103° to 143° is a shadow zone for which seismic waves?

Question

Accepted Answer

Option 3 is correct because the shadow zone between 103° and 143° from the earthquake's focus exists for both Primary (P) and Secondary (S) waves, though for different reasons.

*   **Why P-waves have a shadow zone:** P-waves are refracted (bent) as they pass through the Earth's liquid outer core. This refraction causes a shadow zone between approximately 103° and 143° from the earthquake's epicenter where direct P-waves are not received. Some P-waves do arrive in this zone, but they are much weaker and have been diffracted at the core-mantle boundary.

*   **Why S-waves have a shadow zone:** S-waves cannot travel through liquids. Since the Earth's outer core is liquid, S-waves are completely blocked by it. This creates a much larger shadow zone for S-waves, starting at approximately 103° from the earthquake's epicenter. No direct S-waves are received beyond this point.

Therefore, the region between 103° and 143° is a shadow zone for *both* P-waves (due to refraction) and S-waves (due to being blocked), making Option 3 the correct answer.

Now, let's explain why the other options are incorrect:

*   **Option 1 (Primary Waves):** While P-waves *do* have a shadow zone in this range, it's not the *only* type of wave affected. S-waves are also affected, making this option incomplete.

*   **Option 2 (Secondary Waves):** Similar to Option 1, S-waves *do* have a shadow zone in this range, but P-waves are also affected. This option is also incomplete.

*   **Option 4 (All options are wrong):** This is incorrect because Option 3 *is* correct. The shadow zone between 103° and 143° is indeed a shadow zone for both primary and secondary waves.