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Time-Domain Determination of Regional Wave Propagation Characteristics and Earthquake Source Spectra: Application to the Ridgecrest, California Earthquakes

Description: 

Earthquake catalogs are growing exponentially in size thanks to the development of advanced detection and location methods. However, most deep catalogs contain only rudimentary information that quantifies the source, such as a Local Magnitude. We have developed a straightforward method that can measure the seismic moment and corner frequency from simple measurements of the seismogram. We analyze the spectral content of S waves using time-domain measurements of the horizontal shear wave maximum displacement amplitude, passed through a set of one-octave band-pass filters between 0.25-32 Hz. Following the development of Richter’s 1935 Local Magnitude scale, we derive a functional form of maximum amplitude versus distance for each frequency band and use it to estimate wave attenuation as a function of frequency and distance. Using events in the Ridgecrest, California earthquake sequence, we derive frequency dependent attenuation curves for epicentral distances between 10 and 120 km. Attenuation is a strong function of frequency, inversely proportional to frequency, at a specific epicentral distance. The path averaged Q also increases at all frequencies (attenuation decreasing) with increasing epicentral distance. At 50 km distance Q(1-2 Hz) ~100 and Q(16-32 Hz) ~300, while at 100 km Q(1-2 Hz) ~ 200 and Q(16-32 Hz) ~500. If the maximum amplitude S wave samples increasingly greater depths in the crust with increasing distance, then Q increases significantly with depth. We use these empirical attenuation curves to correct the time-domain peak amplitudes to a reference distance of 10 km to recover the source spectrum. We find that the spectral shape is consistent with the Brune model, with a well-defined plateau at low frequency and a decay at high frequency inversely proportional to frequency squared. This time-domain method provides a reliable and efficient way to accurately measure earthquake source parameters.

Session: Earthquake Source Parameters: Theory, Observations and Interpretations

Type: Oral

Date: 4/18/2023

Presentation Time: 08:15 AM (local time)

Presenting Author: William L. Ellsworth

Student Presenter: No

Invited Presentation: Yes

Authors