8/29/08
Since January 2007, a small area a few miles southeast of Maupin, Oregon, has produced over 300 earthquakes, 13 with magnitudes of 3.0 or larger. The earthquakes are located near where a large earthquake swarm took place over 30 years ago, the largest of which was a magnitude 4.8 on 13 April, 1976. The onset of the Maupin swarm coincided in time with the regional deployment of the EarthScope USArray experiment which temporarily added many stations in the region. At first seismologists were uncertain as to whether the 2007-2008 Maupin seismicity represented a new swarm or simply the better sensitivity provided by the new stations, allowed the PNSN to locate earthquakes that would have been missed prior to USArray.
As the swarm continued to produce larger quakes, it became apparent that seismicity had increased in the Maupin area. Seismologists debate the tectonic significance of swarm behavior, where earthquake temporal patterns deviate from the more common pattern of a large mainshock followed by numerous aftershocks that become less frequent in time. In swarms seismicity can be fickle and fitful, and the largest earthquake may not be at (or even near) the beginning of the swarm. In fact many swarms haven’t one significantly larger earthquake in them, and may just “go out”, or may fade out gradually. Many seismologists see these features as representing the effects of fluid movements in Earth’s crust. But others do not, and there is often no clear answer as to why a particular swarm happens. While it is true that swarm behavior is common near volcanoes, the Maupin swarm is far enough from the nearest active volcano (Mt. Hood), that magmatic activity may be ruled out as a driving force for the swarm. Swarm characteristics are, in fact, a feature of much of the seismicity east of the Cascades in Oregon and Washington.
In the last week of April 2008, the PNSN installed five portable broadband seismographs near the Maupin Swarm. These seismographs were able to record 3 dimensions of very long period ground motions (>50 s) to very high frequencies (25 Hz) with high fidelity. The portable array remained in the area for at least two months. Installing stations close to, and on all sides of, the epicenters allowed us to locate the earthquakes with much better resolution, and to carry out studies to determine their source characteristics, including the causative fault and the stresses that drove them. The sites were not telemetered so the data needed to be retrieved manually. We retrieved two data sets, the first on June 17th and the second on July 8th. Click here to view the results. Once we process these data we hope to answer a few questions:
Mag Date Time(UTC) Depth