by
| Kenneth M. Cruikshank Department of Geology |
Arvid M. Johnson Harry Fielding Reid Earthquake
Research Laboratory |
| Robert W. Fleming Geologic Hazards Team |
Robert L. Jones Survey Division, |
U. S. Geological Survey
Open-File Report 96-698
Denver, Colorado
1996
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposed and does not imply endorsement by the U.S. Government.
Plates 2 & 3 have not yet been converted. Check Back Soon!
Figure 1. Location of the Northridge earthquake,
showing traces of various faults that have been recognized in the area.
Figure 2. Location of the Northridge earthquake,
showing partial traces of various faults that have been recognized in the area.
Figure 3. Detailed contours of differential vertical
displacement between 1980 and 1994
Figure 4. Photograph of graben just south of Malden,
and east of Tampa Ave.
Figure 5. Photograph of thrust fracture in Malden.
Figure 6. Relation between length of street segment
and absolute value of normalized length change (strain) of street segment..
Figure 7. Details of normalized length changes and
damage to streets and sidewalks in the northeastern end of the Winnetka area.
Figure 8. Computed extension of streets in Canoga
Park-Winnetka-Northridge area between the 1950’s to 1970’s and 1985, and after
the Northridge earthquake sequence.
Figure 9. Computed strain figures (shmoos and nerds)
of streets in Winnetka/Northridge area between the 1950’s to 1970’s and 1985,
after the Northridge earthquake sequence.
Figure 10. Interpretative structural cross-section of
San Fernando Valley area, showing a dish-shaped fault that underlies the center of the
valley and that ends beneath the Santa Monica Mountains to the south and the Santa Susana
Mountains to the north
Figure 11. Heart structure, a type of faulted fold
Figure A-1. Lengths and angles of street segments used to compute deformations at
intersections in the Winnetka area
Table 1. Relation between relative depth, d/a, of tip of fault beneath ground surface to factor, f, that determines the magnitude of the maximum strain at the ground surface. 14
Plate 1. Differential vertical displacements between
1980 and 1994 in San Fernando Valley, Los Angeles, California.
Plate 2. Extension (or shortening) of streets
calculated from surveys of monuments in 1970'S and 1995 in the Winnetka area, following 17
January 1994 Northridge, California earthquake.
Plate 3. Strains calculated from changes in lengths of
streets and angles between streets in the northeastern part of the Winnetka area.
Measurements of normalized length changes of streets over an area of 9 km2 in San Fernando Valley of Los Angeles, California, define a distinctive strain pattern that may well reflect blind faulting during the 1994 Northridge earthquake. Strain magnitudes are about 3´10–4, locally 10–3. They define a deformation zone trending diagonally from near Canoga Park in the southwest, through Winnetka, to near Northridge in the northeast. The deformation zone is about 4.5 km long and 1 km wide. The northwestern two–thirds of the zone is a belt of extension of streets and the southeastern one–third is a belt of shortening of streets. On the northwest and southeast sides of the deformation zone the magnitude of the strains is too small to measure, less than 10–4. Complete states of strain measured in the northeastern half of the deformation zone show that the directions of principal strains are parallel and normal to the walls of the zone, so the zone is not a strike–slip zone. The magnitudes of strains measured in the northeastern part of the Winnetka area were large enough to fracture concrete and soils, and the area of larger strains correlates with the area of greater damage to such roads and sidewalks. All parts of the pattern suggest a blind fault at depth, most likely a reverse fault dipping NW but possibly a normal fault dipping SE. The magnitudes of the strains in the Winnetka area are consistent with the strains produced at the ground surface by a blind fault plane extending to depth on the order of 2 km and a net slip on the order of 1 m, within a distance of about 100 to 500 m of the ground surface. The pattern of damage in the San Fernando Valley suggests a fault segment much longer than the 4.5 km defined by survey data in the Winnetka area. The blind fault segment may extend several kilometers in both directions beyond the Winnetka area.
This study of the Winnetka area further supports observations that a large earthquake sequence can include rupture along both a main fault and nearby faults with quite different senses of slip. Faults near the main fault that approach the ground surface or cut the surface in an area have the potential of moving coactively in a major earthquake. Movement on such faults is associated with significant damage during an earthquake. The fault that produced the main Northridge shock and the faults that moved coactively in the Northridge area probably are parts of a larger structure. Such interrelationships may be key to understanding earthquakes and damage caused by tectonism.
