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The Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, is a 2.8 km thick clastic sedimentary sequence deposited in a continental basin closely related to volcanic activity. It consists of three formations: the Pozo Duro (oldest), the Anita, and the Escalante (youngest). Petrographic study, conglomerate pebble counts, and U–Pb geochronology of detrital zircons were performed to determine the source and age of this sequence, and to interpret its tectonic setting. In the sandstones of all three formations, the most abundant grains are those of volcanic composition (Q38F22L40, Q35F19L46, and Q31F22L47, respectively). The Pozo Duro Formation includes well-rounded quartz-arenite clast conglomerates, whereas conglomerates of the two upper units have clasts predominantly of andesitic and rhyolitic composition. The most likely source for these sediments was the Jurassic volcanic arc exposed in northern Sonora and southern Arizona. Zircons from five sandstone samples define two main age groups, Proterozoi...

We studied the attenuation of local and regional earthquake body waves recorded by the seismic network of northeastern Sonora, Mexico (Red Sismica del Noreste de Sonora [RESNES]). The source-station paths of the selected data set cover the epicentral region of the May 1887 ( M w 7.5) Sonora earthquake. We used the best-located earthquakes recorded by RESNES between 2003 and 2004 to analyze the spectral amplitude decay of body waves with hypocentral distance. The database consists of 44 earthquakes with magnitudes between 1.8 and 3.2 and hypocentral distances between 25 and 150 km. We calculated Fourier spectral amplitudes from more than 400 records and defined attenuation functions with a nonparametric model by inverting the observed spectral amplitudes at 23 individual frequencies between 0.4 and 63.1 Hz. We determined regional attenuation curves based on events located more than 100 km northwest of the center of the network and local curves based on earthquakes located near the fault zone that ruptured in the 1887 earthquake. The obtained attenuation curves indicate that the spectral amplitudes decay faster in the area of the 1887 rupture than in the surrounding region. To analyze the possible sources of attenuation, we parameterized the attenuation functions considering three terms: one that accounts for geometrical spreading, another one for anelastic attenuation, and a third for near-surface attenuation. We found the S -wave quality factor Q to be similar in both the general region and near the 1887 rupture. However, in the region of the 1887 rupture, the near-surface attenuation exceeds the regional average. Finally, in the region of the 1887 rupture, the nonparametric attenuation functions and the estimates of Q indicate that at between 0.6 and 63.1 Hz, P waves attenuate more than S waves ( Q P =20.8 f 1.1, whereas Q S =83.8 f 0.9).