bounding bimaterial interface is 10-15 per cent. The trapping structure is associated with a width of ∼200 m, S-wave velocity reduction of ∼35 per cent with respect to the surrounding rock, Q-value of ∼20 and depth of ∼3.5 km. The imaging results suggest that the main seismogenic fault is near the SW end of the JF array, in agreement with a prominent geomorphologic feature. The existence of intense local damage on the crustal block with faster larger scale velocity at depth is consistent with common propagation of earthquake ruptures in the area to the NW.

“…Other researchers, utilizing some of the same or similar FZTW data, have proposed that the low velocity zone terminates at a much shallower 5 km depth or less (e.g. Ben-Zion & Sammis 2003;Peng et al 2003;Fohrmann et al 2004;Lewis et al 2005). Part of the difficulty in determining the depth is because of the high level of uncertainty and non-uniqueness associated with FZTWs (Michael & Ben-Zion 1998).…”

Section: Introductionmentioning

confidence: 99%

The effect of gradational velocities and anisotropy on fault-zone trapped waves

Gulley

Eccles

Kaipio

et al. 2017