Aftershocks and Images of South Central Chile: Results from the Analysis of the IMAD Data Set
Abstract
In response to the Mw=8.8, Maule earthquake that occurred off the coast of Chile on February 27, 2010, seismologists from France, Germany, Great Britain, and the United States joined their Chilean colleagues to install seismic stations between 33-38.5S, from the coast to the foothills of the Andes to produce the International Maule Aftershock Deployment (IMAD) data set. These stations were deployed starting in mid-March with some stations pulled out in late September while others remained recording until the end of December of 2010. We used procedures developed by the USGS National Earthquake Information Center to generate earthquake locations, magnitudes, phase readings and regional moment tensors solutions. The catalog consists of some 44,000 events to approximately M2.0. We use the catalog locations and travel times as a starting point we use double difference techniques to investigate relative locations and earthquake clustering. We generated Receiver Functions (RFs) from teleseismic P and PP phases and construct common conversion point stacks to image the structure of the slab and forearc region down to a depth of ~120 km. The migrated RF image the oceanic slab Moho on several E-W and N-S profiles at 40 to 60 km depth beneath the array and several discontinuities above the slab in the forearc. We also image a prominent Ps conversion that we interpret as the continental Moho at ~35 km under the foothills of the Andes and decreasing to 25-30 km under the central valley of Chile. We observe a prominent Ps conversion at ~40-50 km depth on a NNE-SSW cross-section parallel to the strike of the slab that we interpret as the oceanic Moho in the down-going slab. We created an Ambient Noise Tomography image combining IMAD stations with other temporary and permanent broadband seismic stations in Chile and Argentina to obtain absolute shear velocities in the crust. Phase velocity maps between periods of 8 and 40 sec are inverted for a 3-D shear-wave velocity at 0.1° grid spacing. At shallow depths the basins in the backarc of the Sierras Pampeanas in Argentina show clear low shear wave velocities. The Chile forearc has higher shear velocities (>3.8 km/s) at all depths between 32 and 38S. Much of the active arc shows lower shear wave velocities (<3.5 km/s) at mid and lower crustal depths consistent with warm temperatures. We used two spectral ratio methods to determine shear wave attenuation in the earthquake rupture zone. Both methods reveal diminution of the ratio of spectral amplitudes of P and S waves observed along a given source-station path. The first calculates 400 individual Qs values for the phase pair. In the second method, spectra of each of the 400 iterations are stacked, yielding a Qs value obtained from the slope of the composite spectrum. Preliminary measurements, deemed acceptable when the two methods agree, show a variation of Qs values from ~100 to 1000.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2013
- Bibcode:
- 2013AGUSM.S23C..06R
- Keywords:
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- 7230 SEISMOLOGY / Seismicity and tectonics;
- 7240 SEISMOLOGY / Subduction zones;
- 7218 SEISMOLOGY / Lithosphere;
- 7205 SEISMOLOGY / Continental crust