Creep events slip less than ordinary earthquakes (original) (raw)
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Can observations of earthquake scaling constrain slip weakening?
Geophysical Journal International, 2005
We use observations of earthquake source parameters over a wide magnitude range (M W ∼ 0-7) to place constraints on constitutive fault weakening. The data suggest a scale dependence of apparent stress and stress drop; both may increase slightly with earthquake size. We show that this scale dependence need not imply any difference in fault zone properties for different sized earthquakes. We select 30 earthquakes well-recorded at 2.5 km depth at Cajon Pass, California. We use individual and empirical Green's function spectral analysis to improve the resolution of source parameters, including static stress drop (σ) and total slip (S). We also measure radiated energy E S. We compare the Cajon Pass results with those from larger California earthquakes including aftershocks of the 1994 Northridge earthquake and confirm the results of Abercrombie (1995): µE S /M 0 σ (where µ = rigidity) and both E S /M 0 and σ increase as M 0 (and S) increases. Uncertainties remain large due to model assumptions and variations between possible models, and earthquake scale independence is possible within the resolution. Assuming that the average trends are real, we define a quantity G = (σ − 2µE S /M 0)S/2 which is the total energy dissipation in friction and fracture minus σ 1 S, where σ 1 is the final static stress. If σ 1 = σ d , the dynamic shear strength during the last increments of seismic slip, then G = G, the fracture energy in a slip-weakening interpretation of dissipation. We find that G increases with S, from ∼10 3 J m −2 at S = 1 mm (M1 earthquakes) to 10 6-10 7 J m −2 at S = 1 m (M6). We tentatively interpret these results within slip-weakening theory, assuming G ≈ G. We consider the common assumption of a linear decrease of strength from the yield stress (σ p) with slip (s), up to a slip D c. In this case, if either D c , or more generally (σ p − σ d) D c , increases with the final slip S we can match the observations, but this implies the unlikely result that the early weakening behaviour of the fault depends on the ultimate slip that the fault will sustain. We also find that a single slip-weakening function σ F (s) is able to match the observations, requiring no such correlation. Fitting G over S = 0.2 mm to 0.2 m with G ∝ S 1+n , we find n ∼ 0.3, implying a strength drop from peak σ p − σ F (S) ∝ S n. This model also implies that slip weakening continues beyond the final slip S of typical earthquakes smaller than ∼ M6, and that the total strength drop σ p − σ d for large earthquakes is typically >20 MPa, larger than σ. The latter suggests that on average a fault is initially stressed below the peak strength, requiring stress concentration at the rupture front to propagate slipping.
Slow slip events in the early part of the earthquake cycle
Journal of Geophysical Research: Solid Earth
In February 2014 a M w = 7.0 slow slip event (SSE) took place beneath the Nicoya Peninsula, Costa Rica. This event occurred 17 months after the 5 September 2012, M w = 7.6, earthquake and along the same subduction zone segment, during a period when significant postseismic deformation was ongoing. A second SSE occurred in the middle of 2015, 21 months after the 2014 SSE and 38 months after the earthquake. The recurrence interval for Nicoya SSEs was unchanged by the earthquake. However, the spatial distribution of slip for the 2014 event differed significantly from previous events, having only deep (~40 km) slip, compared to previous events, which had both deep and shallow slip. The 2015 SSE marked a return to the combination of deep plus shallow slip of preearthquake SSEs. However, slip magnitude in 2015 was nearly twice as large (M w = 7.2) as preearthquake SSEs. We employ Coulomb Failure Stress change modeling in order to explain these changes. Stress changes associated with the earthquake and afterslip were highest near the shallow portion of the megathrust, where preearthquake SSEs had significant slip. Lower stress change occurred on the deeper parts of the plate interface, perhaps explaining why the deep (~40 km) region for SSEs remained unchanged. The large amount of shallow slip in the 2015 SSE may reflect lack of shallow slip in the prior SSE. These observations highlight the variability of aseismic strain release rates throughout the earthquake cycle. Plain Language Summary We analyzed small signals in continuous GPS time series. By averaging many GPS measurements over a day, we are able to get very precise measurements of the motion of the ground. We found two events in the Nicoya Peninsula of Costa Rica where the GPS changed direction and began moving toward the oceanic trench in the opposite direction of subduction plate motion. These events are called slow slip events and have been found in other regions such as Cascadia, Alaska, Japan, and New Zealand. In Nicoya, a large earthquake of magnitude 7.6 on the Richter scale occurred in 2012. The two slow slip events occurred in 2014 and 2015. We explored the relationship between the earthquake and the slow slip events and looked to see if the earthquake changed the behavior of the slow slip events. We found that the slow slip events have a regular timing before and after the earthquake, but the behavior of the slow slip events since the earthquake is different with slip taking place along different portions of the plate interface then was previously seen.
