Large [moment magnitude (M(w)) ≥ 7] continental earthquakes often generate complex, multifault ruptures linked by enigmatic zones of distributed deformation. Here, we report the collection and results of a high-resolution (≥nine returns per square meter) airborne light detection and ranging (LIDAR) topographic survey of the 2010 M(w) 7.2 El Mayor-Cucapah earthquake that produced a 120-kilometer-long multifault rupture through northernmost Baja California, Mexico. This differential LIDAR survey completely captures an earthquake surface rupture in a sparsely vegetated region with pre-earthquake lower-resolution (5-meter-pixel) LIDAR data. The postevent survey reveals numerous surface ruptures, including previously undocumented blind faults within thick sediments of the Colorado River delta. Differential elevation changes show distributed, kilometer-scale bending strains as large as ~10(3) microstrains in response to slip along discontinuous faults cutting crystalline bedrock of the Sierra Cucapah.
For the first time on the basis of direct observations (June 2003 and June 2005), the characteristics of the summer‐time coastal poleward current off SW Mexico are reported. Only the surface evidence of this coastal current has previously been described, from ship drift compilations. In June 2003 the current was 90–180 km wide, 400 m deep, with speed ∼0.3 ms−1 and transport ∼4 Sv (1 Sv = 106 m3 s−1). In June 2005, its width was ∼90 km, it was 250–300 m deep, with mean speed ∼0.15 m s−1 and transport ∼2.5 Sv. California Current water (CCW) and equatorward flow were found further offshore. Mesoscale eddies significantly affected the coastal current, and transported CCW into the coastal zone.
The 4 April 2010 M w 7.2 El Mayor-Cucapah (EMC) earthquake provides the best opportunity to date to study the lithospheric response to a large (>M6) magnitude earthquake in the Salton Trough region through analysis of Global Positioning System (GPS) data. In conjunction with the EarthScope Plate Boundary Observatory (PBO), we installed six new continuous GPS stations in the months following the EMC earthquake to increase station coverage in the epicentral region of northern Baja California, Mexico. We modeled the pre-EMC deformation field using available campaign and continuous GPS data for southern California and northern Baja California and inferred a pre-EMC secular rate at each new station location. Through direct comparison of the pre-and post-EMC secular rates, we calculate long-term changes associated with viscoelastic relaxation in the Salton Trough region. We fit these velocity changes using numerical models employing an elastic upper crustal layer underlain by a viscoelastic lower crustal layer and a mantle half-space. Forward models that produce the smallest weighted sum of squared residuals have an upper mantle viscosity in the range 4-6 × 1018 Pa s and a less well-resolved lower crustal viscosity in the range 2 × 10 19 to 1 × 10 22 Pa s. A high-viscosity lower crust, despite high heat flow in the Salton Trough region, is inconsistent with felsic composition and might suggest accretion of mafic lower crust associated with crustal spreading obscured by thick sedimentary cover.
[1] Two overflows, originating at sills in the northern Gulf of California, are marked by strong average downstream slopes of 17 and 4%. Near-bottom stratification upstream of both sills is relatively strong, but the near-bottom water is very well mixed downstream of the sills. The homogenization of a thick bottom layer downstream, albeit close to the sills, is indicative of very strong mixing. Waters in the downstream basins are well ventilated with relatively high oxygen concentrations. These hydrographic patterns were observed during three different seasons. Time series of temperature, salinity, and potential density show that water, flowing over the sills, is essentially subsurface, subtropical water from the Pacific Ocean. The time series also indicate that water is nearly always denser at the sills compared to water in the downstream basins, despite the two-fold ($800 m) and more than three-fold ($1500 m) increase in depth. The long-term (more than a year) average current at both sills is bottom intensified with maximum average speeds of 33 and 17 cm/s. There are also strong tidal currents at the sills, although the subinertial currents near the bottom are quite persistent with nearly no flow reversals. Transports, estimated exclusively with the mean flow, are about 0.08 and 0.09 Sv (1 Sv = 1Â 10 6 , m 3 /s), which agree well with previous estimates based on shorter time series and a lower vertical resolution. These results indicate that the deep water at both downstream basins are renewed by these overflows, which enter at sill depths 400 m and are capable of reaching the deepest part of both basins. The overflows appear to be responsible for the main mechanisms that transform subsurface, subtropical water into Gulf of California water.
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