and U.S. Army
Earth Science Applications, National Training Center, Fort Irwin

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Applying geology to hazards

Floods

Young (Quaternary) materials provide information on erosional, transport, and depositional processes, and their history. For instance, young alluvial fans contain deposits and display landforms that indicate deep, gully-confined flow during flooding at their upper reaches, distributed sheet floods in their middle parts, and intermittent standing water (ponding) near their toes next to dry lakes. Similar information can be gleaned from dry wash deposits, form eolian (wind-transported) deposits, and from landslide and rockfall deposits.

• Erosion: wind, water, mass wasting
• Transport: wind, sheetflood, confined flood, rockfall and avalanche
• Deposition: sand dunes, alluvial fans, playas (dry lakes)

Flood susceptibility map of Fort Irwin basin [under construction]

Windblown dust and sand

The mapped distribution of windblown dust and sand from two aerial photographs taken 36 years apart shows many differences. We can see these environmental changes as broader distributions of wind-deposited sand, mostly east and northeast of the Main Post. It was probably caused by increased vehicular traffic and loss of vegetation, which made more land vulnerable to wind erosion.

Earthquakes

The Mojave Desert contains many active and recently active (Quaternary) faults,some of which have ruptured in historic time. In fact, the west desert is bounded by two of the biggest and most dangerous faults in the U.S., the San Andreas and Garlock faults.

Quaternary Faults in the Mojave Desert

Quaternary fault map

The map shows ruptures along faults by four age categories:

• Cracks formed during 1993: blue lines and circles
• Known Holocene rupture (within the last 11,000 years): purple lines
• Known Pleistocene rupture (within the last few 100,000 years): red lines
• Permissible young fault (age uncertain): black lines

Faults are solid lines where well known, dashed where less certain, and dotted where covered.

Active faults and topography: Faults on this map tend long mountains and ridges. A plot of faults on a shaded relief map shows this relationship.

Mapping methods: Faults were located and studied in the field and by using aerial photographs. Where possible, excavations and natural stream cuts were used to determine the youngest surficial deposits broken by fault movement, and the oldest deposits unbroken. These two ages bracket the age of the last fault rupture. Ages of deposits, however, are only broadly known through correlation with dated deposits elsewhere in the desert. Details on several faults.

Locations of some large faults also are easily defined by satellite images that give information on rock types and by geophysical methods such as aeromagnetic information.

What needs to be done: Faults close to key facilities and those showing evidence of Holocene rupture should be trenched for more detailed evaluation of the times of rupture. A key is to date the deposits directly by using isotopic systems.

Applications:

• Earthquake hazard zones: The western Coyote Canyon fault very likely ruptured in early to middle Holocene time, which by definition makes it an active fault and capable of rupturing again.
• Possible creep hazards: Studies thus far suggest that cracks that formed in 1993 were caused by creep triggered by the 1992 Landers and Big Bear earthquakes, which added small components of left-lateral shear stress and fault-normal tensional stress to faults. This scenario suggests that all of the Quaternary faults, and even the older faults, at Fort Irwin are capable of creep and possibly of seismic activity. Accordingly, it is important to acquire as many data as possible pertaining to surface and subsurface faults in order to evaluate potential future activity and the hazards posed by aseismic creep, surface ruptures owing to seismic events, and groundshaking caused by nearby earthquakes. With more data available, plans to mitigate the effects of seismic and aseismic tectonism can be developed for critical facilities and utility corridors at Fort Irwin.
• Major faults under the Main Post. Faults of Pleistocene age having at least 10 km of net slip are mapped to the edge of the Fort Irwin basin, and must extend under unbroken surface materials of the basin. Bedrock steps as great as 100 m high within the basin have been documented by drilling and are likely caused by one or more of these faults. On this basis, we show possible locations of buried faults on the fault map. A key question is how recently these buried faults last ruptured, because this information is needed to estimate potential for future rupture. This question can be answered by shallow, high-resolution, geophysical studies coupled with trenching if needed.

Download published paper on part of the map:

Preliminary assessment of the recency of faulting at southwestern Fort Irwin, north-central Mojave Desert, California
by David M. Miller, James C. Yount, Elizabeth R. Schermer, and Tracey J. Felger: Special Publication 94-1, San Bernardino County Museum Association, Redlands, CA

Recent earthquake locations on line

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