This paper is part of a broader study, a collaboration of the City of Los Angeles Department of Surveys, the U. S. Geological Survey and Purdue University through grants from the Nuclear Regulatory Commission (Fleming), the Federal Emergency Management Agency (Jones), the National Science Foundation EAR 9416760 (Johnson and Cruikshank) the Department of Energy DE FG02 93ER14365 (Johnson) and the Southern California Earthquake Center NTP2898 (Johnson). We greatly appreciate the support of these organizations.
This research would not have been possible without the generous and enthusiastic efforts of the Survey Division, City of Los Angeles (Bureau of Engineering, Department of Public Works, City of Los Angeles, 16th floor, 600 S. Spring, Los Angeles, California 90014). In particular, we wish to thank Robert Packard, (ret.) Engineer of Surveys, Robert Renison, Supervisor of Surveying and Robert Taylor, Acting Engineer of Surveys. All these people have been extremely helpful as we worked with their data. Jim Gardner, technical editor in Earth and Atmospheric Sciences, and Ansel Johnson, Portland State University, edited the manuscript. We thank these people for contributing to the manuscript.
This is the second in a series of papers on the Northridge, California earthquake , dealing with evidence for coactive faulting. In addition to the fault that produced the main shock, other faults moved during the same event and produced high, localized ground deformation. Some coactive faults appear to cause extensive localized damage to structures, utilities, highways, and other lifelines. Although such damage typically is attributed to elastic ground shaking, such shaking cannot explain the large, permanent ground deformations that we measure .
The main shock of the Northridge earthquake (6.7 Ms), was at 12:31 UTC (4:31 a.m., Pacific Standard Time), 17 January 1994 . The hypocenter was about 18 km beneath the town of Northridge in the San Fernando Valley, and significant damage was caused up to 64 km from the epicenter . The sense of differential displacement on the main fault was predominantly reverse, with the southern block upthrown. The slip near the epicenter of the main fault was about 1 m with a maximum slip of about 2.2 m at a depth of 12.4 km , with the hanging wall pushed upward. The earthquake producing fault strikes N 70°–80° W and dips 35°–45° S . Had it propagated with this orientation to the ground surface, the fault rupture would have appeared in the vicinity of the crest of the Santa Susana Mountains to the north of the San Fernando Valley. Although attention centers on the main shock in an earthquake event, the Northridge earthquake event was a sequence of hundreds, or thousands of earthquakes, many of which were not on the fault that produced the main shock .
This study focuses on the Winnetka deformation zone, a zone of relatively high strains consisting of a belt of extension on one side and a belt of shortening on the other, about 500–1000 m wide, extending at least 4500 m in the NE–SW direction through Canoga Park in the southwest, Winnetka in the center and Northridge in the NE (Figure 1, Plate 1). The Winnetka deformation zone was discovered during a preliminary field examination of ground fracturing – examining fractures in sidewalks, streets and houses – in the Northridge area shortly after the January 1994 earthquake sequence. We also examined damage in the area for five days during the spring and summer of 1995. A single site within the Winnetka deformation zone has been examined by Holzer, Bennett, Tinsley and others who have concluded that the deformation there is a result of superficial deformation of soils by movement on a weaker layer at depth, not of deep-seated tectonism.
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Figure 1. Location of San Fernando Valley, between Santa Susana Mountains to the north and Santa Monica Mountains to the south. The epicenter of the main shock at Northridge is in the center of the valley. Contours of differential vertical displacement between 1980 and 1994 show general, fan-shaped tilting of San Fernando Valley and Santa Susana Mountains relative to an assumed fixed point in the SW corner of the valley. The Granada Hills and Winnetka deformation zones are shown with bold outlines. |
The study of the Winnetka deformation zone reported here supports the general conclusion of our study of the Granada Hills area : that the Granada Hills deformation zone is the surface expression of coactive slip on a blind blade of the Mission Hills fault, at the foot of the Santa Monica Mountains. The principal evidence that leads to this conclusion for the Granada Hills deformation zone is as follows:
Nowhere in the San Fernando Valley have we directly observed the surface expression of a fault breaking the ground surface so the evidence is circumstantial. Saul , however, mapped the Mission Hills fault near Rinaldi and Amestoy. Perhaps a blade of this fault moved to produce the Granada Hills deformation zone. A blind in the Granada Hills zone fault would have to be a very short blade, because the zone of deformation is only about 500 m wide (NW-SE direction) and only 500 to 800 m long (NE-SW direction). The belt of anomalously large strains centered on the Winnetka area is about 1 km wide (NW-SE direction) but extends at least 4.5 km, from near Canoga Park in the southwest through Winnetka to at least Northridge in the northeast (Plate 2).The concern about the size of the belt is minimized in the Winnetka area.
