Plate Reconstructions The evolution of the San Andreas fault system over the las
ID: 235156 • Letter: P
Question
Plate Reconstructions
The evolution of the San Andreas fault system over the last 30 million years is shown below. The Pacific plate currently is moving past the North American plate at a rate of about 3.5 cm/year.
1. During what time (in millions of years) would you expect volcanoes near Los Angeles?
2. Why would there be volcanoes at this time (i.e., what would create them)?
3. Should the volcanoes be active today? Why or why not?
4. Using the scale on the map and the rate of plate motion above, determine how long it will take Los Angeles to reach San Francisco.
Seismicity
Seismicity is simply another way to say earthquakes. Areas of high seismisity levels are typically, but not always, coincident with those of recent volcanic eruptions. Differences in earthquake depth tend to indicated plate boundary differences. Below is a seismicity map of the world, focused on the Pacific Plate – also called the ring of fire due to the large occurance of volcanic activity.
5. What is the difference in seismic activity between the California coast of the U.S. and the Peruvian / Chilean coasts of South America?
6. What is the reason for this difference? (refer to your notes on plate boundary types)
7. Describe the pattern of earthquake depths that you see along the coast of Chile.
8. Why is this pattern occuring?
Explanation / Answer
1. It's hard to say. That part of the subduction of the Pacific Plate is inactive, and I have not heard any time frame for it to resume. Currently, there is transform motion undergoing southern to central California, while northern California has volcanoes due to the active subduction of the Pacific Plate.
2.The molten rock (magma) that feeds volcanoes comes from much closer to the surface than the core, which is about 2,900 kilometers (about 1,750 miles) deep. Volcanoes are located where there is a source of magma. Lots of times this is at plate boundaries and that's also where there are lots of faults and earthquakes. The San Andreas Fault is a place where two plates are sliding PAST one another, so there are lots of faults and earthquakes. One of the main places where rock is melted is where one plate slides UNDER another. That happens further north in the Cascades of Washington, Oregon, and northern California and that's why they have some active volcanoes (like Mount St. Helens) there. Los Angeles and southern California may have a lot of potential for earthquakes, but are probably safe from volcanoes for a while.
3. The closest volcanic area to Los Angeles is the Coso Volcanic Field that lies just north of Ridgecrest, California, about 181 miles north of Los Angeles. So May be there are chances of a volcano.
4.100 million years approx
5 and 6. Earthquakes along the shallow South American subduction zone exhibit heterogeneous rupture characteristics, going back several centuries of the earthquake record in this area. This heterogeneity is manifest in several ways, such as changes in rupture mode from magnitude N8 events during one century followed by smaller ones in other time periods, as well as unusual tsunami events. There is also an apparent interaction between earthquake rupture and subducting plate complexity in this region. Significant complexity exists on the subducting Nazca plate, including fracture zones and ridges such as the large Nazca Ridge. Several large magnitude earthquakes have occurred in the region of ridge subduction, but no earthquake rupture has ruptured through these features into adjacent regions, suggesting that these subducting features act to segment the margin. Other features, such as fracture zones and variable sediment thickness on the Nazca Plate, appear to influence earthquake behavior over a wide range of magnitude scales. Upper plate features such as crustal faults also lead to heterogeneous earthquake behavior. Here I provide an overview of seismicity along the margin since 1850 in the context of the subduction zone structures. This includes great earthquakes such as the 1906 Ecuador and 1960 Chile events. I also present results showing increased rupture complexity in moderate magnitude earthquakes that can also be linked to certain Nazca Plate features.
Most of the world’s largest earthquakes occur along subduction megathrusts. Study of the evolution mechanism of seismogenic locking and strain accumulation along the subducting interface is crucial for estimating recurrence of these destructive events. The Costa Rica region is ideal for investigating megathrust earthquakes because of the region’s proximity to the subducting interface and abundance of existing and new seismic and geodetic data. For this project, I have manually located more than 5000 local earthquakes that occurred in 2009 in the Costa Rica region by using the Antelope seismic analysis software. I have applied the local magnitudes of these events to demonstrate the spatial variability of frequency-magnitude (FM) along the subduction interface of the Nicoya Peninsula. Preliminary results show the current spatial FM distribution has changed compared with Nicoya seismicity FM (between late-1999 and mid-2001) maps produced by previous studies. This change is most likely due to either slow slip events or tremor, or a variation in an interface property. Future work includes generating a best-fit 3-D interface model for the Costa Rican subduction zone by using the seismicity distribution along the approximate interface. I will then use the geometry of this newly generated subduction interface model, combining with GPS surface deformation data (from 1996 to 2010) of the Nicoya Peninsula, to generate the inversion results of the slip distribution along the interface. This analysis can be used as a good proxy for locating accumulated stress along the megathrust interface. If the simulated interface model works well, I will also apply it to specifically examine the stress accumulation pattern changes before and after the 2007 slow slip event by calculating the inversion results of GPS movement data. I will then evaluate the impact of slow slip events on potential regions of future megathrust earthquakes with highly accumulated stress.
7 and 8. Using a technique called seismic profiling, researchers have found evidence of ancient earthquake faults under Portland, Oregon. The faults may still be active, a USGS [United States Geological Survey] seismologist will announce tomorrow. The research also turned up a 250-foot deep layer of silt and mud, deep under the city, which may have been caused by a catastrophic ice dam break some 15,000 years ago. The two findings could together mean bad news, as soft sediment is known to amplify ground shaking during strong earthquakes. In the 1989 San Francisco earthquake, much of the damage to buildings was caused by liquefaction, a shaking and sinking of sandy, water saturated soil along waterways. . . .