Traditional models of slow slip events (SSEs) often oversimplify fault geometry, yet imaging studies show that real subduction faults are segmented and complex. We investigate how fault interactions influence slip behavior using 3D quasi-dynamic earthquake sequence simulations of two parallel faults with uniform rate-weakening friction, accelerated with hierarchical matrices. Our results identify four slip regimes-periodic earthquakes, coexisting SSEs and earthquakes, only SSEs, and complex sequences-while a single planar fault under the same conditions produces only earthquakes. We quantify fault interaction using the maximum Coulomb stress induced on a target fault by unit, spatially uniform stress drop on a neighboring fault. Because the source stress drop is normalized, the metric depends only on geometry and is independent of friction and nucleation length, and it can be extended to arbitrary fault configurations. The occurrence of SSEs is confined to an intermediate range of interaction strength. We also reproduce the observed moment-duration scaling and show that it depends on event detection thresholds. These results demonstrate that complex fault geometry can naturally generate both slow and fast earthquakes through evolving traction heterogeneities.
It has long been recognized that earthquake-related slip accounts for only a fraction of the overall slip budgets within plate tectonics. As continuous geodetic networks have improved, researchers have discovered slow slip events (SSEs) in various tectonic environments, for example in subduction zone: Cascadia subduction zone (Hirose et al. 1999;Dragert et al. 2001), in continental strike-slip fault systems e.g., Haiyuan fault (Jolivet et al. 2013), San Andreas fault (Rousset et al. 2019). These events involve episodic, slow shear motion along faults (a few orders of magnitude faster than plate motion velocity) with no or minimal seismic activity. These events can range from small to large magnitudes, sometimes comparable to earthquakes of the same magnitude. Although SSEs generate little seismic radiation, large-magnitude events can still cause significant stress perturbations and accumulated slip on the fault, affecting its behavior. They are closely linked spatially and temporally with low-frequency earthquakes (LFEs), very low-frequency earthquakes (VLFEs), and tremors, exhibiting a lower frequency of seismic radiation compared to regular earthquakes of same magnitudes. Therefore, seismological instruments can indirectly detect slow slip events by tracking the migration of tremors (Ito et al. 2007;Shelly et al. 2007;Michel et al. 2018), repeating earthquakes (Kato et al. 2012;Uchida 2019), or LFEs (Bouchon et al. 2011;Frank & Brodsky 2019), which improve detection capabilities for slow slip events. Slow slip events are ubiquitous in subduction zones and exhibit a diverse range of spatiotemporal complexities. Sometimes, they can be observed in shallow depths or below the seismogenic zone. In Nankai Trough, shortterm SSEs are discovered with a duration spanning from days to weeks and 3-6 months recurrence time (Obara et al. 2004;Hirose & Obara 2006) and long-term SSEs are observed in deep areas with around a 1-year duration and 6-year recurrence time (Ozawa et al. 2001). Shallow VLFEs and tremor, megathrust earthquake, long-term SSEs, and short-term SSEs are observed from trough to the deep (Obara & Kato 2016). This pattern of depthdependent SSEs is also seen in the Mexican subduction zone (El Yousfi et al. 2023). In Hikurangi, shallow SSEs are accompanied by microearthquakes, and deep SSEs are long-term with a duration of 2-3 months and a recurrence interval of 2 years with no tremors (Wallace & Eberhart-Phillips 2013).
Slow slip events (SSEs) have a complex relationship with earthquakes in space and time. In the San Andreas fault, slow and fast rupture can coexist on the same section of the fault (Shelly 2009;Veedu & Barbot 2016). Tremor signals were observed 18 months prior to the 2004 Mw 6.0 Parkfield earthquake. There are also examples of SSEs that can occur before earthquakes (Bürgmann 2018;Martínez-Garzón & Poli 2024). The 1999 Mw7.6 Izmit earthquake was preceded by a 44-minute slow slip (Bouchon et al. 2011). A slow slip event in Guerrero triggered the 2014 Mw 7.3 Papanoa earthquake (Radiguet et al. 2016). In the Cascadia subduction zone, GPS observations suggest that the merging of slow slip event fronts, potentially leading to a major earthquake, maybe a possible mechanism for earthquake occurrence (Bletery & Nocquet 2020).
Earthquakes can also trigger SSEs. The 2016 Mw7.8 Kaikoura earthquake triggered a slow slip on the southern Hikurangi subduction zone (Wallace et al. 2018). The 2017 Chiapas earthquake in Mexico triggered a slow slip event on the southern San Andreas Fault, located 3000 km away from the earthquake’s epicenter (Tymofyeyeva et al. 2019). SSEs can also occur periodically without earthquakes, like in Cascadia (Rogers & Dragert 2003) and Hikurangi subduction zone (Wallace et al. 2016). The relationship between earthquakes and SSEs is still unclear and needs more studies and investigation.
Those slow phenomena significantly influence fault behavior by altering the stress field and having intricate relationships with earthquakes (Avouac 2015;Bürgmann 2018;Obara & Kato 2016). Understanding slow slip events is crucial for gaining insights into earthquake mechanisms. There are several explanations for the mechanism of SSEs. SSEs can emerge from the transition of rate and state friction stability from velocityweakening to velocity-strengthening (Liu & Rice 2005;Rubin 2008). Heterogeneous frictional properties, such as varying the proportions of velocity-weakening and velocity-strengthening patches, can produce stable, slow, or dynamic slip events along the fault (Skarbek et al. 2012;Luo & Ampuero 2017;Nie & Barbot 2021). Moreover, fault width plays a role in limiting rupture nucleation and stabilizing the faults (Liu & Rice 2007). Mechanisms like dilatant strengthening (Segall et al. 2010;Liu & Rubin 2010) or frictional restrengthening at high slip speeds (Kato 2003;Shibazaki & Shimamoto 2007;Im & Avouac 2021) can modulate fault stabilization and instigate slow slip events. Additiona
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