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Figure 2. Location of the Northridge earthquake, showing partial traces of various faults that have been recognized in the area. Time of latest movement during an earthquake are indicated with a date. (After Ziony and Jones, 1989). |
The belt extends into areas of relatively high damage to buildings to the southwest in Canoga Park and in the northeast in Northridge. For example, the California State University at Northridge campus would be in the center of the belt of damage if it extended further to the NE. The campus suffered heavy damage during the earthquake, including a collapsed parking structure at the NE corner of campus. The Northridge Fashion Mall, where several stores collapsed would be the NW side of an extension of the Winnetka belt. The Northridge Apartments, where most of the earthquake fatalities occurred, would also be on the NW side of the belt. Farther still to the NE along the line of the belt is the collapsed Kaiser Permanante administration building. To the SW of the Winnetka area along the trend of the belt is Canoga Park’s Topanga Plaza, another shopping mall that suffered heavy damage. Including these structures in the Winnetka deformation zone would make the zone greater than 10 km long (Plate 2).
A belt of damage that might coincide with the Winnetka deformation zone shows up in the maps of distributions of pipe breakage or red-tagged buildings in San Fernando valley . Examination of these maps indicates that the belt of damage could extend from Sylmar in the NE corner of the valley to Hidden Hills in the SW. The Granada Hills area lies to the north of this zone, and probably represents movement on a blade of the Mission Hills fault . Thus the belt of localized damage is much longer and relatively narrower in the Winnetka area than at Granada Hills. In several ways, however, the belts are similar.
Herein we infer that the Winnetka deformation zone is a surface expression of movement on a blind fault. We informally refer to this fault as the Winnetka fault. The Winnetka area includes the epicentral area of the main shock and is in the hanging–wall block of the Pico thrust fault. As far as we know, nobody has previously recognized the Winnetka fault. For example, it is not shown in the most recent geologic map of the Valley . There is a nearby fault, the Chatsworth fault, in the vicinity of the Chatsworth Reservoir (Figure 2 & Plate 1), and trending generally northeasterly . There is no historic movement on the Chatsworth fault, although there is recognized Quaternary movement according to the preliminary fault map of California .
Besides horizontal deformation of the earth’s surface in the Winnetka area, movement on the Winnetka fault appears to be reflected in perturbations in the pattern of uplift and tilting of the San Fernando Valley (Figure 3 & Plate 1). As we have indicated elsewhere , there are several perturbations in the pattern of uplift, perhaps associated with movement on smaller faults during the earthquake sequence, in the Winnetka Chatsworth, and Granada Hills areas (Figure 3 & Plate 1). Movement on the Winnetka fault appears to be reflected in an area of flattening of the surface of differential vertical uplift south of a NE–trending belt of steepening through Canoga Park, Winnetka and Northridge and an outward bowing of contours south of Northridge. The details of the contours shown in Plate 1, though, are highly interpretive because the data are from lines 5-6 km apart. The location of level lines is indicated by small numbers along streets in Plate 1.
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Figure 3. Detailed contours of differential vertical displacement between 1980 and 1994, showing perturbations in the general tilting at Winnetka (W), Northridge, Granada Hills (GH), Chatsworth. There probably are blind faults at these areas. |
While examining earthquake damage in the San Fernando Valley following the January 17th 1994 Northridge earthquake it became apparent that the damage was not uniformly distributed throughout the valley, but rather was concentrated locally. Areas that appeared to have more damage than surrounding areas were more carefully examined. One of these areas contained a rift that had formed in a vacant lot off Malden Street (Figure 4). The rift was adjacent to a large section of Tampa avenue that had been repaved shortly after the earthquake. The rift was about 4 m wide, and oriented about N 40° E. Each side consisted of a series of open fractures with two preferred orientations. Each side was displaced vertically about one dm. The view in Figure 4 is to the SW. To the NE the rift projected into Malden Street. There the rift was reduced to a few tension cracks in the curb.
Although damage in the area of Malden Street was noticeably greater than in the surrounding area the level of damage was less than in the Balboa Avenue area in Granada Hills .
On the west side of Tampa Avenue (Plate 3), there are several of closely spaced N-S streets. North of Chase Street the sidewalks and roads are in good repair. South of Chase there are no sidewalks, and the road surface is not in good condition. Even though recognition of earthquake damage was nearly impossible south of Chase there was running water in the streets from numerous broken water pipes. Otherwide extensive damage to water mains was been observed in areas of extensive surface damage, such as Granada Hills .