Bulletin of the Seismological Society of America, 2008
Coseismic slip is observed to increase with earthquake rupture length for lengths far beyond the length scale set by the seismogenic layer. The observation, when interpreted within the realm of static dislocation theory and the imposed limit that slip be confined to the seismogenic layer, implies that earthquake stress drop increases as a function of rupture length for large earthquakes and, hence, that large earthquakes differ from small earthquakes. Here, a three-dimensional elastodynamic model is applied to show that the observed increase in coseismic slip with rupture length may be satisfied while maintaining a constant stress drop across the entire spectrum of earthquake sizes when slip is allowed to penetrate below the seismogenic layer into an underlying zone characterized by velocity-strengthening behavior. Is this deep coseismic slip happening during large earthquakes? We point to a number of additional associated features of the model behavior that are potentially observable in the Earth. These include the predictions that a substantial fraction, on the order of onethird of the total coseismic moment, is due to slip below the seismogenic layer and that slip below the seismogenic layer should be characterized by long rise times and a dearth of high-frequency motion.
Creep on seismogenic faults: Insights from analogue earthquake experiments
Tectonic faults display a range of slip behaviors including continuous and episodic slip covering rates of more than 10 orders of magnitude (m/s). The physical control of such kinematic observations remains ambiguous. To gain insight into the slip behavior of brittle faults we performed laboratory stick-slip experiments using a rock analogue, granular material. We realized conditions under which our seismogenic fault analogue shows a variety of slip behaviors ranging from slow, quasi continuous creep to episodic slow slip to dynamic rupture controlled by a limited number of parameters. We explore a wide parameter space by varying loading rate from those corresponding to interseismic to postseismic rates and normal loads equivalent to hydrostatic to lithostatic conditions at seismogenic depth. The experiments demonstrate that significant interseismic creep and earthquakes may not be mutually exclusive phenomena and that creep signals vary systematically with the fault’s seismic poten...
The effect of slip variability on earthquake slip-length scaling, Geophysical Journal International
Geophysical Journal International, 2005
There has been debate on whether average slip D in long ruptures should scale with rupture length L, or with rupture width W . This scaling discussion is equivalent to asking whether average stress drop σ , which is sometimes considered an intrinsic frictional property of a fault, is approximately constant over a wide range of earthquake sizes. In this paper, we examine slip-length scaling relations using a simplified 1-D model of spatially heterogeneous slip. The spatially heterogeneous slip is characterized by a stochastic function with a Fourier spectrum that decays as k −α , where k is the wavenumber and α is a parameter that describes the spatial smoothness of slip. We adopt the simple rule that an individual earthquake rupture consists of only one spatially continuous segment of slip (i.e. earthquakes are not generally separable into multiple disconnected segments of slip). In this model, the slip-length scaling relation is intimately related to the spatial heterogeneity of the slip; linear scaling of average slip with rupture length only occurs when α is about 1.5, which is a relatively smooth spatial distribution of slip. We investigate suites of simulated ruptures with different smoothness, and we show that faults with large slip heterogeneity tend to have higher D/L ratios than those with spatially smooth slip. The model also predicts that rougher faults tend to generate larger numbers of small earthquakes, whereas smooth faults may have a uniform size distribution of earthquakes. This simple 1-D fault model suggests that some aspects of stress drop scaling are a consequence of whatever is responsible for the spatial heterogeneity of slip in earthquakes.
The evolving interaction of low-frequency earthquakes during transient slip
Observed along the roots of seismogenic faults where the locked interface transitions to a stably sliding one, low-frequency earthquakes (LFEs) primarily occur as event bursts during slow slip. Using an event catalog from Guerrero, Mexico, we employ a statistical analysis to consider the sequence of LFEs at a single asperity as a point process, and deduce the level of time clustering from the shape of its autocorrelation function. We show that while the plate interface remains locked, LFEs behave as a simple Poisson process, whereas they become strongly clustered in time during even the smallest slow slip, consistent with interaction between different LFE sources. Our results demonstrate that bursts of LFEs can result from the collective behavior of asperities whose interaction depends on the state of the fault interface.
Repeating earthquakes, episodic tremor and slip: Emerging patterns in complex earthquake cycles?
