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What is an Earthquake?The basic facts of seismology |
Activity #1: Rescue in Pinevale
Activity #2: Fault Rupture Analogies
Testing Simple Rupture Analogies
Answers will vary.
The final criterion -- slip decreases to zero at the ends of the rupture -- was the most difficult to meet. Only the rug example even comes close, though if you imagine ripping a piece of paper from the center, without breaking an edge, this would work. It was specified, however, that the paper be ripped in half -- so none of our models met this criterion.
The Rupture Animation
Looking across the fault, the objects on the far side moved toward the right, relative to the near side. (This is called right-lateral strike-slip, as we will learn later.)
Answers may vary. The most limiting feature of this animation is that we can only see the surface rupture, which appears as a line, rather than the entire area of the fault that ruptures at depth.
Answers will vary. The rupture front looks and propagates something like a circular wave, perhaps similar to a ripple you might make by disturbing a still body of water.
The depth to the Northridge hypocenter appears to be something like 18 to 19 km. Yes, there might have been more downward rupture, since part of the wave seems to head downward at the very beginning of the rupture.
Answers will vary. The average velocity of rupture propagation was roughly 3 km per second, plus or minus 1 km/sec or so.
Answers will vary. The average slip appears to have been roughly 1 meter or less.
About seven seconds.
The area shown in this animation is 18 by 14 kilometers, or 252 square kilometers, in size.
Yes, the rupture seemed to propagate in a wave-like manner. The key difference in the way the two animations look is probably the fact that the Landers rupture primarily is propagating laterally and, after the initial stages, begins to look like a planar wave (helped by the fact that the rupture front propagates for a very long time). This happens because the rupture reaches two confining boundaries -- the lower limit of the brittle crust and the surface -- well before the rupture is finished. The Northridge rupture propagated more vertically, and was smaller, so that we see only what is analogous to the initial stages of the Landers earthquake, when it too looked like a circular wave. The rupture's average velocity was about 3 km per second, similar to that of the Northridge earthquake.
If your MPEG player is good enough, you should note that there are a few very obvious changes in rupture velocity.
This rupture lasted over 23 seconds.
The area shown in this animation is about 80 by 15 kilometers, or roughly 1200 square kilometers.
The values for slip, duration, and area are all roughly 3 or 4 times greater than that of the Northridge earthquake. (Technically: maximum slip is about 2.7 times that of Northridge, duration is about 3.3 times greater, and area (of the animation rectangle) is nearly 5 times greater!)
The Tamarack fault might extend under Forest Springs, since its dip is to the northeast. You could have figured this out without the barbs marking the direction of dip by studying the surface trace of the Tamarack fault as it crosses topography.
No, the Bayside fault could not possibly dip to the southwest. The two possibilities for its direction of dip are northwest and southeast. Either the article or your memory is incorrect (or perhaps the article was referring to a different Bayside fault!).
Exercise 2
This supports the findings of the group with which you're working.
This would not completely rule out the first group's calculation.
Activity #5: Revealing a Fault Plane with Hypocenters
Answers will vary. Most should find an angle of about 45°, dipping toward the northeast.
Depending on how you drew your line, you should find it comes closest to either the Garnet Hill fault zone or the Banning fault zone (South Branch San Andreas fault). The assumption that this is the fault that produced the mainshock may be invalid if that fault changes dip before it reaches the surface, or if that fault is truncated by another fault.
Answers will vary. Some possibilities are: inaccuracies from the crudeness of this cross-section, inaccuracies in the data due to uncertainties in locating hypocenters, choosing the cross-section in an orientation not exactly perpendicular to the strike of the fault, and/or that the aftershocks are not actually occurring on the mainshock fault, but on small faults adjacent to it.
Some examples of correct answers:
Scarps can be formed by water: e.g., wave-cut platforms and marine
terraces, or the edges of river and stream channels. This kind of feature
could also be the edge of a landslide (a debris flow) or a
glacial deposit (a moraine). It could even have been man-made.
[no answer required]
Activity #7: Changing the Dimensions of the Crust
You could drill through the crust near the normal fault more easily; the crust is thinner here.