North of Chase Street, along Aura, the first street to the west of Tampa avenue, there were thrusting and extension features in the curb. A 60 m long section of the road was also replaced, starting about 60 m north of Chase. Cracks in the curb along Aura were open from 0.5 to 3 cm.
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Figure 4. (a) Photograph of rift just south of Malden, and east of Tampa Ave. This large rift went across two vacant lots. View is to the SW. The trend of the rift is about N40°E, and it is made up of biconjugate faults. (b) Details of the east side of the rift. The scarp is almost a decimeter high.
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Figure 5. Photograph of thrust or shortening fracture in Malden, an E-W street just east of the N-S Tampa Avenue. The thrust is about 20 m west of where the trend of the rift shown in Figure 4 would cross Malden. Offset markers in the center of the road indicate 4.5 cm of thrusting.
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Repeated surveys of streets throughout the Los Angeles area provide unusually detailed information about horizontal components of ground deformation during earthquakes. Throughout large parts of the city, the relative horizontal positions of monuments at most street intersections have been measured on a periodic basis. The first surveys were made by contractors in the 1950’s and 1960’s. After the 1971, San Fernando earthquake sequence, intersections throughout the northern part of the city were resurveyed by the City of Los Angeles.
In 1995 the City of Los Angeles resurveyed the Granada Hills area and part of the Winnetka area. The surveys were completed and angle measurements closed over an area of about 2 km long and 2 km wide. They re-measured lengths between subsurface monuments in an adjacent area about 2 km wide and 4 km long, but did not turn angles and close surveys closed that entire area.
The survey data provide unprecedented details of deformation. Monuments that are in the subsurface beneath the street and sidewalk levels were relocated and surveyed. Details of the monuments, surveying techniques and accuracy of measurements are discussed in Appendix 2. The points are located at the centerlines of roads and at the centers of intersections. Most of these are subsurface monuments, which are of two types, both of which are anchored to ground beneath the road–fill prism. One is a special target hole punched into a cap on a steel pipe, encased in concrete below road level; (generally the top of the target is about 0.3 m below the road surface). These targets are accessed through steel covers, about 10 cm in diameter, at road level. The second is a series of four punch marks in the sides of concrete sewer-access vaults. The access vaults extend below the road–fill material to the sewer level. The punches are at a depth of about 0.5 m. The point where lines connecting opposing punches cross is the target in these cases.
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Figure 6 Relation between length of street segment and absolute value of normalized length change (strain) of street segment. The street segments tend to be 400, 200 and 100 m long. The larger strain values, for street segments shorter than 50 m, are based on surface monuments, and so are discounted. For street segments longer than 50 m, the strains appear to range widely, regardless of length |
The City of Los Angeles regards their determination of distances to be accurate to 3.1 mm. For a street length of 100 m, the normalized error would be 3´ 10–5. Angle measurements should have an error of less than three seconds. The corresponding error in shear strain is the tangent of the angle, so the error is 1.45´ 10–5. Thus angle measurements have an error that is less than distance measurements, and shear strains can be determined to about 10–5. We use 10–4 as a cutoff for strain determinations, so strains judged to be significant are three to seven times larger than the estimated instrument error.
The 1995 re-survey by the City of Los Angeles measured street lengths over the entire study area, however angles were only closed for the NE part of the study area. Where only street lengths were measured the complete state of strain cannot be determined. We will first discuss the pattern of length changes, and then the pattern of strains in the area where angles were closed.
Over an area of about 8 km2 in the Winnetka area we can calculate changes in lengths of street segments using re-surveys of street lengths. The lengths of the segments range from about 10 to 500 m, but they tend to be 400, 200 or 100 m (Figure 6). We determine the strains of street segments by calculating the extension, the current length minus the reference length divided by the reference length. The reference lengths were generally measured in the 1970’s, but could include data from the 1950’s and 60’s.
There is a potential source of bias in the data in the reference lengths of streets. Is there a length over which strains are maximized? If deformation were highly localized, some short street segments should show large strains and other short street segments should show no strain. Also, for short segments the reference lengths can be so short that errors in length measurement can overshadow the strains.