Complexity, 2007
The lack of regularity in earthquake cycles continues to be a confounding issue in earthquake science. Lately, observations of episodic nonvolcanic tremor and slip (ETS) along a few well-instrumented tectonic plate boundaries are intriguing: these features recur together with predictable time intervals. Data now trace recurring ETS back to 1990 and no significant earthquake ever followed an ETS episode. This observation and the fact that stress drops associated with episodic slips are low, only on the order of 0.01 MPa, suggests that repeated ETS has little cumulative effects in priming the fault for the next large earthquake. Another known regularity in seismic activity is the so-called repeating earthquakes that rupture the same patch of fault repetitively. Current hypotheses for repeating earthquakes point to the interaction of continual, aseismic fault slip with locked, seismogenic patches of the fault. Interestingly, ETS also recur near where the transition between brittle faulting and plastic flow is expected, although it is not clear how and why regularities in space and time are interconnected. New data show that ETS recurs throughout the entire length of the Cascadia subduction zone, thus ruling out any special, local factors as necessary conditions for ETS. Recurrence intervals of ETS vary along the Cascadia. Such variations are not governed by the rate of plate motion that ultimately drives the earthquake process, but they do coincide with variations in the geology of the overriding plate that can influence the rheology along the plate interface. To this end, we call attention to the Portevin-Le Chatelier effect (PLC, or jerky flow) as a potential analog to the earthquake process. The dynamics of the PLC has been extensively studied and shows many intriguing features as the system goes from chaotic to self-organized critical regimes as strain rate increases. In particular, the PLC exhibits not only stick-slip behavior (stress serration) over time but also spatial interactions over extended regions-features that are necessary to account for complex spatio-temporal variations associated with earthquake activities. Wiley Periodicals, Inc. Complexity 12: 33-44, 2007 large earthquakes reaffirmed the basic concept of earthquake cycles [1] or a direct extension of the elastic rebound theory of Reid . During the long interval (often decades to centuries) between large earthquakes-the inter-seismic stage-the seismogenic fault has little displacement across it ("locked") while two blocks of lithosphere that straddles the fault continue to slide relative to each other in the far-field [ ]. The slip rate in the far-field is controlled ultimately by motion of tectonic plates over geologic time scales.
Can slip heterogeneity be linked to earthquake recurrence
The rupture process of two M4 repeating earthquake sequences in eastern Taiwan with contrasting recurrence behavior is investigated to demonstrate a link between slip heterogeneity and earthquake recurrence. The M3.6–3.8 quasiperiodic repeating earthquakes characterized by 3 years recurrence interval reveal overlapped slip concentrations. Inferred slip distribution for each event illustrates two asperities with peak slip of 47.7 cm and peak stress drop of 151.1 MPa. Under the influence of nearby M6.9 event, the M4.3–4.8 repeating earthquakes separated only by 6–87 min, however, reveal an aperiodic manner. There is a distinct rupture characteristic without overlap in the slip areas, suggesting that shortening of the recurrence interval by the nearby large earthquake may change the slip heterogeneity in a repeatedly ruptured asperity. We conclude that the inherent heterogeneity of stress and strength could influence the distribution of coseismic slip, which is strongly tied to the recurrence behavior.
The effect of slip variability on earthquake slip-length scaling
Geophysical Journal International, 2005
There has been debate on whether average slip D in long ruptures should scale with rupture length L, or with rupture width W . This scaling discussion is equivalent to asking whether average stress drop σ , which is sometimes considered an intrinsic frictional property of a fault, is approximately constant over a wide range of earthquake sizes. In this paper, we examine slip-length scaling relations using a simplified 1-D model of spatially heterogeneous slip. The spatially heterogeneous slip is characterized by a stochastic function with a Fourier spectrum that decays as k −α , where k is the wavenumber and α is a parameter that describes the spatial smoothness of slip. We adopt the simple rule that an individual earthquake rupture consists of only one spatially continuous segment of slip (i.e. earthquakes are not generally separable into multiple disconnected segments of slip). In this model, the slip-length scaling relation is intimately related to the spatial heterogeneity of the slip; linear scaling of average slip with rupture length only occurs when α is about 1.5, which is a relatively smooth spatial distribution of slip. We investigate suites of simulated ruptures with different smoothness, and we show that faults with large slip heterogeneity tend to have higher D/L ratios than those with spatially smooth slip. The model also predicts that rougher faults tend to generate larger numbers of small earthquakes, whereas smooth faults may have a uniform size distribution of earthquakes. This simple 1-D fault model suggests that some aspects of stress drop scaling are a consequence of whatever is responsible for the spatial heterogeneity of slip in earthquakes.
Journal of Structural Geology, 2006
The spatial and temporal accumulation of slip from multiple earthquake cycles on active faults is poorly understood. Here, we describe a methodology that can determine the time period of observation necessary to reliably constrain fault behaviour, using a high-resolution longtimescale (the last 17 kyr) fault displacement dataset over the Rangitaiki Fault (Whakatane Graben, New Zealand). The fault linked at ca. 300 ka BP and analysis of time periods within the last 17 kyr gives insight into steady-state behaviour for time intervals as short as ca. 2 kyr. The maximum displacement rate observed on the Rangitaiki Fault is 3.6G1.1 mm yr K1 measured over 17 kyr. Displacement profiles of the last 9 ka of fault movement are similar to profiles showing the last 300 ka of fault movement. In contrast, profiles determined for short time intervals (2-3 kyr) are highly irregular and show points of zero displacement on the larger segments. This indicates temporal and spatial variability in incremental displacement associated with surface-rupturing slip events. There is spatial variability in slip rates along fault segments, with minima at locations of fault interaction or where fault linkage has occurred in the past. This evidence suggests that some earthquakes appear to have been confined to specific segments, whereas larger composite ruptures have involved the entire fault. The short-term variability in fault behaviour suggests that fault activity rates inferred from geodetic surveys or surface ruptures from a single earthquake may not adequately represent the longer-term activity nor reflect its future behaviour. Different magnitude events may occur along the same fault segment, with asperities preventing whole segment rupture for smaller magnitude events. q