Question

An earthquake occurred on the Erie fault 5 kilometers ( 3 miles) beneath San Gabriel. Damage from the earthquake was greatest in nearby Fremont. The farthest report of shaking was recorded in Stockton. Where was the earthquake's epicenter? a) The Erie fault c) Fremont b) San Gabriel d) Stockton

Short answer

The earthquake's epicenter was in San Gabriel.

Step-by-step solution

Understanding the Epicenter

The epicenter of an earthquake is the point on the Earth's surface directly above where the earthquake originates. It is typically located at the surface location directly above the earthquake's focus, which is the point within the Earth where the earthquake starts.

Identifying Earthquake Information

From the problem, we know that the earthquake occurred on the Erie fault, 5 kilometers (3 miles) beneath San Gabriel. This provides two critical pieces of information: the fault on which it occurred and its depth beneath a location.

Determining the Epicenter

Given that the earthquake's focus was beneath San Gabriel, the epicenter would be the point on the surface directly above the focus. Since it is stated that the earthquake occurred beneath San Gabriel, this makes San Gabriel the epicenter.

Key concepts

Fault Lines

Fault lines are significant cracks in the Earth's crust where tectonic plates meet and stress builds up over time due to their movement. During an earthquake, the relief of this stress causes the two plates to slip past each other along these lines.

• They serve as common boundaries for tectonic plates.
• Faults are areas where earthquakes are most likely to occur, due to the pressure and potential for movement between the plates.
• Understanding fault lines helps in predicting possible areas for earthquakes.

The Erie fault mentioned in the exercise is an active fault. It's essential to monitor and study fault lines for safety and prediction efforts.

Seismic Focus

The seismic focus, also known as the hypocenter, is the point inside the Earth where an earthquake begins. It is the site of initial fracture in the Earth's crust, causing the energy release that we feel as an earthquake.

• Located below the Earth's surface, often at depths ranging from a few kilometers to several hundred kilometers.
• The seismic waves generated at this point spread outwards and are felt as vibrations on the surface.
• In the exercise example, the seismic focus is located 5 kilometers beneath San Gabriel, where the earthquake initiated.

The location of the focus is important for understanding how and why an earthquake occurs, and it directly influences the location of the epicenter on the surface.

Earthquake Damage

Earthquake damage refers to the destruction or alteration caused by the seismic waves produced during an earthquake. The severity of damage often depends on several factors:

• Magnitude of the earthquake: Higher magnitudes typically result in more significant damage.
• Depth of the focus: Shallower earthquakes tend to cause more damage to the surface structures.
• Distance from the epicenter: Areas closer to the epicenter usually experience more intense shaking.
• Building structures: Materials and designs used in construction affect vulnerability.
• Geology of the area: Certain soil types can amplify seismic waves.

In the exercise, Fremont experienced the most damage due to its proximity to the focus beneath San Gabriel.

Surface Location Determination

Determining the surface location of the earthquake's epicenter involves identifying the point on the Earth's surface directly above the seismic focus. This process is essential for understanding the affected area and planning responses.

• Seismologists use data from multiple seismic stations to triangulate and locate the epicenter.
• The location is crucial for emergency services to assess impact areas and to mobilize resources efficiently.
• The epicenter provides insights into the fault lines and potential future seismic activity.
• In the exercise, understanding that San Gabriel is the surface location directly above where the earthquake occurred helped identify it as the epicenter.

Knowing the epicenter allows for better preparedness and response to earthquake events.