Grant No. 98-HQ-GR-1026
GEO-HAZ Consulting, Inc.
P. O. Box 1377
Estes Park, CO 80517
OFFICE: (970) 586-3217
FAX: (970) 577-0041
E-MAIL: mccalpin@geohaz.com
URL: http://www.geohaz.com/geohaz
Program Element II.5: Identify active faults, define their geometry, and determine the characteristics and dates of past earthquakes.
Keywords: slope failures, paleoseismology, trench investigations, site effects
Ridgetop and ridgeflank scarps have appeared after most M>6.5 earthquakes in Southern California, as well as in the 1989 Loma Prieta earthquake. These scarps and fissures are often superimposed on larger, older scarps. Trenching at Summit Ridge after the Loma Prieta earthquake showed that the ridgetop scarps, troughs, and depressions had been created by a series of prehistoric displacements similar in style and amount to that caused by the Loma Prieta earthquake. Several geologists suggested that the ridgetop troughs and scarps might contain a record of paleoearthquakes comparable to that of fault traces. To test this hypothesis, we mapped ridgetop troughs and scarps on the mountains north of the Los Angeles Basin, in anticipation of trenching some scarps that appear to have a long record of successive displacements. The spatial and temporal distribution of ridgetop scarps can be compared to that of historic and prehistoric earthquakes on the region's larger active faults. We expect that episodes of ridgetop spreading will correlate well with the largest known paleoearthquakes on nearby faults, but there will also be deformation not contemporaneous with known earthquakes. These episodes may represent shaking from paleoearthquakes on buried or poorly-expressed reverse faults, such as the Sierra Madre fault. In addition, the California Division of Mines and Geology will use our results to decide if ridgetop scarps should be zoned as "other ground failures" under the Seismic Hazards Mapping Act.
During the Spring of 1998 we performed photogeologic reconnaissance over the San Gabriel Mountains and the eastern part of the Santa Susana Mountains, mapping ridgetop depressions and associated landslides. In May 1998 both PIs spent 2 weeks field checking the better-developed and more easily accessible ridgetop depressions and describing local lithology and structures.
Physiography, lithology, and structure define three sub-provinces within our study area. In the San Gabriel Mountains proper, slopes are very steep, ridgetops are narrow, local relief is 100s-1000 m, rocks are dominantly intrusives or gneissic rocks, and local shearing and hydrothermal alteration zones are abundant and control local physiography. In the vicinity of the San Andreas fault, the flanking ridges (Upper Lytle Creek ridge and Circle Mountain ridge) have broad, flattened crests, local relief is a few hundred meters, rocks are dominantly foliated schist ((Upper Lytle Creek ridge) and granitic and gneissic rocks (Circle Mountain ridge), and shattering is pervasive. In the eastern Santa Susana Mountains slopes are moderately steep, landslides are abundant, rocks are mainly weakly cemented upper Cenozoic sedimentary rocks, and broad folding is the major structure. The abundance and distribution of ridgetop depressions, and their relationship to landslides, is different in each sub-province.
The most common landform mapped is a linear trough or swale that strongly parallels the ridgeline, even where strike of foliation and beds does not. Some of these troughs are bounded by linear scarps. Some troughs are occupied by drianages and have distinct gradient, but most have flat segments or closed depressions with as much as 1-2 m of clousre.
The second most abundant landforms are linear uplsope-facing scarps, sidehill benches, and troughs just below ridgcrests. They too parallel the ridgelines and in places have slightly closed depressions. They tend to occur above massive landslides and may be incipient head scarps. Some benches are not associated with landslides, although some slopes are too vegetated to tell. These landforms resemble the ridgecrest landforms described previously, but are not on the ridge crest.
A third landform are the large closed depressions that are roughly
equidimensional. These depressions exhibit up to 3 m of topographic closure where
best-developed (Fig. 1). These landforms, many of
which are not associated with landslides, are inferred to represent ridgetop
collapse related to deep-seated gravitational spreading.
A fourth landform is the flattened to slightly rounded ridge with subdued
hummocks and swales with no preferred orientation. These look like laterally
spread ridges. Some are associated with landslides, and some are not. We did
not map all of these features.
A fifth landform, perhaps an extreme variation of the fourth, are the spurs off of Upper Lytle Creek Ridge that appear to have flowed into the San Andreas rift valley. These ridges in Pelona Schist were previously mapped by Doug Morton.
Finally, almost all of our ridgetop depressions lie along fire breaks created by bulldozers. In most cases the bulldozers appear to have scraped off 30-50 cm of loose material (regolith, colluvium, and sag pond deposits) incidental to removing trees and brush along the ridgecrests. This means that the late Holocene record of sedimentation in the depressions is probably missing.
Ridgetop spreading landforms in the San Gabriel Mountains sub-province tend vary according to the proximity to landslides and the degree of shearing/fracturing of the rock. Where rocks are sparsely fractured, and landslides are distant, the dominant landform is moderate-to-large (ca. 0.2-1 ha) individual depressions bounded by one or more linear scarps(Fig. 1).. Where landslides are closer or rocks are more shattered, landforms tend toward swarms of closed depressions or linear troughs. Based on an inventory of 61 sites of anomalous ridgetop topography (Table 1; includes closed depressions, troughs, and scarps), 20% are definitely associated with landslides, 30% are probably associated, 23% are probably not associated, and 27% are definitely not associated with slides. Overall, the 50% of ridgetop landforms that are not spatially associated with landslides form the best candidates for future trenching, because they are most probably related to deep-seated gravitational spreading, and they are closed depressions bounded by linear scarps that probably overlie discrete planar zones of displacement.