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Figure 7. Details of normalized length changes and damage to streets and sidewalks in the northeastern end of the Winnetka area. The lower part of the diagram is a map of the area, showing the magnitudes, directions and signs of the extensions in the area, as well as the types of damage to streets and sidewalks, and broken water lines. The large extension (inset figure) is for the misaligned street segment of Malden Street, which is about 30 m long. It was measured using surface monuments (spike and washer). All the measurements along Malden are based on surface monuments and therefore must be discounted. All five strain figures in the detail at the top of the figure is for Malden Street and is based on length changes of "+" marks chiseled into sidewalks by the builder at the times of construction to mark long boundaries. The large shortening of a lot west of Beckford apparently reflects a thrust fault that is visible in the street (Figure 5). The relatively uniform extensions of the four lots east of Beckford and north of Malden apparently do not reflect the tension crack) in the south side of Malden. |
Long street segments should show smaller strains than the highly strained short segments because the segments can average out the localized extensions and shortenings. Figure 6 shows the relation between values of strain, as defined above, and lengths of street segments. Strains smaller than 10–4 are negligible, so data in the lower part of the diagram can be neglected. The results show that strain of street segments longer than about 50 m ranges widely, from about 10–5 to 10–3 regardless of the lengths of the segments.
For street segments of about 30 m long, the strains range to a somewhat higher value of about 3´ 10–3. Examination of the original data (Appendix 3) indicates that the data for the short street segments were all based on measurements of surface monuments founded in the pavement or in sidewalks. The largest strains were obtained along Malden Street, adjacent to a rift (Figure 4) in a vacant lot between Malden and Chase, and between Tampa Avenue and Van Alden (Figure 7). Figure 4 shows the rift and Figure 7 shows the measurements of strains of streets in the area of the large deformation. The larger extensive strain was measured with surface monuments (spike & washer or spike & tin monuments, see Appendix 2 for explanation of monument types) in a street segment about 30 m long that is a jog connecting two straight segments. We observed an opening fracture in this street segment. The largest compressive strain was calculated for a lot west of Beckford (insetmap in Figure 7) using measurements of the change in length between "+" marks chiseled into the sidewalk to mark lot boundaries. The largest compressive strain was calculated for lengths measured immediately adjacent to a thrust fault in pavement (Figure 5) and an adjacent transform fault that had accommodated 4.5 cm of right–lateral offset (Figure 7).
Thus, if we discount the very large deformations measured over short street segments or over widths of individual lots, the deformations appear to be independent of the lengths of street segments. The very large strains are due to localized deformation in pavement and cement, and do not represent some form of bias in the survey procedure, or choice of street length.
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Figure 8. Computed extension of streets in Canoga Park-Winnetka-Northridge area between the 1950’s to 1970’s and 1985, and after the Northridge earthquake sequence. Magnitude of the largest extension (regardless of sign) indicated by radius of heavy circle. Where extensions were smaller than error, measurement indicated with circle with diagonal line. Significant strains form a deformed zone trending NE-SW, about 1 km wide and 4.5 km long. Extension predominates in NW part (area shown with a "+") and compression in the SE part (area shown with a "-"). The deformation zone may continue to SE, as indicated in Figure 1, into area with many red-tagged buildings. |
The measurements of normal strain, defined as the normalized length changes, measured along street segments, are shown in Figure 8 and Plate 2. The extended streets are indicated with darker circles and the compressed streets with lighter circles. The magnitude of the strain can be determined by comparing the size of a circle with the size of scaled circles shown in the Explanation. The direction of the street segment that was measured is indicated by the direction of the arrows. The circles with a diagonal line represent measurements where the strains are in the range of error, less than 10–4.
A smaller-scale map of the distribution of strains (Plate 2) shows that trending diagonally through the Canoga Park, Winnetka and Northridge area there is a deformation zone, containing an extension belt and a shortening belt (Figure 8). The belt of predominantly extension is perhaps 0.6 km wide and about 4 or 4.5 km long, in the northwesterly part of the deformation zone. The belt of predominantly shortening is about 0.4 km wide and about 4 km long, in the southeasterly part of the deformation zone. On the northwest and southeast sides of the deformation zone the magnitude of the strains is too small to measure, that is, less than about 10–4.
The pattern of extension and shortening indicates that there are relatively large deformations within a zone about 1 km wide (NW–SE) and 4 or 4.5 km long (NE–SW), and that the deformations in the NW part of the zone are dominated by extension whereas the deformations in the SE part of the zone are dominated by shortening. This pattern suggests a blind fault at depth, either a reverse fault dipping NW or a normal fault dipping SE .
The magnitudes of extension and shortening decrease from northeast to southwest. Even if, for reasons explained above, we discount the unusually large circles in the upper right of the area, near the intersection of Van Alden and Parthenia, we see that the deformations tend to be larger in the area of Tampa Avenue and Chase than elsewhere in the area of measurement. The larger deformations in the same general area–on either side of Tampa Avenue and between Parthenia and Chase–correlate with greater damage to structures such as roads and sidewalks.
