skip to main content


Title: Fault Interactions Enhance High‐Frequency Earthquake Radiation
Abstract

Fault complexity has been linked to high‐frequency earthquake radiation, although the underlying physical mechanisms are not well understood. Fault complexity is commonly modeled with rough single faults; however, real‐world faults are additionally complex, existing within networks of other faults. In this study, we introduce two new ways of defining fault complexity using mapped fault traces, characterizing fault networks in terms of their degree of alignment and density. We find that both misalignment and density correlate with enhanced high‐frequency seismic radiation across Southern California, with misalignment showing a stronger correlation. This robust correlation suggests that high‐frequency radiation may arise in part from fault‐fault interactions within networks of misaligned faults. Fault‐fault interactions may therefore have important consequences for earthquake rupture dynamics, energetics and earthquake hazards and should not be overlooked.

 
more » « less
NSF-PAR ID:
10366558
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
48
Issue:
20
ISSN:
0094-8276
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Tectonic faults fail through a spectrum of slip modes, ranging from slow aseismic creep to rapid slip during earthquakes. Understanding the seismic radiation emitted during these slip modes is key for advancing earthquake science and earthquake hazard assessment. In this work, we use laboratory friction experiments instrumented with ultrasonic sensors to document the seismic radiation properties of slow and fast laboratory earthquakes. Stick‐slip experiments were conducted at a constant loading rate of 8 μm/s and the normal stress was systematically increased from 7 to 15 MPa. We produced a full spectrum of slip modes by modulating the loading stiffness in tandem with the fault zone normal stress. Acoustic emission data were recorded continuously at 5 MHz. We demonstrate that the full continuum of slip modes radiate measurable high‐frequency energy between 100 and 500 kHz, including the slowest events that have peak fault slip rates <100 μm/s. The peak amplitude of the high‐frequency time‐domain signals scales systematically with fault slip velocity. Stable sliding experiments further support the connection between fault slip rate and high‐frequency radiation. Experiments demonstrate that the origin of the high‐frequency energy is fundamentally linked to changes in fault slip rate, shear strain, and breaking of contact junctions within the fault gouge. Our results suggest that having measurements close to the fault zone may be key for documenting seismic radiation properties and fully understanding the connection between different slip modes.

     
    more » « less
  2. Abstract

    A fundamental question of earthquake science is what produces damaging high‐frequency ground motion, with the classic Brune‐Haskell model postulating that abrupt fault slip causes it. However, even when amended with heterogeneous rupture, the model fails to explain observations of different sized repeating earthquakes and has challenges explaining high‐frequency radiation patterns. We propose an additional cause for high‐frequency earthquake spectra from elastic collisions of structures within a rupturing fault zone. The collision spectrum is set by an impact contact time proportional to the size of colliding structures so that spectra depend on fundamentally different physical parameters compared with slip models. When added to standard models, collisions can reconcile the discrepant observations since the size, shape, and orientation of structures vary between different fault zones but remain constant within a fault segment. High‐frequency earthquake ground motions and damage may therefore be an outgrowth of fault‐zone structure rather than sudden initiation of slip.

     
    more » « less
  3. Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed using the stacked P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016). We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-100km in the 2004 Parkfield and 2019 Ridgecrest earthquake regions. First, we group earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. We applied our method to the 2019 Ridgecrest earthquake sequence, and our preliminary results show that stations close to the fault strike tend to record more high-frequency energies around the characteristic frequency of fault zone reflections. The frequency bands in which amplified high-frequency energies are observed may be used to estimate the width and velocity contrast of the fault damage zone. We aim to develop a robust and versatile method that can be used to search for fault damage zone structures and estimate their material properties, in order to shed light on earthquake source processes. 
    more » « less
  4. SUMMARY

    Earthquakes come in clusters formed of mostly aftershock sequences, swarms and occasional foreshock sequences. This clustering is thought to result either from stress transfer among faults, a process referred to as cascading, or from transient loading by aseismic slip (pre-slip, afterslip or slow slip events). The ETAS statistical model is often used to quantify the fraction of clustering due to stress transfer and to assess the eventual need for aseismic slip to explain foreshocks or swarms. Another popular model of clustering relies on the earthquake nucleation model derived from experimental rate-and-state friction. According to this model, earthquakes cluster because they are time-advanced by the stress change imparted by the mainshock. This model ignores stress interactions among aftershocks and cannot explain foreshocks or swarms in the absence of transient loading. Here, we analyse foreshock, swarm and aftershock sequences resulting from cascades in a Discrete Fault Network model governed by rate-and-state friction. We show that the model produces realistic swarms, foreshocks and aftershocks. The Omori law, characterizing the temporal decay of aftershocks, emerges in all simulations independently of the assumed initial condition. In our simulations, the Omori law results from the earthquake nucleation process due to rate and state friction and from the heterogeneous stress changes due to the coseismic stress transfers. By contrast, the inverse Omori law, which characterizes the accelerating rate of foreshocks, emerges only in the simulations with a dense enough fault system. A high-density complex fault zone favours fault interactions and the emergence of an accelerating sequence of foreshocks. Seismicity catalogues generated with our discrete fault network model can generally be fitted with the ETAS model but with some material differences. In the discrete fault network simulations, fault interactions are weaker in aftershock sequences because they occur in a broader zone of lower fault density and because of the depletion of critically stressed faults. The productivity of the cascading process is, therefore, significantly higher in foreshocks than in aftershocks if fault zone complexity is high. This effect is not captured by the ETAS model of fault interactions. It follows that a foreshock acceleration stronger than expected from ETAS statistics does not necessarily require aseismic slip preceding the mainshock (pre-slip). It can be a manifestation of a cascading process enhanced by the topological properties of the fault network. Similarly, earthquake swarms might not always imply transient loading by aseismic slip, as they can emerge from stress interactions.

     
    more » « less
  5. Abstract

    Because quartz veins are common in fault zones exhumed from earthquake nucleation temperatures (150°C–350°C), quartz cementation may be an important mechanism of strength recovery between earthquakes. This interpretation requires that cementation occurs within a single interseismic period. We review slip‐related processes that have been argued to allow rapid quartz precipitation in faults, including: advection of silica‐saturated fluids, coseismic pore‐fluid pressure drops, frictional heating, dissolution‐precipitation creep, precipitation of amorphous phases, and variations in fluid and mineral‐surface chemistry. We assess the rate and magnitude of quartz growth that may result from each of the examined mechanisms. We find limitations to the kinetics and mass balance of silica precipitation that emphasize two end‐member regimes. First, the mechanisms we explore, given current kinetic constraints, cannot explain mesoscale fault‐fracture vein networks developing, even incrementally, on interseismic timescales. On the other hand, some mechanisms appear capable, isolated or in combination, of cementing micrometer‐to‐millimeter thick principal slip surfaces in days to years. This does not explain extensive vein networks in fault damage zones, but allows the involvement of quartz cements in fault healing. These end‐members lead us to hypothesize that high flux scenarios, although more important for voluminous hydrothermal mineralization, may be of subsidiary importance to local, diffusive mass transport in low fluid‐flux faults when discussing the mechanical implications of quartz cements. A renewed emphasis on the controls on quartz cementation rates in fault zones will, however, be integral to developing a more complete understanding of strength recovery following earthquake rupture.

     
    more » « less