13.9: Geologic Hazards of the Mojave and Colorado Desert Provinces

Seismic Hazard

Like other provinces in California, earthquakes are a primary geologic hazard in the Mojave and Colorado Desert Provinces; you can learn more about earthquakes in California in 19: California's Earthquake Hazards. Other hazards are the result of the desert environment. In particular, dust storms (discussed in 8: Basin and Range), debris flows and flash floods are matters of concern. 

The Mojave and Colorado Desert Provinces include a continuation of the Eastern California Shear Zone (Walker Lane) described in the chapter on the Basin and Range, and are bounded by the San Andreas Fault itself (Figure \(\PageIndex{1}\)). As such, it’s a region of seismic risk.  

Figure \(\PageIndex{1}\): A map of southern California faults highlighting the Eastern California Shear zone in the eastern part of the state, along the Nevada border, and the San Andreas Fault System, which trends northwest and, with the exception of the western Transverse Ranges, is generally adjacent to the California Coast. The approximate locations of the epicenters for the Hector Mine and Landers earthquakes in the Mojave Desert are shown with stars. This work by M. House after BrucePL, is licensed under CC BY-SA 4.0 / a derivative of the original work.

Two notable earthquakes have occurred in this region since 1990: The Landers Earthquake of 1992 and the Hector Mine Earthquake of 1999 (Figure \(\PageIndex{2}\)). Both of these earthquakes ruptured areas in the Mojave Desert and had relatively large magnitudes (7.3 for the Landers quake and 7.1 for the Hector Mine quake) but were relatively low in damage due to their remote locations. The system of faults ruptured in these events were within the Eastern California shear zone. 

Figure \(\PageIndex{2}\): Map showing the relative location of the 1999 Hector Mine.  “Hector Mine Earthquake Map” by SCEDC, is licensed under CC BY-SA 4.0. View a detailed description of this map.

In the case of the Landers earthquake, five separate faults ruptured: Johnson Valley, Landers, Homestead Valley, Emerson, and Camp Rock faults. The total rupture length was ~85 km (53 miles), and the faults slipped from 2 meters (~6 ft) to a maximum of 6 meters (~18 ft). Nearby faults also experienced triggered slip and minor surface rupture. The Hector Mine earthquake was located relatively near, and ruptured a set of faults that were similarly oriented. 

Other notable seismic hazards are found in the Brawley region in the Imperial Valley, south of the Salton Sea (Figure \(\PageIndex{3}\)). The Brawley Seismic Zone (BSZ) extends from the northern end of the Imperial fault to the southern end of the San Andreas fault. The BSZ represents the northernmost extension of the spreading center axis associated with the East Pacific Rise which runs up the axis of the Gulf of California and is in the process of rifting the Baja California Peninsula away from the mainland of Mexico. 

Figure \(\PageIndex{3}\): Map of CISN / SCSN Relocations for Brawley seismic zone events. “Map of CISN / SCSN Relocations for Brawley seismic zone events” by USGS, is licensed under public domain. View a detailed description of this map.

In the 1960's and 1970's, the southern part of the BSZ was very active. Several swarms included many thousands of events, but with a largest magnitude of only M5.0 or less; most occurred along Northeast striking faults.  From 1981 to 2000, the Brawley Seismic Zone was quiet, although an event in 2005 occurred along the south shore of the Salton Sea, with a magnitude of M5.1.

Flooding

Although these desert provinces are generally dry, periodic storms have a profound impact. For example, strong summer monsoonal storms in 2022 washed out roads throughout the Mojave National Preserve and demonstrated the process of landscape modification in action (Figure \(\PageIndex{4}\)).  The impact of flash floods and debris flows that result are discussed in  13.8: Water in the Desert.

Figure \(\PageIndex{4}\): Extensive road damage following summer monsoons. A road surface is broken and undermined by a flood that is now over. Sediment channels indicate the path of the floodwaters across the road surface “Extensive road damage following summer monsoon” by US Park Service, is licensed under public domain.

References

1. Carlowicz, M. (2019, July 24). Measuring Movement from the Ridgecrest Quake. NASA Earth Observatory. Retrieved August 31, 2023, from https://earthobservatory.nasa.gov/images/145318/measuring-movement-from-the-ridgecrest-quake 
2. General Geologic Setting. (2007). In Geology and Mineral Resources of the East Mojave National Scenic Area, San Bernardino County, California (pp. 10-11). USGS.
3. Geology of Joshua Tree National Park | U.S. Geological Survey. (n.d.). USGS.gov. Retrieved September 3, 2023, from https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-joshua-tree-national-park 
4. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
5. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved August 31, 2023, https://pubs.usgs.gov/of/2004/1007/intro.html 
6. Walker, A. S. (1996). Deserts: Geology and Resources. USGS.