The kinds of rupturing of roads and sidewalks are consistent with the kinds of strain. The strains in the NE part of the area, between Tampa and Van Alden, and north of Chase, are predominantly extensile. The rupturing is similarly predominantly extensile. The rift between Malden and Chase, at Beckford, indicates NW extension. The tension cracks along Aura, north of Chase, indicate roughly N-S extension. The thrusting of sidewalks at street corners, between Strathern and Arminta (Plate 2) is consistent with shortening strains in this area.
This pattern of shortening and extension and the correlation of the types of damage to the types of strains, suggest a blind fault at depth, either a reverse fault dipping NW or a normal fault dipping SE.
In the northeastern part of the area shown in Plate 2 we have complete information required to determine strains at many street intersections. That is, we have pre- and post-earthquake measurements of street lengths and angles. This allows us to compute the principal strains and their orientations at an intersection. We use the measurements of changes of lengths of streets and angles between streets to determine strains . The procedure is summarized in Appendix 1.
We have developed a way of displaying the state of strain near a point via an extension figure we call a shmoo. A special case is a nerd. The shmoo or nerd shows, in one diagram, the absolute magnitude of the largest principal strain as well as the directions of the maximum and minimum, principal strains (see Explanation in Appendix 1 and on Plate 3).
Shmoos and nerds graphically display the strain state near a point. The magnitude of the strain is indicated by the radius of the larger–extension circle, which can be compared to a scale of such circles. The directions of the maximum and minimum principal strains correspond to the directions of the maximum and minimum dimensions of the curvilinear part of the strain figure, so the shmoo or nerd indicates the direction of the principal strains.
To determine the magnitudes of the strains, the size of the circle is compared to a series of calibrated circles in the Explanation in Plate 3. The circles are for 10–2.5 (@ 0.003), 10–3 (0.001), 10–5.5 (@ 0.0003) and 10–4 (=0.0001). Where the multi–shaped line is inside the circle there is shortening, and where it is outside the circle there is extension. For example, the shmoo immediately SE of the intersection of Rosco and Shirley in Plate 3 (see also Figure 9) shows the multi–shaped, light line is outside the heavy circle, indicating that there is extension in all directions, although the extension is larger in the direction N 45° E. The magnitude of the larger principal strain is on the order of ± 0.0005 (about 10–3.3), the principal extensions are positive, and the strains are largely dilational for this example. The shmoo on the SE of the intersection of Strathern and Corbin shows a common, figure–eight shape of the light line, and indicates that there is extension in the NW–SE direction, where the light line is outside the circle, and shortening in the NE–SW direction, where the light line extends slightly inside the circle. In this example the magnitude of the larger principal strain is on the order of ± 0.0003 (10–3.6). The roughly equal values of maximum shortening and extension in this example suggest shear without area change. Simple shear relative to the orientation of Strathern would be left–lateral. Simple shear relative to the orientation of Corbin would be conjugate, that is, right–lateral.
Thus, the direction of maximum extension (or minimum shortening) is defined, to within an unknown rigid–body rotation, by the long dimension of the multi–shaped line. The sense of shear (relative to the east–west or north–south streets) can be read from the inclination of the long dimension of the shmoo.
The shmoo extension figure near the intersection of Strathern and Tampa in Plate 3 is a nerd, indicating approximately NW–SE maximum shortening and very minor NE–SW extension. The magnitude of the larger principal strain is on the order of +0.0006 (10–3.2). Similar nerds, with similar orientations, are shown nearby.
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Figure 9 Computed strain figures (shmoos and nerds) of streets in Winnetka/Northridge area between the 1950’s to 1970’s and 1985, after the Northridge earthquake sequence. The strain figures show that the belt of extension in the Winnetka deformation zone is dominated by principal extensions in the NW and SE direction, and that the belt of compression is diminated by a principal compression in the same direction. The pattern indicates that the Winnetka deformation zone would not be a strike-slip fault, but rather a reverse or normal fault. |
The strain figures, the shmoos and nerds, clarify the internal structure of the deformation zone in the Winnetka area defined by the extension and shortening figures shown for the larger area in Plate 2. The strains in the Winnetka deformation zone are defined by NW–SE extension in the NW belt of the zone and by NW–SE shortening in the SE belt of the zone (Figure 9). The walls of the belts, of course, are only roughly defined, and the belts defined on the basis of strains shown in Figure 9 have not been forced to match those defined on the basis of normalized extensions, shown in Figure 8. The critical observation is that the directions of principal strains are parallel and normal to the walls of the deformation zone, indicating that the zone is not a strike–slip zone, but, most likley is a reverse zone.
