Crustal structure of the Borderland‐Continent Transition Zone of southern California adjacent to Los Angeles

Abstract

We use data from the onshore‐offshore component of Los Angeles Region Seismic Experiment (LARSE) to model the broad‐scale features of the midcrust to upper mantle beneath a north‐south transect that spans the continental borderland in the Los Angeles, California, region. We have developed an analysis method for wide‐angle seismic data that consists primarily of refractions, lacks near‐offset recordings, and contains wide gaps in coverage. Although the data restrict the analysis to the modeling of broad‐scale structure, the technique allows one to explore the limits of the data and determine the resolving power of the data set. The resulting composite velocity model constrains the crustal thickness and location and width of the continent‐Borderland transition zone. We find that the mid to lower crust layer velocities of the Inner Borderland are slightly lower than the corresponding layers in the average southern California crust model, while the upper mantle velocity is significantly higher. The data require the Moho to deepen significantly to the north. We constrain the transition zone to initiate between the offshore slope and the southwest Los Angeles Basin. If the Inner Borderland crust is 22 km thick, then the transition zone is constrained to initiate within a 2 km wide region beneath the southwest Los Angeles Basin, and have a width of 20–25 km. The strong, coherent, and continuous Pn phase suggests the Moho is coherent and laterally continuous beneath the Inner Borderland and transition zone. The Inner California Borderland seems to be modified and thickened oceanic crust, with the oceanic upper mantle intact beneath it.

Characteristics of the crustal structure and hypocentral tectonics in the epicentral area of Nan’ao earthquake (M7.5), the northeastern South China Sea

“…The onshore-offshore seismic survey, through triggering by air-gun in the sea and receiving the signals on the land, has been conducted in recent years. The models of the crustal structure of land and sea were successfully combined to explore the crustal structure of the land-sea transitional zone [18,19] .…”

Rayleigh wave phase velocities, small‐scale convection, and azimuthal anisotropy beneath southern California

“…In this area, Zhu and Kanamori [2000] had no crustal control, because there are no stations where converted phases could be detected. Consequently, our interpolated crustal model has average crustal thicknesses of close to 30 km, but there is every reason to expect that the $22 km thick crust adjacent to Catalina [Nazareth and Clayton, 2003] continues farther south along the coast, as the Catalina schist belt, representing middle crustal rocks uplifted and exposed during extension, continues at least as far as 31°S [Bohannon and Geist, 1998]. Thus the velocity anomalies in the uppermost mantle layer and lower crust simply indicate that model crust should be thinner in this area.…”

The application of large volume airgun sources to the onshore-offshore seismic surveys: implication of the experimental results in northern South China Sea

“…the LARSE in 1994 in southern California, the TAICRUST in 1995 in Taiwan of China and the SIGHT in 1996 on South Island of New Zealand. The published experiment results indicate that LARSE, TAICRUST and SIGHT identified rather clear airgun signals at 195 km, 215 km and 265 km offset distances respectively [5][6][7]17,18] . All these airgun signals at far distances were recorded by portable seismic stations.…”

“…Generally speaking, Pn is the first arrival at offset distance over 200 km and should be identified easier, or Pn and PmP appear together [3,5,7,17,18] . But sometimes there are also the examples in which only PmP appears or Pn is very weak [6,[24][25][26][27] , similar to the situation of Zhaoqing Station.…”

“…Furthermore, most deep seismic surveys of previous studies are separated between offshore and onshore areas, such that an imaging gap exists in the land-sea transitional zones, which are probably tectonic boundaries [3,4] . Recently some successful onshore-offshore seismic experiments, using marine airgun sources and land-based seismometers, were carried out, so that the onshore and offshore crustal structure models could be linked together [5][6][7] . However, such onshore-offshore experiments are rare, especially those using permanent land stations to receive marine airgun signals.…”