Seismic Waves: Northridge Earthquake - 1994

At 4:30 a.m. on January 17, 1994, the shaking of an earthquake awakened 10 million people in the Los Angeles region of Southern California. The earthquake's epicenter was in Northridge, CA, and it was a magnitude 6.7 shock that proved to be the most costly earthquake in United States history. The shaking heavily damaged communities throughout the San Fernando Valley and Simi Valley, and the surrounding mountains north and west of Los Angeles, causing estimated losses of $20 billion. Fifty-seven people died, more than 9,000 were injured, and more than 20,000 were displaced from their homes by the effects of the quake.

When an earthquake occurs, rock is fractured. This fracturing causes nearby rock to move, producing waves that travel through and across the world. There are three main types of seismic waves: P (Primary), S (Secondary), and surface waves. The P-waves are characterized by particle motion in the direction of the wave's propagation. S-waves, on the other hand, produce particle motion at right-angles to the propagation direction. Surface waves, as their name implies, travel over the surface of Earth.

The dataset includes a PIP that shows what is going on inside Earth as the waves propagate from the point of origin. This origin is called the hypocenter, while the area on the surface of Earth directly above the hypocenter is called the epicenter. The interior of Earth has three regions under the crust: the mantle, the outer core, and the inner core. These regions were discovered through the study of seismic waves, which travel through the interior of Earth and arrive at seismic stations around the world.

S-waves can only travel in a solid, so as an S-wave arrives at the core-mantle boundary, it is reflected back into the mantle as both an S-wave and a P-wave. In addition, some of the energy continues into the outer core as only a P-wave. This is because the outer core is a liquid made of mostly of molten iron. The inner core is also mostly iron, but is solid due to the higher pressures. Therefore, both P-waves and S-waves can propagate through the inner core.