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.
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
- 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
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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.
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SUMMARY A large non-double-couple component of a tectonic earthquake indicates that its rupture likely was complex and likely involved multiple faults. Detailed source models of such earthquakes can add to our understanding of earthquake source complexity. The 2007 Martinique earthquake in the Caribbean Sea is one of the largest recent earthquakes with a known large non-double-couple component. It was an intermediate depth intraslab earthquake within the South American plate where it is subducting beneath the Caribbean plate. We applied potency density tensor inversion (PDTI) to teleseismic P waves generated by the 2007 Martinique earthquake to model its source processes and focal mechanism distribution. We identified two focal mechanisms: a strike-slip mechanism with a north–south tension axis (T-axis), and a downdip extension (DDE) mechanism with an east–west T-axis. Rupture by the DDE mechanism was predominant in the northern part of the source region and strike-slip rupture in the southern part. These two focal mechanisms had approximately parallel pressure axes (P-axes) and approximately orthogonal T-axes. The seismic moments released by both types of rupture were almost equal. These results indicate that the 2007 Martinique earthquake had a large non-double-couple component. We identified five subevents with two predominant directions of rupture propagation: two strike-slip subevents propagated to the southeast and three DDE subevents propagated to the east. Although the directions of propagation were consistent for each focal mechanism, each subevent appears to have occurred in isolation. For example, the rupture of one DDE subevent propagated from the edge of the source region back towards the hypocentre. Complex ruptures that include multiple subevents may be influenced by high pore fluid pressure associated with slab dehydration. Our results show that PDTI can produce stable estimates of complex seismic source processes and provide useful information about the sources of complex intermediate depth intraslab earthquakes for which fault geometry assumptions are difficult.
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