A problem that we did not address at the Granada Hills area is the large magnitudes of strains . There the strains were commonly 3´ 10–3 and ranged up to 10–2, and the strains were closely reflected in extensive and intensive surface damage to man–made structures. In the Winnetka area, the strains are about an order of magnitude smaller, commonly 3´ 10–4 and range up to about 10–3.
Two questions, besides those addressed above, arise concerning the strains. First, how do such strains compare to those required to fracture unconsolidated soil. Second, how could these very large strains be generated.
Concerning the first question, we note that a brittle rock, such as granite , fails under atmospheric confining pressures at compressive axial strain values of between –2 to –6 x 10–3. In the Aspen Grove landslide in Utah the soil in a developing landslide toe cracked at strains of about –1.4 to –2´ 10–2 and in a newly forming pull–apart, the strains were about 6´ 10–3 before a tension crack formed and 3.5´ 10–2 after the tension crack had formed.
Comparing these values with those measured in the Granada Hills area, we note that the typical strains of 3´ 10–3, with maximum strains ranging in magnitude up to 10–2, should have been large enough to produce tension cracks in concrete and soil and reverse faults, at least in concrete. Such fracturing was abundant there . Comparing these values with those measured in the Winnetka area, 3´ 10–4 and ranging up to 10–3, it would seem that the strains should not have produced much fracturing in sidewalks or in soil. Again, this is what we observed. Furthermore, since the strains decrease from NE to SW in the Winnetka area, we would expect the ground deformation to decrease in that direction, as we observed. Thus, the conclusions are consistent with our observations in both areas.
The second question concerns the origin of such large strains. In order to address this question, we consider an idealized fault in an elastic material. Let us determine the order of magnitude of strains produced at the ground surface, a distance x, from the mid-length of the fault, along a plane passing through the fault (y = 0). We emphasize that this is a rough calculation, because we have not matched the boundary conditions along the free surface, but our experience indicates that, to first order, we would obtain the same answer, to within a factor of two, from the exact solution. Also, the actual strains will be normal strains rather than shear strains because the ground surface is free of shear.
According to Tada, Parris and Irwin , the shear strain is a function of the distance, x, from mid-length of the fault according to the relation,
(1a)
in which umax is the maximum slip due to a stress drop on the fault, a is the half–length of the fault, and n is Poisson’s ratio. For incompressible material, Poisson’s ratio is 0.5; for granite, it is about 0.2. Assuming incompressibility, eq. (1a) becomes
(1b)
We use the form of this equation to calculate, approximately, the strains at the ground surface,
(1c)
in which e is strain, the factor, f(d/a), is determined by the distance, d, (where x = d + a), from the end of the fault to the ground surface. Values are given in Table 1.
Table 1. Relation between relative depth, d/a, of tip of fault beneath ground surface to factor, f, that determines the magnitude of the maximum strain at the ground surface.
Relative Depth |
Fault depth factor (d/a) |
| 0.5 | 0.31 |
| 0.05 | 2.80 |
| 0.005 | 12.20 |
| 0.0005 | 43.00 |
Let us assume a fault with a normalized maximum slip of umax/a = 10–3. This value would correspond to a maximum slip of 1 m for a fault with a length of 2 km, or a slip of 10 cm for a fault with a length of 200 m. The former seems a more reasonable estimate for a fault to produce a zone of high deformation between 500 and 1000 m wide. For a maximum normalized slip of umax/a = 10–3, the strain near the ground surface would be on the order of 3´ 10–4 if the tip of the blind fault were a distance equal to the half length of the fault from the ground surface (d/2a = 0.5). It would be an order of magnitude larger, or about 3´ 10–3, if the tip of the fault were a distance of one–five hundredths the length of the fault from the ground surface (d/2a = 0.05). The maximum strain near the ground surface would be about 10–2 if the tip of the fault were a distance of one–fiftieth the length of the fault from the ground surface (d/a = 0.005). Thus we see that it is quite reasonable to expect a blind fault, near the ground surface, to produce strains on the order of those measured in the Winnetka area as well as in the Granada Hills area. Presumably the fault slip was less or the fault was more deeply buried in the Winnetka area than in the Granada Hills area.
Thus, the magnitudes of the strains in the Winnetka area are consistent with the strains produced at the ground surface by a blind fault plane with a length on the order of 2 km and a net slip on the order of 1 m, within a distance of 0.5 to 0.1 of the half length of the fault to the ground surface. The localization of fracturing damage in the northeastern end of the known part of the Winnetka deformation zone is consistent with the localization of larger strains in that area. The strains are on the order of strains required to fracture concrete and soil. By way of counterproof, where the strains are smaller, in the southwest end of the Winnetka deformation zone, there was relatively little damage to streets and sidewalks.