Yes, changing the dip of these faults would make a difference in how they lengthen or shorten the crust -- as the angle of dip decreases, the amount of lengthening or shortening increases. If you change the angle of dip to increase the change in length, the change in thickness will decrease. This kind of change will also tend to increase the area over which crustal thickness is altered.
You would expect faults in an area of strong tectonic compression or extension to be low-angle faults.
Tectonic extension (rifting) should allow volcanism to take place more easily than would tectonic compression, since extension decreases the distance between the top of the mantle and the surface of the Earth.
Yes, all the examples displayed nearly equal dip-slip and strike-slip components.
Activity #9: Recurrence Interval
The recurrence interval should be about 7500 years.
Given that the previous major rupture occurred only 400 years ago, this long recurrence interval should reassure people who live very close to the surface trace of the Salt Wash fault, since it seems unlikely the fault would experience another major rupture very soon.
According to this average recurrence interval, this section of the San Andreas fault appears due -- even slightly overdue -- for a major rupture.
The longest interval between events is 332 years -- 200 years longer than the average recurrence interval. The shortest interval is 44 years -- 88 years shorter than the average recurrence interval. This should give you less confidence in your answer to question #2.
Graphs will vary slightly in appearance, but should have an overall look similar to the example of a completed graph given.
The pattern should be apparent if the graph is constructed properly. It is more likely that we are in a "gap", not a "cluster", since the gaps average about 250 years, while the time between earthquakes in a cluster averages about 68 years -- and it has been over 140 years since the last major rupture at Pallett Creek. This finding opposes the conclusion reached in question #2, because this graph suggests that the San Andreas fault in this area is not yet due to rupture for many decades.
The Mojave section of the San Andreas fault should have a rupture interval of about 130 years. The northern half of the Imperial fault should have a rupture interval of about 35 years.
According to these calculations, the northern half of the Imperial fault would rupture almost four times as often as the Mojave section of the San Andreas fault. This shows that a higher slip rate does not always produce more frequent damaging earthquakes, and thus, should not be the only factor considered when determining fault hazards.
Activity #10: Different Data, Different Rates?
Average recurrance interval is about 2600 years (2566.67). This yields a slip rate of about 1.2 mm/yr.
They are not similar -- the rate from Study #2 is significantly smaller than those from Study #1.
These studies imply that the slip rate of the Brighton fault is decreasing over time. You should probably use the slip rate value from Study #2 (1.2 mm/yr) to estimate its current slip rate. If you were modelling its behavior from between 2 to 4 million years ago, you should probably choose the 2.9 mm/yr value, as provided for that time period by Study #1. You could even choose to use a slightly higher average value, assuming the rate was decreasing during the period (2 to 4 million years) you were modelling.
The other component is a lateral motion perpendicular to the strike of the fault, at roughly 0.4 mm/yr. This agrees with Study #1 if you assume that this component results from the shortening of the crust in this area, as accomplished by (reverse) dip slip along this fault. Since both studies seem to have their facts straight, the true sense of slip for the Windy Valley fault should be left-reverse.
Study #1 implies a slip rate of 2.2 mm/yr. Study #2 implies a slip rate of 3.2 mm/yr.
Using these two values as vertical and horizontal components yields a resultant slip rate of 3.2 mm/yr -- the same as found by Study #2.
Another fault or zone of deformation could account for the disparity. The diagram shows an obvious fold belt (producing a chain of hills) to the east (left) of the Alder Thrust; this could be the surficial expression of a blind thrust fault.
Yes; the horizontal component of motion from such a fault would be 1.3 mm/yr.
The horizontal component of the slip rate along the Montane Thrust is 2.5 mm/yr. The vertical component of its slip rate is 2.0 mm/yr.
The angle of dip of the Montane Thrust must change south of the Torn Valley fault to match the components of vertical and horizontal slip found in Question #2. The slip rate of this southern section of the fault is 3.2 mm/yr; its dip is about 39° to the east.
Activity #11: How Tectonic Forces Affect Faults
This fault is a right-lateral reverse fault.
Fault A is a thrust fault. Fault B is a left-lateral oblique slip fault. The tectonic forces push on the east and west faces of the block.
