Internal structure of the San Jacinto fault zone at the Ramona Reservation, north of Anza, California, from dense array seismic data

Abstract

Summary We image the internal structure of the San Jacinto fault zone (SJFZ) near Anza, California, with seismic data recorded by two dense arrays (RA and RR) from ∼42,000 local and ∼180 teleseismic events occurring between 2012–2017. The RA linear array has short aperture (∼470 m long with 12 strong motion sensors) and recorded for the entire analyzed time window, whereas the RR is a large three-component nodal array (97 geophones across a ∼2.4 km x 1.4 km area) that operated for about a month in September-October 2016. The SJFZ at the site contains three near-parallel surface traces F1, F2, and F3 from SW to NE that have accommodated several Mw > 6 earthquakes in the past 15,000 years. Waveform changes in the fault normal direction indicate structural discontinuities that are consistent with the three fault surface traces. Relative slowness from local events and delay time analysis of teleseismic arrivals in the fault normal direction suggest a slower SW side than the NE with a core damage zone between F1 and F2. This core damage zone causes ∼0.05 second delay at stations RR26–31 in the teleseismic P arrivals compared with the SW-most station, and generates both P- and S- type fault zone trapped waves. Inversion of S trapped waves indicates the core damaged structure is ∼100 m wide, ∼4 km deep with a Q value of ∼20 and 40 per cent S-wave velocity reduction compared with bounding rocks. Fault zone head waves observed at stations SW of F3 indicate a local bimaterial interface that separates the locally faster NE block from the broad damage zone in the SW at shallow depth and merges with a deep interface that separates the regionally faster NE block from rocks to the SW with slower velocities at greater depth. The multi-scale structural components observed at the site are related to the geological and earthquake rupture history at the site, and provide important information on the preferred NW propagation of earthquake ruptures on the San Jacinto fault.

General Seismic Architecture of the Southern San Andreas Fault Zone around the Thousand Palms Oasis from a Large-N Nodal Array

“…High correlation coefficients between neighboring stations imply similar large-scale structure beneath those stations. Low coefficients pinpoint a transition between two dissimilar media (e.g., Qin et al, 2021).…”

“…Next, we inspected the delay-time results from all teleseismic and regional events to help quantify the general P-wave velocity (V P ) variations across the profile. For each event, the picked arrival times were corrected for array geometry by subtracting the predicted travel times to individual stations using the TauP algorithm (Crotwell et al, 1999) and the IASP91 global velocity model (e.g., Share, Allam, et al, 2019;Qin et al, 2021;Qiu et al, 2021). A correction was then made for elevation above sea level by subtracting the excess travel times to elevated stations calculated with a reference near-surface V P 2 km=s (from Ajala et al, 2019).…”

“…Active source seismic exploration and potential field results indicate similar near-surface geometries for the surrounding SSAF (e.g., Catchings et al, 2009;Fuis et al, 2017). To understand these and other features better, we apply a range of tools well-suited for imaging internal fault zone features (e.g., Ben-Zion et al, 2015;Share, Allam, et al, 2019;Qin et al, 2021;Qiu et al, 2021). In this article, we focus on general aspects of the recorded data such as the arrival time, frequency, amplitude, and phase variations across the array from direct, reflected, and transmitted seismic phases.…”

Denoising Surface Waves Extracted From Ambient Noise Recorded by 1‐D Linear Array Using Three‐Station Interferometry of Direct Waves

“…First, we project all stations to the straight line connecting RR01 and RR47 (cyan dashed line in Figure 1a). Second, travel time measurements associated with each virtual source i at the target frequency max f are extracted and interpolated (e.g., black curve in Figure 6a) with a regular grid size of Δ = 50 m. Since variations in topography (Figure 2b of Qin et al, 2021) have a negligible effect (<0.5%) on the results, the eikonal tomography can be simplified as:…”

“…We apply this method to data recorded by a dense linear array deployed at the Ramona Reservation (RR) site across surface traces of the San Jacinto fault, near Anza (Figure 1), California. Seismic waveforms from the RR array have been analyzed for fault zone internal structures in Qin et al (2021). ANCs were computed for each station pair of the RR array and the corresponding Rayleigh wave phase velocities for periods from 0.3 to 0.8 s were derived from double beamforming tomography in Wang et al (2019).…”