Toggling of seismicity by the 1997 Kagoshima earthquake couplet: A demonstration of time‐dependent stress transfer

Abstract

[1] Two M $ 6 well-recorded strike-slip earthquakes struck just 4 km and 48 days apart in Kagoshima prefecture, Japan, in 1997, providing an opportunity to study earthquake interaction. Aftershocks are abundant where the Coulomb stress is calculated to have been increased by the first event, and they abruptly stop where the stress is dropped by the second event. This ability of the main shocks to toggle seismicity on and off argues that static stress changes play a major role in exciting aftershocks, whereas the dynamic Coulomb stresses, which should only promote seismicity, appear to play a secondary role. If true, the net stress changes from a sequence of earthquakes might be expected to govern the subsequent seismicity distribution. However, adding the stress changes from the two Kagoshima events does not fully capture the ensuing seismicity, such as its rate change, temporal decay, or migration away from the ends of the ruptures. We therefore implement a stress transfer model that incorporates rate/state friction, in which seismicity is treated as a sequence of independent nucleation events that are dependent on the fault slip, slip rate, and elapsed time since the last event. The model reproduces the temporal response of seismicity to successive stress changes, including toggling, decay, and aftershock migration. Nevertheless, the match of observed to predicted seismicity is quite imperfect, due perhaps to inadequate knowledge of several model parameters. However, to demonstrate the potential of this approach, we build a probabilistic forecast of larger earthquakes on the expected rate of small aftershocks, taking advantage of the large statistical sample the small shocks afford. Not surprisingly, such probabilities are highly time-and location-dependent: During the first decade after the main shocks, the seismicity rate and the chance of successive large shocks are about an order of magnitude higher than the background rate and are concentrated exclusively in the stress triggering zones.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7223 Seismology: Seismic hazard assessment and prediction; 7230 Seismology: Seismicity and seismotectonics; 7260 Seismology: Theory and modeling; KEYWORDS: Coulomb stress, stress triggering, seismicity Citation: Toda, S., and R. Stein, Toggling of seismicity by the 1997 Kagoshima earthquake couplet: A demonstration of timedependent stress transfer,

Remotely Triggered Earthquakes Following Moderate Mainshocks (or, Why California Is Not Falling into the Ocean)

“…In fact, isolating the immediate San Simeon aftershock zone defined by (somewhat arbitrary) spatial limits 35.35°-35.9°N and 121.2°-120.65°W, one finds an apparently abrupt decrease in seismicity at the time of the Parkfield mainshock ( Figure 6). The result shown in Figure 6 is reminiscent of the "toggling" of seismicity observed by Toda and Stein (2003) and, more fundamentally, the stress shadow hypothesis (e.g., Harris and Simpson, 1992;Jaume and Sykes, 1996). Such an effect would presumably be related to the static stress change caused by the Parkfield mainshock in the San Simeon region, an issue well beyond the scope of this paper.…”

“…The association of triggered earthquakes with dynamic stress changes is in contrast to aftershocks, which have been assumed to be caused primarily by local, static stress changes associated with fault movement (e.g., Das and Scholz, 1981;King et al, 1994;Toda and Stein, 2003). (According to convention, aftershocks are generally, albeit vaguely, assumed to be events within one to two fault lengths of a mainshock.)…”

“…In addition to developing theories of static stress transfer (e.g., Das and Scholz, 1981;King et al, 1994;Toda and Stein, 2003), remotely triggered earthquakes were first identified in 1992 (Hill et al, 1993) and have subsequently been observed following large (generally M > 7) earthquakes in California as well as in other regions. Investigations of "earthquake interactions" remain at the forefront of earthquake science, with many key questions still unanswered: What is the mechanism by which remote triggering occurs?…”

Scenario for a Short-Term Probabilistic Seismic Hazard Assessment (PSHA) in Chiayi, Taiwan

“…For application of the rate-and-state friction model, previous studies (e.g., Toda and Stein 2003;Toda et al 2005;and Catalli et al 2008) suggested that the physically reasonable range for Aσ was between 0.1 and 0.4 bars. In this study, a fixed Aσ of 0.2 bars was assumed.…”

Fluid-induced seismicity: comparison of rate- and state- and critical pressure theory

“…Whereas CPT is designed for changes in normal stress by changing pore pressure, Dieterich (1994) derives a general equation for seismicity changes, as compared to the background seismicity if shear and/ or normal stresses are changed. The RST has been used in the simulation of aftershocks and earthquake swarms (Catalli et al 2008;Daniel et al 2011;Dieterich et al 2000;Kilb et al 2002;Toda et al 2002Toda et al , 2003 and recently also utilized for seismicity modelling in geothermal reservoirs (Hakimhashemi et al 2014). In this model, the change in Coulomb failure stress (King 2007) is combined with the RST theory.…”