What is the "shadow zone" and why is it important in seismic studies?
Figure 1: Cross section of the Earth's interior. Cream color is the inner core, yellow the outer core and orange the mantle. Black lines represent the pathways of seismic waves travelling through the Earth from an epicenter at 0°. Note how the waves are refracted as they travel through the liquid outer core, creating the P-wave shadow zone. (image © USGS)
When large earthquakes occur on the other side of the world from Southern California they can sometimes be difficult to detect if they occur approximately 100° to 140° away. For example, compare the following events recorded on the Anza broadband array during 2003:
- ~70° away: Magnitude 6.20 quake near Hihifo, Tonga
- ~80° away: Magnitude 6.20 quake near Primorye, Russia
- ~90° away: Magnitude 6.7 quake near Algiers, Algeria
- ~100° away: Magnitude 7.0 quake South Island, New Zealand
- ~110° away: Magnitude 7.0 quake near Mindanao, Philippines
Comparison of each plots vertical component (HHZ) from each event shows a widely contrasting amplitude of the waveforms recorded (note the scale of each waveform plot). This section of the earth of poor seismic information, or 'shadow zone', is defined as the area of the earth from angular distances of 104° to 140° to the epicenter that, for a given earthquake, does not experience seismic waves (Figure 1). The shadow zone results from S waves being unable to transmit through the liquid core and P waves being bent (refracted) by the liquid outer core. Also see our glossary. Although direct P waves and S waves are not observed in this distance range, we still do see signals from refracted P waves that have lower frequency content and are significantly attenuated. We also clearly receive seismic information from surface waves and reflected phases.
The presence of an inner/outer core boundary at around 3200 miles depth was proved by Inga Lehmann, one of the first women seismolgists. It separates the liquid iron outer core from the solid iron inner core. The existence of a solid inner core was demonstrated by Inga Lehmann's detailed computer studies of the weak secondary P waves generated in the liquid outer core as S waves from the inner core fall on it.
Does this mean that some earthquakes go undetected?
No, although earthquakes often occur around the world in locations that position the Anza broadband array in the epicenter's shadow zone, earthquakes are measured by multiple broadband arrays across the globe. Each earthquake is recorded in database catalogs which we compare with our database of earthquakes measured by the Anza broadband array in Southern California.
URL: http://eqinfo.ucsd.edu/faq/shadow_zone.php [Last updated: 2015-10-22 (295) 23:07:44 UTC]