Scaling Relationships of Source Parameters for Slow Slip Events

Abstract

To better understand the physical mechanisms of slow slip events (SSEs) detected worldwide, we explore the scaling relationships of various source parameters and compare them with similar scaling laws for earthquakes. These scaling relationships highlight differences and similarities between slow slip events and earthquakes and hold implications for the degree of heterogeneity and fault-healing characteristics. The static stress drop remains constant for different-sized events as is observed for earthquakes. However, the static stress drop of slow slip events is within a range of 0.01-1.0 MPa, 1-2 orders of magnitude lower than that found for earthquakes, which could be related to the low stress state on the fault. The average rupture velocity, ranging from kilometers per second to kilometers per day, decreases linearly with increasing seismic moment in log-log space, unlike earthquakes that are nearly constant. This inverse relationship of rupture velocity with seismic moment could be related to the heterogeneity of fault properties. Slow slip events typically have ratios of event duration over dislocation rise time less than 3, while earthquakes have ratios greater than 3. This indicates that slow slip events are less pulselike than earthquakes in their mode of propagation and suggests that the healing behind the rupture front is delayed. The recurrence statistics of slow slip events on the northern Cascadia subduction zone are weakly time predictable and moderately antislip predictable (that is, the event size and preevent recurrence interval are anticorrelated), which may indicate that healing between events strengthens the fault with time.

Effect of stress state on slow rupture propagation in synthetic fault gouges

“…Nevertheless, the apparent velocities of slow rupture propagation within the liz/ctl fault gouges, which range from 0.07 to 5.43 m/s at effective normal stresses of 114 to 268 MPa (Figure 5), are largely consistent with those documented for short-term SSEs in Japan and Cascadia (Table 1) (e.g., Hirose and Obara 2010;Gao et al 2012). It is important to note that the hydration of the mantle wedge in subduction zones is expected to form antigorite rather than liz/ctl (e.g., Hirauchi et al 2010b).…”

“…Despite the dry conditions, we still observed slow slip events during our experiments on the low-μ fault zone material (i.e., liz/ctl) at low confining pressure (i.e., 60 to 140 MPa; Figure 3C). This suggests that variations in the duration and propagation velocity of SSEs (Gao et al 2012) are primarily controlled by the balance between the effective normal stress conditions and the intrinsic properties of the fault zone material. However, we cannot exclude the possibility that other factors, such as dilatancy, neutral velocity dependence of (a − b), and nonmonotonic dependence of steady state friction on slip velocity, also contribute to the emergence of SSEs in subduction zones (e.g., Liu and Rice 2009;Rubin 2008;Liu and Rubin 2010;Hawthorne and Rubin 2013).…”

“…Recent numerical and experimental studies have shown that, depending on friction and stress conditions, slow rupture modes occur either as isolated events or in conjunction with much faster modes (Rubinstein et al 2004;Ben-David et al 2010a;Nielsen et al 2010;Kaneko and Ampuero 2011;Bar Sinai et al 2012). However, previous laboratory experiments on rocks have focused on stick-slip events for bare surfaces of granite and dunite under a relatively narrow range of normal stress conditions (equivalent to less than approximately 10 MPa) (e.g., Johnson and Scholz 1976;Okubo and Dieterich 1984;Kato et al 1992;Ohnaka and Shen 1999), although the values of duration and propagation velocity for the SSEs (Gao et al 2012) may be influenced by variations in effective normal stress on the plate interface. In the present study, we report observations of unstable slip events generated in synthetic fault zones, each of which represents a suitable analog for studying slow slip regions (Kato et al 2010) through triaxial loading at confining pressures ranging from 60 to 170 MPa in order to explore how the properties of slow rupture change across a much wider range of physical conditions.…”

Role of multiscale heterogeneity in fault slip from quasi-static numerical simulations

“…We note that our fixed �τ may be smaller than that inferred for standard earthquakes. For example, Gao et al (2012) infer a range between 0.01 and 1 MPa from SSEs in Cascadia, and Maury et al (2014) infer a value of ~0.1 MPa from the large 2010 SSE in Guerrero, Mexico. We fix a value of �τ = 0.5 MPa for our study.…”

Rheology and stress in subduction zones around the aseismic/seismic transition

“…Slow slip events involve geodetically determined surface displacements of a few centimeters over periods of a few days to~1 year (Peng and Gomberg 2010;Gao et al 2012); these appear to reflect the release of elastic strain by displacement within the subduction zone at rates that are sub-seismic, but one to two orders of magnitude faster than plate motion rates. Slow slip is commonly accompanied by tremor, a non-impulsive seismic signal that may last for periods up to the duration of the slow slip event (Obara 2002;Rogers and Dragert 2003;Schwartz and Rokosky 2007).…”