Faults A and B are both normal faults.
Activity #12: "Find That Fault Slip!"
Possible answers include: Alamo, Buena Vista, Dry Creek, Frazier Mountain, Oak Ridge, Pleito, Red Mountain, San Cayetano, Santa Susana, Wheeler Ridge
Possible answers include: Big Pine, Mission Ridge/Arroyo Parida/Santa Ana, Santa Cruz Island, Santa Rosa Island, Santa Ynez, White Wolf
Possible answers include: Cady, Cleghorn, Coyote Lake (?), Grass Valley, Manix, Tunnel Ridge (?)
Possible answers include: Avawatz Mountains, North Frontal, Santa Ana
Possible answers include: Alamo, Big Mountain, Chatsworth, Clamshell-Sawpit Canyon, Clearwater, Cucamonga, Dry Creek, Eagle Rock, El Modeno, Frazier Mountain, Holser, Lion Canyon, Llano, Malibu Coast, Mission Hills, Northridge Hills, Oak Ridge, Peralta Hills, Pine Mountain, Red Hill, San Cayetano, San Fernando, Santa Susana, Simi, Verdugo
Possible answers include: Elsinore, Newport-Inglewood, San Gabriel, San Jacinto
Possible answers include: Blue Cut, Elmore Ranch, Morongo Valley, Pinto Mountain, Salton Creek, San Antonio, San Jose, Stoddard Canyon
Possible answers include: Crafton Hills, Point Loma, Rico
Possible answers include: Airport Lake, Brown Mountain, Furnace Creek, Goldstone, Hunter Mountain, Wilson Canyon
Owl Lake
Avawatz Mountains
Activity #13: Regional Distribution of Slip
Reverse and thrust faults are primarily concentrated in a wide band (the Transverse Ranges) which runs roughly east-west at about the same latitude as the "Big Bend" of the San Andreas fault. Because the "Big Bend" generates tectonic compression, this distribution is as expected.
Yes, something different, tectonically speaking, appears to be happening north of the Garlock fault zone. Normal slip is quite common here, yet nearly absent on the rest of the map. This area appears to be experiencing extensional tectonics; it is being "pulled apart".
The slip rates of these faults tend to decrease with increasing distance from the San Andreas fault. The slip rates at the northern ends of these faults seem to decrease.
Answers may vary. The assumption that blind faults do not reach the surface because they are not as active is not a reasonable assumption, according to the map. These blind thrust faults seem just as active as, and possibly even more active than, nearby faults which reach the surface.
Activity #14: Partitioning Slip
Fault A has a slip rate of 9.1 mm/yr. Fault B has a slip rate of 2.6 mm/yr. Fault C has a slip rate of 1.3 mm/yr.
Neither team was correct -- the actual recurrence interval should be 500 years.
The single fault would be a right-lateral strike-slip fault striking at about N39E and slipping at a rate of about 11 mm/yr (the same figure as in Question #1, above). Answers involving normal sense dip-slip faults may seem valid, but they involve vertical displacement, which was not mentioned in the exercise, so they are not technically correct.
The rate of uplift at point X is 3.0 mm/yr.
The rate of uplift would decrease if the angle of dip of the thrust faults were decreased.
Points 1 and 2 are moving toward each other at a rate of 6.0 mm/yr. This rate would be unaffected (it would stay the same) if the dip of the thrust faults were decreased.
The rate of the component of right-lateral slip parallel to the plate motion is 26 mm/yr.
The fault is bent 30° away from its "proper" trend.
The slip rates tend to get smaller as you move away from the San Andreas fault zone. Answers may vary.
No, the east-west trend in slip rates is still the same.
Yes, the slip rates have changed along strike. With the exception of the San Jacinto fault, the rates are increasing toward the south.
Yes, the recombined slip rate is equal to the sum of the two branches.
The east-west trend in slip rates is the same as before. The slip rates have continued to increase toward the south, again with the exception of the San Jacinto fault.
In both diagrams, all the faults lie on the "inside corner" of the San Andreas fault's Big Bend -- you would intersect the bend if you followed their trend.
