Statistical Relationship Between the Decrease of Major Seismicity and Drought in Southern California Since 1900

The seismicity in Southern California significantly decreased over the last decades. The decrease went in parallel with the reduction of meteoric groundwater recharge, which is a well-known factor capable of affecting seismicity. In this work the existence of a systematic statistical relationship was investigated by comparing the time density of Mw ≥ 5.7 earthquakes since 1900 with the time series of the Palmer Drought Severity Index (PDSI), an indicator of soil moisture roughly correlated with groundwater recharge. Given the non-stationarity of the two signals, the formal comparison was performed using both binomial logistic regression and cointegration testing. The analysis showed a significant statistical relationship, with peaks of seismicity 8 years behind those of PDSI. This finding suggests the hypothesis that groundwater recharge might affect earthquakes at a multi-year time scale. Proving this theory requires accurate measures and hydrogeological modeling, which is behind the scope of this work. Nonetheless, according to previous studies, the observed time lag might be explained by the slow propagation of pore pressure from the surface to the seismogenic volume. The ongoing trend towards an arid climate, made more evident by the recent severe droughts, might have contributed to the earthquake reduction of the last decades. The connection is particularly evident in the Salton Trough, with possible implications for the interpretation of its paleoseismicity.

Redefining the age of the lower Colorado River, southwestern United States

Sanidine dating and magnetostratigraphy constrain the timing of integration of the lower Colorado River (southwestern United States and northern Mexico) with the evolving Gulf of California. The Colorado River arrived at Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the Thvera subchron). The river reached the proto–Gulf of California once between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across newly identified strands of the Earthquake Valley fault zone (California) probably explains the discrepancy. The data also imply the start of focused plate motion and basin development in the Salton Trough (California) at 6–6.5 Ma and relative tectonic stability of the southernmost part of the lower Colorado River corridor after its integration. After integration, the Colorado River quickly incised through sediment-filled basins and divides between them as it also likely excavated Grand Canyon (Arizona). The liberated sediment from throughout the system led to deposition of hundreds of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as the river adjusted to its lower base level.

Nature of the crust in the northern Gulf of California and Salton Trough

In the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.