Model with deeper rupture (M2)
5.5 Seismicity and fault heterogeneity at the transitional depth
coseismic
(potential deep slip) postseismic interseismic
slip budget
Dgeod Drupt Dlock
Fault depth
SL
Dseis
Figure 5.11: Illustration of slip partition andDgeod,DseisDlock, andDruptin a seismic cycle.
The slip budget is split between the co-, post- and inter-seismic periods of fault. The gray profile indicates the depth distribution of stressing rates, which determinesDlock(horizontal red line) and corresponds to Drupt (horizontal black line) right after the earthquake. The red, orange and blue lines represent fault slip during the co-, post- and inter-seismic periods, with the red and blue dashed lines corresponding to the depth of dislocation model with the near-equivalent surface expression. The red and blue band indicate the approximate depth range forDseis and possible range forDgeod.
behavior, as illustrated in Fig. 5.12. More mature faults, characterized by large cumulative displacement and simple geometry, presumably would have more uniform along-strike fault properties and some heterogeneity at the transitional depth, which motivate our models M1-M4 (Fig. 5.12A1). However, it could be speculated that for faults with geometrical, lithological and structural complexity, variation of fault properties along the strike and at the transitional depth would be expected (Sibson, 1984, Fig. 5.12B1) .
Motivated by the consideration above, we develop a fault model that features along- strike variations in large VW regions, and VW patches that represent fault heterogeneity, which are not limited to the large VW region, but distributed in the deeper VS region (Fig. 5.12B2). The long-term fault behavior consists of frequent microseismicity (Mw ∼4) in the interseismic periods of occasional larger events (Mw 6-6.5) (Fig. 5.13). The apparent interseismic locking depth for the segment, inverted from stacked profiles of the surface velocity field across the fault based on a dislocation model in a homogeneous half-space (Fig. 5.14), stays with time at around 6-8 km, while most seismicity occur at 12 km and even deeper due to loading from the surrounding creeping regions, suggesting that such a model with deep creep can indeed reproduce the discrepancy of seismicity and Drupt
(Wdowinski, 2009). It might be questioned whether a large number of isolated VW patches can exist in VS regions, since they should produce repeating earthquakes predominantly, which is not observed for the San Jacinto Fault (Lindsey et al., 2013). In our model, while most of the isolated VW patches produce repeating earthquakes, nearby patches also interact and produce complicated behavior. Therefore, it might still be possible to have these VW patches with high spatial density, so that they frequently interact and do not simply repeat by themselves.
There remains a possibility that the region below the geodetic locking depth could still be
Along strike distance (km) 10
20
−40 −30 −20 −10 0 10 20 30 40
VW patches VW + DW
Depth (km) VS 10
20
-40 -30 -20 -10 0 10 20 30 40
Depth (km)
VS
VW patches VW + DW VW 10
20
-40 -30 -20 -10 0 10 20 30 40
Depth (km)
VS creeping
locked
10 20
-40 -30 -20 -10 0 10 20 30 40
Depth (km)
VS creeping
locked
Conceptual models
Faults with less heterogeneity Faults with more heterogeneity
Rate-and-state fault models
Figure 5.12: Conceptual and physical models for faults with other heterogeneity. (Top) Con- ceptual understanding of fault heterogeneity. More mature faults have along-strike uniform properties, narrower transitional region at depth with less rheological heterogeneities, while less mature faults have structural complexities, potentially characterized by along-strike variations of fault properties and broader transitional region at depth. (Bottom) Rate- and-state fault models motivated by the conceptual models above. Model M2 on the left, together with other models in Fig. 5.3, represent faults with less heterogeneity, while the model (M5) on the right represent faults with more heterogeneity and complexity.
VW, but with a much larger nucleation size, e.g., due to fluid overpressure (Suppe, 2014), or inelastic dilatancy (Segall and Rice, 1995; Segall et al., 2010). Microseismicity could occur at highly stressed local heterogeneity. Further study is needed to explore whether this model could produce fault locking that is compatible with geodetic observations.
In either case, the occurrence of microseismicity depends more on the surrounding creep rates (just like seismicity indicated by blue circles in Fig. 5.5), than the location of the stress concentration front. Thus perhaps to a great extent Dseis would depend on the combined effects of statistical properties of fault heterogeneity distribution and the fault slip rate profiles in the LC transition, while Dlock only delineate the upper bound of the seismicity band. In all, the relation betweenDseis and Dlock would be complicated by other forms of fault heterogeneity or additional physical mechanisms at the transitional depth.
10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
interseismic coupling # 2
−12−11−10−9 −8 −7 −6 −5 −4 −3 −2 −1 0 1 log(m/s) 10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
interseismic coupling # 1
−12−11−10−9 −8 −7 −6 −5 −4 −3 −2 −1 0 1 log(m/s) 10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
earthquake rupture
−12−11−10−9 −8 −7 −6 −5 −4 −3 −2 −1 0 1 log(m/s)
10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
microseismicity
−12−11−10−9 −8 −7 −6 −5 −4 −3 −2 −1 0 1 log(m/s)
(A) Large earthquake rupture (B) Microseismicity
103.02 yr 87.97 yr
(C1) Interseismic period
10
Depth (km) 20
−40 −30 −20 −10 0 10 20 30 40
Along-strike distance (km)
−12−11−10−9 −8 −7 −6 −5 −4 −3 −2 −1 0 1 log(m/s)
96.08 yr
103.99 yr 10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
0.20.40.60.80
Coupling
0.0 0.2 0.4 0.6 0.8 1.0
ISC
10
20
Z (km)
−30 −20 −10 0 10 20 30
X (km)
0 0.40.2 0.60.8
Coupling
0.0 0.2 0.4 0.6 0.8 1.0
ISC
(C2) Interseismic period
(D1) Interseismic period (D2) Interseismic period
Figure 5.13: Complex fault behavior and variation of fault coupling in model M5. Snapshots of a large earthquake rupture (A), microseismicity (B) and interseismic periods (C1,D1) are shown from the long-term behavior of model M5 with logarithmic slip rate in color. The distributions of ISC corresponding to (C1) and (D1) are shown in (C2) and (D2).
(A)
(C1) (C2)
(C3) (B)
Figure 5.14: Shallower locking depth and deeper seismicity in model M5. Stacked profiles of fault-parallel slip rate (A) sampled from the 2D surface velocity field across the fault (B) in the interseismic period of the model M5 are used to invert the apparent Dgeod for the segment. The fault is assumed fully locked at depth<5 km. Dashed blue lines are sampling profiles. Time evolution of the apparent geodetic locking depth for this segment and the depth of seismicity is shown in (C1), with the magnitudes and overall depth distribution of seismicity in (C2) and (C3), respectively, suggesting thatDseis is deeper thanDgeod in this model.