IMPROVED GROUND TRUTH IN SOUTHERN ASIA USING IN-COUNTRY DATA, ANALYST WAVEFORM REVIEW, AND ADVANCED ALGORITHMS

A new catalog of seismicity at magnitudes above 2.5 for the period 1923-2008 in the Iran region is assembled from arrival times reported by global, regional, and local seismic networks. Using in-country data we have formed new events, mostly at lower magnitudes that were not previously included in standard global earthquake catalogs. The magnitude completeness of the catalog varies strongly through time, complete to about magnitude 4.2 prior to 1998 and reaching a minimum of about 3.6 during the period 1998-2005. Of the 25,722 events in the catalog, most of the larger events have been carefully reviewed for proper phase association, especially for depth phases and to eliminate outlier readings, and relocated. To better understand the quality of the data set of arrival times reported by Iranian networks that are central to this study, many waveforms for events in Iran have been re-picked by an experienced seismic analyst. Waveforms at regional distances in this region are often complex. For many events this makes arrival time picks difficult to make, especially for smaller magnitude events, resulting in reported times that can be substantially improved by an experienced analyst. Even when the signal/noise ratio is large, re-picking can lead to significant differences.more » Picks made by our analyst are compared with original picks made by the regional networks. In spite of the obvious outliers, the median (-0.06 s) and spread (0.51 s) are small, suggesting that reasonable confidence can be placed in the picks reported by regional networks in Iran. This new catalog has been used to assess focal depth distributions throughout Iran. A principal result of this study is that the geographic pattern of depth distributions revealed by the relatively small number of earthquakes (~167) with depths constrained by waveform modeling (+/- 4 km) are now in agreement with the much larger number of depths (~1229) determined using reanalysis of ISC arrival-times (+/-10 km), within their respective errors. This is a significant advance, as outliers and future events with apparently anomalous depths can be readily identified and, if necessary, further investigated. The patterns of reliable focal depth distributions have been interpreted in the context of Middle Eastern active tectonics. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, less than about 25-30 km in depth, with the crustal shortening produced by continental collision apparently accommodated entirely by thickening and distributed deformation rather than by subduction of crust into the mantle. However, intermediate-depth earthquakes associated with subducted slab do occur across the central Caspian Sea and beneath the Makran coast. A multiple-event relocation technique, specialized to use different kinds of near-source data, is used to calibrate the locations of 24 clusters containing 901 events drawn from the seismicity catalog. The absolute locations of these clusters are fixed either by comparing the pattern of relocated earthquakes with mapped fault geometry, by using one or more cluster events that have been accurately located independently by a local seismic network or aftershock deployment, by using InSAR data to determine the rupture zone of shallow earthquakes, or by some combination of these near-source data. This technique removes most of the systematic bias in single-event locations done with regional and teleseismic data, resulting in 624 calibrated events with location uncertainties of 5 km or better at the 90% confidence level (GT590). For 21 clusters (847 events) that are calibrated in both location and origin time we calculate empirical travel times, relative to a standard 1-D travel time model (ak135), and investigate event to station travel-time anomalies as functions of epicentral distance and azimuth. Substantial travel-time anomalies are seen in the Iran region which make accurate locations impossible unless observing stations are at very short distances (less than about 200 km) or travel-time models are improved to account for lateral heterogeneity in the region. Earthquake locations in the Iran region by international agencies, based on regional and teleseismic arrival time data, are systematically biased to the southwest and have a 90% location accuracy of 18-23 km, with the lower value achievable by applying limits on secondary azimuth gap. The data set of calibrated locations reported here provides an important constraint on travel-time models that would begin to account for the lateral heterogeneity in Earth structure in the Iran region, and permit seismic networks, especially the regional ones, to obtain in future more accurate locations of the earthquakes in the region. « less