The 3 August 2009 Mw 6.9 Canal de Ballenas Region, Gulf of California, Earthquake and Its Aftershocks

Abstract

On 3 August 2009 an earthquake of magnitude M w 6.9 occurred near Canal de Ballenas, in the north-central region of the Gulf of California, Mexico. The focal mechanism of the main event, reported in the Global Centroid Moment Tensor (CMT) catalog, is right lateral strike-slip with a strike of 216°and a dip of 78°. The initial location reported by the National Seismological Service of Mexico [Servicio Sismólogico Nacional (SSN)] and the Array Network Facility (ANF) suggested that the epicenter was on the North American plate near the Tiburón fault, which is considered inactive. This earthquake was preceded by a magnitude m b 5.5 event that occurred about 5 min before. In the next 40 min after the main event two aftershocks with magnitudes m b 4.9 and M w 6.2 occurred, and on 5 August a third aftershock of M w 5.7 was located in the Canal de Ballenas region. The events of August 2009 were recorded by the regional stations of the broadband network Red Sismólogica de Banda Ancha (RESBAN) that Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) operates and by stations of the SSN also located in the region of the Gulf of California. We used body-wave arrivals to determine precise epicentral locations and to estimate the rupture area of this important sequence of earthquakes. The resulting hypocentral coordinates indicate that the main event of this sequence occurred along the Canal de Ballenas transform fault, with a rupture length of 50 km. Based on the aftershock distribution, we estimate that the main event had a rupture area of approximately 600 km 2 , an average slip of 1.3 m, and a stress drop of 2.2 MPa.

Space geodetic observation of the deformation cycle across the Ballenas Transform, Gulf of California

“…Initially we solve for uniform slip but later derive spatially variable slip for our preferred model geometry. The dislocation model uses eight parameters, with inversion boundaries (Table 4) approximating the central moment tensor solution of the main shock, precise epicentral locations and the subsequent estimated rupture area [Castro et al, 2011], the mapped fault geometry [Lonsdale, 1989], and the azimuth of Baja California-North America geodetic plate motion [Plattner et al, 2007]. Simultaneously with the deformation source, we solve for phase ramps for each averaged interferogram representing longwavelength tropospheric delay variations [Fattahi and Amelung, 2014].…”

“…Most parameters are tightly constrained; only rupture depth and slip show larger [Castro et al, 2011].…”

“…Furthermore, it has been suggested that the discrepancy may be explained by slow rupture processes, not well recorded in the seismic data [Castro et al, 2011]. In this case, the geodetic slip model should show a greater moment because the deformation measurements are independent of the rupture speed causing them.…”

“…Our model fault propagates into the Salsipuedes/Ballenas Basin, striking along the basin's eastern margin, and does not terminate at the ridge-transform intersection. Clustering of several smaller aftershocks was found north of the Ballenas Transform, at and beyond the Lower Delfin Basin (Figure 13), while the rupture process started by a moderate size foreshock south of the Salsipuedes/Ballenas Basin [Castro et al, 2011] (Figure 10). …”

“…In 2004 we installed a campaign GPS network across the Ballenas Fault to monitor the interseismic motions ( Figure 1). A M w 6.9 earthquake occurred along Ballenas Channel on 3 August 2009 [Castro et al, 2011], allowing us to also study the coseismic and postseismic deformation from this event. To increase the spatial resolution of the coseismic observation we also acquired synthetic aperture radar (SAR) data for interferometric analysis.…”