Earthquake Initiation From Laboratory Observations and Implications for Foreshocks

Abstract

This paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3‐m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale Lc, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a “kick” above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate‐dependent “cascade up” model for earthquake initiation. This model simultaneously accounts for foreshocks that are a by‐product of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth's crust, ranging from slip limited environments with Lc < 1 m, to ignition‐limited environments with Lc > 10 km. In the latter case, Lc fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture.

The Role of Shear Fabric in Controlling Breakdown Processes During Laboratory Slow‐Slip Events

“…The trajectories of phase diagram (highly reproducible across experiments under the same boundary conditions) show points of consensus and some discrepancies when compared to theoretical models of earthquake rupture based on RSF or slip weakening laws. For quartz gouges the occurrence of the peak slip velocity approximately at the end of the breakdown stage (Figure 12) is consistent with theoretical models of earthquakes and stick‐slip events observed on planar surfaces (e.g., Fang et al, 2010; McLaskey, 2019; Mclaskey & Yamashita, 2017). Similarly, the assumption in these theoretical models of a medium that is perfectly elastic with an infinitesimal surface of discontinuity is quite similar with the sharp zone of localization observed in quartz gouges and our interpretation that the coseismic acceleration takes place along these planes (Figure 9).…”

Characteristics of Frequent Dynamic Triggering of Microearthquakes in Southern California

“…The combined effects of reduced nucleation sizes and increased stressing rates from the aseismic events would be effective at promoting microearthquake triggering. Earthquake nucleation length also inversely scales with the initial stress, suggesting an even smaller nucleation size for deep earthquakes during a creeping event (Guérin‐Marthe et al., 2019; Latour et al., 2013; McLaskey, 2019). With a smaller nucleation size, these earthquakes would be preferentially triggered if the fracture surface energy scales with the nucleation length (Ide & Aochi, 2005).…”

Short-Term Foreshocks as Key Information for Mainshock Timing and Rupture: The Mw6.8 25 October 2018 Zakynthos Earthquake, Hellenic Subduction Zone

“…Seismicity studies performed in several tectonic environments have shown that foreshocks increase towards the mainshock as the inverse of time and in a timeframe varying from hours to a few days (imminent foreshocks) up to 5-6 months (short-term foreshocks), e.g., [ 4 , 5 , 6 , 16 , 19 , 56 , 57 , 58 , 59 , 60 ]. The acceleration of the fracturing process is supported by laboratory material fracture experiments, e.g., [ 1 , 61 , 62 ], numerical modeling in spring-block models, e.g., [ 63 , 64 , 65 ] and analytical damage mechanics modeling, e.g., [ 66 ]. Interestingly, the seismicity often accelerates in a long-term sense as well, e.g., in a time window of several years before strong or large earthquakes [ 67 , 68 , 69 ], while in some instances gradual long-term drop of b-value has been also observed [ 70 ].…”

“…Τhis implies that foreshock sequences are relatively enriched in higher magnitude events. The b-value depends on several factors including stress loading conditions and tectonic regime [ 1 , 61 , 79 ] as well as crustal heterogeneity, e.g., [ 26 ]. High b indicates low stress is asymmetrically distributed, while low b is an evidence of concentrated high stress.…”