A Map View of the Basics
Exercise 1A Map View of the Basics
The map at right shows a hypothetical landscape in a region with
active faults. Three faults in particular are shown, and two towns
are labelled. North, as always (unless otherwise specified), is at top.
Elevation is shown as given by the scale bar.
The ocean can be seen at the southwestern (lower left) corner of the map.
Study this map and use it to answer the questions below.
The Tamarack thrust fault is a low-angle fault to the northeast
of the Splitridge fault, shown in black. Under which town, Ridgeview or
Forest Springs, might the Tamarack fault extend? (That is, toward which
town does it dip?) Could you have figured that out without the barbs
marking the direction of dip? If so, how?
The Bayside fault, shown in dark purple, is located in
an area of low relief. This is the first time
you've seen a map view of it, but you remember reading an article
which stated that the Bayside fault dips steeply to the southwest.
Assuming your memory is correct, can that statement in the article
be true? Why or why not?
Exercise 2Confirming Fault Dip with an Earthquake
The diagrams on the right show the location of an earthquake
hypocenter (or focus) -- that point at which the
breaking of a fault (i.e. an earthquake) begins. The top diagram
shows a map view of the area in which the earthquake occurred,
with a large fault (called the Skylight fault) and the epicenter --
that point on the Earth's surface directly above the hypocenter -- labelled.
The diagram below it shows a cross section of the area, with the hypocenter,
sea level, and the Skylight fault labelled.
Currently, there is some debate as to the actual dip of the Skylight fault. One team of researchers claims, on the basis of their studies, that it dips at 32°, and to the west. The group you're working with has reasoned, by another means of analysis, that the dip is actually 41°, and to the west. This latest earthquake was moderately large, and thus, most likely occurred on the Skylight fault, since no other fault large enough to produce the earthquake is known to exist in the area. The earthquake is located very accurately because of the brand new network of seismic instruments in the area. This accuracy gives you a chance to (potentially) settle the debate.
The facts you have are these:
At its closest point, the surface trace of the Skylight fault is 8.7 km from
the epicenter of the earthquake (upper diagram). The difference in elevation
between the surface trace at that closest point and the depth of the
hypocenter is 7.5 km (lower diagram).
Which group's claim does this earthquake seem to support?
We initially said that the measurements were very accurate, but suppose the figures given were much less accurate. In fact, suppose that both the depth of the hypocenter and the distance to epicenter were only known to an accuracy of 1.5 km in either direction. In other words, the distance from the fault to the epicenter could be as short as 7.2 km or as great as 10.2 km, and the depth to the hypocenter could be as shallow as 6.0 km or as deep as 9.0 km.
Would this completely rule out the first group's calculation
of a dip of 32° for the Skylight fault?
The exercise above could easily be applied to a situation where
there were actually two different "overlapping" faults in an area,
as shown in the simple diagram at right. In this case, the challenge
would be to determine on which fault the earthquake had occurred.
Typically, the method used above would only be one of several tests
used to determine the fault responsible for the earthquake, and a
very basic one at that. Two ways to decide, with greater certainty,
which fault was responsible involve the use of aftershock hypocenters
and focal mechanisms. Any large earthquake generally triggers
numerous smaller earthquakes, called aftershocks. By plotting the hypocenters
of each of these aftershocks in a way similar to what you did above, it is
sometimes possible to produce an "image" of the fault responsible.
(This can work for non-aftershocks, but aftershocks are more efficient
because several thousand of them can occur in a short period of time,
providing a huge data set.)
You will see how this works in Activity #5 of Section 1. In Section
3, you will learn about focal mechanisms -- how you can determine
them, and how they can help identify faults at depth. For both
techniques, however, knowing something about the dip of a fault is
essential in making the identification.
