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Localized seismic deformation in the upper mantle revealed by dense seismic arrays

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Science  07 Oct 2016:
Vol. 354, Issue 6308, pp. 88-92
DOI: 10.1126/science.aaf1370
  • Fig. 1 Earthquake density, seismicity, and helium ratios.

    This figure shows the spatial distribution of seismicity that occurred between 1980 and 2011 and was recorded by the Southern California Seismic California (SCSN) (14), as well as helium ratios (3He/4He) in the LA basin, which were measured and corrected for air contamination by Boles et al. (25). (A) Earthquake density as a function of location. We used the color bar labeled “Rate” to indicate the spatially smoothed number of events over a 30-year period, binned in 9-km2 squares. The locations of helium measurements, seismic stations, and dense seismic arrays are denoted by green inverted triangles, gray triangles, and blue polygons, respectively. The dashed rectangle indicates the region from which we extracted the earthquakes used to construct panels (B) and (C). Red curves denote the surface trace of active faults. (B) SCSN catalog seismicity depth distribution along the Newport-Inglewood fault (NIF) and in southern California (CA). z is the depth below sea level. (C) Depths of NIF seismicity and the Moho as function of location along line A-A′ in (A). The Moho (45, 46) is indicated by the green curve. The depths (d) above which 50 and 95% of the earthquakes occur in the SCSN- and back-projection–derived catalogs are indicated by the orange and red dashed curves and squares, respectively. Gray triangles represent the projection of SCSN seismic stations onto the line A-A′; blue triangles denote the arrays. ROSE indicates the Rosecrans Array and LB denotes the Long Beach Array. (D) Helium ratios within the area enclosed by the dashed rectangle in (A), as a function of distance along A-A′. The dashed curve indicates the polynomial best fit to the observations. Green inverted triangles are the same as in (A). Ra is the 3He/4He ratio in air; Rc is the same for the crust. (E) Helium ratios as a function of distance normal to A-A′.

  • Fig. 2 Spatial distribution of earthquake density derived from a catalog spanning 93 nights of the LB Array data set.

    (A to C) Map view of event density in the following depth ranges: (A) 5 to 12 km, (B) 12 to 20 km, and (C) 20 to 32 km. Densities in each panel were normalized by their maximum value. Areas with intense seismicity are shown in orange and red; areas devoid of seismicity appear in yellow and white. The NIF surface trace and the local oilfields are denoted by black and green dashed lines, respectively. LB, Long Beach oilfield; LBA, Long Beach Airport oilfield; WI, Wilmington oilfield. (D) Vertical cross section showing event density along the B-B′ line in (A). We normalized the counts in each 2-km depth bin by their maxima. The Moho depth (47) is indicated by a green curve, and the uncertainty on this estimate was determined using previously published results (26, 38, 48). (E) Seismicity depth distribution in the LB Array data set.

  • Fig. 3 Temporal analysis and earthquake size distribution in Long Beach.

    (A) Distribution of earthquake magnitudes. The blue squares and red circles denote shallow (<15 km) and deep (>25 km) events, respectively. The black squares are for the SCSN catalog and are normalized according to the LB Array spatiotemporal coverage. The slopes of black and gray dashed lines are equal to –1 and –2, respectively. (B) Distribution of earthquake signal amplitudes, which we define as the maximum of the downward-continued, migrated stack in a 5-s window containing the event, scaled by the maximum of the synthetic stack computed for a collocated source with Mw = 1. The best-fitting exponential model, which appears linear in this semi-logarithmic scale, is indicated by the red curve. The blue curve shows a power law. r, correlation coefficient. (C) Autocorrelation as a function of lag-time between earthquake rate time series for shallow (<10 km) and deep (>25 km) clusters. The blue and red curves indicate the average values for 112 shallow and 52 deep clusters, respectively. The gray shaded area denotes the 1σ uncertainties. The black dashed curve represent a synthetic earthquake catalog with a random, Poissonian distribution of earthquake occurrences.

Supplementary Materials

  • Localized seismic deformation in the upper mantle revealed by dense seismic arrays

    Asaf Inbal, Jean Paul Ampuero, Robert W. Clayton

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