Properties of the Aftershock Sequence of the 1999 Mw 7.1 Hector Mine Earthquake: Implications for Aftershock Hazard

Abstract

We investigate the spatial and temporal seismicity parameters and the related probabilistic aftershock hazard for the aftershock sequence of the 1999 M w 7.1 Hector Mine mainshock and compare it with the neighboring 1992 M w 7.3 Landers sequence. Using a catalog of 11,000 earthquakes, we determine the earthquake size distribution (b-value), the aftershock decay rate (p-value), and the seismic activity rate (a-value). The b-values are high (b Ͼ 1.2) within the rupture area, significantly lower (b Ϸ 0.7) north of the rupture area, and increase with time since the mainshock. Probabilistic aftershock hazard maps, computed automatically as early as 4 days after the mainshock, identified the northernmost part of the sequence as the highest-hazard region. These maps show a good agreement between the forecasts and the recorded large aftershocks. Based on the asymmetrical b-value and hazard patterns for both the Hector Mine and Landers sequences, we hypothesize that the mainshock rupture directivity and slip distribution influence aftershock hazard. Current static or dynamic stress triggering models cannot resolve this spatial and temporal evolution of the hazard. Stress tensor inversions of 1400 relocated first-motion focal mechanisms show predominantly a strike-slip stress state with a SW-NE trend of the greatest principal stress. The heterogeneity of the stress field is unusually high near the Hector Mine and Landers mainshock ruptures, particularly near patches of large slip.

The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia, Turkey

“…Low variance (0.17, Table 3) indicates a good fit of homogeneous stress tensor to the observed focal mechanism, and shows low heterogeneity of the stress field (Michael et al, 1990;Lu et al, 1997;Wiemer et al, 2002;Bohnhoff et al, 2006;Görgün et al, 2010). The results from the stress tensor inversions based on 23 focal mechanisms are shown in map view in Fig.…”

“…10 (blue bars). Wiemer et al (2002) concluded that a low degree of heterogeneity in stress field (variance 0.2) indicates uniformity. There is no significant variance in stress tensor orientations (variance 0.2).…”

A Method for Calibration of the Local Magnitude Scale Based on Relative Spectral Amplitudes, and Application to the San Juan Bautista, California, Area

“…They are used when examining earthquake statistics, as in b-value studies (e.g., Wiemer et al, 2002;Wyss et al, 2004;Ghosh et al, 2008), when interpreting earthquake rates, as in studies of repeating earthquakes (e.g., Nadeau and Johnson, 1998;Templeton et al, 2008;Werner and Sornette, 2008;Taira et al, 2014), and when comparing geodetic and seismic observations, as in studies of postseismic slip (e.g., Bell et al, 2012;Fattahi et al, 2015;Hawthorne et al, 2016). Here, we use an empirical Green's function approach to estimate the relative moments of about 1600 earthquakes near San Juan Bautista, California.…”

Aftershock Hazard Magnitude, Time, and Location Probability Forecasting

“…Large aftershocks can be particularly hazardous if they occur in populated areas such as the M w 5.80 aftershock to the 1999 M w 7.40 Izmit, Turkey, earthquake. This aftershock killed 7 and injured a further 420 people (Wiemer et al 2002). Toda et al (1998), and Wiemer and Katsumata (1999) systematically analyzed the spatial distribution of seismicity within aftershock zones.…”