SCEC Award Number 12042 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title A Dense 400-Sensor Survey in Los Angeles
Name Organization
Paul Davis University of California, Los Angeles
Other Participants
SCEC Priorities 6d, 6c SCEC Groups Seismology, GMP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
Seismic-scattering theories describe high-frequency coda waves as a combination of waves from random scatterers superimposed on direct waves from the source. The direct waves are expected to be spatially coherent whereas the scattered waves, ar- riving with random phase, will be spatially incoherent. Our objective is to use data from an extreme high-resolution seismic experiment in Long Beach, California, to de- termine the transition from coherent to incoherent coda. The network, deployed by Nodal Seismic, comprises 5400 vertical component instruments, spaced every 100 m over an area of 5 × 7 km2. It was deployed for a period of six months with the pri- marily target to image the geological structure of the area for oil exploration. During the deployment, several thousand earthquakes and microearthquakes were recorded. We examine coda waves from the two largest events that occurred in the vicinity of the array.We compute frequency–wavenumber diagrams to determine the sources of coda and their evolution in time. Entropy analysis of the propagation of seismic waves through the array indicates the transition between the coherent direct body waves and the onset of incoherent coda waves. Our analysis illustrates that after the arrival of the body waves, the seismic coda is initially dominated by a dispersing wave train com- posed of spatially coherent body waves, forward scattered from 1D crustal layering.
This is then followed by omni-directional, spatially incoherent coda waves that can be described as scattered waves from 3D random sources.
Intellectual Merit For the first time coda from earthquakes recorded by an unaliased array of 5000 stations 100 m apart has been used to track the transition from coherent forward-scattered waves to incoherent back-scattered waves. Back-scattered waves are used for determining site effects as they are thought to be incident from multiple directions and so do not suffer defocusng/focusing effects or amplitude modulation from the radiation pattern of direct waves. However when this transitio occurs has perviously relied on theoretical syntheses. We use an entropy measure to detect this transition, which aoccurs at approximately twice the S wave lapse time.
Broader Impacts Understanding scattering is important for estimating ground motions and and seismic hazard at high frequencies where it is unlikey we will have ether the computing power or the detailed structural information to construct determininstic waveforms, and so must rely on stochastic propertiesas categorized here.
Exemplary Figure Figure 4: Entropy–energy analysis. (a) Map and classification of the stations. Red circles mark the central stations; yellow circles indicate the neighbor stations of size L = αλ; and open circles denote stations not used in this computation; (b) normalized entropy decay; (c) average energy computed using equation (2); and (d) sample seismo- gram randomly chosen from the network. The solid line marks the transition from coherent arrivals to noise-like random state.FOr th efirst itme an unaliased seismic array of 5000 stations 100