The 1994, Northridge, California earthquake sequence illustrates again that a large earthquake can include rupture along both a main fault and nearby faults with quite different kinematic signatures. Faults near the main fault that approach the ground surface or cut the surface in an area, have the potential of moving coactively in a major earthquake sequence .
The fault that produced the main shock and the faults that moved coactively in the Northridge area are probably parts of a larger, growing structure . However, Stein and others suggest that the Northridge earthquake was a consequence of the change in stress conditions due to earlier major earthquake in the Los Angeles area. Thus, Stein and others would interpret coactive movement of other faults in the Northridge sequence as a result of the change in stress caused by the main shock rather than coactive movement in a growing structure.
Coactive faulting produces the high, localized ground deformation along blind or visible faults. In the case of Northridge (Figure 10), the earthquake fault appears to be part of a large, heart structure (Figure 11). The heart structure includes a horst block about 35 km wide and 20 km deep bounded by listric reverse faults (Figure 11). In the center of the heart structure is the basin of San Fernando Valley, and on either side are broad anticlinal highs, the Santa Monica Mountains to the south and the Santa Susana Mountains to the north (Figure 10). The anticlinal high in the Santa Susana Mountains is complicated by a large thrust block, overlying the Santa Susana thrust fault, that is overriding that limb of the heart structure (Figure 11). A heart structure is a type of fault–related fold subjected to horizontal compression.
The tectonic origin of the horizontal compression in the San Fernando Valley is well known. The Los Angeles area is immediately south of the Transverse Ranges, where the broad zone of generally right–lateral San Andreas fault systems south of Los Angeles take a left jog and reorganize into a narrower zone of strike–slip faults north of the Transverse Ranges, producing an area of roughly N-S compression within the Transverse Ranges. The San Fernando Valley heart structure is a result of this compression. The tilting of the San Fernando Valley during the Northridge earthquake is, we believe, a result of larger slip on the Pico thrust fault than on the Santa Monica fault (Figure 11) .
| Figure 10. Interpretative structural cross–section of San Fernando Valley area, showing a dish–shaped fault that underlies the center of the valley and that ends beneath the Santa Monica Mountains to the south and the Santa Susana Mountains to the north . The epicenter of the Northridge earthquake was at about 19 km depth, apparently along the Pico thrust fault. The Pico thrust fault is interpreted to be listric in order to explain the tilting of the San Fernando Valley toward the south. |
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Figure 11 Heart structure, a type of faulted fold.. The heart structure is produced by a dish fault in homogeneous flowing material with passive markers. The flowing material is incompressible, so the cross–section is balanced. A. Loading conditions, consisting of uniform shortening and thickening. B. Form of passive layering after 10% shortening. Heart structure is starting to take form. C. Form of passive layering, defining a clear heart structure, after 30% shortening |
The coseismic deformation in the Winnetka area supports a growing body of evidence that any active fault approaching or at the ground surface in an area has the potential of moving coactively at the time of a major earthquake. If, in fact, further research supports the notion that some earthquakes are results of seismogenic slip on faults within a large tectonics structure and that rapid growth of the structure involves coactive faults within the structure , then predictions of damage during earthquake sequences in such structural blocks must broaden, from a narrow focus on the main fault producing the main shock and the ground shaking attendant to the main shock, to include permanent deformation along fault zones that might move coactively with the main fault. These may be faults or shear zones that generate aftershocks, or faults or shear zones that shift aseismically. Conversely, in studying the setting of seismogenic faults, we may need to narrow our view in some cases, from a broad tectonic region such as a plate boundary or a subduction zone to tectonic structures within such regions. In any case, we should recognize that damage to man–made structures may be caused by permanent ground deformation accompanying slip on coactive faults, as well as by transient ground shaking.
On the basis of our study in the Winnetka area of the Northridge earthquake sequence we draw the following conclusions:
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Plate 1. Differential vertical displacements between 1980 and 1994 in San Fernando Valley, Los Angeles, California. [8½ x 11 JPEG Image at 300 dpi ( 653 Kb)]
Plate 2. Extension (or shortening) of streets calculated from surveys of monuments in 1970'S and 1995 in the Winnetka area, following 17 January 1994 Northridge, California earthquake. [8½ x 11 JPEG Image at 300 dpi ( Kb)]
Plate 3. Strains calculated from changes in lengths of streets and angles between streets in the northeastern part of the Winnetka area. [8½ x 11 JPEG Image at 300 dpi ( Kb)]
