The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue

The Hector Mine, California, earthquake (M_w 7.1) struck the Mojave Desert at 09:46 UTC, 16 October 1999. The earthquake occurred approximately 55 km northwest of the town of Twentynine Palms, California, and about 200 km east-northeast of Los Angeles (Fig. 1). The shock was widely felt throughout southern California, southern Nevada, western Arizona, and northernmost Baja California, Mexico. The Hector Mine earthquake, like the M_w 7.3 Landers earthquake seven years earlier, was associated with fault rupture in the eastern California shear zone (ECSZ) (Fig. 1), which is an approximately 80-km-wide zone of deformation that accommodates about 24% of the relative Pacific–North American plate motion (Sauber et al., 1986, 1994; Dokka and Travis, 1990; Savage et al., 1990, 2001; Gan et al., 2000; Miller et al., 2001). A block diagram highlighting some of the basic aspects of the Hector Mine earthquake is presented in Figure 2. A preliminary summary of the Hector Mine earthquake, its effects, and the response of the geoscience community is presented by Scientists from the U.S. Geological Survey; Southern California Earthquake Center, and California Division of Mines and Geology (USGS, SCEC, and CDMG, 2000)

The 1997 Kagoshima (Japan) earthquake doublet: A quantitative analysis of aftershock rate changes

We quantitatively map relative rate changes for the aftershock sequence following the second mainshock of the 1997 earthquake doublet (M_W = 6.1, M_W = 6.0) in the Kagoshima province (Japan). Using the spatial distribution of the modified Omori law parameters for aftershocks that occurred during the 47.8 days between the two mainshocks, we forecast the aftershock activity in the next 50 days and compare it to the actually observed rates. The relative rate change map reveals four regions with statistically significant relative rate changes - three negative and one positive. “Our analysis suggests that the coseismic rate changes for off-fault aftershocks could be explained by changes in static stress. However, to explain the activation and deactivation of on-fault seismicity, other mechanism such as unusual crustal properties and the presence of abundant crustal fluids are required.

The static stress change triggering model: Constraints from two southern California aftershock sequences

Static stress change has been proposed as a mechanism of earthquake triggering. We quantitatively evaluate this model for the apparent triggering of aftershocks by the 1992 M_W 7.3 Landers and 1994 M_W 6.7 Northridge earthquakes. Specifically, we test whether the fraction of aftershocks consistent with static stress change triggering is greater than the fraction of random events which would appear consistent by chance. Although static stress changes appear useful in explaining the triggering of some aftershocks, the model's capability to explain aftershock occurrence varies significantly between sequences. The model works well for Landers aftershocks. Approximately 85% of events between 5 and 75 km distance from the mainshock fault plane are consistent with static stress change triggering, compared to ∼50% of random events. The minimum distance is probably controlled by limitations of the modeling, while the maximum distance may be because static stress changes of <0.01 MPa trigger too few events to be detected. The static stress change triggering model, however, can not explain the first month of the Northridge aftershock sequence significantly better than it explains a set of random events. The difference between the Landers and Northridge sequences may result from differences in fault strength, with static stress changes being a more significant fraction of the failure stress of weak Landers-area faults. Tectonic regime, regional stress levels, and fault strength may need to be incorporated into the static stress change triggering model before it can be used reliably for seismic hazard assessment

Active tectonics in the Gulf of California and seismicity (M > 3.0) for the period 2002–2014

We present a catalog of accurate epicenter coordinates of earthquakes located in the Gulf of California (GoC) in the period 2002–2014 that permits us to analyze the seismotectonics and to estimate the depth of the seismogenic zone of this region. For the period April 2002 to December 2014 we use body-wave arrival times from regional stations of the Broadband Seismological Network of the GoC (RESBAN) operated by CICESE to improve hypocenter locations reported by global catalogs. For the northern region of the GoC (30°N–32°N) we added relocated events from the 2011-Hauksson-Yang-Shearer, Waveform Relocated Earthquake Catalog for Southern California (Hauksson et al., 2012; Lin et al., 2007). From October 2005 to October 2006 we incorporated hypcenters located by Sumy et al. (2013) in the southern GoC combining an array of ocean-bottom seismographs, of the SCOOBA experiment, with onshore stations of the NARS-Baja array. This well constrained catalog of seismicity highlights zones of active tectonics and seismic deformation within the North America-Pacific plate boundary. We estimate that the minimum magnitude of completeness of this catalog is Mc = 3.3 ± 0.1 and the b = 0.92 ± 0.04 value of the Gutenberg-Richter relation. We find that most earthquakes in the southern GoC are generated by transform faults and this region is more active than the central GoC region. However, the northern region, where most deformation is generated by oblique faults is as active as the southern region. We used the ISC catalog to evaluate the size distribution of seismicity of these regions, and the b value of the Gutenberg-Richter relation and found that b is slightly lower in the central GoC (b = 0.86 ± 0.02) compared to the northern (b = 1.14 ± 0.04) and the southern (b = 1.11 ± 0.04) regions. We observed seismicity that occurs in the Stable Central Peninsular Province, despite the fact that significant active deformation has not been identified in this region