Table 1. Number of Anomalous Ridgetop Landforms, by Quadrangle, and Their Association With Landslides
|
Sub-province |
Quadrangle |
Depressions Landslide-Associated? |
||||
|
|
|
Yes |
Yes? |
No? |
No |
Total |
|
San Gabriel |
Azusa |
3 |
1 |
2 |
1 |
7 |
|
|
Chilao Flat |
0 |
0 |
1 |
0 |
1 |
|
|
Condor Peak |
2 |
2 |
0 |
5 |
9 |
|
|
Cucamonga Peak |
0 |
0 |
2 |
0 |
2 |
|
|
Crystal Lake |
0 |
1 |
0 |
1 |
2 |
|
|
Devore |
1 |
3 |
1 |
1 |
6 |
|
|
Glendora |
1 |
1 |
1 |
0 |
3 |
|
|
Mt. Baldy |
0 |
2 |
1 |
0 |
3 |
|
|
Mt. San Antonio |
1 |
0 |
1 |
1 |
3 |
|
|
Mt. Wilson |
1 |
3 |
2 |
0 |
6 |
|
|
Pasadena |
0 |
0 |
0 |
2 |
2 |
|
|
San Fernando |
1 |
1 |
2 |
1 |
5 |
|
|
Sunland |
2 |
3 |
1 |
2 |
8 |
|
|
Telegraph Peak |
0 |
1 |
0 |
2 |
3 |
|
|
Waterman Mtn. |
0 |
0 |
0 |
1 |
1 |
|
|
|
12 (20%) |
18 (30%) |
14 (23%) |
17 (27%) |
61 (100%) |
|
San Andreas |
Cajon |
2 |
0 |
0 |
2 |
4 |
|
|
Telegraph Peak |
10 |
2 |
2 |
3 |
17 |
|
|
|
12 (57%) |
2 (10%) |
2 (10%) |
5 (23%) |
21 (100%) |
|
Santa |
Oat Mtn. |
2 |
2 |
0 |
1 |
5 |
|
Susana |
Simi Valley E |
1 |
1 |
1 |
2 |
5 |
|
|
Val Verde |
5 |
3 |
3 |
1 |
12 |
|
|
|
8 (36%) |
6 (27%) |
4 (18%) |
4 (18%) |
22 (100%) |
The overall morphology of ridges flanking the San Andreas
rift valley southeast of Wrightwood is that of broad, flattened ridge crests and
spatulate spur ridges that broaden into lobe-shapes at their bases. Morton and
Sadler (1991) term these lower bedrock spur ridges "exploded ridges",
because it appears that the rocks comprising them have been shattered as if by
explosives, after which they "flowed" in a semi-solid state down into
the rift valley.
Depressions on the ridge crests tend to be shallow and all have been scraped by
bulldozers, so the chance of finding thick stratigraphic sections of
fine-grained fills appears remote. However, some depressions do have linear
scarps on their margins that may be underlain by fault planes with associated
colluvial wedge stratigraphy. The depressions and scarps on spurs extending
from the Upper Lytle Creek Ridge have not been graded but are vegetated and not
easily accessible.
Ridgetop spreading in the weak sandstones of the eastern Santa Susana Mountains (Pico Formation) is more spatially associated with the pervasive landsliding than in the other sub-provinces we studied. 63% of anomalous ridgetop landforms are definitely or probably associated with slides (Table 1). Some of these larger, thicker slides were probably triggered by prehistoric earthquake shaking, as suggested by the Martinez Grande slide, the largest landslide triggered by the 1994 Northridge earthquake (Jibson and Harp, 1997). However, the adjacent unnamed ridgetop features and associated prehistoric landslide were not reactivated in 1994, suggesting that the ridgetop features are not always reactivated in weak sedimentary rocks.
In the remaining time of our contract we will construct a database listing ridgetop depression locality number, airphoto reference, distance to nearest active fault, associated historic ruptures, local relief of ridge, rock type/formation, comments, and field observations. The locality numbers will be linked to a digital map of each locality, probably to be based on a digital raster graphic (DRG) of the appropriate quadrangle. Alternatively, we may convert the mapping of landform elements to vector GIS format (ArcView) that can be overlaid onto a DRG.
The San Gabriel and eastern Santa Susana Mountains contain anomalous ridgetop depressions that resemble depressions formed or reactivated during large earthquakes elsewhere in California and worldwide. These depressions may constitute a ground-rupture hazard to buildings and utilities during earthquake shaking, so our maps will show planners and engineers what ridgetop areas to avoid. The depressions may also preserve a record of prehistoric earthquake shaking in the form of displaced or deformed sediments. This possibility, to be explored in a follow-on grant, would add these landforms to the suite of sites where paleoearthquake histories can be deduced.
Reports Published: none to date.
Figures
NOTE: Figures 1 is a high resolution aprox. 640x480 pixel 24-bit color, jpeg
file. File size is 130 Kb.
Fig. 1. Photograph of the largest closed ridgetop depression observed in the San Gabriel Mountains, in the extreme southern part of the Chilao Flat quadrangle. View is northwest. The depression is roughly 100 m wide and 200 m long, and is asymmmetric, with the floor tilting south toward the larger south margin scarp.
